Luteal and follicular populations in the ovary of the opossum (Didelphis virginiana) after ovulation.

The maximum diameters of all luteal and antral follicular structures were measured in opossum ovaries on Days 3, 7 and 11 after oestrus, and follicles were classed as developing or atretic. Ovarian weights and luteal diameters were equivalent in comparisons of pregnant and non-pregnant animals on each day. The number of CL (range 45-85) per animal per cycle indicated a very high ovulation rate for a mammal. Luteinized follicles (1-4 per ovary) were identified in all Day-3 ovaries. Ovarian weight, luteal diameter and follicular diameter were greater on Day 7 than on Days 3 or 11. More antral follicles occurred on Day 11 (120 +/- 10.7, s.e.m.) than Day 3 (77.8 +/- 8.8), although the percentage of atretic antral follicles also increased from 20% to 50 and 57% on Days 7 and 11. These increases were not accompanied by an increase in the number of developing antral follicles (58.1 +/- 4.0), thus indicating a mid-luteal increase in the rate of follicular recruitment, of growth and of atresia in the opossum.