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.