Decline in serum follicle-stimulating hormone concentrations alters key intrafollicular growth factors involved in selection of the dominant follicle in heifers

The oocyte: the key player in the success of assisted reproduction technologies

The ovulation of a mature oocyte at metaphase II of meiosis, with optimal potential to undergo fertilisation by a sperm cell, complete meiosis and sustain the switch to mitotic division, and support early embryo development, involves a protracted and disrupted/delayed series of processes. Many of these are targeted for exploitation in vivo , or recapitulation in vitro , by the livestock industry. Reproductive technologies, including AI, multiple ovulation embryo transfer, ovum pick-up, in vitro embryo production, and oestrus and ovulation synchronisation, offer practitioners and producers the opportunity to produce offspring from genetically valuable dams in much greater numbers than they would normally have in their lifetime, while in vitro oocyte and follicle culture are important platforms for researchers to interrogate the physiological mechanisms driving fertility. The majority of these technologies target the ovarian follicle and the oocyte within; thus, the quality and capability of the recovered oocyte determine the success of the reproductive intervention. Molecular and microscopical technologies have grown exponentially, providing powerful platforms to interrogate the molecular mechanisms which are integral to or affected by ART. The development of the bovine oocyte from its differentiation in the ovary to ovulation is described in the light of its relevance to key aspects of individual interventions, while highlighting the historical timeline.

Effect of follicular ablation and gonadotropin priming on the recovery and quality of oocytes in Boran cows

The objective of the study was to evaluate the effect of follicle ablation, exogenous FSH application, and different coasting time prior to ovum pick-up (OPU) on the number of follicles suitable for aspiration, oocyte quality, and cleavage rate in Ethiopian Boran cows. The experiment was carried out in three parts, (I) cows were synchronized using 500 µg PGF2α given 11 days apart. Cows were then subjected to a biweekly ovum pickup session before ovulation (n = 5) or starting day 7 after ovulation (n = 4) for 3 weeks. (II) Cows were synchronized, and all visible follicles were ablated on the first days of overt estrus, and cows were grouped into those that received a divided dose of 350 IU FSH (n = 5) or 175 IU FSH (n = 5) over 3 days. In both groups, OPU was carried out weekly starting 48 h after the last FSH for 6 weeks. (III) Protocol was similar to part II, but in group with 350 IU FSH (n = 5), coasting period was increased to 72 h. The covariates of follicles and oocyte were not affected (P > 0.05) by corpus luteum presence at OPU. The mean number of medium (7.36 ± 0.57) and large (8.28 ± 0.96) follicles were significantly higher (P < 0.05) in the group that received divided 350 IU FSH. Similarly, the mean number of grade-1 (4.19 ± 0.24) and grade-2 (4.32 ± .27) oocytes, maturation rate (70.41%), and cleavage rate (47.5%) were significantly higher (P < 0.05) in the group that received 350 IU FSH. COC quality was significantly (P < 0.05) influenced by coasting period. However, both maturation and cleavage rates were not affected by the coasting period. This study demonstrated that follicular ablation and treatment with FSH improves follicular population and oocyte recovery rate in Boran cows.

Defining the gonadotrophin requirement for the selection of a single dominant follicle in cattle

The aim was to define the pattern and physiological concentrations of FSH and LH required for the selection of a single dominant follicle in mono-ovulatory species. A series of five experiments was carried out using gonadotrophin-releasing hormone agonist-induced hypogonadal heifers. Animals were infused with different patterns of either FSH and/or LH followed by an ovulatory dose of human chorionic gonadotrophin. Follicular response was monitored by ultrasound scanning and blood samples were collected to measure concentrations of FSH, LH, oestradiol and progesterone. The main findings were: (1) physiological concentrations of FSH given as a continuous infusion and for an adequate duration, in the presence of basal LH, with or without LH pulses, are capable of inducing a superovulatory response, (2) initial exposure to FSH followed by LH pulses alone stimulate the development of multiple preovulatory follicles, confirming that ovarian follicles are capable of transferring dependence on gonadotrophins from FSH to LH, (3) while LH pulses appear not to have a major effect on the pattern of preovulatory follicle development, adequate LH pulsatile support is required for full oestradiol synthesis and (4) the duration of initial exposure to FSH and the ability to transfer the dependence from FSH to LH are critical for the selection of a single dominant follicle. In conclusion, this experimental series confirms that the duration of initial exposure to FSH and the ability of the selected follicle to transfer its gonadotrophic dependence from FSH to LH are critical for the selection of a single dominant follicle in cattle.

Cellular and Molecular Adaptation of Bovine Granulosa Cells and Oocytes under Heat Stress

Simple Summary

Heat stress can have large effects on most aspects of reproductive function in dairy cows. A hot environment can increase blood, rectal, and uterine temperatures, alter ovarian folliculogenesis, suppress fertility, oogenesis, and embryogenesis and ultimately reduce conception and pregnancy rates. Among the components of the female reproductive tract, the ovarian pool of follicles and their enclosed granulosa cells and oocytes are highly sensitive to hyperthermia. Many effects of elevated temperature on granulosa cells and developing oocytes involve increased production of reactive oxygen species, subsequently induce cellular apoptosis, and decrease the developmental ability of oocytes to be fertilized. Furthermore, heat stress-associated reproductive disorders are associated with altered progesterone and reduced estradiol production by ovarian follicles. The review mainly focuses on the follicle-enclosed granulosa cells and oocytes, provides new insights into the cellular and molecular adaptations of granulosa cells and oocyte under heat stress, depicts the role of the follicle microenvironment, and discusses some mechanisms that might underlie oocyte impairment. This study provides a possible way for the genetic adaptation to heat stress both for the regulation of body temperature and cellular resistance to elevated temperature.

Abstract

Heat stress has long been recognized as a challenging issue that severely influences the reproductive functions of dairy cattle, disrupting oocyte development during fetal growth. These detrimental effects of heat stress are the result of either the hyperthermia associated with heat stress or the physiological adjustments made by the heat-stressed animal to regulate body temperature. In addition, elevated temperatures have been implicated in increasing the production of reactive oxygen species. Thus, understanding the impact of heat stress on reproductive functions, from a cellular to molecular level, might help in selecting heat-resilient dairy cattle and developing heat stress mitigation strategies. In the present paper, we have attempted to describe the changes in the reproductive system and function of dairy cattle in response to heat stress by reviewing the latest literature in this area. The review provides useful knowledge on the cellular and genetic basis of oocyte and granulosa cells in heat-stressed dairy cattle, which could be helpful for future research in this area.

Trio a novel bovine high-fecundity allele: II. Hormonal profile and follicular dynamics underlying the high ovulation rate

The newly discovered Trio high-fecundity allele produces multiple ovulations in cattle. This study evaluated (1) size and growth rates of follicles in Trio carriers during a synchronized follicular wave, induced by follicle aspiration; (2) follicle-stimulating hormone (FSH) patterns associated with the follicular wave; (3) size of corpora lutea (CL) and circulating progesterone; and (4) intrafollicular estradiol concentrations prior to normal deviation. Trio carriers had mean dominant follicles that were significantly smaller in diameter and volume than noncarriers. Onset of diameter deviation occurred at ∼3 days after the last follicle aspiration in both genotypes despite Trio carriers having much smaller individual follicles. Follicles of Trio carriers grew at a slower rate than noncarrier follicles (∼65% in mm/day or ∼30% in mm3/day) resulting in much smaller individual dominant follicles (∼25% volume). However, total dominant follicle volume, calculated as the sum of all dominant follicles in each animal, was similar in carriers and noncarriers of Trio throughout the entire follicular wave. Circulating FSH was greater in Trio carriers during the 24 h encompassing deviation. Trio carriers had significantly more ovulations than noncarriers, and individual CL volume was smaller, although total luteal tissue volume and circulating P4 were not different. Thus, increased ovulation rate in Trio carriers relates to smaller individual follicles (one-third the volume) near the time of deviation due to slower follicle growth rate, although time of deviation is similar, with increased circulating FSH near deviation leading to selection of multiple dominant follicles in Trio carriers with similar total follicle volume.

Resumption of reproductive activity in the early postpartum period of cows

Following parturition, there is an early resumption of sequential but transient FSH increases of 2-3 days duration in dairy and beef cows. The first increase results in the emergence of the first postpartum follicle wave and the decline in FSH results in selection of a dominant follicle (DF). The ovulatory fate of this DF is dependent on LH pulse frequency and IGF-I concentrations. The energy status of the cows affects the degree of anoestrus. High yielding cows in prolonged negative energy balance (NEB) have a greater incidence of anoestrous, where the DF is smaller and fails to produce sufficient oestradiol to induce ovulation. Thus, there are sequential follicle waves in anoestrus prior to first ovulation. A small percent of DFs in dairy cows fail to ovulate and they continue to grow due to high LH pulse frequency and form follicular cysts. These cysts produce oestradiol for a variable period and then become physiologically defunct, despite their morphological presence for variable periods before final regression. During their physiological active state, new follicle wave emergence and ovulation are suppressed. A further problem in high yielding cows is the increased incidence of abnormal ovarian cycles after ovulation, and specifically, the high incidence of maintained corpora lutea (CL), probably related to uterine problems in the peri-parturient period. The ovulation of smaller DFs and the high liver metabolic rate may adversely affect oestradiol and progesterone concentrations. The long-term effects of prolonged NEB on oocyte competence, DF physiology and follicular and luteal steroidogenesis all may have detrimental effects on conception rates to AI. Hormonal methods to induce ovulation should be carried out in conjunction with corrective management problems, once diagnosed The pre-treatment of anoestrous cows with progesterone for 5-9 days is a prerequisite for the concomitant expression of oestrus at first ovulation, but whether or not further hormonal therapy is necessary to ensure the ovulation of the DF, is dependent on energy status, body condition score and postpartum interval, which regulate LH pulse frequency.

Intrafollicular oestradiol production, expression of the LH receptor (LHR) gene and its isoforms, and early follicular deviation in Bos indicus

The aim of the present study was to characterise the roles of intrafollicular oestradiol production and granulosa cell (GC) expression of the LH receptor (LHR) gene and its isoforms during follicular deviation in Bos indicus. Follicular wave emergence was synchronised in heifers from a Bos taurus dairy (Holstein; n = 10) and a B. indicus dairy breed (Gir; n = 10). Follicles were aspirated individually at sizes corresponding to the periods of predeviation, deviation and postdeviation. Intrafollicular oestradiol (IF-E2) and progesterone (IF-P4) concentrations were determined in the follicular fluid (FF) by radioimmunoassay, and relative expression of P450 aromatase (CYP19A1) and LHR forms was evaluated in GC using real-time quantitative–polymerase chain reaction. Despite differences in the size of the dominant follicle at deviation, changes in CYP19A1 expression and IF-E2 concentrations were similar in follicles of the same diameter in both breeds. A peak in total LHR expression occurred after follicular deviation in association with low expression of LHR isoforms. The results suggest that regulation of LHR function by sequential changes in the expression pattern of LHR isoforms may play a role in the early deviation of the dominant follicle, as observed in B. indicus breeds.

Follicular fluid biomarkers for human in vitro fertilization outcome: Proof of principle

Background

Human follicular fluid (FF) is a unique biological fluid in which the oocyte develops in vivo, and presents an optimal source for non-invasive biochemical predictors. Oocyte quality directly influences the embryo development and hence, may be used as a predictor of embryo quality. Peptide profiling of FF and its potential use as a biomarker for oocyte quality has never been reported.

Methods

This study screened FF for peptide biomarkers that predict the outcome of in vitro fertilization (IVF). Potential biomarkers were discovered by investigating 2 training datasets, consisting both of 17 samples and validating on an independent experiment containing 32 samples. Peptide profiles were acquired by nano-scale liquid chromatography coupled to tandem mass spectrometry (nano LC-MS/MS).

Results

From the training datasets 53 peptides were found as potential biomarker candidates, predicting the fertilization outcome of 24 out of the 32 validation samples blindly (81.3% sensitivity, 68.8% specificity, AUC = 0.86). Seven potential biomarker peptides were identified. They were derived from: insulin-like growth factor binding protein-5, alpha-2-antiplasmin, complement component 3, inter-alpha-trypsin inhibitor heavy chain H1, serum albumin, protein diaphanous homolog 1 and plastin-3.

Conclusions

The MS-based comprehensive peptidomic approach carried out in this study, established a novel panel of potential biomarkers that present a promising predictive accuracy rate in fertilization outcome, and indicates FF as an interesting biomarker resource to improve IVF clinic routine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12953-016-0106-9) contains supplementary material, which is available to authorized users.

Lack of FSH support enhances LIF-STAT3 signaling in granulosa cells of atretic follicles in cattle

Subordinate follicles (SFs) of bovine follicular waves undergo atresia due to declining FSH concentrations; however, the signalling mechanisms have not been fully deciphered. We used an FSH-induced co-dominance model to determine the effect of FSH on signalling pathways in granulosa cells of the second-largest follicles (SF in control cows and co-dominant follicle (co-DF2) in FSH-treated cows). The SF was smaller than DF in control cows while diameters of co-DF1 and co-DF2 in FSH-treated cows were similar. The presence of cleaved CASP3 protein confirmed that granulosa cells of SFs, but not of DFs and co-DFs, were apoptotic. To determine the effect of FSH on molecular characteristics of the second-largest follicles, we generated relative variables for the second largest follicle in each cow. For this, variables of SF or co-DF2 were divided by the variables of the largest follicle DF or co-DF1 in each cow. There was higher transcript abundance of MAPK1/3 and AKT1/2/3 but lower abundance of phosphorylated MAPK3/1 in SF than co-DF2 granulosa cells. Abundance of mRNA and phosphorylated protein of STAT3 was higher in granulosa cells of control SF than FSH-treated co-DF2. SF granulosa cells had higher levels of LIFR and IL6ST transcripts, the two receptors involved in STAT3 activation. Further, lower transcript abundance of interleukin 6 receptor (IL6R), another receptor involved in STAT3 activation, indicated that STAT3 activation in SF granulosa cells could be mainly due to leukemia inhibitory factor (LIF) signalling. These results indicate that atresia due to lack of FSH is associated with activated LIF-STAT3 signalling in SF granulosa cells, as FSH treatment reversed such activation.

Endocrinology of number of follicular waves per estrous cycle and contralateral or ipsilateral relationship between corpus luteum and preovulatory follicle in heifers

A 3-d extension of the luteal phase occurs in interovulatory intervals (IOIs) with a contralateral relationship between the corpus luteum (CL) and preovulatory follicle with 3 follicular waves (Contra-3W group). Concentrations of FSH, progesterone, LH, and estradiol-17β for the ipsilateral versus contralateral CL and/or follicle relationship and 2 versus 3 waves per IOI were studied in 14 heifers. Follicular waves and FSH surges were designated 1, 2, or 3, according to order of occurrence in the IOI. The day (day 0 = ovulation) of the FSH peak in surge 2 occurred earlier (P < 0.02) in 3-wave IOIs (day 6.3 ± 0.5) than in 2-wave IOIs (day 8.5 ± 0.5). Mean FSH was higher in 3-wave than in 2-wave IOI on 82% of the days in the IOI. Repeatability or individuality in FSH concentration was indicated by a correlation (r = 0.54, P < 0.04) in FSH concentrations between ovulations at the beginning and at the end of the IOI. Concentrations of LH and estradiol increased (P < 0.05) near the beginning of the luteolytic period in 2-wave IOI regardless of the CL and/or follicle relationship. In the Contra-3W group, LH and estradiol remained at basal concentrations concurrently with FSH surge 3 and extension of the luteal phase. The hypotheses were supported that FSH surge 2 occurs earlier in 3-wave IOIs than in 2-wave IOIs and that the development of 3-wave IOIs occurs in individuals with greater FSH concentrations. Extension of the luteal phase in the Contra-3W group was temporally associated with lower concentrations of LH and estradiol.

Computational modelling of bovine ovarian follicle development

Background

The development of ovarian follicles hinges on the timely exposure to the appropriate combination of hormones. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) are both produced in the pituitary gland and are transported via the blood circulation to the thecal layer surrounding the follicle. From there both hormones are transported into the follicle by diffusion. FSH-receptors are expressed mainly in the granulosa while LH-receptors are expressed in a gradient with highest expression in the theca. How this spatial organization is achieved is not known. Equally it is not understood whether LH and FSH trigger distinct signalling programs or whether the distinct spatial localization of their G-protein coupled receptors is sufficient to convey their distinct biological function.

Results

We have developed a data-based computational model of the spatio-temporal signalling processes within the follicle and (i) predict that FSH and LH form a gradient inside the follicle, (ii) show that the spatial distribution of FSH- and LH-receptors can arise from the well known regulatory interactions, and (iii) find that the differential activity of FSH and LH may well result from the distinct spatial localisation of their receptors, even when both receptors respond with the same intracellular signalling cascade to their ligand.

Conclusion

The model integrates the large amount of published data into a consistent framework that can now be used to better understand how observed defects translate into failed follicle maturation.

Follicular characteristics and luteal development after follicle-stimulating hormone induced multiple ovulations in heifers

A protocol based on small doses of FSH was examined for the induction of double or triple (multiple) ovulations in cattle. Ovulation rate, follicular characteristics, and luteal responses were determined. In Exp. 1, three groups of estrous-synchronized, cyclic Holstein heifers were treated once daily, on d 3 to 6 of the cycle, with a FSH product (Folltropin-V): large FSH dose (total of 150 mg; n=18), medium FSH dose (total of 130 mg, n=12), and small FSH dose (total of 80 mg; n=7). Controls received saline (n=6). Prostaglandin F(2α) was injected on d 6, ultrasound-guided aspiration of surplus follicles (if needed) was performed on d 7, and GnRH was injected on d 8 to induce ovulation. The large FSH dose induced growth of more (2.6±0.3, P<0.05) large follicles than controls on d 8; medium and small FSH doses insufficiently stimulated growth of <2 large follicles. Ovulation rates were determined in subgroups of heifers (n=10, 13, 4, and 6, respectively). The large FSH dose induced greater rates (P<0.01) of mostly double and triple ovulations (90% multiple ovulations, 70% double ovulations), most of which (89%) were bilateral, with only 2 out of 10 heifers requiring aspiration of surplus follicles. Medium and small FSH doses induced fewer multiple ovulations (38% and 25%, respectively). Estradiol concentrations on d 8 did not differ among treatments, but the concentration per large follicle in controls was greater (P<0.05) than in FSH treatments. Mean corpus luteum (CL) volume in single-ovulation controls was greater (P<0.05) than that of multiple ovulations in the large FSH group and total CL volume and progesterone concentrations were numerically greater in multiple ovulations. In Exp. 2, the characteristics of follicles aspirated on d 7 from large FSH (n=11) and control heifers (n=10) were compared. Based on estradiol-to-progesterone ratio, 57% of the large FSH-treated follicles were classified as codominant/healthy follicles and 43% as subordinate/early atretic. Although concentrations of estradiol and androstenedione in FSH-treated codominant follicles were less (P<0.05) than in controls, estradiol-to-progesterone ratio indicated that those follicles were steroidogenically active. Finely tuned small doses of FSH administered during the first follicular wave can induce a large incidence of double/triple, mainly bilateral, ovulations in cattle, which may serve as a basis for treatment aimed at promoting twinning in beef cattle.

Characterization of serum-free buffalo granulosa cell culture and analysis of genes involved in terminal differentiation from FSH- to LH-responsive phenotype

In the present study, buffalo granulosa cells were cultured under serum-free conditions and characterized to study the changes in gene expression associated with the transition of granulosa cells from estrogen- to progesterone-secreting phenotype. The cells were cultured in vitro under completely serum-free conditions for 8 d. Gene expression and hormone analysis showed that on day 4 granulosa cells exhibit FSH responsiveness with preovulatory phenotype having highest CYP19 gene expression and 17β-estradiol production, whereas a significant increase in transcript abundance of STAR, CYP11, and HSD3B genes accompanied with an increase in progesterone production was observed on day 8. Cells treated with LH on day 4 followed by gene expression analysis at 1, 2, 4, 6, 18, and 24 h showed significant increase in transcripts of LH-responsive genes. In conclusion, culture condition used in the present study showed that granulosa cells were FSH responsive and attained attributes of granulosa cells of dominant follicles at day 4 with highest CYP19 and LHR gene expression beyond which they acquired the ability to luteinize and thus were more LH responsive. In addition, after LH treatment, analysis of early LH-responsive genes (EGR2, RUNX1, and NR4A1) on day 4 showed that granulosa cells at this stage in culture exhibits phenotype similar to that of preovulatory follicles before LH surge in vivo and corresponds to the in vivo transition of well-orchestrated gene expression profile after LH surge. The characterized culture conditions represent a suitable in vitro model for analysis of genes involved in terminal differentiation of granulosa cells from FSH- to LH-responsive phenotype during folliculogenesis in buffalo.

Gene expression profiling of preovulatory follicle in the buffalo cow: effects of increased IGF-I concentration on periovulatory events

The preovulatory follicle in response to gonadotropin surge undergoes dramatic biochemical, and morphological changes orchestrated by expression changes in hundreds of genes. Employing well characterized bovine preovulatory follicle model, granulosa cells (GCs) and follicle wall were collected from the preovulatory follicle before, 1, 10 and 22 h post peak LH surge. Microarray analysis performed on GCs revealed that 450 and 111 genes were differentially expressed at 1 and 22 h post peak LH surge, respectively. For validation, qPCR and immunocytochemistry analyses were carried out for some of the differentially expressed genes. Expression analysis of many of these genes showed distinct expression patterns in GCs and the follicle wall. To study molecular functions and genetic networks, microarray data was analyzed using Ingenuity Pathway Analysis which revealed majority of the differentially expressed genes to cluster within processes like steroidogenesis, cell survival and cell differentiation. In the ovarian follicle, IGF-I is established to be an important regulator of the above mentioned molecular functions. Thus, further experiments were conducted to verify the effects of increased intrafollicular IGF-I levels on the expression of genes associated with the above mentioned processes. For this purpose, buffalo cows were administered with exogenous bGH to transiently increase circulating and intrafollicular concentrations of IGF-I. The results indicated that increased intrafollicular concentrations of IGF-I caused changes in expression of genes associated with steroidogenesis (StAR, SRF) and apoptosis (BCL-2, FKHR, PAWR). These results taken together suggest that onset of gonadotropin surge triggers activation of various biological pathways and that the effects of growth factors and peptides on gonadotropin actions could be examined during preovulatory follicle development.

IGF1 induces up-regulation of steroidogenic and apoptotic regulatory genes via activation of phosphatidylinositol-dependent kinase/AKT in bovine granulosa cells

IGF1, a potent stimulator of cellular proliferation, differentiation and development, regulates granulosa cell steroidogenesis and apoptosis during follicular development. Depending upon species and stage of follicular growth, IGF1 acts on granulosa cell steroidogenesis either alone or together with FSH. We examined the mechanism of action of IGF1 in bovine granulosa cells in serum-free culture without insulin to determine its potential role in the regulation of steroidogenic and apoptotic regulatory gene expression and to investigate the interaction of FSH with IGF1 on this mechanism. Bovine granulosa cells treated with IGF1 demonstrated a significant increase in 17beta-oestradiol (OE(2)) production, cell number and in mRNA expression of CYP11A1, HSD3B1, CYP19A1, BAX, type 1 IGF receptor (IGF1R) and FSHR, while FSH alone had no significant effects. IGF1 or FSH alone or both together had no effect on BCL2 expression. IGF1 with FSH resulted in a synergistic increase in granulosa cell number and in mRNA expression of CYP19A1 and IGF1R without altering OE(2) production. IGF1 stimulated the phosphoinositide 3'-OH kinase (PI3K) but not the MAPK pathway in granulosa cells, as evidenced by increased phosphorylation of AKT but not extracellular-regulated kinase 1/2. Addition of the PI3K pathway inhibitor LY294002 (but not the MAPK pathway inhibitor PD98059) abrogated the increased expression of genes induced by IGF1. IGF1 therefore up-regulates the steroidogenic and apoptotic regulatory genes via activation of PI3K/AKT in bovine granulosa cells. The synergistic action of IGF1 with FSH is of likely key importance for the development of small antral follicles before selection; subsequently, other factors such as LH may also become necessary for continued cell survival.

Modulating peripheral gonadotrophin levels affects follicular expression of mRNAs encoding insulin-like growth factor binding proteins in sheep

Evidence suggests that the insulin-like growth factor binding proteins (IGFBPs) are involved in modulating the role that IGF-I and -II play in regulating follicular growth and development in sheep. However, little information exists as to the role that key peripheral factors play in regulating the expression of IGFBP components within the follicle. The present study investigated the regulatory effects of FSH and LH on gene expression for IGFBP-2 to -6 in ovine follicles, using bovine follicular fluid (bFF) and gonadotrophin-releasing hormone antagonist (GnRHa) model systems to perturb endogenous gonadotrophin secretion. Gene expression studies were carried out using in situ hybridisation with sheep-specific ribonucleotide probes. Treatment of ewes with bFF had few consistent effects on IGFBP-3 and -4 mRNA expression in follicles, whereas bFF treatment resulted in significant decreases in IGFBP-2 and IGFBP-6 mRNA expression in only medium follicles 60 h after onset of bFF, and in follicles 12h after the onset of bFF, respectively. Treatment of ewes with GnRHa resulted in transient significantly increased IGFBP-2 mRNA in healthy follicles in comparison to early atretic or atretic follicles 12h post-GnRHa (P<0.05), in IGFBP-3 mRNA in early atretic and atretic follicles 36 h post-GnRHa (P<0.001), and IGFBP-5 mRNA in follicles 12h post-GnRHa (P<0.001). In contrast, GnRHa treatment resulted in significant decreases in IGFBP-4 (P<0.001) and IGFBP-6 (P<0.01) mRNA expression in large follicles 36 h post-GnRHa, and atretic follicles by 60 h post-GnRHa, respectively. These data highlight that FSH and LH are involved, at least in part, in mediating the proliferative and differentiative changes in intrafollicular IGFBP levels that are observed during follicular growth and atresia in the sheep.

Variation in the ovarian reserve is linked to alterations in intrafollicular estradiol production and ovarian biomarkers of follicular differentiation and oocyte quality in cattle

The mechanisms whereby the high variation in numbers of morphologically healthy oocytes and follicles in ovaries (ovarian reserve) may have an impact onovarian function, oocyte quality, and fertility are poorly understood. The objective was to determine whether previously validated biomarkers for follicular differentiation and function, as well as oocyte quality differed between cattle with low versus a high antral follicle count (AFC). Ovaries were removed (n = 5 per group) near the beginning of the nonovulatory follicular wave, before follicles could be identified via ultrasonography as being dominant, from heifers with high versus a low AFC. The F1, F2, and F3 follicles were dissected and diameters determined. Follicular fluid and thecal, granulosal, and cumulus cells and the oocyte were isolated and subjected to biomarker analyses. Although the size and numerous biomarkers of differentiation, such as mRNAs for the gonadotropin receptors, were similar, intrafollicular concentrations of estradiol and the abundance of mRNAs for CYP19A1 in granulosal cells and ESR1, ESR2, and CTSB in cumulus cells were greater, whereas mRNAs for AMH in granulosal cells and TBC1D1 in thecal cells were lower for animals with low versus a high AFC during follicle waves. Hence, variation in the ovarian reserve may have an impact on follicular function and oocyte quality via alterations in intrafollicular estradiol production and expression of key genes involved in follicle-stimulating hormone action (AMH) and estradiol (CYP19A1) production by granulosal cells, function and survival of thecal cells (TBC1D1), responsiveness of cumulus cells to estradiol (ESR1, ESR2), and cumulus cell determinants of oocyte quality (CTSB).

Cocaine- and amphetamine-regulated transcript accelerates termination of follicle-stimulating hormone-induced extracellularly regulated kinase 1/2 and Akt activation by regulating the expression and degradation of specific mitogen-activated protein kinase phosphatases in bovine granulosa cells

Pleiotropic actions of cocaine- and amphetamine-regulated transcript (CART) are well described in the central nervous system and periphery, but the intracellular mechanisms mediating biological actions of CART are poorly understood. Although CART is not expressed in mouse ovaries, we have previously established CART as a novel intracellular regulator of estradiol production in bovine granulosa cells. We demonstrated that inhibitory actions of CART on estradiol production are mediated through inhibition of FSH-induced cAMP accumulation, Ca(2+) influx, and aromatase mRNA expression via a G(o/i)-dependent pathway. We also reported that FSH-induced estradiol production is dependent on Erk1/2 and Akt signaling, and CART may regulate other signaling proteins downstream of cAMP essential for estradiol production. Here, we demonstrate that CART is a potent inhibitor of FSH-stimulated Erk1/2 and Akt signaling and the mechanisms involved. Transient CART stimulation of bovine granulosa cells shortens the duration of FSH-induced Erk1/2 and Akt signaling whereas a prolonged (24 h) CART treatment blocks Erk1/2 and Akt activation in response to FSH. This CART-induced accelerated termination of Erk1/2 and Akt signaling is mediated both by induced expression and impaired ubiquitin-mediated proteasome degradation of dual specific phosphatase 5 (DUSP5) and protein phosphatase 2A. Results also support existence of a negative feedback loop in which CART via a G(o/i)-MAPK kinase dependent pathway activates Erk1/2, and the latter induces DUSP5 expression. Moreover, small interfering RNA mediated ablation of DUSP5 and/or protein phosphatase 2A prevents the CART-induced early termination of Erk1/2 and Akt signaling. Results provide novel insight into the intracellular mechanism of action of CART in regulation of FSH-induced MAPK signaling.

Differentiation of the bovine dominant follicle from the cohort upregulates mRNA expression for new tissue development genes

This study was designed to identify genes that regulate the transition from FSH- to LH-dependent development in the bovine dominant follicle (DF). Serial analysis of gene expression (SAGE) was used to compare the transcriptome of granulosa cells isolated from the most oestrogenic growing cohort follicle (COH), the newly selected DF and its largest subordinate follicle (SF) which is destined for atresia. Follicle diameter, follicular fluid oestradiol (E) and E:progesterone ratio confirmed follicle identity. Results show that there are 93 transcript species differentially expressed in DF granulosa cells, but only 8 of these encode proteins known to be involved in DF development. Most characterised transcripts upregulated in the DF are from tissue development genes that regulate cell differentiation, proliferation, apoptosis, signalling and tissue remodelling. Semiquantitative real-time PCR analysis confirmed seven genes with upregulated (P≤0.05) mRNA expression in DF compared with both COH and SF granulosa cells. Thus, the new genes identified by SAGE and real-time PCR, which show enhanced mRNA expression in the DF, may regulate proliferation (cyclin D2;CCND2), prevention of apoptosis or DNA damage (growth arrest and DNA damage-inducible, β;GADD45B), RNA synthesis (splicing factor, arginine/serine rich 9;SFRS9) and unknown processes associated with enhanced steroidogenesis (ovary-specific acidic protein; DQ004742) in granulosa cells of DF at the onset of LH-dependent development. Further studies are required to show whether the expression of identified genes is dysregulated when abnormalities occur during DF selection or subsequent development.

Progesterone induces the expression of vascular endothelial growth factor (VEGF) 120 and Flk-1, its receptor, in bovine granulosa cells

Vascular endothelial growth factor (VEGF) isoforms (VEGF 120 and VEGF 164) secreted by granulosa cells are involved in thecal angiogenesis during follicular development in the bovine ovary. The follicular fluid in the developing follicle includes a slight amount of the progesterone. However, the progesterone (P4) effects on VEGF120 and VEGF164 isoforms have not been well characterized in the bovine granulosa cells. We investigated the effects of progesterone on the gene expression of hypoxia-inducible factor 1alpha (HIF-1alpha, transcription factor), VEGF120, VEGF164 and Flk-1, its receptors, in cultured bovine granulosa cells. Messenger RNA expression for HIF-1alpha, VEGF120, VEGF164 and Flk-1 was quantified using real-time PCR methods. The levels of VEGF120, HIF-1alpha and Flk-1 mRNAs were increased significantly by P4 at a concentration of 10 ng/ml. In contrast, the expression of VEGF 164 gene is inhibited by P4. The level of VEGF120 and Flk-1 mRNAs in the granulosa cells treated with 10 ng/ml progesterone plus 1 ng/ml estradiol significantly decreased compared with progesterone alone. In contrast, the addition of 1 ng/ml estradiol to the culture medium increased the expression of VEGF164 gene. In conclusion, our data demonstrated that progesterone might stimulate the expression of the VEGF120 via HIF-1alpha, transcription factor, in bovine granulosa cells. These results suggest the hormone-dependent expression pattern of VEGF isoforms during follicular development. Thus, our study suggested the expression of VEGF isoforms in granulosa cells might be controlled by a different pathway during follicle development in cow.

Cocaine- and amphetamine-regulated transcript regulation of follicle-stimulating hormone signal transduction in bovine granulosa cells

Regulation of estradiol production, central to ovarian follicular development and reproductive function, is mediated by a complex interaction of pituitary gonadotropins such as FSH with locally produced regulatory molecules. We previously demonstrated a negative association of expression of cocaine-and amphetamine-regulated transcript (CART) with follicle health status and a novel local negative role for CART in regulation of basal estradiol production by bovine granulosa cells. However, effects of CART on FSH-induced estradiol production and the underlying mechanism(s) mediating the physiological actions of CART on granulosa cells are not known. Objectives of the present study were to determine effects of CART on basal and FSH-induced intracellular cAMP levels, aromatase mRNA, estradiol accumulation, calcium signaling, and the intracellular signaling pathways involved using primary cultures of bovine granulosa cells. CART treatment potently inhibits the FSH-induced rise in granulosa cell cAMP levels, estradiol accumulation, and aromatase mRNA. Furthermore, results show that calcium is essential for FSH-induced cAMP and estradiol accumulation, and CART significantly inhibits FSH-induced calcium influx. Select G protein and protein kinase inhibitors were used to elucidate pathways involved in CART actions. The inhibitory actions of CART on FSH signaling and estradiol production are mediated via a G(o/i)-dependent pathway, whereas none of the other signaling inhibitors had any effect on CART actions. Results demonstrate novel potent inhibitory effects of CART on multiple components of the FSH signaling pathway linked to estradiol production and follicular development and shed new insight into the mechanism of action of CART potentially pertinent within and beyond the reproductive system.

Follicle selection in cattle and horses: role of intrafollicular factors

The eminent event in follicle selection during a follicular wave in monovular species is diameter deviation, wherein one follicle continues to grow (developing dominant) and other follicles (subordinates) begin to regress. In cattle, the IGF system, oestradiol and LH receptors are involved in the intrafollicular events initiating deviation as indicated by the following: (1) concentrations of free IGF-I and oestradiol in the follicular fluid and number of LH receptors in the follicular wall increase more dramatically in the future dominant follicle than in the future subordinate follicles before the beginning of deviation and (2) ablation of the largest follicle (LF) or injection of recombinant human IGF (rhIGF)-I into the second LF at the expected beginning of deviation increases the concentrations of oestradiol in second LF before the expected beginning of deviation between second LF and third LF. In horses, an increase in free IGF-I, oestradiol, inhibin-A and activin-A is greater in the future dominant follicle than in other follicles before the beginning of deviation. However, free IGF-I is the only one of these four factors needed for the initiation of deviation in horses as indicated by the following: (1) ablation of LF at the expected beginning of deviation increases the concentrations of free IGF-I in second LF before the beginning of deviation between second LF and third LF but does not increase the other factors; (2) injection of rhIGF-I into second LF at the expected beginning of deviation causes second LF to continue to grow and become a codominant follicle and (3) injection of IGF-binding protein-3 into LF at the expected beginning of deviation causes LF to regress and second LF to become dominant. Thus, the dramatic changes in the IGF system in LF compared to other follicles before the beginning of deviation play a crucial role in the events that lead to the beginning of diameter deviation in both cattle and horses. Oestradiol and LH receptors also play a role in cattle. These intrafollicular events prepare the selected follicle for the decreasing availability of FSH and increasing availability of LH. The other follicles of the wave have the same future capability but do not have adequate time to attain a similar preparatory stage.

Akt and Erk signal transduction pathways are early markers of differentiation in dominant and subordinate ovarian follicles in cattle

Dominant follicles are those that continue to develop and have the potential to ovulate while subordinate follicles regress. Characteristics of dominant follicles include a larger diameter, higher intrafollicular estradiol, and lower IGF-binding protein (IGFBP)-4 concentrations compared with other cohort follicles. Follicle development is regulated by endocrine hormones that act via intracellular signaling pathways. Here, we show the differences in Akt, Erk, c-Jun N-terminal protein kinase, and p-38 signaling pathways between dominant and subordinate follicles at the dominance stage of the follicle wave. However, earlier in the follicle wave (dominant follicle selection), there were only differences in the levels of Akt and Erk signal transduction proteins among dominant and subordinate follicles. Using this profile of Akt and Erk protein expression in granulosa and theca cells of selected dominant follicles compared with subordinate follicles, we suggest a predictive model to identify future dominant and subordinate follicles from the pool of otherwise similar cohort follicles at the time of follicle wave emergence. We conclude that the Erk and Akt signal transduction pathways are important for dominant follicle selection and development and, furthermore, that the observed differences in these pathways mark the future dominant follicle from subordinate follicles before differences in follicular diameter, follicular fluid estradiol, and IGFBP-4 concentrations are apparent.

Effects of cobalt/vitamin B12 status in ewes on ovum development and lamb viability at birth

Scottish Blackface ewes from cobalt-deficient farmland were fed a diet containing 0.06 mg cobalt per kg dry matter from approximately 30 days before embryo recovery/transfer until lambing. Ewes remained untreated (–Co; n = 82) or were given an intraruminal cobalt-containing bolus to compensate for the dietary deficit (+Co; n = 82). Ewes used as embryo donors (–Co, n = 17; +Co, n = 16) were artificially inseminated with semen from a single Suffolk sire. Day 6 embryos obtained from –Co and +Co donors were transferred in singleton to –Co and +Co recipients in a 2 × 2 factorial-designed experiment to determine the effects of cobalt/vitamin B12 status during the periconception period (factor 1) and pregnancy (factor 2) on lamb viability at birth. Mean (± s.e.m.) circulating concentrations of vitamin B12 in –Co and +Co donors at ovum recovery were 182 ± 10 and 1288 ± 64 pmol L–1, respectively (P < 0.001), and the number of corpora lutea per ewe ovulating was 9.9 ± 1.6 and 14.4 ± 1.3, respectively (P < 0.05). Treatment did not affect the proportion of recovered ova that contained >32 cells (viable) or the median stage of development (late morula), but viable ova recovered from –Co v. +Co ewes had a better morphological grade (2.0 ± 0.1 v. 2.20 ± 0.04, respectively; P < 0.01). There was no effect of treatment on the proportion of recipient ewes that became pregnant. Circulating concentrations of vitamin B12 were lower in –Co than +Co ewes during pregnancy (P < 0.001) and at birth in lambs born to –Co ewes compared with those born to +Co ewes (P < 0.001). There was no effect of donor or recipient cobalt/vitamin B12 status on lamb birthweight, neonatal vigour or neonatal rectal temperatures, but lambs derived from +Co v. –Co embryo donors were more active in the first 3 days after birth (P < 0.05). Results show that sub-clinical cobalt/vitamin B12 deficiency reduces ovulatory response in superovulated ewes and that periconception nutrition can affect neonatal lamb behaviour.

Molecular Evidence That Growth of Dominant Follicles Involves a Reduction in Follicle-Stimulating Hormone Dependence and an Increase in Luteinizing Hormone Dependence in Cattle1

The bovine dominant follicle (DF) model was used to identify molecular mechanisms potentially involved in initial growth of DF during the low FSH milieu of ovarian follicular waves. Follicular fluid and RNA from granulosa and theca cells were harvested from 10 individual DF obtained between 2 and 5.5 days after emergence of the first follicular wave of the estrous cycle. Follicular fluid was subjected to RIA to determine estradiol (E) and progesterone (P) concentrations and RNA to cDNA microarray analysis and (or) quantitative real-time PCR. Results showed that DF growth was associated with a decrease in intrafollicular E:P ratio and in mRNA for the FSH receptor, estrogen receptor 2 (ER beta), inhibin alpha, activin A receptor type I, and a proliferation (cyclin D2) and two proapoptotic factors (apoptosis regulatory protein Siva, Fas [TNFRSF6]-associated via death domain) in granulosa cells. In contrast, mRNAs for the LH receptor in granulosa cells and for two antiapoptotic factors (TGFB1-induced antiapoptotic factor 1, LAG1 longevity assurance homolog 4 [Saccharomyces cerevisiae]) and one proapoptotic factor (tumor necrosis factor [ligand] superfamily, member 8) were increased in theca cells. We conclude that the bovine DF provides a unique model to identify novel genes potentially involved in survival and apoptosis of follicular cells and, importantly, to determine the FSH-, estradiol-, and LH-target genes regulating its growth and function. Results provide new molecular evidence for the hypothesis that DF experience a reduction in FSH dependence but acquire increased LH dependence as they grow during the low FSH milieu of follicular waves.

Evidence of a local negative role for cocaine and amphetamine regulated transcript (CART), inhibins and low molecular weight insulin like growth factor binding proteins in regulation of granulosa cell estradiol production during follicular waves in cattle

The ability of ovarian follicles to produce large amounts of estradiol is a hallmark of follicle health status. Estradiol producing capacity is lost in ovarian follicles before morphological signs of atresia. A prominent wave like pattern of growth of antral follicles is characteristic of monotocous species such as cattle, horses and humans. While our knowledge of the role of pituitary gonadotropins in support of antral follicle growth and development is well established, the intrinsic factors that suppress estradiol production and may help promote atresia during follicular waves are not well understood. Numerous growth factors and cytokines have been reported to suppress granulosa cell estradiol production in vitro, but the association of expression of many such factors in vivo with follicle health status and their physiological significance are not clear. The purpose of this review is to discuss the in vivo and in vitro evidence supporting a local physiological role for cocaine and amphetamine regulated transcript, inhibins and low molecular weight insulin like growth factor binding proteins in negative regulation of granulosa cell estradiol production, with emphasis on evidence from the bovine model system.

Alterations in follicular IGFBP mRNA expression and follicular fluid IGFBP concentrations during the first follicle wave in beef heifers

The objective was to determine the pattern of IGFBP-2, -3 and -4 gene expression and follicular fluid concentrations of IGFBP-2, -3, -4 and -5 during emergence, selection and dominance of the first follicle wave of the estrous cycle in cattle and during exogenous steroid treatment. Heifers (n = 35) were ovariectomized at 36 (n = 7), 66 (n = 8), 84 (n = 12) and 108 (n = 8) h after the onset of estrus. Heifers in the 84 h ovariectomy group were sub-divided to receive either no treatment (n = 6) or were treated with a progesterone-releasing intravaginal device (n = 6, PRID) and 0.75 mg estradiol benzoate i.m. at the approximate time of ovulation, 30 h post estrus until ovariectomy. Within heifers the four largest follicles recovered following ovariectomy were ranked on size (F1, F2, F3 and F4). At 36 h IGFBP gene expression and follicular fluid IGFBP concentrations were similar in all follicles (F1-F4). Mean diameter of the F1 follicle increased (P < 0.05) between 36 and 84 h with no difference between 84 and 108 h. The F1 follicle had the highest (P < 0.05) concentration of estradiol compared with the F2, F3 and F4 at 84 and 108 h. There was no granulosa cell IGFBP-2 mRNA in F1 follicles at 84 or 108 h. Intrafolliclar IGFBP-2 concentrations were lower (P < 0.05) in the F1 compared with F3 and F4 follicles at 108 h. There was no difference in theca cell IGFBP-4 mRNA expression at 108h, but amounts of follicular fluid IGFBP-4 were lower (P < 0.05) in F1 follicles compared with F3 and F4 follicles at 108 h. IGFBP-3 mRNA was localized in the theca layer of all follicles examined with no difference in expression or follicular fluid concentrations during emergence, selection and dominance of the first follicle wave. IGFBP-5 concentrations were higher (P < 0.05) in follicular fluid of F3 follicles at 108 h compared with the F3 at 36 h. In conclusion follicular dominance was associated with low or decreased follicular fluid concentrations of IGFBP-4 and -5, increased estradiol and differential regulation of IGFBP production.

Identification of downregulated messenger RNAs in bovine granulosa cells of dominant follicles following stimulation with human chorionic gonadotropin

Molecular determinants and mechanisms involved in ovarian follicular growth, ovulation, and luteinization are not well understood. The objective of this study was to identify genes expressed in bovine granulosa cells (GC) of dominant follicles (DF) and downregulated after hCG-induced ovulation, using the suppression subtractive hybridization (SSH). GC were collected from DF at Day 5 of the estrous cycle and from ovulatory follicles (OF) obtained 23 h following injection of hCG. A subtracted cDNA library (DF-OF) was generated and screened using unsubtracted (DF, OF) and subtracted (DF-OF, OF-DF) cDNAs as complex (32)P-probes. A total of 32 nonredundant cDNAs were identified: 23 cDNAs matched with sequences of known biological function and 9 cDNAs with complete or partial sequences of undefined biological function. Detection of genes known to be downregulated during the periovulatory period in the bovine species, such as CPD, CYP11A1, CYP19A1, FSHR, LRP8/ ApoER2, and SERPINE2, validated the physiological model and analytical techniques used. For a subset of genes, such as ARFGAP3, CYP11A1, CYP19A1, FSHR, FST, GJA1, IDH3, INHBA, LHCGR, LHCGR lacking exon 10, PRC1, PRG1, RPA2, SCD, and TRIB2, gene expression profiles were compared by virtual Northern blot or reverse transcriptase-polymerase chain reaction from follicles obtained at different developmental stages. Results confirmed a downregulation of the respective mRNAs in GC of OF compared with that of DF. We conclude that we have identified novel genes that are downregulated by hCG in bovine GC of DF during the periovulatory period, which may contribute to follicular growth, ovulation, and/or luteinization.

Reproductive Hormones and Follicular Growth During Development of One or Multiple Dominant Follicles in Cattle1

The mechanisms regulating ovulation rate under natural conditions are not yet defined, particularly for monovular species. In the present study, we evaluated ovarian structures (every 12 h by ultrasonography) and circulating hormones (every 6 h) to determine the differences between cows that developed one (single dominant; n = 16), two (double dominant; n = 8), or three (triple dominant; n = 3) dominant follicles. The four largest follicles were tracked retrospectively, and the data were normalized to the time of expected follicular deviation (F1 >/= 8.5 mm; hour 0). Follicular dynamics from emergence to deviation were similar, whereas after deviation, expected subordinate follicles continued to grow at a rate similar to the dominant follicle. Triple dominants had greater FSH than double dominants (hour -24 to hour -12) and single dominants (hour -42 to hour -6), and double dominants had greater FSH than single dominants (hour -24 to hour -12). Increased circulating estradiol but lower inhibin were observed in cows that developed multiple follicles. In addition, double dominants had greater LH than single dominants (hour -42 to hour -24 and hour -6 to hour 0) and lower progesterone than single dominants (hour -12 and hour -6). Luteal volume was similar between groups, but milk production was greater for codominant than for single-dominant cows. Thus, selection of multiple dominant follicles during high milk production is related to a transient increase in circulating FSH and LH during the 24 h before follicular selection, producing continued postdeviation growth of follicles that ordinarily would have regressed. Increased FSH and LH probably result from decreased circulating inhibin and progesterone in cows that develop codominant follicles.

Numbers of antral follicles during follicular waves in cattle: evidence for high variation among animals, very high repeatability in individuals, and an inverse association with serum follicle-stimulating hormone concentrations

The extent, causes, and physiological significance of the variation in number of follicles growing during ovarian follicular waves in human beings and cattle are unknown. Therefore, the present study examined the variability and repeatability in numbers of follicles 3 mm or greater in diameter during the follicular waves in bovine estrous cycles, and we determined if the variation in number of follicles during waves was associated with alterations in secretion of FSH, estradiol, inhibin, and insulin-like growth factor I (IGF-I). Dairy cattle were subjected to twice-daily ultrasound analysis to count total number of antral follicles 3 mm or greater in diameter throughout 138 different follicular waves. In another study, blood samples were taken at frequent intervals from cows that consistently had low or very high numbers of follicles during waves and were subjected to immunoassays. Results indicate the following: First, despite an approximately sevenfold variation in number of follicles during waves among animals and marked differences in age, stage of lactation, and season of the year, a very highly repeatable (0.95) number of follicles 3 mm or greater in diameter is maintained during the ovulatory and nonovulatory follicular waves of individuals. Second, variation in number of follicles 3 mm or greater in diameter during waves and the inverse association of number of follicles during waves with FSH are not directly explained by alterations in the patterns of secretion of estradiol, inhibin, or IGF-I. Third, ovarian ultrasound analysis can be used reliably by investigators to identify cattle that consistently have low or high numbers of follicles during waves, thus providing a novel experimental model to determine the causes and physiological significance of the high variation in antral follicle number during follicular waves among single-ovulating species, such as cattle or humans.

Developmental Pattern of Small Antral Follicles in the Bovine Ovary1

The study was designed to characterize the developmental pattern of 1- to 3-mm follicles and to determine the stage at which the future dominant follicle first attains a size advantage among its cohorts. In experiment 1, heifers (n = 18) were examined every 24 h by transrectal ultrasonography for one interovulatory interval (IOI). In experiment 2, cows (n = 9) were examined every 6 h from 5 to 13 days after ovulation to monitor precisely the diameter changes of individual follicles >/=1 mm during emergence of wave 2. Results revealed a change over days (P < 0.05) in the number of 1- to 3-mm follicles, with a maximum (P < 0.05) 1 or 2 days before wave emergence (conventionally defined as the time when the dominant follicle is first detected at 4 mm), followed 3-4 days later by a maximum (P < 0.05) in the number of >/=4-mm follicles. The profiles of small (1-3 mm) and large (>/=4-mm) follicles were inversely proportional (r = -0.79; P = 0.01). The profile of the number of 1- to 3-mm follicles during wave emergence was similar (P = 0.63) between waves in two-wave IOI, but differed (P < 0.01) among waves in three-wave IOI as a result of a greater number of follicles in the ovulatory wave (P < 0.04). As well, the number of follicles in the ovulatory wave tended to be greater (P < 0.06) in three-wave IOI than in two-wave IOI. The future dominant follicle was first identified at a diameter of 1 mm and emerged 6-12 h earlier than the first subordinate follicle (P < 0.01). After detection of the dominant follicle at 1 mm (0 h), its diameter differed from that of the first and second subordinate follicles at 24 h (P = 0.04) and 12 h (P = 0.01), when the dominant follicle was 2.4 +/- 0.17 mm and 1.7 +/- 0.14 mm, respectively. The growth rate of the dominant follicle differed from that of the first and second subordinate follicles at 120 h (P = 0.03) and 108 h (P = 0.02), when the dominant follicle was 9.5 +/- 0.30 mm and 8.8 +/- 0.49 mm, respectively. Emergence of the future dominant (r = 0.71), first (r = 0.73), and second (r = 0.76) subordinate follicles was temporally associated (P < 0.01) with a rise in circulating concentrations of FSH. Transient, nocturnal elevations in plasma FSH concentration were followed within 6 h by an increase in the growth rate of 1- to 3-mm follicles. We conclude that 1) 1- to 3-mm follicles develop in a wave-like manner in association with surges in plasma concentrations of FSH, 2) 1- to 3-mm follicles are exquisitely responsive to transient elevations in FSH, and 3) selection of the dominant follicle is manifest earlier than previously documented and is characterized by a hierarchical progression over a period encompassing the entire FSH surge (5 days).

Short intervals between ultrasonographically guided follicle aspiration improve oocyte quality but do not prevent establishment of dominant follicles in the Gir breed (Bos indicus) of cattle

The aim of the present study was to evaluate oocyte recovery and embryo yield using two different ovarian follicular aspiration schedules in donor cows of the Gir breed. Pluriparous, non-lactating Gir cows (n = 14) were randomly allocated to one of two groups, one of which had aspirations of ovarian follicular contents conducted once a week (TVFA-1x) and the other twice weekly (TVFA-2x), for nine consecutive weeks. Before follicle aspiration, follicular population was recorded in three classes according to size (> 6 mm, 6-9 mm and > 9 mm). The cumulus-oocyte complexes (COCs) recovered were identified, morphologically classified and in vitro matured, fertilized with Gir sperm and cultured in CR2 medium for 7 days. There was no difference (P > 0.05) in the size of the largest follicle, number of follicles identified or follicular content aspirations between TVFA-1x and TVFA-2x groups. Large follicles (> 9 mm) were observed for all the aspiration intervals considered (3, 4 or 7 days). More oocytes were recovered per session in TVFA-1x as compared with TVFA-2x (8.9 +/- 0.8 versus 7.0 +/- 0.7, P < 0.01), resulting in a greater recovery rate in this group (74.3% versus 58.7%, P < 0.01). More COCs of Grade I were recovered from TVFA-2x (22.6% versus 13.3%, P < 0.01). There was no difference in cleavage rate between groups, but the percentage of embryos that reached the blastocyst stage was greater in TVFA-2x as compared with the TVFA-1x (31.8% versus 21.6%, P < 0.01). The greater in vitro performance qualities of TVFA-2x oocytes compensates for the greater oocyte recovery rate in TVFA-1x, demonstrating a greater embryo production potential. Despite showing uncommon follicular dynamics characteristics when subjected to follicular aspiration, Gir cows can be successfully used as oocyte donors for in vitro embryo production.

Follicular development: the role of the follicular microenvironment in selection of the dominant follicle

The importance of endocrine signals in the regulation of follicular development has long been recognized. However, the follicular microenvironment also plays a critical role in determining follicular fate. This review summarizes our studies on the role of the intrafollicular IGF system in selection of the dominant follicle (DF) in cattle. During the bovine estrous cycle, the largest antral follicles develop in two or three successive waves of follicular recruitment and selection of a DF. High concentrations of estradiol in the follicular fluid are the hallmark of dominant and preovulatory follicles and are associated with lower concentrations of low molecular weight (MW) insulin-like growth factor binding proteins (IGFBP-2, -4, and -5), which can prevent binding of IGF to its receptor. Our studies have shown that dominant and preovulatory follicles also have much higher levels of an IGFBP-4/-5 protease activity, which is the bovine equivalent of the human IGFBP-4 protease, pregnancy-associated plasma protein-A (PAPP-A). Studies of follicles isolated just after the emergence of the DF showed that PAPP-A is present in the follicular fluid of the DF as soon as it can be detected as morphologically dominant. To examine whether higher levels of PAPP-A in one follicle of the cohort (the future DF) precedes morphological dominance, the four largest follicles were isolated from pairs of bovine ovaries obtained before one follicle of the cohort was significantly larger the others, around the time that one follicle was first detected as morphologically dominant and after dominance was well established. Analysis of the temporal sequence of changes in estradiol, low MW IGFBPs, free IGF, and PAPP-A in the follicular fluid suggested that an increase in PAPP-A is the earliest biochemical difference yet detected in the future DF and that follicular selection is the result of a progressive series of changes beginning with the acquisition of PAPP-A, which leads to a decrease in IGFBP-4 and -5 and an increase in free IGF, which synergizes with FSH to increase estradiol production. Co-dominant follicles, induced by injection of small doses of recombinant bovine (rb) FSH, both had levels of PAPP-A similar to the single DF of control heifers, supporting the hypothesized role of FSH in the induction of PAPP-A in the DF. Taken together, these results suggest a critical role for FSH-induced PAPP-A, and thus for free IGF, in the selection of the DF. In contrast, other experiments provided evidence for a deleterious effect of IGF on the initiation of bovine follicular growth and the survival of primordial and primary follicles in vitro. These results underscore the importance of the follicular microenvironment in determining follicular fate and indicate that its effects can be stage-specific.

In vivo effects of an intrafollicular injection of insulin-like growth factor 1 on the mechanism of follicle deviation in heifers and mares

In cattle and mares, free insulin-like growth factor 1 (IGF-1) is higher in the future dominant follicle (F1) than in the future largest subordinate follicle (F2) before deviation in diameter or selection is manifested between the two follicles. The effect of IGF-1 on other follicular-fluid factors and on the destiny of F2 were studied in two experiments in each species, using a total of 40 heifers and 42 mares. An injection of IGF-1 was made into F2 at the expected beginning of deviation (heifers, F1 >or= 8.5 mm; mares, F1 >or= 20.0 mm; Hour 0). In heifers, follicular fluid was taken from F2 at Hours 3, 6, 12, or 24; each heifer was sampled only once. In mares, sequential F2 samples were taken from each mare at Hours 0, 6, and 24 or at Hours 12 and 24. Transvaginal ultrasound guidance was used for treatment and sample collection. In heifers, IGF-1 treatment of F2 stimulated the secretion of estradiol (P < 0.05) between Hours 3 and 6 and androstenedione (P < 0.05) between Hours 3 and 12. In F2 of control heifers, estradiol decreased (P < 0.05) and androstenedione did not change significantly. In mares, IGF-1 treatment of F2 did not affect the concentrations of estradiol during the 24-h posttreatment period; androstenedione decreased (P < 0.04) in the IGF-1 group and increased (P < 0.006) in the controls. Compared with control mares, the IGF-1 group had higher (P < 0.04) activin-A at Hours 12 and 24 and higher (P < 0.0006) inhibin-A at Hour 24. After ablating F1 at Hour 24 in mares, F2 became dominant and ovulated in more mares (P < 0.0002) in the IGF-1 group (12/14) than in the control group (2/14). These results are consistent with reported temporal relationships among follicular factors during deviation in both species and indicate that IGF-1 plays a key role in controlling the temporal relationships; however, no indication was found that IGF-1 stimulated estradiol production in mares during the 24 h after treatment.

Endocrine regulation of ovarian antral follicle development in cattle

Antral follicle growth in cattle occurs in two distinct phases; the first 'slow' growth phase spans the time from antrum acquisition to a size of approximately 3 mm detectable by transrectal ultrasound, and the second 'fast' phase is gondadotrophin-dependent and includes cohort growth, dominant follicle (DF) selection, and DF growth. This review summarises current concepts of the relative roles FSH and LH, ovarian and metabolic hormones play mainly in the second phase of antral follicle growth in animals of different reproductive and nutritional states. It is proposed that differential FSH response may enable one cohort follicle to become selected, and that follicular secretions, particularly inhibin, suppress FSH and thus are responsible for DF selection and dominance. Acute dependence of the DF on LH pulses will determine DF lifespan, and the LH pulse profile can be influenced by metabolic hormones such as leptin, providing one possible link for nutritional state and reproduction. Direct ovarian effects of acute and chronic changes in growth hormone, insulin and insulin-like growth factor (IGF)-I have been described on cohort follicles, DF oestrogen activity and on DF growth. Influences of metabolic hormones on early antral follicles undergoing their first 'slow' growth phase are less well described, yet metabolic hormones appear to enhance growth into the cohort available for FSH-induced emergence, and may influence subsequent developmental competence of oocytes.

Selection of the dominant follicle and insulin-like growth factor (IGF)-binding proteins: evidence that pregnancy-associated plasma protein A contributes to proteolysis of IGF-binding protein 5 in bovine follicular fluid

Development of a dominant follicle is associated with decreased intrafollicular low molecular weight IGF-binding proteins (namely IGFBP-2, -4, and -5) and increased proteolysis of IGFBP-4 by pregnancy-associated plasma protein A (PAPP-A). In addition to IGFBP-4 proteolytic activity, bovine follicular fluid contains strong proteolytic activity for IGFBP-5, but not for IGFBP-2. Here we show that the IGFBP-5 protease present in bovine follicular fluid is a neutral/basic pH-favoring, Zn(2+) metalloprotease very similar to the previously described IGFBP-4 protease. We hypothesized that immunoneutralization and immunoprecipitation with anti-PAPP-A antibodies would result in abrogation of the IGFBP-4, but not the IGFBP-5, proteolytic activity in follicular fluid. As expected, anti-PAPP-A antibodies were able to neutralize and precipitate the IGFBP-4, but not the IGFBP-5, proteolytic activity of human pregnancy serum, which was used as a positive control for PAPP-A. Surprisingly, immunoneutralization and immunoprecipitation of follicular fluid from bovine preovulatory follicles with anti-PAPP-A antibodies abrogated both IGFBP-4 and IGFBP-5 proteolysis. Quantitative results derived from phosphorimaging revealed a complete inhibition of both IGFBP-4 and -5 proteolysis by follicular fluid incubated for 2 or 5 h in the presence of anti-PAPP-A antibodies. After 18 h of incubation, anti-PAPP-A antibodies still inhibited IGFBP-5 degradation, although with an efficiency lower than that for IGFBP-4 degradation. Both proteolytic activities have identical electrophoretic mobility, and a single band ( approximately 400 kDa) was detected by Western immunoblotting of bovine follicular fluid with anti-PAPP-A antibodies. Proteolysis of IGFBP-5 was readily detectable in follicular fluid from dominant follicles and was negligible in subordinate follicles from the same cohort. These results suggest that an active intrafollicular IGFBP-4/-5 proteolytic system, in which PAPP-A is the major protease involved, is an important determinant of follicular fate.

Efeito da pré-estimulação ovariana sobre características de oócitos após punção folicular em bovinos

Avaliaram-se os efeitos da pré-estimulação ovariana com FSH e da imunização ativa contra inibina suína sobre o número e classificação morfológica de oócitos recuperados por punção folicular. Catorze vacas da raça Gir, pluríparas e não-lactantes, foram distribuídas em: TI - submetidas a pré-estimulação com 250 UI de FSH (n=4), TII - submetidas a imunização ativa contra inibina suína (n=4) e TIII - não tratadas (n=6; controle). Todos os animais tiveram seus folículos com diâmetro superior a três milímetros aspirados uma vez por semana, durante 10 semanas consecutivas. O TI apresentou aumento na população folicular total (P<0,05) e no número de folículos puncionados (P<0,001), porém a taxa de recuperação foi inferior à dos tratamentos II e III (68,8% vs. 78,4% e 76,3%, respectivamente; P<0,01). O TII não afetou as principais características ovarianas, exceto pelo aumento no número de folículos de tamanho médio (P<0,01). O TI produziu maior percentual de oócitos viáveis (77,2% vs. 69,8%; P<0,05) e de oócitos de Grau I (19,8% vs. 11,6%; P<0,001) do que o TIII. Os tratamentos I e II reduziram o percentual de oócitos degenerados (16,4% e 16,8% vs. 22,3%; P<0,05). A pré-estimulação ovariana com FSH demonstrou ser a alternativa mais consistente para aumentar a qualidade dos oócitos recuperados de vacas zebuínas.

Mouse ovarian follicles secrete factors affecting the growth and development of like-sized ovarian follicles in vitro

A series of experiments have been carried out to determine whether follicles secrete factors able to affect the growth and development of other, like-sized follicles. Late preantral mouse ovarian follicles were either cocultured or cultured in media conditioned by previously cultured follicles. In particular, the experiments examined whether follicles do secrete such factors, whether the level of FSH in the culture media can affect that process, and what the nature of such secretory factor(s) might be. First, pairs of follicles were cocultured across a polycarbonate membrane containing pores. This showed that communication between the follicles resulted in the stimulation of growth and that the stimulation was due, at least in part, to the production of secretory factor(s). In subsequent experiments, follicles were cultured in media that had been preconditioned by previously cultured follicles. The concentration of FSH in the cultures determined the effect of the conditioned media: conditioned media was stimulatory to follicle growth when levels of FSH remained high throughout the culture, but inhibitory when FSH levels were dropped midway through the cultures. Heat inactivation removed this inhibitory effect, showing that the factor was likely to be a protein; addition of follistatin to the conditioned media did not alter its effect, indicating that the factor was unlikely to be activin. We have shown through a series of culture experiments that mouse follicles secrete factor(s) that can affect the development of other like-sized follicles when cultured from the late preantral to Graafian stages. Furthermore, we have shown that the effect (or production) of such factors is dependent on the FSH environment of the follicles.

Follicle wave growth in cattle

Ovarian follicle growth in cattle culminates in the selection of a single dominant follicle which attains the ability for final maturation and ovulation once or twice during the luteal phase and at the end of the oestrous cycle, as well as during other reproductive states. This review will describe in detail the first follicle wave of the cycle leading to selection of the first wave dominant follicle, indicating the specific gonadotrophin dependencies of cohort and dominant follicles, and relating follicle fate to steroidogenesis. As a differential gonadotrophin response of growing antral follicles during the follies-stimulating hormone (FSH) decline may determine which follicle becomes selected, first wave follicles are also characterized in relation to intrafollicular growth factors, which may modify the gonadotrophin response, such as inhibins and members of the insulin-like growth factor (IGF) family. Subsequently, the follicular control of the transient FSH rise and decline so crucial to dominant follicle selection will be discussed. It is concluded that successful hormonal manipulation of follicle wave growth and dominant follicle selection will depend on our detailed understanding of the gonadotrophin requirements of differentiating wave follicles.

Activin A, estradiol, and free insulin-like growth factor I in follicular fluid preceding the experimental assumption of follicle dominance in cattle

In cattle, the two largest follicles of a wave (F1, F2) begin to deviate into a dominant follicle and a subordinate follicle when F1 is a mean of 8.5 mm in diameter. After the beginning of deviation, F1 and F2 are diameter-defined dominant and subordinate follicles. Changes associated with the conversion of F2 into a future dominant follicle were studied by ablating F1 at the expected beginning of deviation (F1, 8.5 mm; Hour 0) and assessing the follicular-fluid factors in F2. Follicles were designated F1C and F2C in controls and F2A in F1-ablated heifers. Follicular-fluid collections were made at Hours 0, 4, 8, or 12 (n = 7 heifers per hour; fluid from F1C, F2C, and F2A; experiment 1) or at Hours 4, 6, 8, 10, or 12 (n = 9 heifers per hour; fluid from F2A; experiment 2). Postablation concentrations of circulating FSH increased (P < 0.05) between Hours 2 and 6. Diameter of F2A increased (P < 0.05) after Hour 8 in both experiments so that the diameter of F2A at Hours 10 or 12 was not different (P > 0.1) from the diameter of F1 at Hour 0. A transient elevation (P < 0.05) in follicular-fluid activin A occurred in F2A at Hour 8 in both experiments. Concentrations of estradiol (P < 0.05) and insulin-like growth factor I (IGF-I; P < 0.1) decreased in F2C by Hour 8. In F2A, the concentrations of both factors began to increase (P < 0.05) after Hours 4 or 8 so that there was no difference (P > 0.1) between F1C and F2A at Hour 12. Concentrations of IGF-I and IGF binding protein 2 (IGFBP-2) in F2A changed in opposite directions at the same hours. No differences between follicles were found for concentrations of progesterone, androstenedione, inhibin A, and inhibin B. The order of events in the conversion of a future subordinate follicle to a future dominant follicle was an increase in systemic FSH, a transient elevation in follicular-fluid activin A, and a simultaneous increase in follicular-fluid estradiol and restoration of an apparent growth-compatible balance of free IGF-I and IGFBP-2.

The final stages of dominant follicle selection in cattle

The final stages of ovarian follicle growth in cattle are typically characterized by the ultrasound-detectable emergence of a cohort of small (3-5mm in diameter) antral follicles, followed by a selection process during which the number of follicles continuing to grow decreases. Finally, only one follicle (the dominant follicle; DF) shows an enhanced growth rate and estradiol synthesis when it attains 8.5mm compared to its closest competitor (the largest subordinate follicle; SF). Cohort emergence is caused by a transient FSH rise, while DF selection occurs during declining FSH indicating differential FSH dependence of DF and SF. In order to elucidate the mechanisms underlying DF survival or SF atresia, this review aims to (i) describe follicular changes in the local production and regulation of members of the inhibin family of proteins and the insulin-like growth factor (IGF) system in relation to FSH deprivation leading to DF selection, and (ii) develop a model for DF selection outlining the putative involvement of inhibins, activin and follistatin on the one hand, and bioavailable IGFs regulated by IGF binding proteins (IGFBPs) and IGFBP proteases on the other hand. It is concluded, that the first indications of differential FSH dependence are seen within 33h of the FSH peak, and high amounts of precursor forms of inhibin and free activin, and low amounts of the lower molecular weight (MW) IGFBPs are related to follicle survival in terms of enhanced growth and estradiol synthesis, and suppression of granulosa cell apoptosis. In addition, maintenance of low amounts of intrafollicular IGFBP4 may constitute an important mechanism in the future DF to attain FSH independence.

Changes in concentrations of follicular fluid factors during follicle selection in mares

The temporal relationships in the changes in concentrations of follicular fluid factors during follicle selection were characterized in mares. All follicles > or =5 mm were ablated 10 days after ovulation, followed by follicular fluid collection from the three largest follicles (F1, F2, and F3) when F1 of the new wave reached a diameter of 8.0-11.9, 12.0-15.9, 16.0-19.9, 20.0-23.9, 24.0-27.9, or 28.0-31.9 mm (n = 4-8 mares/range). Diameter deviation between F1 and F2 began during the 20.0- to 23.9-mm range, as indicated by a greater difference in diameter between the two follicles at the 24.0- to 27.9-mm range than at the 20.0- to 23.9-mm range. Androstenedione concentrations increased in F1, F2, and F3 between the 16.0- to 19.9- and 20.0- to 23.9-mm ranges. In contrast, estradiol, free insulin-like growth factor (IGF)-1, activin-A, and inhibin-A concentrations increased only in F1 beginning at the 16.0- to 19.9-mm range. As a result, the concentrations of all four factors were higher in F1 than in F2 and F3 at all the later ranges, including the 20.0- to 23.9-mm range (beginning of diameter deviation). Concentrations of progesterone differentially increased in F1, concentrations of androstenedione and IGF-binding protein (IGFBP)-2 increased only in F2 and F3, and concentrations of inhibin-B differentially decreased in F2 and F3 simultaneous with the beginning of deviation. Concentrations of FSH, LH, pro-alphaC inhibin, and total inhibin did not change differentially among follicles. Results indicated that, on a temporal basis, estradiol, free IGF-1, activin-A, and inhibin-A may have played a role in the initiation of follicle deviation. In addition, these four factors as well as progesterone, androstenedione, IGFBP-2, and inhibin-B may have been involved in the subsequent differential development of the follicles.

Follicle selection in cattle: dynamics of follicular fluid factors during development of follicle dominance

Follicle diameter deviation during follicular waves in cattle begins with a reduction in growth rates of developing subordinate follicles, in contrast to the maintenance of a constant growth rate by a developing dominant follicle. In experiment 1, the temporal changes encompassing deviation in concentrations of follicular fluid factors relative to one another in the three largest follicles (F1, F2, and F3) were studied. Follicular fluid samples were collected when F1 reached diameter ranges of 7.0-7.9, 8.0-8.9, 9.0-9.9, and 10.0-10.9 mm (n = 12 per range). The first increase (P < 0.05) in the difference between F1 and F2 for estradiol occurred at the 8.0- to 8.9-mm range, which was one range earlier than for diameter (P < 0.05). Free insulin-like growth factor (IGF)-1 concentrations in F1 were similar among diameter ranges, but concentrations in F1 were higher (P < 0.05) than in F2 for each range except 7.0-7.9 mm. Concentrations of free IGF-1 in F2 decreased (P < 0.05). No significant differences were detected in concentrations of progesterone, androstenedione, total inhibin, and inhibin-A. Averaged over follicles, inhibin-B decreased (P < 0.05) between the 8.0- to 8.9- and 10.0- to 10.9-mm ranges, and activin-A increased (P < 0.05) between the 7.0- to 7.9- and 9.0- to 9.9-mm ranges. However, no differences were found among follicles. In experiment 2, changes associated with the development of dominance by F2 were studied using ablation of F1 at the beginning of expected deviation (F1, 8.5 mm; Hour 0) as the reference point. Follicular fluid factors were compared at Hour 12 between F2 of a control group (F1 intact; n = 10) and an ablated group (F1 ablated; n = 10). Diameter (P < 0.02), estradiol (P < 0.001), free IGF-1 (P < 0.002), and progesterone (P < 0.003) were greater and IGF-binding protein-2 was lower (P < 0.01) in F2 of the ablated group at Hour 12. No differences were detected in concentrations of androstenedione, total inhibin, and inhibin-A. The results of the two experiments indicated, on a temporal basis, that intrafollicular changes in estradiol and the IGF system, but not in the inhibin/activin system, could account for a reported greater FSH responsiveness by the future dominant follicle than by the future subordinate follicles by the beginning of diameter deviation in cattle.

Embryo transfer in Bos indicus cattle

In the present short review superovulation treatments commonly used for Bos taurus and/or Bos indicus will be addressed with emphasis in recent superstimulation protocols associated with pharmacological manipulation of the follicular dynamics to improve donor management and potentially embryo yield. Results obtained after superovulation treatments in which the time of LH surge is selectively delayed as an attempt to improve embryo yield are presented and discussed.

Combined effect of follicle-follicle interactions and declining follicle-stimulating hormone on murine follicle health in vitro

Follicle selection occurs throughout an adult female's reproductive life, with selected, dominant follicle(s) developing to the preovulatory stage whereas the remaining, subordinate follicles within the growing cohort instead undergo atresia and die. To date, most research into follicle dominance has concentrated on its endocrine regulation, although it seems likely that intraovarian mechanisms are also involved in its regulation. We demonstrate here that the response of singly cultured murine follicles to declining concentrations of FSH depends on their developmental stage, with follicles at an earlier stage of development being much more susceptible than mature follicles to a lowering of FSH levels. We then extrapolate this information to follicle cocultures, in which a large dominant follicle was grown with a small subordinate follicle in a manner that maintained a dominant/subordinate relationship, with follicle health assessed by a terminal transferase-mediated 2'-deoxyuracil 5'-triphosphate nick end-labeled reaction on whole-follicle mounts. Our investigations show a combined negative effect of coculture and FSH withdrawal on small subordinate follicles, such that subordinate follicles cocultured with dominant follicles and subjected to a lowering of FSH levels during the culture period exhibit a greatly increased incidence of apoptosis in the granulosa cells (750% increase) compared with that exhibited by the dominant follicles (97% increase). We suggest that a similar interaction between endocrine and intraovarian factors regulates follicular dominance in vivo, such that dominant follicles, in addition to bringing about a fall in FSH levels via the hypothalamic-pituitary axis, exert local, direct effects on subordinate follicles, with both of these influences combining to induce atresia in subordinate follicles.