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Uterine histology and reproductive cycles in pregnant and non-pregnant opossums, Didelphis virginiana

Summary. Uteri (N = 18) were collected on Days 3, 7, and 11 of the oestrous cycle or gestation. Reproductive cycles were monitored by vaginal cytology in captive opossums. There was no difference in length of cycles between reproductive states (29·2 ± 1·0 (s.e.m.) days). All uteri on Day 3 or Day 7 were similar in total volume, endometrial width, and glandular abundance. However, on Day 11 endometrial widths were significantly less in pregnant animals and stroma appeared diffuse and flocculent in the non-pregnant uteri. The chorio-vitelline membrane induced no change in the size, conformation, or structure of the uterine epithelium. In some areas, trophoblast cells or their cellular fragments were attached to the epithelium, indicating that the embryonic-maternal relationship might entail adhesion or interdigitation as well as apposition. A mucopolysaccharide layer also characterized this interface of heterologous tissues.