Studies of Follicular Vascularity Associated with Follicle Selection and Ovulation in Cattle

Improving Milk Yield, Milk Quality, and Follicular Functionality Behavior in Dairy Cows from the Implementation of Microencapsulated Chili Pepper Supplements in Their Diets

The present study evaluates the effect of including microencapsulated hot chili pepper (MHCP) in the diet of crossbred dairy cows on the volume and quality of milk and on ovarian morphofunctionality. Twenty-four crossbred females in their lactating period were used. The cows were divided into two experimental groups, a control (CT) and an MHCP -supplemented group (CP) given 1 g a day per animal of microencapsulated hot chili in concentrate for 42 days. Over seven weeks of daily milk production was measured, and sample milk was collected weekly for composition analysis. Animals were subject to an ovulation synchronization protocol on day 0 (D0), and an intravaginal progesterone (P4) implant, estradiol benzoate, and prostaglandin (PGF2α) were administered. On D8, the P4 implant was removed and PGF2α, equine chorionic gonadotropin, and estradiol cypionate were administered to the animals. The ovarian dynamics were evaluated in B mode and color Doppler. There were significant differences (p < 0.05) in the group X time interaction, the volume of milk produced, and the amount in kg/day of milk components. There was a higher percentage of vascularization in the preovulatory follicle in the CP group (p ≥ 0.10). The findings show that the inclusion of MHCP in the diet of dairy cows does influence their milk production and reproduction.

FSHR-mTOR-HIF1 signaling alleviates mouse follicles from AMPK-induced atresia

The majority of activated ovarian follicles undergo atresia during reproductive life in mammals, and only a small number of follicles are ovulated. Though hormone treatment has been widely used to promote folliculogenesis, the molecular mechanism behind follicle selection and atresia remains under debate due to inconsistency among investigation models. Using a high-throughput molecular pathology strategy, we depicted a transcriptional atlas of mouse follicular granulosa cells (GCs) under physiological condition and obtained molecular signatures in healthy and atresia GCs during development. Functional results revealed hypoxia-inducible factor 1 (HIF1) as a major effector downstream of follicle-stimulating hormone (FSH), and HIF1 activation is essential for follicle growth. Energy shortage leads to prevalent AMP-activated protein kinase (AMPK) activation and drives follicular atresia. FSHR-mTOR-HIF1 signaling helps follicles escape from the atresia fate, while energy stress persists. Our work provides a comprehensive understanding of the molecular network behind follicle selection and atresia under physiological condition.

Effect of Exogenous Melatonin on the Development of Mice Ovarian Follicles and Follicular Angiogenesis

In mammalian, the periodic growth and development of ovarian follicles constitutes the physiological basis of female estrus and ovulation. Concomitantly, follicular angiogenesis exerts a pivotal role in the growth of ovarian follicles. Melatonin (N-acetyl-5-methoxytryptamine, Mel), exists in follicle fluid, was suggested to affect the development of follicles and angiogenesis. This research was conducted to investigate the effects and mechanisms of Mel on the development of ovarian follicles and its angiogenesis. In total, 40 ICR mice at age of 3 weeks were allocated into four groups at liberty: control, Mel, FSH and FSH + Mel for a 12-day trial. Ovaries were collected at 8:00 a.m. on Day 13 for detecting the development of ovarian follicles and angiogenesis. Results indicated that Mel promoted the development of ovarian follicles of 50–250 μm (secondary follicles) and periphery angiogenesis, while FSH remarkably increased the number of antral follicles and periphery angiogenesis. Mechanically, Mel and FSH may regulate the expression of VEGF and antioxidant enzymes in different follicular stages. In conclusion, Mel primarily acted on the secondary follicles, while FSH mainly promoted the development of antral follicles. They both conduced to related periphery angiogenesis by increasing the expression of VEGF. These findings may provide new targets for the regulating of follicular development.

Prediction of Ovarian Follicular Dominance by MRI Phenotyping of Hormonally Induced Vascular Remodeling

In the mammalian female, only a small subset of ovarian follicles, known as the dominant follicles (DFs), are selected for ovulation in each reproductive cycle, while the majority of the follicles and their resident oocytes are destined for elimination. This study aimed at characterizing early changes in blood vessel properties upon the establishment of dominance in the mouse ovary and application of this vascular phenotype for prediction of the follicles destined to ovulate. Sexually immature mice, hormonally treated for induction of ovulation, were imaged at three different stages by dynamic contrast-enhanced (DCE) MRI: prior to hormonal administration, at the time of DF selection, and upon formation of the corpus luteum (CL). Macromolecular biotin-bovine serum albumin conjugated with gadolinium-diethylenetriaminepentaacetic acid (b-BSA-GdDTPA) was intravenously injected, and the dynamics of its extravasation from permeable vessels as well as its accumulation in the antral cavity of the ovarian follicles was followed by consecutive T₁-weighted MRI. Permeability surface area product (permeability) and fractional blood volume (blood volume) were calculated from b-BSA-GdDTPA accumulation. We found that the neo-vasculature during the time of DF selection was characterized by low blood volume and low permeability values as compared to unstimulated animals. Interestingly, while the vasculature of the CL showed higher blood volume compared to the DF, it exhibited a similar permeability. Taking advantage of immobilized ovarian imaging, we combined DCE-MRI and intravital light microscopy, to reveal the vascular properties of follicles destined for dominance from the non-ovulating subordinate follicles (SFs). Immediately after their selection, permeability of the vasculature of DF was attenuated compared to SF while the blood volume remained similar. Furthermore, DFs were characterized by delayed contrast enhancement in the avascular follicular antrum, reflecting interstitial convection, whereas SFs were not. In this study, we showed that although DF selection is accompanied by blood vessel growth, the new vasculature remained relatively impermeable compared to the vasculature in control animal and compared to SF. Additionally, DFs show late signal enhancement in their antrum. These two properties may aid in clinical prediction of follicular dominance at an early stage of development and help in their diagnosis for possible treatment of infertility.

Postpartum nutrition affects the insulin-like growth factor system in dominant follicles and plasma of anestrous beef cows

Effects of nutrition on insulin-like growth factor-I (IGF-I), IGF binding proteins (IGFBP), and insulin in plasma and dominant follicles were evaluated at day 72 and 56 (Exp. 1, n = 12 and Exp. 2, n = 28, respectively) postpartum in anovulatory primiparous beef cows. Cows were stratified based on body condition score at calving and randomly assigned to nutritional treatments: maintain (M), 2.27 kg of a 40 % CP supplement per day and ad libitum hay; or gain (G), ad libitum access to a 50 % concentrate diet and ad libitum hay. Blood samples were collected twice weekly starting 30 days postpartum. Ovarian follicles were evaluated using ultrasonography commencing 42 (Exp. 1) or 30 (Exp. 2) days postpartum. Body weight and condition score were greater (P < 0.05) for cows of G than M groups and postpartum interval to luteal function was longer for cows of the M than G group. Insulin and IGF-I concentrations in follicular fluid (FF) and plasma were greater (P < 0.05) for cows of the G than M group at follicular aspiration. Plasma and FF IGFBP4 and IGFBP5 concentrations were greater (P <  0.05) in Exp. 2, and IGFBP5 was greater in Exp. 1 for cows of the G than M group. Treatment did not affect FF steroid concentrations or granulosal cell CYP19A1, PAPPA, IGFBP4, and IGFBP5 mRNA abundance. These results indicate concentrations of IGF-I, insulin, IGFBP4, and IGFBP5 in FF and plasma are affected by nutritional intake and may be related to follicular function.

Transcriptome Analysis of circRNA and mRNA in Theca Cells during Follicular Development in Chickens

Development of ovarian follicles requires interactions between granulosa cells, theca cells, and oocytes. Multiple transcription levels are involved but information about the role of noncoding RNAs, especially circular RNAs (circRNAs), is lacking. Here, we used RNA sequencing to profile circRNAs and mRNAs in theca cells from three types of follicle: small yellow follicles (SYF), the smallest hierarchical follicles (F6), and the largest hierarchical follicles (F1). Using bioinformatics analysis, we identified a total of 14,502 circRNAs in all theca cells, with 5622 widely distributed in all stages of development. Differential expression analysis suggested that some genes display differential isoforms during follicular development. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed enrichment of both differentially expressed circRNAs and mRNAs in pathways associated with reproduction, including the TGF-β signaling pathway, oocyte meiosis, and vascular smooth muscle contraction. Our study provides the first visual information about circRNAs and mRNAs in theca cells during follicle development in chickens and adds to the growing body of knowledge about theca cells.

Regulation of the transcription factor E2F8 gene expression in bovine ovarian cells

Overexpression of the transcription factor, E2F8, has been associated with ovarian cancer. Objectives of this study were to determine: 1) if E2F8 gene expression in granulosa cells (GC) and theca cells (TC) change with follicular development, and 2) if E2F8 mRNA abundance in TC and GC is hormonally regulated. Using real-time PCR, E2F8 mRNA abundance in GC and TC was greater (P < 0.05) in small than large follicles. FGF9 induced an increase (P < 0.05) in E2F8 mRNA abundance by 1.6- to 7-fold in large-follicle (8-20 mm) TC and GC as well as in small-follicle (1-5 mm) GC. Abundance of E2F8 mRNA in TC was increased (P < 0.05) with FGF2, FGF9 or VEGFA treatments alone in vitro, and concomitant treatment of VEGFA with FGF9 increased (P < 0.05) abundance of E2F8 mRNA above any of the singular treatments; BMP4, WNT3A and LH were without effect. IGF1 amplified the stimulatory effect of FGF9 on E2F8 mRNA abundance by 2.7-fold. Collectively, our studies show for the first time that follicular E2F8 is developmentally and hormonally regulated indicating that E2F8 may be involved in follicular development.

Hormonal regulation of vascular endothelial growth factor A (VEGFA) gene expression in granulosa and theca cells of cattle1

Vascular endothelial growth factor A (VEGFA) stimulates angiogenesis and is associated with increased vascularity in ovarian follicles of cattle. The objectives of this study were to investigate the developmental and hormonal regulation of VEGFA expression in ovarian granulosa and theca cells (TC) of cattle. Bovine ovaries were collected from a local slaughterhouse and granulosa cells (GC) and TC were collected from small (SM; 1 to 5 mm) and large (LG; 8 to 20 mm) follicles. Cells were collected fresh or cultured in serum-free medium and treated with various factors that regulate angiogenesis and follicular development. RNA was collected for analysis of VEGFA mRNA abundance via quantitative PCR. In SM-follicle GC (SMGC), prostaglandin E2 (PGE2) and FSH decreased (P < 0.05) VEGFA mRNA abundance by 30 to 46%, whereas in LG-follicle GC (LGGC), PGE2 and FSH were without effect (P > 0.10). In SMGC, dihydrotestosterone (DHT), sonic hedgehog (SHH), and growth differentiation factor-9 (GDF9) decreased (P < 0.05) VEGFA expression by 30 to 40%. Fibroblast growth factor-9 (FGF9) and estradiol (E2) were without effect (P > 0.10) on VEGFA mRNA in both SMGC and LGGC, whereas progesterone increased (P < 0.05) VEGFA mRNA in LGGC but had no effect in LGTC. Bone morphogenetic protein-4 (BMP4), LH, and FGF9 increased (P < 0.05) abundance of VEGFA mRNA by 1.5- to 1.9-fold in LGTC. Insulin-like growth factor-1 (IGF1) was without effect (P > 0.10) on VEGFA mRNA in both TC and GC. An E2F transcription factor inhibitor, HLM0064741 (E2Fi), dramatically (i.e., 8- to 13-fold) stimulated (P < 0.01) the expression of VEGFA mRNA expression in both SMGC and LGTC. Abundance of VEGFA mRNA was greater (P < 0.05) in LGGC and SMGC than in LGTC. Also, SMTC had greater (P < 0.05) abundance of VEGFA mRNA than LGTC. In conclusion, VEGFA mRNA abundance was greater in GC than TC, and VEGFA expression decreased in TC during follicle development. Some treatments either suppressed, stimulated, or had no effect on VEGFA expression depending on the cell type. The inhibition of E2F transcription factors had the greatest stimulatory effect of all treatments evaluated, and thus, E2Fs may play an important role in regulating angiogenesis during follicle growth in cattle.

Follicle vascularity coordinates corpus luteum blood flow and progesterone production

Colour Doppler ultrasonography was used to compare the ability of preovulatory follicle (POF) blood flow and its dimensions to predict the size, blood flow and progesterone production capability of the subsequent corpus luteum (CL). Cows (n=30) were submitted to a synchronisation protocol. Follicles ≥7mm were measured and follicular wall blood flow evaluated every 12h for approximately 3.5 days until ovulation. After ovulation, cows were scanned daily for 8 days and similar parameters were evaluated for the CL. Blood samples were collected and plasma progesterone concentrations quantified. All parameters were positively correlated. Correlation values ranged from 0.26 to 0.74 on data normalised to ovulation and from 0.31 to 0.74 on data normalised to maximum values. Correlations between calculated ratios of both POF and CL in data normalised to ovulation and to maximum values ranged from moderate (0.57) to strong (0.87). Significant (P<0.0001) linear regression analyses were seen in all comparisons. In conclusion, higher correlations were observed between the dimensions of POF and/or CL and blood flow of both structures, as well as POF and/or CL blood flow with plasma progesterone concentrations of the resultant CL. These findings indicate that follicle vascularity coordinates CL blood flow and progesterone production in synchronised beef cows.

Ovarian Folliculogenesis

The ovary, the female gonad, serves as the source for the germ cells as well as the major supplier of steroid sex hormones. During embryonic development, the primordial germ cells (PGCs) are specified, migrate to the site of the future gonad, and proliferate, forming structures of germ cells nests, which will eventually break down to generate the primordial follicles (PMFs). Each PMF contains an oocyte arrested at the first prophase of meiosis, surrounded by a flattened layer of somatic pre-granulosa cells. Most of the PMFs are kept dormant and only a selected population is activated to join the growing pool of follicles in a process regulated by both intra- and extra-oocyte factors. The PMFs will further develop into secondary pre-antral follicles, a stage which depends on bidirectional communication between the oocyte and the surrounding somatic cells. Many of the signaling molecules involved in this dialog belong to the transforming growth factor β (TGF-β) superfamily. As the follicle continues to develop, a cavity called antrum is formed. The resulting antral follicles relay on the pituitary gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) for their development. Most of the follicles undergo atretic degeneration and only a subset of the antral follicles, known as the dominant follicles, will reach the preovulatory stage at each reproductive cycle, respond to LH, and subsequently ovulate, releasing a fertilizable oocyte. The remaining somatic cells in the raptured follicle will undergo terminal differentiation and form the corpus luteum, which secretes progesterone necessary to maintain pregnancy.

Temperature gradients in vivo influence maturing male and female gametes in mammals: evidence from the cow

Since 1980 several reports have indicated that temperatures vary between preovulatory follicles and other ovarian tissues in rabbit, cow, pig and human. However, these observations did not achieve prominence; they were regarded as artefacts due to the use of anaesthetics and open surgery (laparotomy). Recently, without resorting to anaesthesia or surgery, direct measurements of temperature in preovulatory follicles have been performed in the cow by means of a thermistor probe introduced into the antrum under ultrasonic guidance. Such follicles revealed a mean antral (follicular fluid) temperature 0.74°C and 1.54°C cooler than uterine surface and rectal temperatures respectively in ovulating cows, whereas no such temperature differences were detected in non-ovulating cows. Cows are predominantly monovular and preovulatory follicles attain a diameter of 15–22 mm or more. These features and the timescale of response to the preovulatory gonadotrophin surge make them a valuable model for the human preovulatory follicle. Temperature gradients are interpreted primarily in a context of final maturation of gametes immediately before the onset of fertilisation. Preovulatory follicular temperature in women could be assessed by a comparable approach and might become a valuable selection guide for oocyte viability.

Vascular endothelial growth factor and angiopoietins during hen ovarian follicle development

Growth and maturation of ovarian follicles in the hen (Gallus gallus) requires a network of blood vessels that increases in complexity during development. The present studies investigate expression of vascular endothelial growth factor A (VEGF), angiopoietin1 (ANGPT1) and ANGPT2 mRNAs together with their associated receptors (VEGFR and TIE2, respectively) during maturation. Elevated expression of VEGF and its receptors is associated with healthy, compared to atretic, follicles. Levels of VEGF significantly increase, while antagonistic ANGPT2 decrease, in granulosa cells (GC) at follicle selection. By comparison, levels of VEGF, VEGFR1, VEGFR2, ANGPT1, ANGPT2 and TIE2 within the theca layer do not change (P>0.05) relative to developmental stages surrounding follicle selection (6-8mm versus 9-12mm follicles). Prior to selection, treatment with transforming growth factor β1 (TGFβ1) significantly increases levels of VEGF in undifferentiated GC from prehierarchal (6-8mm) follicles and actively differentiating GC from selected (9-12 and F4) follicles. Moreover, subsequent to selection follicle stimulating hormone (FSH) increases VEGF expression in GC from 9 to 12mm follicles, and eventually luteinizing hormone (LH) promotes VEGF expression in GC from more mature preovulatory follicles. It is concluded that prior to follicle selection VEGF expression is regulated by autocrine and paracrine actions of TGFβ1 (but not FSH), and that a comparatively limited extent of vasculature is sufficient to maintain prehierarchal follicles in a viable and undifferentiated state. At follicle selection, FSH- and subsequently LH-induced VEGF production within the GC layer enhance angiogenesis within the theca layer, which facilitates the rapid growth of preovulatory follicles via enhanced incorporation of yellow yolk.

Metabolic profile of serum and follicular fluid from postpartum dairy cows during summer and winter

This study was designed to monitor the biochemical profiles of serum and follicular fluid (FF) of postpartum dairy cows during the summer (n=30) and winter (n=30). Blood and FF (follicles ≥ 9 mm) were obtained from Girolando cows at 30, 45, 60, 75 and 90 days postpartum. The samples were collected and analysed to determine glucose, total cholesterol (TC), triglyceride (TG), urea, sodium (Na), potassium (K) and calcium (Ca) levels. Throughout the study, the following clinical variables were measured: rectal temperature (RT), respiratory rate (RR) and body condition score (BCS). In addition, the temperature humidity index (THI) was calculated for each season. During the summer season, THI was higher, BCS decreased, there was an increase in RT, and glucose, urea, Na and K serum levels were decreased (P<0.05). The levels of TC, TG, urea, K and Ca in follicular fluid increased (P<0.05). Positive correlations (P<0.05) were observed between the serum and FF levels for glucose (r=0.29), TC (r=0.24) and Ca (r=0.30). Therefore, the biochemical profile of serum and FF of dairy cows under summer heat-stress conditions demonstrates marked changes that may impair fertility during lactation.

Formation of the ovarian follicular antrum and follicular fluid

The formation of the follicular antrum and follicular fluid has received scant attention from researchers, yet both are important processes in follicular development. The central hypothesis on follicular fluid formation suggests that production by granulosa cells of hyaluronan and the chondroitin sulfate proteoglycan versican generates an osmotic gradient. This gradient draws in fluid derived from the thecal vasculature. Inter-alpha-trypsin inhibitor is also present in follicular fluid at least in species with large follicles, and inter-alpha-trypsin inhibitor and versican could additionally bind or cross-link with hyaluronan, resulting in the retention of these molecules within the follicular antrum. Barriers to the movement of fluid across the membrana granulosa are apparently minimal, as even relatively large serum proteins are present in follicular fluid. Despite the relative permeability of the follicular wall, aquaporins are present in granulosa cells and could be actively involved in the transport of water into the follicle. The formation of an antrum also requires movement of granulosa cells relative to each other to allow the fluid to accumulate. This presumably involves remodeling of cell-cell junctions and in species with small follicles may involve death of centrally located granulosa cells. Remodeling of the stroma and thecal layers also accompanies growth and expansion of the antrum and presumably involves similar processes that accompany growth of other glands.

Blood vessel remodeling in pig ovarian follicles during the periovulatory period: an immunohistochemistry and SEM-corrosion casting study

BACKGROUND

The present research aims to describe the process of vascular readjustment occurring in pig ovary during the periovulatory phase (from LH surge to ovulation) that drives the transformation of the follicle, a limited blood supplied structure, into the corpus luteum, a highly vascularised endocrine gland required to maintain high levels of progesterone in pregnancy. The swine model was chosen because it is characterized by a long periovulatory window (about 40-44 hrs-similar to human) that permits to recover follicles at a precise endocrinological timing.

METHODS

By validated hormonal protocol (eCG+hCG), able to mimic the physiologic gonadotropin stimulation, preovulatory follicles (PreOFs, 60 h-eCG), follicles in the middle (early periovulatory follicles, EPerOFs, 18 h-hCG) or late (LPerOFs, 36 h-hCG) periovulatory phase were isolated from prepubertal gilts. To understand the angiogenic process, morphological/morphometrical analyses were performed by combining immunohistochemistry (IHC) and SEM of vascular corrosion casts (VCC) techniques.

RESULTS

PreOFs showed a vascular plexus with proliferating endothelial cells (EPI). This plexus was characterized by a dense inner capillary network, with angiogenic figures, connected to the outer network by anastomotic vessels (arterioles and venules of the middle network). EPerOFs decreased their EPI, blood vessel extension in the outer network, and evidenced a reduced compactness of blood vessels. In LPerOFs, a rapid neovascularization was associated to an intensive tissue remodeling: the follicle acquired an undulated aspect presenting arterioles/venules near the basal membrane, increased vascular extension by EPI, sprouting and non-sprouting angiogenesis.The analysis of vascular geometric relations and branching angles evidenced similar values at all stages.

CONCLUSION

These data allow us to hypothesize that EPerOFs are in a quiescent status. LPerOFs represent the "metamorphic" follicles that rapidly turn-on angiogenesis to sustain a successful corpus luteum formation. Particularly, it is interesting to underlie that the non-sprouting angiogenesis, typical of structures in rapid neovascularization, occurred only in the LPerOFs. Moreover, vascular geometric relations showed as blood vessel remodeling occurs with the "maximum output and the minimum energetic expense".This knowledge will allow to better understand the mechanisms regulating the reproductive success and to clarify the complex physiological angiogenic process in adult tissues.

Genital blood flow and endometrial gene expression during the preovulatory period after Prostaglandin F(2alpha)-induced luteolysis in different luteal phases in cows

The aim of this study was to examine the hypothesis that follicular and uterine perfusion as well as endometrial gene expression during the ovulatory period differs after induction of luteolysis during the 1(st) follicular wave compared with the 2(nd) wave or in intact cycle. Nine healthy non-lactating Holstein-Friesian cows were examined during three estrous cycles. A prostaglandin F(2alpha) analogue (PGF) was administered randomly either on Day 7 (1(st) wave cycle) or Day 11 (2(nd) wave cycle) after detection of ovulation (=Day 1). No hormonal treatment was used in the intact cycle with spontaneous ovulation. Transrectal Doppler sonography was conducted daily after PGF injection and in the intact cycle beginning on Day 18 of the estrous cycle until ovulation. Follicular blood flow (FBF) was determined by measuring the maximum area of colour pixels on digitalized images of the follicles. Uterine blood flow was quantified by the time-averaged maximum velocity (UTAMV) and pulsatility index (PI) in both uterine arteries. Blood flow measurements were carried out on Days -1 and 0. Endometrial biopsy specimens were taken on Day 1 and analyzed for the gene expressions of estrogen, progesterone, oxytocin and VEGF receptors and eNOS and iNOS using RT-PCR. The interval from PGF injection to ovulation was shorter (P<0.05) in 1(st) wave cycles than in 2(nd) wave cycles. On Days 0 and -1, FBF was greater (P<0.05) in 1(st) wave cycles than in intact and 2(nd) wave cycles. On Day -1, UTAMV was greater (P<0.05) in 1(st) wave cycles than in intact and 2(nd) wave cycles. There were no differences (P>0.05) in FBF and UTAMV values between 2(nd) wave and intact cycles. No differences (P>0.05) were detected in the gene expressions of endometrial receptors and enzymes between intact, 1(st) and 2(nd) wave cycles. The results show that follicular and uterine perfusion during the ovulatory phase are higher after induction of luteolysis during the 1(st) follicular wave compared with the 2(nd) wave or intact cycle. There were no effects on endometrial gene expression.

Early apoptosis is associated with improved developmental potential in bovine oocytes

The poor quality of oocytes may be the main reason for the low efficiency of the current in vitro embryo production. However, efforts are required to understand the mechanisms of oocyte development, which is believed to be largely regulated by apoptosis in vivo. The aim of this study was to investigate the levels of apoptosis in bovine immature oocytes with different developmental potentials and to determine whether early apoptosis in bovine oocytes is correlated with their subsequent development. Cumulus-oocyte complexes (COCs) were selected and classified into four groups according to oocyte cytoplasm and cumulus status. Early and late stages of apoptosis were detected by Annexin-V and TUNEL staining, respectively. Developmental competence was evaluated by nuclear maturation (MII) after in vitro maturation and development rates in different stages following in vitro fertilization. Meanwhile, the transcripts of Bcl-2 and Bax genes were carried out in immature oocytes by real-time RT-PCR. Results indicated that Annexin-V-positive oocytes were detected in various groups at different percentages, and Group III showed the highest positive ratio. No TUNEL-positive oocytes were found in any immature COCs. Group III oocytes demonstrated the highest nuclear maturation, cleavage, blastocyst, and hatching blastocyst rates. Meanwhile, Group III oocytes exhibited the highest Bax (initiating apoptosis) transcriptional level and the lowest Bcl-2 (preventing apoptosis) transcriptional level. Taken together, Annexin-V and quantitative PCR results indicated that early apoptosis was beneficial for developmental competence, while TUNEL staining showed that none of the immature oocytes were undergoing late-stage apoptosis. This is the first time that Bax and Bcl-2 transcripts were characterized in the immature bovine oocyte, and results indicated that the genes are good markers of early apoptosis and embryo development. This research overthrows the traditional view that oocytes undergoing apoptosis have poor developmental competence, and the findings will facilitate oocyte selection and improvement of in vitro embryo production.