DEVELOPMENTAL COMPETENCE OF GOAT OOCYTES FROM FOLLICLES OF DIFFERENT SIZE CATEGORIES FOLLOWING MATURATION, FERTILIZATION AND CULTURE IN-VITRO

The aim of the present study was to investigate the effect of size of follicle from which goat oocytes originate on their subsequent ability to be fertilized and to undergo early embryonic development ia vitro. Nonatretic follicles larger than 2 mm in diameter were dissected and distributed into three groups according to size (small: 2-3 mm; medium : 3.1-5 mm; large: > 5 mm). Cumulus-oocyte complexes were isolated fro m the follicles and only those with a compact multilayered cumulus wer e selected for in vitro maturation. After maturation, 70%, 83% and 97% of oocytes from small, medium and large follicles, respectively, were at metaphase II. After in vitro fertilization, no significant differe nce was observed in the cleavage rate 40 h after insemination between oocytes from small (46%) and medium; (55%) follicles, and between oocy tes from large follicles (69%) and ovulated oocytes (75%). After in vi tro culture, significantly more embryos from small follicles arrested before or at the 8-16 cell stage (84% compared with 53%, 45% and 39% o f embryos from medium and large follicles and ovulated oocytes, respec tively). The proportion of morulae and blastocysts obtained was 10% an d 6% from small follicles, 35% and 12% from medium follicles, 29% and 26% from large follicles and 20% and 41% from ovulated oocytes. Oocyte s from small and medium follicles yielded a significantly lower propor tion of hatched blastocysts (0% and 3%, respectively) than did those f rom large follicles and from ovulated oocytes (15% and 34%, respective ly). These results indicate that developmental competence of goat oocy tes is acquired progressively during follicular growth and that only a small proportion of oocytes, those isolated from large antral follicl es, have the capacity to progress to the blastocyst stage following in vitro maturation, fertilization and culture.