The aims of this study were to investigate whether the number of antral follicles (AF) in the ovaries of Nelore cows is influenced with the developmental competence of oocytes to reach the blastocyst stage and to quantify the mRNA abundance of genes associated with folliculogenesis and oogenesis in granulosa and cumulus cells. A total of 168 cows were distributed into two experimental groups according to the number of AF, low (≤31) and high AF (≥92), which were determined based on the mean number of AF (61.14) ± SD (30.43). Granulosa and cumulus cells were used to assess the mRNA expression of 16 genes. Cumulus cells from cows with low AF had higher mRNA expression of genes involved in meiosis resumption (NPR-2, NPR-3) and cumulus cell expansion (FGF10), as well as a transcription factor involved in the regulation of oocyte maturation and cell proliferation (STAT3). Conversely, granulosa cells from females with high AF had higher expression of PGR and AMHR2a, which are involved in meiosis resumption and cumulus cell expansion. Cumulus-oocyte complexes (COCs) were collected from 356 cows with low and high AF populations to evaluate embryo development. Cleavage and blastocyst rates did not differ between the groups. In conclusion, our findings revealed that genes involved in folliculogenesis and oogenesis are differently expressed in cumulus and granulosa cells of cows having low and high numbers of AF. These molecular differences suggest that the regulation of oocyte maturation, meiotic resumption and cumulus expansion may be influenced by the number of AFs. However, the variations in gene expression were not associated with in vitro oocyte developmental competence to reach the blastocyst stage, which confirms that oocytes from Nelore cows with low and high numbers of AF are similarly able to mature, regulate the fertilization process and support pre-implantation embryo development.

Natriuretic peptides (NPs) are known to regulate reproductive events in polyovulatory species, but their function and regulation in monovulatory species remain to be fully characterized. Using a well-established in vivo model, we found that bovine granulosa cells from follicles near the deviation stage express mRNA for the three NP receptors (NPR1, NPR2 and NPR3), but not for NP precursors (NPPA, NPPB and NPPC). The abundance of NPR3 mRNA was higher in dominant compared to subordinate follicles at the expected time of follicular deviation. After deviation, mRNA for all NP receptors was significantly more abundant in the dominant follicle. Intrafollicular inhibition of oestrogen receptors downregulated NPR1 mRNA in dominant follicles. In granulosa cells from preovulatory follicles, NPPC mRNA increased at 3 and 6 h after systemic GnRH treatment, but decreased at 12 and 24 h to similar levels observed in samples collected at 0 h. After GnRH treatment, NPR1 mRNA was upregulated at 24 h, NPR3 mRNA gradually decreased after 3 h, while NPR2 mRNA was not regulated. The mRNA expression of the enzyme FURIN increased at 24 h after GnRH treatment. These findings revealed that the expression of mRNA encoding important components of the NP system is regulated in bovine granulosa cells during follicular deviation and in response to GnRH treatment, which suggests a role of NP system in the modulation of these processes in monovulatory species.

The LH surge induces functional and morphological changes in granulosa cells. Mechanistic target of rapamycin (mTOR) is an integrator of signalling pathways in multiple cell types. We hypothesized that mTOR kinase activity integrates and modulates molecular pathways induced by LH in granulosa cells during the preovulatory period. Cows were ovariectomized and granulosa cells collected at 0, 3, 6, 12 and 24 hr after GnRH injection. While RHEB mRNA levels increased at 3 and 6 hr, returning to basal levels by 12 hr after GnRH treatment, RHOA mRNA levels increased at 6 hr and remained high thereafter. Western blot analyses revealed increased S6K phosphorylation at 3 and 6 hr after GnRH injection. Similarly, mRNA levels of ERK1/2, STAR and EGR-1 were higher 3 hr after GnRH treatment. Rapamycin treatment inhibited mTOR activity and increased AKT activity, but did not alter ERK1/2 phosphorylation and EGR1 protein levels in cultured bovine granulosa cells. Rapamycin also inhibited LH-induced increase in EREG mRNA abundance in granulosa cells in vitro. However, intrafollicular injection of rapamycin did not suppress ovulation. These findings suggest that mTOR is involved in the control of EREG expression in cattle, which may be triggered by LH surge stimulating RHEB and S6K activity.

Oocyte competence and reproductive biology in prepubertal heifer calves are not fully understood. Multiple publications have reported high oocyte yields recovered from calves aged 2–6 months old but low embryo development rates following in vitro embryo production. The objective of this study was to characterise the developmental competence of oocytes from young calves. We report herein the oocyte/embryo yields obtained from 6 Holstein heifer calves that were subjected to gonadotropin stimulation and laparoscopic ovum pick-up (LOPU) every 2 weeks, starting at 2 months of age and ending at 5 months of age. The LOPU was conducted under general anaesthesia with the animal lying in dorsal recumbency on a table with a 45-degree angle to facilitate the visualisation of reproductive organs. Briefly, looking through the laparoscope, the ovarian surface was exposed by pulling from the fimbria with an atraumatic grasping forceps. The follicle contents were aspirated using a 20 G needle mounted on a pipette, which was connected to a collection tube and a vacuum pump. Media and procedures for aspiration, in vitro maturation (IVM), IVF, and in vitro culture (IVC) were standard in use for commercial adult bovine embryo production. Because of the small number of animals and the multifactorial variables in play (age, number of previous treatments and aspirations, etc.), in this preliminary study we focused on the overall oocyte/embryo yield and the potential effects of LOPU on ovarian integrity. A total of 766 follicles were aspirated (avg. 17/calf per session) resulting in 625 cumulus-oocyte complexes (COC) recovered (avg. 14/calf per session; 82% recovery rate). A total of 457 (73%) COC were graded eligible for IVM, of which 353 cleaved (77%) and 109 (24%) reached a viable blastocyst stage at the end of IVC, of which 42 (38.5%) were graded as freezable. In balance, ~2 viable blastocysts/calf per session were produced. No adhesions or sequels were observed in the animals up to the last LOPU session, as well as 2 weeks after the last LOPU when the animals were evaluated by rectal palpation by an experienced OPU veterinary practitioner. Further studies will look into other aspects of oocyte developmental competence to better understand this biological process.