275 FOLLICULAR ATRESIA IN SMALL, NON-FSH-DEPENDENT BOVINE FOLLICLES IS ASSOCIATED WITH INCREASED DEVELOPMENTAL POTENTIAL OF OOCYTES FOLLOWING IVP

Abstract

Oocytes from small, non-FSH-dependent follicles are associated with reduced developmental competence following in vitro embryo production (IVP) compared to oocytes from larger follicles. It has been suggested that, for small follicles, oocytes derived from atretic follicles are more developmentally competent than those from healthy follicles (Blondin P and Sirard MA, 1995 Mol. Reprod. Dev. 41, 54–62). Little is known of the characteristics of small follicles that support developmentally competent oocytes. Here we examine the development to blastocyst stage of oocytes collected from histologically-assessed bovine 2–5 mm follicles. Ovaries were obtained at a local abattoir;; 4 follicles were dissected from each ovary and oocytes were recovered. A section of each follicle wall was taken and fixed in 2.5% glutaraldehyde for histological assessment of the follicle and characterization of the morphology of the follicular basal lamina by electron microscopy (Irving-Rodgers HF and Rodgers RJ, 2000 J. Reprod. Fert. 118, 221–228). Oocytes recovered from follicles underwent IVP utilizing a novel single IVP system. Oocytes were matured for 24 h (10 μL per COC) in TCM199, supplemented with FSH, hCG, FCS, cysteamine and pyruvate. Mature oocytes were inseminated with 1 × 10⁶ sperm mL⁻¹ for an additional 24 h using Bovine Fertilization Medium (10 μL per COC;; Cook, Australia). Following insemination, putative zygotes were stripped of remaining cells and placed within individual micro-wells prepared in 1% agar in Bovine Early Cleavage Medium, Cook, Australia. The agar (350 μL) was prepared within wells of a 4-well plate and small plugs of agar were removed to form micro-wells. The agar was over-laid with 450 μL of Early Cleavage Medium and 250 μL mineral oil, and equilibrated overnight before putative zygotes were placed individually within micro-wells. Culture was performed under 7% O₂, 6% CO₂, and 87% N₂ at 39°C. On Day 5 following insemination, fetal calf serum (final concentration 10% v/v) was added to facilitate blastocyst development. Blastocyst formation was assessed on Day 8. A total of 211 oocytes were cultured and 69% were from healthy follicles;; 67 oocytes (32%) had developed to the blastocyst stage by Day 8. Forty-three percent of oocytes recovered from atretic follicles (28/65) had developed to the blastocyst stage by Day 8, as compared to only 27% (39/146) oocytes recovered from healthy follicles, this difference was significant (P < 0.05, chi-square analysis). Seventy-eight percent (14/18) of oocytes from healthy follicles with additional follicular basal lamina material (Irging-Rodgers HF and Rodgers RJ, 2000 J. Reprod. Fert. 118, 221–228) failed to develop, whereas only 44% (4/9) of oocytes from healthy follicles with a normal basal lamina failed to develop (P > 0.08). The present study finds a direct association between the follicle morphology and oocyte maturational potential within non-FSH dependent follicles, revealing that high levels of development (>40%) can be obtained from atretic follicles. Furthermore, differences between healthy follicles may also contribute to developmental variation.