The relationship of follicle atresia to follicle size, oocyte recovery rate on aspiration, and oocyte morphology in the mare.

Oocytes were collected by aspiration of follicles from horse ovaries obtained at surgery or post-mortem. The oocytes were classified according to morphology of the ooplasm and cumulus. The size of the corresponding follicles was measured, and sections of the follicles were fixed and examined histologically to determine the stage of viability or atresia. In Part 1, 11 pairs of ovaries were examined and all follicles were sectioned; in Part 2, 9 pairs of ovaries were examined and only those follicles from which oocytes were recovered were sectioned. The number of follicles examined per pair of ovaries in Part 1 (average +/- SD) was 12.9 +/- 4.1. The proportion of follicles that were viable increased with increasing follicular size (P < 0.01); the percentage of viable follicles was 21, 42 and 83% for follicles < 10 mm, 10 to 19 mm, and >/= 20 mm in diameter, respectively. The overall oocyte recovery rate on aspiration of follicles was 46%. There was no significant difference in the oocyte recovery rate between viable and atretic follicles. A significantly higher proportion of oocytes recovered from viable follicles had granular ooplasm (64 vs 39%; (P < 0.05); whereas significantly more oocytes from atretic follicles had a misshapen or dense ooplasm (23 vs 6%; P < 0.05), or an expanded or pyknotic cumulus (24 vs 6%; P < 0.05). The most common cumulus morphology (63% of oocytes from viable follicles and 48% of oocytes from atretic follicles) was presence of only the corona radiata. Only 11% of oocytes from viable follicles and 9% of oocytes from atretic follicles had a complete cumulus present.