Expression of messenger ribonucleic acid (mRNA) encoding 3beta-hydroxysteroid dehydrogenase delta4,delta5 isomerase (3beta-HSD) during recruitment and selection of bovine ovarian follicles: identification of dominant follicles by expression of 3beta-HSD mRNA within the granulosa cell layer

High copper levels in follicular fluid affect follicle development in polycystic ovary syndrome patients: Population-based and in vitro studies

Although the adverse effects of copper overexposure on the liver, kidney, spleen and intestinal organs are well known, information about the impact of copper toxicity on human reproduction is limited. A total of 348 infertile patients were enrolled in our present study, including 89 with polycystic ovary syndrome (PCOS), 145 with fallopian tube obstruction and 114 controls. The follicular fluid concentrations of 22 trace elements were measured by inductively coupled plasma mass spectrometry (ICP-MS). Principal component analysis was used to identify trace element profile alterations in different groups. The mRNA levels of steroidogenesis-related genes were measured by real-time PCR. Our results showed that the trace element profile in follicular fluid was obviously altered in PCOS patients. Copper concentrations were significantly (p < .05) higher in the PCOS group than in the other two groups. Increased copper levels in follicular fluid were associated with a higher number of retrievable oocytes in the PCOS group (B = 1.785, p = .001) but a lower rate of high-quality embryos (B = -6.360, p = .050). Moreover, follicular fluid copper levels were positively correlated with follicular fluid progesterone levels (r = 0.275, p = .010) and testosterone levels (r = 0.250, p = .022). Cultured human granulosa cells overexposed to copper showed significantly (p < .05) increased estradiol secretion and decreased testosterone levels. Real-time quantitative PCR revealed a significant (p < .05) increase in CYP19A1 and HSD3b mRNA expression. Our results indicate that increased copper levels in follicular fluid could affect follicle development in PCOS patients, and the mechanism may be related to copper-induced abnormalities in steroidogenesis.

Factors affecting folliculogenesis in ruminants

This review addresses the reasons for the lack of progress in the control of superovulation and highlights the importance of understanding the mechanisms underlying follicular development. The present inability to provide large numbers of viable embryos from selected females still restricts genetic improvement, whilst variability in ovarian response to hormones limit the present capacity for increasing reproductive efficiency.

Females are born with a large store of eggs which rapidly declines as puberty approaches. If these oocytes are normal then there is scope for increasing the reproductive potential of selected females. Oocytes must reach a certain size before they can complete all stages of development and the final changes that occur late in follicular development. It is likely that oocytes that do not produce specific factors at precise stages of development will not be viable. Hence, it is important to characterize oocyte secreted factors since there are potential indicators of oocyte quality.

The mechanisms that determine ovulation rate have still not been fully elucidated. Indeed follicular atresia, the process whereby follicles regress, is still not known. A better understanding of these processes should prove pivotal for the synchronization of follicular growth, for more precise oestrous synchronization and improved superovulatory response.

Nutrition can influence a whole range of reproductive parameters however, the pathways through which nutrition acts have not been fully elucidated. Metabolic hormones, particularly insulin and IGFs, appear to interact with gonadotrophins at the level of the gonads. Certainly gonadotropins provide the primary drive for the growth of follicles in the later stages of development and both insulin and IGF-1, possibly IGF-2, synergize with gonadotrophins to stimulate cell proliferation and hormone production. More research is required to determine the effects of other growth factors and their interaction with gonadotropins.

There is evidence, particularly from studies with rodents, that steroids can also modulate follicular growth and development, although information is very limited for ruminants. There may be a rôle for oestrogens in synchronizing follicular waves, to aid in oestrous synchronization regimes and for removing the dominant follicle to achieve improved superovulatory responses. However more information is required to determine whether these are feasible approaches.

Heritability for litter size is higher in sheep than in cattle. Exogenous gonadotropins are a commercially ineffective means of inducing twinning in sheep and cattle. Although there are differences in circulating gonadotropin concentrations, the mechanism(s) responsible for the high ovulation appear to reside essentially within the ovaries. The locus of the Booroola gene, a major gene for ovulation rate, has been established but not specifically identified. However sheep possessing major genes do provide extremely valuable models for investigating the mechanisms controlling ovulation rate, including a direct contrast to mono-ovulatory species such as cattle.

In conclusion, the relationship between oocyte quality, in both healthy follicles and those follicles destined for atresia, must be resolved before the future potential for increasing embryo yield can be predicted. In addition, a greater understanding of the factors affecting folliculogenesis in ruminants should ensure that the full benefits ensuing from the precise control of ovarian function are achieved. The improved use of artificial insemination and embryo transfer that would ensue from a greater understanding of the processes of folliculo genesis, coupled with the new technologies of genome and linkage mapping, should ensure a more rapid rate of genetic gain.

Physiology and Endocrinology Symposium: Harnessing basic knowledge of factors controlling puberty to improve synchronization of estrus and fertility in heifers

The development of replacement heifers is a major economic investment for all beef and dairy operations. The costs associated with heifer development cannot be recovered if heifers do not conceive and remain productive in the herd; therefore, heifers need to conceive early in the breeding season or risk being culled. Previous research has reported up to a 21% increase in fertility from pubertal estrus to the third estrus of a heifer. The use of reproductive tract scores to determine pubertal status has demonstrated that peripubertal and pubertal heifers have increased pregnancy success to estrous synchronization compared with heifers that were prepubertal. The development of RIA has allowed accurate measurement of peripheral blood hormone concentrations associated with the pubertal process and luteal formation. This basic knowledge has increased our understanding of the mechanisms that control puberty in heifers. In addition, understanding the hormonal changes that occur during the estrous cycle has allowed for the development of estrous synchronization protocols that result in increased control of follicular growth, regression of luteal tissue, and ovulation. Transrectal ultrasonography has increased our understanding of follicular waves; this understanding led to research investigating the endocrine regulation of follicular waves and development of methods to synchronize follicular waves for purposes of fixed-time AI. Current topics of research include the effect of antral follicle count on fertility and the effect of maternal nutrition (on the fetus in utero) on subsequent reproductive potential of a heifer (i.e., fetal programming). Advancements in genomic technologies will likely provide a powerful tool for selecting heifers at birth that will have a greater probability of being reproductively successful if managed correctly. Therefore, knowledge gained through basic research on factors that control puberty has improved and will continue to improve heifer development and fertility.

Down-regulation of genes encoding steroidogenic enzymes and hormone receptors in late preovulatory follicles of the cow coincides with an accumulation of intrafollicular steroids

The transformation of the dominant follicle into a functional corpus luteum is accompanied by a profound molecular and morphological reorganization of somatic cell layers. Several studies have focused on gene expression during early processes of follicular differentiation as it relates to recruitment and selection of dominant follicles. However, little information exists on changes of gene expression profiles in late preovulatory follicles. This lack of information is addressed here to elucidate molecular mechanisms behind the LH-induced transition from the large dominant estrogen-active to the preovulatory follicle, an intermediate stage toward full luteinization. Transcripts encoding key molecules for the biosynthesis of steroid hormones and prostaglandins, as well as receptors for gonadotropic and growth hormones (Star, Cyp11a1, Hsd3b, Cyp17, Cyp19, Ptgs2, Fshr, Lhr, and Ghr), were quantified by real-time polymerase chain reaction (PCR) in the granulosa and theca of large dominant and late preovulatory follicles. The steroid hormones progesterone (P4) and estradiol-17beta (E2) were monitored to distinguish estrogen-active and estrogen-inactive follicles. We found that (1) independent of the follicular stage, the gene expression profile was very different in granulosa and theca; (2) the abundance of several key transcripts was lower in estrogen-inactive, compared with estrogen-active, dominant follicles; (3) in the granulosa of late preovulatory follicles, transcripts encoding steroidogenic enzymes and hormone receptors were largely down-regulated, whereas (4) progesterone and E2 were found at high concentrations in the follicular fluid. Collectively, our data show that late preovulatory follicles have a transient and unique gene expression profile and are clearly different from both the preceding and subsequent (follicular and luteal, respectively) stages.

Identification of differentially expressed markers in human follicular cells associated with competent oocytes

BACKGROUND

The development of an accurate method for selection of high-quality embryos is essential to achieve high pregnancy rates with single embryo transfer in human IVF. The developmental competence of the oocyte is acquired during follicle maturation and strong communication also exists between the follicular cells (FCs) and the oocytes; thus oocyte developmental competence may be determined by markers expressed in the surrounding FCs.

METHODS

From consenting patients (n = 40), FCs were recovered on a per follicle basis by individual follicle puncture. Hybridization analyses using a custom-made complementary DNA microarray containing granulosa/cumulus expressed sequence tags (ESTs) from subtracted libraries and an Affymetrix GeneChip were performed to identify specific genes expressed in follicles leading to a pregnancy. The selected candidate genes were validated by quantitative-PCR (Q-PCR).

RESULTS

Subtractive libraries prepared from pooled samples representing pregnant versus non-pregnant patients produced 1694 ESTs. Hybridization data analysis discriminated 115 genes associated with competent follicles. Selected candidates were confirmed by Q-PCR: 3-beta-hydroxysteroid dehydrogenase 1 (P = 0.0078), Ferredoxin 1 (P = 0.0203), Serine (or cysteine) proteinase inhibitor clade E member 2 (P = 0.0499), Cytochrome P450 aromatase (P = 0.0359) and Cell division cycle 42 (P = 0.0396).

CONCLUSIONS

Microarray technologies are useful to mine the transcriptome of FCs expressed in follicles associated with competent oocytes and could be used to improve embryo selection with the objective of successful single embryo transfer.

Elaboration of a working model for the involvement of inhibin A as a mediator of the preovulatory rise of progesterone levels

During the final days of follicular development, exogenously administered follicle-stimulating hormone (FSH) produces a rise in serum progesterone level. The aim of the present study was to investigate the possible source and regulation of this preovulatory progesterone surge. Four sets of matching treatments with gonadotropins for in vitro fertilization and intracytoplasmic sperm injection were selected from a cohort of 953 treatments in 244 couples. Half of these four sets of treatments were selected based on the unusual course of the progesterone concentration during follicular development. The first set of 11 cycles with early termination of gonadotropin administration for prolonged coasting were compared with a set of 12 cycles with similar estradiol levels but with uninterrupted ovarian stimulation. Another set of 12 cycles with low preovulatory progesterone levels (<2 nmol/l) were matched with ten cycles with normal preovulatory progesterone levels (>2 nmol/l). The sera of these four selected sets of treatments were stored for subsequent measurement of the concentrations of inhibin A, inhibin B, activin A and leptin. During ovarian hyperstimulation serum levels of inhibin A correlated significantly with those of progesterone (p < 0.001), whereas this correlation disappeared after the withdrawal of FSH administration. The rapid fall of progesterone levels during prolonged coasting contrasts with the continuing rise of estradiol concentration and indicates that the theca interna, not the granulosa, is the major source of preovulatory progesterone. Women failing to produce any increment of progesterone levels at the end of follicular development had significantly lower levels of inhibin A (p < 0.05), indicating that inhibin A may well be involved in mediating the signal of FSH from the granulosa to the theca interna.

Expression of 3alpha- and 3beta-hydroxy steroid dehydrogenase mRNA in COCs and granulosa cells determines Zearalenone biotransformation

Zearalenone (ZEA) is a mycoestrogen found in diverse food and feed materials, particularly in corn and small grains. Following ingestion, the parent zearalenone is converted predominantly into alpha-zearalenol (alpha-ZOL) and beta-zearalenol (beta-ZOL) by hepatic hydroxy steroid dehydrogenases (HSD). The present study demonstrated by standard RT-PCR the expression of 3alpha- and 3beta-HSD also in porcine cumulus oocyte complexes (COCs) and granulosa cells isolated form cumulus oocyte complexes. Analysis of the rate of bioconversion of zearalenone (ZEA) by the cultured granulose cells showed the extra-hepatic production of both hydroxy metabolites of ZEA with alpha-ZOL being the dominating metabolites as previously observed in incubations with liver microsomes. The endogenous steroids 5alpha-dihydrotestosterone (5alpha-DHT), and progesterone (PGTN), both known substrates for 3alpha-HSD inhibited the conversion of ZEA into alpha-ZOL. In the presence of pregnelonone (PGN), a major substrate for 3beta-HSD only a slight inhibitory effect on the apparent beta-ZOL formation could be observed. In conclusion, these data indicate that both 3alpha- and 3beta-HSDs are expressed in porcine COCs and GCs, whereas the biotransformation experiments confirm the involvement of these enzymes in the extra-hepatic biotransformation of ZEA.

Molecular biology of the 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase gene family

The 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD) isoenzymes are responsible for the oxidation and isomerization of Delta(5)-3beta-hydroxysteroid precursors into Delta(4)-ketosteroids, thus catalyzing an essential step in the formation of all classes of active steroid hormones. In humans, expression of the type I isoenzyme accounts for the 3beta-HSD activity found in placenta and peripheral tissues, whereas the type II 3beta-HSD isoenzyme is predominantly expressed in the adrenal gland, ovary, and testis, and its deficiency is responsible for a rare form of congenital adrenal hyperplasia. Phylogeny analyses of the 3beta-HSD gene family strongly suggest that the need for different 3beta-HSD genes occurred very late in mammals, with subsequent evolution in a similar manner in other lineages. Therefore, to a large extent, the 3beta-HSD gene family should have evolved to facilitate differential patterns of tissue- and cell-specific expression and regulation involving multiple signal transduction pathways, which are activated by several growth factors, steroids, and cytokines. Recent studies indicate that HSD3B2 gene regulation involves the orphan nuclear receptors steroidogenic factor-1 and dosage-sensitive sex reversal adrenal hypoplasia congenita critical region on the X chromosome gene 1 (DAX-1). Other findings suggest a potential regulatory role for STAT5 and STAT6 in transcriptional activation of HSD3B2 promoter. It was shown that epidermal growth factor (EGF) requires intact STAT5; on the other hand IL-4 induces HSD3B1 gene expression, along with IL-13, through STAT 6 activation. However, evidence suggests that multiple signal transduction pathways are involved in IL-4 mediated HSD3B1 gene expression. Indeed, a better understanding of the transcriptional factors responsible for the fine control of 3beta-HSD gene expression may provide insight into mechanisms involved in the functional cooperation between STATs and nuclear receptors as well as their potential interaction with other signaling transduction pathways such as GATA proteins. Finally, the elucidation of the molecular basis of 3beta-HSD deficiency has highlighted the fact that mutations in the HSD3B2 gene can result in a wide spectrum of molecular repercussions, which are associated with the different phenotypic manifestations of classical 3beta-HSD deficiency and also provide valuable information concerning the structure-function relationships of the 3beta-HSD superfamily. Furthermore, several recent studies using type I and type II purified enzymes have elegantly further characterized structure-function relationships responsible for kinetic differences and coenzyme specificity.

The role of thecal androgen production in the regulation of estradiol biosynthesis by dominant bovine follicles during the first follicular wave1,2

The first wave of follicular development following ovulation in cattle is characterized by selection and growth of a large, estrogenic dominant follicle. After the follicle becomes morphologically dominant, concentrations of estradiol in its follicular fluid decrease abruptly. The purpose of this study was to determine whether this decrease in estrogen production is caused by an insufficient supply of androgen from theca interna or decreased aromatization of androgen precursor by granulosa cells. Dominant follicles were collected from Holstein heifers on d 4, 6, or 8 of the first follicular wave (n = 5/d). Amounts of 17alpha-hydroxylase mRNA in theca interna were sevenfold higher (P < 0.01) on d 4 than on d 8. After 3 h in culture, secretion of androstenedione by theca interna collected on d 4 (236 +/- 44 pg/microg of protein) tended to be lower (P = 0.055) compared with d 6 (517 +/- 162 pg/microg protein) and was lower (P < 0.05) compared with d 8 (387 +/- 51 pg/microg of protein). In granulosa cells, amounts of aromatase mRNA decreased (P < 0.05) on d 8 compared with d 6 but not d 4. In vitro secretion of estradiol was higher in granulosa cells collected on d 4 (3.5 +/- 0.8 ng/[10(5) cells x 3 h]) compared with d 6 (1.8 +/- 0.6 ng/[10(5) cells x 3 h]; P < 0.05) and tended to be higher on d 4 than on d 8 (2.2 +/- 0.2 ng/[10(5) cells x 3 h]; P = 0.058). We conclude that the decrease in estradiol production observed during atresia of the dominant follicle is not due to lack of androgen substrate for aromatization or downregulated expression of the aromatase gene, but may be the direct result of decreased activity of the aromatase enzyme within granulosa cells.

Expression of 17beta- and 3beta-hydroxysteroid dehydrogenases and steroidogenic acute regulatory protein in non-luteinizing bovine granulosa cells in vitro

Granulosa cells of small follicles differentiate in vitro in serum-free medium, resulting in increased estradiol secretion and abundance of mRNA encoding cytochrome P450aromatase (P450arom). We tested the hypothesis that differentiation in vitro also involves increased expression of 3beta- and 17beta-hydroxysteroid dehydrogenases (HSD) in the absence of steroidogenic acute regulatory protein (StAR) expression, as has been observed in vivo. Granulosa cells from small (<6 mm diameter) follicles were cultured for up to 6 days, and mRNA levels quantified by Northern hybridization or RT-PCR. Estradiol and progesterone concentrations in medium increased with time in culture, as did mRNA encoding P450arom, 3beta- and 17beta-HSD but not P450scc. Both P450arom and 17beta-HSD were significantly correlated with estradiol accumulation in culture medium. Progesterone secretion was correlated with 3beta-HSD but not P450scc mRNA levels. StAR mRNA was detectable by RT-PCR, did not change with duration of culture and was not correlated with progesterone secretion. FSH significantly stimulated P450arom and 17beta-HSD mRNA levels. Cell origin (from the antral or the basal layer of the membrana granulosa) did not affect steroidogenesis. We conclude that under the present cell culture system granulosa cells do not luteinize, and show expression of key steroidogenic enzymes in patterns similar to those occurring in differentiating follicles in vivo. Further, the data suggest that 17beta-HSD may be as important as P450arom in regulating estradiol secretion, and that 3beta-HSD is more important than P450scc as a regulator of progesterone secretion in non-luteinizing granulosa cells.

Androgen receptor mRNA expression in the bovine ovary

Previous studies have shown that androgen receptor (AR) is expressed in granulosa cells of healthy, growing ovarian follicles in rats and primates. However, AR expression in the bovine ovary has not been examined. Therefore, a 346-base pair segment of the bovine AR was cloned and sequenced. Using a ribonuclease protection assay, AR expression was detected in total RNA from bovine ovarian cortex. Expression (absence or presence) of AR mRNA was detected by in situ hybridization in bovine ovarian cortex. Follicles (n = 32) were classified as follows: type 1 (1 layer of flattened granulosa cells), type 2 (1-1.5 layers of cuboidal granulosa cells), type 3 (2-3 layers of granulosa cells), type 4 (4-6 layers of cuboidal granulosa cells and formation of thecal layer), and type 5 (>6 layers of cuboidal granulosa cells, defined theca layer, and antrum formation). Frequency of AR mRNA expression increased (P < 0.001) as follicles entered the growing pool. Expression of AR mRNA was absent in type 1 follicles (n = 8), but present in the granulosa cells of 41% of type 2 follicles (n = 12). In types 3-5 follicles, AR mRNA expression was present in granulosa cells of 100% of follicles examined (n = 4, 4, and 4, respectively) and was greater than type 1 follicles (P = 0.002). These data provide evidence of AR mRNA expression in bovine follicles and suggest that AR mRNA increases during early follicle development.

A novel basal lamina matrix of the stratified epithelium of the ovarian follicle

Basal laminas are important sheets of specialized extracellular matrix that underlie and surround groups of cells, such as epithelia or endothelia, enabling the cells to orientate their basal/apical polarity and creating a microenvironment for them. Basal laminas can also individually encapsulate whole cells, such as muscle cells, thereby forming a microenvironment but not polarizing the enclosed cells. Other mesenchymal or stromal cells exist with no basal lamina. In the course of studying the bovine follicular basal lamina which underlies the multilayered epithelium of the ovarian follicle, we identified a developmentally regulated novel extracellular matrix (which we call focimatrix for focal intra-epithelial matrix). Focimatrix is composed of basal lamina-like material deposited as plaques or aggregates between the multilayers of the epithelial granulosa cells. The focimatrix does not encapsulate individual or groups of cells and therefore does not form a microenvironment for them. Focimatrix contains collagen type IV subunits alpha1 and alpha2 (but not alpha3-alpha6), and laminin chains alpha1, beta2 and gamma1 (but not alpha2 or beta1), and nidogen-1 and perlecan (but not versican). The amount of focimatrix increases with increasing follicular size, and its appearance precedes the expression by granulosa cells of the enzymes for steroid hormone synthesis, cholesterol side-chain cleavage cytochrome P450 (SCC) and 3beta-hydroxysteroid dehydrogenase (3beta-HSD), in the days preceding ovulation. The expression in granulosa cells of two components examined, nidogen-1 and perlecan, also increases substantially when follicles enlarge to a sufficient size capable of ovulating. Following ovulation the follicular basal lamina is degraded, and presumably focimatrix is too since it is not detected in corpora lutea that develop from the ovulating follicles. During this development the granulosa cells undergo an epithelial-mesenchymal transition (EMT) into luteal cells following ovulation, and substantially increase their expression of steroidogenic enzymes in the process. During EMT epithelial cells lose polarity. Since focimatrix exists on more than one side of the granulosa cells, we propose that it disrupts the polarity induced by the follicular basal lamina in the lead up to ovulation. Hence focimatrix maybe a key part of the follicular/luteal EMT.

Gonadotropin requirements for dominant follicle selection in GnRH agonist-treated cows

A study was conducted to examine the effects of gonadotropins on ovarian follicular development and differentiation in GnRH agonist (GnRHa)-treated cattle. Holstein cows were allotted into two pre-treatment groups: controls (n = 5) and GnRHa-treated (n = 9). Ovaries were removed from control cows on day 5 following a synchronized estrus. Treatment with GnRHa resulted in follicular arrest at <5 mm. Following follicular arrest, GnRHa-treated cows received a constant infusion of FSH for 96 h (GnRHa/FSH), with a randomly selected subset receiving hourly pulses of LH in addition to FSH during the last 48 h of infusion (GnRHa/FSH + LH). At the end of infusion, ovaries were removed, follicles were counted and measured, and follicular fluid samples were collected from large follicles (>10 mm). Differences in expression of mRNA for LH receptor, FSH receptor, cytochrome P450 side-chain cleavage, 3β-hydroxysteroid dehydrogenase, cytochrome P450 17α-hydroxylase (P450c17) and cytochrome P450 aromatase were determined in large follicles using in situ hybridization. The number of large follicles did not differ between GnRHa/FSH-treated and GnRHa/FSH + LH-treated cows (P = 0.64), but was greater than control animals (P ≤ 0.004). Follicular fluid concentrations of estradiol-17β and androstenedione were highest in GnRHa/FSH + LH-treated cows (P ≤ 0.04), intermediate in control cows, and lowest in GnRHa/FSH-treated cows. Hybridization intensity of P450c17 was greater in GnRHa/FSH + LH-treated versus control or GnRHa/FSH-treated cows (P ≤ 0.03). These results indicate that while FSH can support bovine follicular growth >10 mm, LH increases androgen production and expression of P450c17.

Expression of the bovine aromatase cytochrome P450 gene (Cyp19 ) is primarily regulated by promoter 2 in bovine follicles and by promoter 1.1 in corpora lutea

Follicular development and differentiation are closely associated with increasing steroidogenesis. During the present study transcript concentration of Cyp19, Cyp11A1, and 3beta-hydroxysteroid dehydrogenase delta (3beta-HSD) encoding the steroidogenic enzymes P450(arom), P450(SCC), and 3beta-HSD were determined by real-time PCR in bovine granulosa cells (GC) as potential markers for follicular differentiation. Ovaries were collected from a local abattoir (experiment 1) and from synchronized animals at day 4 of estrus cycle (experiment 2). To study effects of luteinization, steroidogenic transcripts were also quantified in corpora lutea (CL) 4 and 20 days after fertilization. In most follicles, all three steroidogenic transcripts were detected, however, at very different concentration. Expression of 3beta-HSD and Cyp11A1 was highly significantly co-regulated and showed a significant correlation with follicular size. Contrary, Cyp19 expression was extremely variable even in follicles of similar size. Cyp19 transcripts were derived predominantly from promoter P2 and less abundant from promoters P1.1 and P1.5. After luteinization, the concentration of 3beta-HSD and Cyp11A1 transcripts increased (75-fold and fivefold, respectively) whereas the Cyp19 transcript level dropped (160-fold). Residual Cyp19 transcripts in CL were almost exclusively derived from P1.1. The data indicate that Cyp19 expression in GC is predominantly regulated by P2 and to a minor extend by P1.1, whereas P1.1 is almost exclusively responsible for residual Cyp19 expression in CL. Correlation analyses suggest that the expression of 3beta-HSD and Cyp11A1 primarily depend on the size of follicles whereas the concentration of P2 derived Cyp19 transcripts in GC is a marker for follicular differentiation towards selection and dominance.

Cholesterol side-chain cleavage cytochrome P450 and 3beta-hydroxysteroid dehydrogenase expression and the concentrations of steroid hormones in the follicular fluids of different phenotypes of healthy and atretic bovine ovarian follicles

Bovine ovarian antral follicles exhibit either one or the other of two patterns of granulosa cell death in atresia. Death can commence either from the antrum and progress toward the basal lamina (antral atresia) or the converse (basal atresia). In basal atresia, the remaining live antrally situated cells appeared to continue maturing. Beyond that, little is known about these distinct patterns of atresia. Healthy (nonatretic) follicles also exhibit either one or the other of two patterns of granulosa cell shape, follicular basal lamina ultrastructure or location of younger cells within the membrana granulosa. To examine these different phenotypes, the expression of the steroidogenic enzymes cholesterol side-chain cleavage cytochrome P450 (SCC) and 3beta-hydroxysteroid dehydrogenase (3beta-HSD) in granulosa cells and concentrations of steroid hormones in follicular fluid were measured in individual histologically classified bovine antral follicles. Healthy follicles first expressed SCC and 3beta-HSD in granulosa cells only when the follicles reached an approximate threshold of 10 mm in diameter. The pattern of expression in antral atretic follicles was the same as healthy follicles. Basal atretic follicles were all <5 mm. In these, the surviving antral granulosa cells expressed SCC and 3beta-HSD. In examining follicles of 3-5 mm, basal atretic follicles were found to have substantially elevated progesterone (P < 0.001) and decreased androstenedione and testosterone compared to healthy and antral atretic follicles. Estradiol was highest in the large healthy follicles, lower in the small healthy follicles, lower still in the antral atretic follicles, and lowest in the basal atretic follicles. Our findings have two major implications. First, the traditional method of identifying atretic follicles by measurement of steroid hormone concentrations may be less valid with small bovine follicles. Second, features of the two forms of follicular atresia are so different as to imply different mechanisms of initiation and regulation.

Distribution of cytochrome P450-side chain cleavage in the theca interna layers of bovine small antral and cystic follicles

Cystic follicle is anovulatory follicular structure that is caused by an endocrine imbalance. The activity of cytochrome P450-side chain cleavage (P450scc) is essential for the initiation of steroidogenesis in the follicle. The present study was designed to compare the frequency of cells containing P450scc between healthy and atretic small antral follicles, and among several types (I, II and III, classified based on the presence of granulosa layer) of cystic follicles. Paraffin sections of healthy (2-5 mm in diameter), atretic (2-5 mm) and cystic follicles (>25 mm) were immunohistochemically stained with rabbit polyclonal antibody to bovine P450scc. The P450scc-positive cells were counted in four different regions of the follicles from the apical to the basal side. In small antral follicles and cystic follicles, P450scc-positive cells were localized in the theca interna layers but not granulosa layers. The P450scc-positive cell populations decreased in the late atretic follicles compared with the early and advanced atretic follicles at all the regions of follicle. Type III cystic follicles showed significantly lower frequencies of P450scc-positive cells than those in the types I and II cystic follicles. These results suggest that in both small and cystic follicles in cows, total loss of granulosa cells may be associated with the reduction of frequency of P450scc-positive cells in theca interna layer.

lmmunohistochemical Localization of 3.BETA.-Hydroxysteroid Dehydrogenase in the Granulosa and Theca Interna Layers of Bovine Cystic Follicles

The aim of the present study was to determine whether the alteration of population of cells containing 3beta-hydroxysteroid dehydrogenase (3beta-HSD) is responsible for the formation of cystic follicles. Paraffin sections of healthy (2 to 5 mm in diameter), atretic (2 to 5 mm) and cystic follicles (more than 25 mm) were immunohistochemically stained with rabbit polyclonal antibody to bovine 3beta-HSD. The 3beta-HSD-positive cells were counted in 4 different regions of the follicles from the apical to the basal side. The frequencies of 3beta-HSD-positive granulosa cells in cystic follicles were significantly higher than those in the healthy follicles (P<0.05), although the number of 3beta-HSD-positive granulosa cells in the cystic follicle were fewer than half the cells (30 to 40%) and was much smaller than that in preovulatory follicles (Conley et al., 1995). The frequencies of 3beta-HSD-positive cells were higher in the granulosa layer and lower in the theca interna layer of the cystic follicles than the atretic follicles. These results suggest that the differentiation of granulosa cells to express 3beta-HSD might be insufficient in cystic follicles and accordingly they fail to ovulate. The differences of frequencies of 3beta-HSD-positive cells in the granulosa and theca interna layers between cystic and atretic follicles may be one of the reasons why regression is delayed in cystic follicles.

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.

Expression of the bovine oestrogen receptor-beta (bERbeta) messenger ribonucleic acid (mRNA) during the first ovarian follicular wave and lack of change in the expression of bERbeta mRNA of second wave follicles after LH infusion into cows

In a previous study, the ERbeta cDNA protein-coding region was utilised to clone bovine ERbeta. The objectives in this study were to examine (1) ERbeta mRNA expression in ovarian follicles throughout the bovine first follicular wave, and (2) effect of LH infusion into cows on bERbeta mRNA expression during the second follicular wave. In experiment 1, heifers (4-5 per time point) were ovariectomized at 12, 24, 36, 48, 60, 72, 84, 96, 144, or 216 h after emergence of the first follicular wave after oestrus. In experiment 2, saline or LH was pulsed hourly (computer-controlled syringe pump) into cows (n = 31; 5-6 per treatment) at wave emergence for 2 or 4 days: wave 1-saline (W1S), wave 2-saline (W2S), or wave 2-LH (25 microg/h; W2LH). Ovaries were removed on day 2 or day 4 after wave emergence. Follicles, 2-19mm in size, were dissected, frozen, and stored at -80 degrees C for in situ hybridisation with two bERbeta cRNA probes. Expression of bERbeta mRNA was localised in granulosa cells of healthy follicles. In experiment 1, bERbeta mRNA expression did not change with time points of the wave showing no association of bERbeta mRNA expression with follicular selection and dominance. However, bERbeta mRNA expression decreased with increase in size of all follicles. Expression of bERbeta mRNA was greater in very small follicles (2-4 mm) than in large (> or = 9 mm) follicles. In experiment 2, expression of bERbeta mRNA in follicles did not differ either between W1S and W2S or between W2S and W2LH. In summary, bERbeta mRNA expression decreased with increasing follicular size. However, neither stage of the wave (selection or dominance), nor pulsatile infusion of LH influenced bERbeta mRNA expression.

Concentrations of steroids and expression of messenger RNA for steroidogenic enzymes and gonadotropin receptors in bovine ovarian follicles of first and second waves and changes in second wave follicles after pulsatile LH infusion

The objectives were to compare expression of mRNA for cytochrome P450 cholesterol side-chain cleavage (P450scc), cytochrome P450 17alpha-hydroxylase (P450c17), cytochrome P450 aromatase (P450arom), 3beta-hydroxysteroid dehydrogenase Delta(4), Delta(5) isomerase (3beta-HSD), FSH receptor (FSHr) and LH receptor (LHr) in bovine ovarian follicles of the first and second waves of the bovine oestrous cycle and to determine if LH infusion changes growth, steroidogenesis and gene expression in second wave follicles. Transrectal ultrasonography was used to examine follicular size changes during the oestrous cycle in non-lactating Holstein cows (n=31). Saline or purified bovine LH was infused intravenously into cows at emergence of follicular waves for 2 or 4 days using a computer-controlled syringe pump (n=5-6 per treatment). Treatments were: wave 1, saline (W1S); wave 2, saline (W2S) or LH (25 microg/h; W2LH). During infusion, blood samples were collected at 12min intervals for 8h via i.v. catheters for measurement of serum LH concentrations. Ovaries were removed from cows on days 2 or 4 after emergence of follicular waves. Follicles were frozen and stored at -80 degrees C. Follicular fluid (FF, 50 microl) was collected for determination of progesterone (P4), oestradiol-17beta (E2) and androstenedione (A4) concentrations. Frozen sections (14 microm) were used for in situ hybridization to measure expression of mRNA (% pixel intensity) for P450scc, P450c17, P450arom, 3beta-HSD, FSHr, and LHr. LH infusion resulted in a serum LH pattern (high frequency) similar to the early luteal phase. There were no significant differences in size of follicles among the three treatment groups. Follicular fluid concentrations of E2 and A4 in W2S were lower than those of W1S on day 2 of a follicular wave. LH infusion into cows during the midluteal phase increased follicular fluid E2 and A4 concentrations in second wave follicles on day 2 of a follicular wave (W2LH) compared to those of W2S. The increase in follicular fluid E2 on day 2 in wave 2 follicles after LH infusion occurred possibly through an increase in mRNA expression of P450c17 and 3beta-HSD. In conclusion, follicular fluid concentrations of E2 and A4 were lower in W2S than in W1S and E2 and A4 concentrations were restored by infusion of LH in W2LH with an increase in mRNA expression of P450c17 and 3beta-HSD.

Differentiation of dominant versus subordinate follicles in cattle

Selection of a dominant follicle, capable of ovulating, from among a cohort of similarly sized follicles is a critical transition in follicular development. The mechanisms that regulate the selection of a species-specific number of dominant follicles for ovulation are not well understood. Cattle provide a very useful animal model for studies on follicular selection and dominance. During the bovine estrous cycle, two or three sequential waves of follicular development occur, each producing a dominant follicle capable of ovulating if luteal regression occurs. Follicles are large enough to allow analysis of multiple endpoints within a single follicle, and follicular development and regression can be followed via ultrasonographic imaging. Characteristics of recruited and selected follicles, obtained at various times during the first follicular wave, have been determined in some studies, whereas dominant and subordinate follicles have been compared around the time of selection in others. As follicular recruitment proceeds, mRNA for P450 aromatase increases. By the time of morphological selection, the dominant follicle has much higher concentrations of estradiol in follicular fluid, and its granulosa cells produce more estradiol in vitro than cells from subordinate follicles. Shortly after selection, dominant follicles have higher levels of mRNAs for gonadotropin receptors and steroidogenic enzymes. It has been hypothesized that granulosa cells of the selected follicle acquire LH receptors (LHr) to allow them to increase aromatization in response to LH, as well as FSH. However, LH does not appear to stimulate estradiol production by bovine granulosa cells, and the role of LHr acquisition remains to be determined. Recent evidence suggests a key role for changes in the intrafollicular insulin-like growth factor (IGF) system in selection of the dominant follicle. When follicular fluid was sampled in vivo before morphological selection, the lowest concentration of IGF binding protein-4 (IGFBP-4) was more predictive of future dominance than size or estradiol concentration. Consistent with this finding, dominant follicles acquire an FSH-induced IGFBP-4 protease activity. Thus, a decrease in IGFBP-4, which would make more IGF available to interact with its receptors and synergize with FSH to promote follicular growth and aromatization, appears to be a critical determinant of follicular selection for dominance.

Dominant bovine ovarian follicular cysts express increased levels of messenger RNAs for luteinizing hormone receptor and 3 beta-hydroxysteroid dehydrogenase delta(4),delta(5) isomerase compared to normal dominant follicles

The objective was to compare ovarian steroids and expression of mRNAs encoding cytochrome P450 side-chain cleavage, cytochrome P450 17 alpha-hydroxylase, cytochrome P450 aromatase, 3 beta-hydroxysteroid dehydrogenase Delta(4),Delta(5) isomerase, LH, and FSH receptors and estrogen receptor-beta in ovaries of cows with dominant and nondominant ovarian follicular cysts and in normal dominant follicles. Estradiol-17 beta, progesterone, and androstenedione concentrations were determined in follicular fluid using specific RIAs. Dominant cysts were larger than young cysts or dominant follicles, whereas nondominant cysts were intermediate. Estradiol-17 beta (ng/ml) and total steroids (ng/follicle) were higher in dominant cysts than in dominant follicles. Expression of LH receptor and 3 beta-hydroxysteroid dehydrogenase mRNAs was higher in granulosa cells of dominant cysts than in dominant follicles. Nondominant cysts had higher follicular concentrations of progesterone, lower estradiol-17 beta concentrations, and lower expression of steroidogenic enzyme, gonadotropin receptor, and estrogen receptor-beta mRNAs than other groups. In summary, increased expression of LH receptor and 3 beta-hydroxysteroid dehydrogenase mRNAs in granulosa and increased follicular estradiol-17 beta concentrations were associated with dominant cysts compared to dominant follicles. Study of cysts at known developmental stages is useful in identifying alterations in follicular steroidogenesis.

Cyclooxygenase-2 and prostaglandin E(2)(PGE(2)) receptor messenger RNAs are affected by bovine oocyte maturation time and cumulus-oocyte complex quality, and PGE(2) induces moderate expansion of the bovine cumulus in vitro

Expression of cyclooxygenase-2 (COX-2) and prostaglandin E(2) (PGE(2)) receptor 2 (EP2) are necessary for rodent cumulus expansion in vivo. Prostaglandin E(2) receptor 3 (EP3) has been detected in bovine preovulatory follicles and corpora lutea. The current experiments examined the effect of PGE(2) on bovine cumulus expansion in vitro and expression of COX-2, EP1, EP2, EP3, and EP4 mRNAs in bovine cumulus-oocyte complexes (COCs) at 0, 6, 12, 18, and 24 h time points during maturation in vitro. Concentrations of PGE(2) above 50 ng/ml resulted in moderate cumulus expansion of bovine COCs, but expansion did not occur in the absence of serum. COX-2 mRNA expression increased in bovine COCs at 6 h and 12 h of maturation, then decreased. EP2 mRNA was detectable by reverse transcription-polymerase chain reaction at all time points. EP3 mRNA expression increased in COCs from 0 to 6 h and remained at this higher level through the culture period. Very low levels of EP4 mRNA expression were detectable, but EP1 was not detected in bovine COCs. Because EP receptor mRNAs and COX-2 mRNA are expressed in bovine COCs, there exists the potential for a prostaglandin autocrine/paracrine regulatory pathway during oocyte maturation. Differential expression of the EP3 mRNA among varying COC classes indicates that this gene product may be a useful marker of oocyte competence. Although the PGE(2) pathway is involved in cumulus expansion, serum factors are required to mediate PGE(2)-induced expansion.

A potential role for insulin-like growth factor binding protein-4 proteolysis in the establishment of ovarian follicular dominance in cattle

A critical transition in ovarian follicular development is the selection of a dominant follicle, capable of ovulating, from a cohort of synchronously growing antral follicles. However, little is known about mechanisms and factors that regulate the selection and growth of dominant ovarian follicles. We have investigated whether a component of the insulin-like growth factor (IGF) system, namely IGFBP-4 protease, is associated with the establishment of follicular dominance in cattle. IGFBP proteases degrade IGFBPs, freeing IGFs to interact with their receptors. In experiment 1, follicular fluid from preovulatory follicles (n = 4) degraded about 80% of the added recombinant human (rh) IGFBP-4 within 18 h of incubation. The IGFBP-4 protease exhibited optimal activity at neutral/basic pH and its sensitivity to various protease inhibitors suggested a metalloprotease. The decline in the intensity of the band corresponding to intact rhIGFBP-4 was accompanied by the appearance of immunoreactive fragments of molecular weights approximately 18 and 14 kDa, which were not detectable by ligand blot analysis. In experiment 2, follicular fluid samples were collected from dominant and subordinate follicles on Day 2 or 3 of the first follicular wave, after ovariectomy (experiment 2a, n = 3/day) or by ultrasound-guided follicular aspiration (experiment 2b, n = 4-5/day). Estradiol concentrations in follicular fluid from dominant vs. subordinate follicles confirmed their identities and indicated that the dominant follicle had been selected by Day 2 of the follicular wave. In both experiments 2a and 2b, IGFBP-4 proteolytic activity was 2- to 3.5-fold (P < 0.05) and 5-fold (P < 0.01) higher in follicular fluid from dominant than subordinate follicles on Days 2 and 3 of the follicular wave, respectively. The finding that IGFBP-4 proteolytic activity is higher in dominant, estrogen-active follicles than in subordinate follicles of the same cohort, as early as Day 2 of the follicular wave, strongly suggests a role for IGFBP-4 protease in the establishment of ovarian follicular dominance.

Follicular-fluid factors and granulosa-cell gene expression associated with follicle deviation in cattle

Intrafollicular changes in the largest follicle (F1) and second-largest (F2) follicle were examined in relation to follicle diameter deviation. Deviation is characterized by continued growth of the largest follicle and the cessation of growth of the smaller follicles. Granulosa cells and follicular fluid were obtained from slaughterhouse ovaries (n = 95 pairs, experiment 1), and follicular fluid was collected in vivo (n = 28 heifers, experiment 2). Several ranges in the diameter of F1 were used to represent the progressive growth of the follicle. The diameter range with the first significant increase in the difference between F1 and F2 was determined for each end point and was used as an indicator of the sequence of events associated with diameter deviation. An increased difference for diameter and for estradiol concentration occurred (P: < 0.05) simultaneously at the 8.5- to 8.9-mm range in both experiments. In experiment 1, the increased difference between F1 and F2 in LH receptor (LHr) mRNA expression occurred (P: < 0.05) at the 8.0- and 8.4-mm range. In F2 of experiment 2, there was a progressive decrease (P: < 0.05) in free insulin-like growth factor (IGF)-1 and a progressive increase (P: < 0.05) in IGF binding protein (BP)-2 across the follicle-diameter ranges (7.5-11.2 mm). No differences were detected between F1 and F2 for 3beta-hydroxysteroid dehydrogenase mRNA expression in experiment 1 and testosterone, total inhibin, and dimeric inhibin-A concentrations in experiment 2. The results indicated that the acquisition of granulosa cell LHrs by F1, as indicated by increased LHr mRNA expression, occurred one diameter range before an increased difference between F1 and F2 for diameter or estradiol concentrations. On a temporal basis, it is concluded that LHr acquisition plays a role in the establishment of diameter deviation. In addition, the reduced growth of F2 may have involved the reduced bioavailability of IGF-1 in association with elevated IGFBPs.

Equine P450 cholesterol side-chain cleavage and 3 beta-hydroxysteroid dehydrogenase/delta(5)-delta(4) isomerase: molecular cloning and regulation of their messenger ribonucleic acids in equine follicles during the ovulatory process

The preovulatory LH rise is the physiological trigger of follicular luteinization, a process during which the synthesis of progesterone is markedly increased. To study the control of follicular progesterone biosynthesis in mares, the objectives of this study were to clone and characterize the equine cholesterol side-chain cleavage cytochrome P450 (P450(scc)) and 3 beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4)-isomerase (3 beta-HSD), and describe the regulation and cellular localization of their transcripts in equine follicles during hCG-induced ovulation. Complementary DNA cloning and primer extension analyses revealed that the equine P450(scc) transcript is composed of a 5'-untranslated region (UTR) of 52 nucleotides, an open reading frame (ORF) of 1560 nucleotides, and a 3'-UTR of 225 nucleotides, whereas the equine 3 beta-HSD mRNA consists of a 5'-UTR of 61 nucleotides, an ORF of 1119 nucleotides, and a 3'-UTR of 432 nucleotides. The equine P450(scc) and 3 beta-HSD ORF encode 520 and 373 amino acid proteins, respectively, that are highly conserved (68-79% identity) when compared to homologs of other mammalian species. Northern blot analyses were performed with preovulatory follicles isolated 0, 12, 24, 30, 33, 36, and 39 h post-hCG, and corpora lutea obtained on day 8 of the cycle. Results showed that levels of P450(scc) mRNA in follicular wall (theca interna with attached granulosa cells) decreased after hCG treatment (30-39 h versus 0 h post-hCG, P: < 0.05), and increased again after ovulation to reach their highest levels in corpora lutea (P: < 0.05). Northern blots on isolated cellular preparations revealed that theca interna was the predominant site of P450(scc) expression in follicles prior to hCG (P: < 0.05). However, transcript levels decreased in theca interna between 30-39 h (P: < 0.05) and increased in granulosa cells at 39 h (P: < 0.05), making the granulosa cell layer the predominant site of P450(scc) expression at the end of the ovulatory process. A different pattern of regulation was observed for 3 beta-HSD, as transcript levels remained constant throughout the luteinization process (P: > 0.05). Also, in contrast to other species, expression of 3 beta-HSD mRNA in equine preovulatory follicles was localized only in granulosa cells and not in theca interna. Thus, this study characterizes for the first time the complete structure of equine P450(scc) and 3 beta-HSD mRNA and identifies novel patterns of expression and regulation of these transcripts in equine follicles prior to ovulation.

Intrafollicular concentrations of steroids and steroidogenic enzymes in relation to follicular development in the mare

The objective of the present study was to determine the changes in follicular fluid steroid concentrations and in granulosa cell steroidogenic enzyme expression during the follicular phase, in relation to follicular size and physiological status in the mare. Follicular fluid and follicular cells were recovered by ultrasound-guided follicular punctures either around the time of emergence of the dominant follicle, at the end of the dominant follicle growth, or at the preovulatory stage, after injection of gonadotropin to induce ovulation. Cellular relative amounts of steroidogenic acute regulatory protein (StAR), P450-side chain cleavage (P450(scc)), 3beta-hydroxysteroid dehydrogenase (3betaHSD), 17alpha-hydroxylase, and aromatase were assessed by semiquantitative Western blot and densitometry. Follicular fluid was assayed for cholesterol concentrations by colorimetric assay and for progesterone, testosterone, and estradiol-17beta concentrations by RIA. Intrafollicular concentrations of progesterone and estradiol-17beta significantly increased in the dominant follicle during growth. After injection of gonadotropin, follicular maturation was characterized by a decrease in estradiol-17beta concentrations and a further increase in progesterone concentrations. Granulosa cells from dominant follicles had increased levels of StAR, P450(scc), 3betaHSD, and aromatase during growth, but decreased levels during maturation. Levels of StAR, P450(scc), 3betaHSD, and aromatase, as well as progesterone and estradiol-17beta, were lower in granulosa cells from subordinate than from dominant follicles. We did not observe a relationship between the steroidogenic activity of follicles and the capacity of their enclosed oocytes to complete meiosis in vitro.

Expression of steroidogenic acute regulatory protein messenger ribonucleic acid is limited to theca of healthy bovine follicles collected during recruitment, selection, and dominance of follicles of the first follicular wave

Expression of mRNA encoding steroidogenic acute regulatory protein (StAR) in bovine follicles during recruitment and selection was examined. Dairy heifers (4-5/time period) were ovariectomized at 12, 24, 36, 48, 60, 72, 84, or 96 h after initiation of the first follicular wave (Time 0) after estrus. Follicles were collected and stored at -80 degrees C until sectioning. Expression of StAR mRNA was localized by in situ hybridization and quantified by image analysis. Expression of StAR mRNA was first detected in theca interna of antral follicles as small as 0.5 mm in diameter and increased with increasing follicular size (>/= 4 mm; r = 0.75; p < 0.001). StAR mRNA was undetectable in granulosa of healthy follicles at any size or stage of follicular wave examined. However, granulosa or luteinized granulosa of some advanced or late atretic follicles expressed StAR mRNA. During recruitment, StAR mRNA expression in theca cells was similar among recruited follicles (4-8 mm). During selection of dominant follicles (36-48 h), StAR mRNA was expressed in theca of more than one follicle (7-9 mm); therefore, expression of StAR mRNA may not be associated with dominant follicle selection. StAR mRNA in theca was higher (p < 0.05) at 48 h after initiation of the first follicular wave than at 12, 24, and 36 h, and it remained elevated thereafter through 96 h. Dominant follicles expressed more (p < 0.01) StAR mRNA in theca than did subordinate healthy follicles. Healthy follicles expressed higher (p < 0.05) StAR mRNA in theca than atretic follicles. In summary, levels of StAR mRNA increased in theca with stage of follicular wave and size of follicles. Follicular atresia was associated with reduced expression of StAR mRNA in theca cells. The results indicate that expression of StAR mRNA in theca may not be the primary limiting factor for follicular recruitment and selection.

Follicular dominance in cattle is associated with divergent patterns of ovarian gene expression for insulin-like growth factor (IGF)-I, IGF-II, and IGF binding protein-2 in dominant and subordinate follicles

A decrease in insulin-like growth factor (IGF) binding protein (BP) amount occurs within the follicular fluid of dominant ovarian follicles. At the same time, concentrations of follicular fluid IGF-I do not change. The mRNA for IGF-I, IGF-II, IGFBP-2, and IGFBP-3, in dominant and subordinate follicles were measured to determine if changes in IGF or IGFBP gene expression are associated with follicular dominance. Heifers were ovariectomized during a follicular wave, either during early-dominance (emerging dominant follicle, 9 mm diameter) or mid-dominance (established dominant follicle, 14-16 mm diameter). Follicles were classified as either dominant (DF), subordinate (SF), or not-recruited (NRF; small antral follicles). mRNA was localized by in situ hybridization and measured by image analyses. The IGF-I mRNA (granulosa cells) was greatest in DF and increased in DF, SF, and NRF from early- to mid-dominance. Likewise, IGF-II mRNA (theca cells) was greatest in DF compared with SF or NRF. The IGFBP-2 mRNA (granulosa cells), however, was nearly undetectable in DF, whereas adjacent SF expressed abundant IGFBP-2 mRNA. The NRF were not uniform in their IGFBP-2 expression because only 5 of 13 NRF had IGFBP-2 mRNA. The IGFBP-3 mRNA (granulosa cells) was found only in two NRF, suggesting that local synthesis is not a predominant source of follicular fluid IGFBP-3. These data show that changes in gene expression for IGFBP-2 are opposite to those for IGF-I or IGF-II. Increased IGF-I and IGF-II mRNA and decreased IGFBP-2 mRNA within the DF may be one mechanism leading to follicular dominance. The opposite pattern of IGFBP-2 gene expression in SF and some NRF may lead to follicular atresia.