Uterine histology of the dasyurid marsupial, Antechinus stuartii: relationship with differentiation of the embryo.

Uterine samples from Antechinus stuartii on days 1, 4, 6, 8, 11, 13, 15, 18, 21 and 23 after ovulation were examined histologically. Animals were pregnant, nonpregnant and unmated, or nonpregnant and mated but found to have only unfertilized eggs on autopsy. The histological parameters used were thickness of the myometrium, endometrial stroma, and endometrial epithelium, and density of uterine stromal glands and of lymphocytes at the endometrial basal lamina. Overall, the fluctuation patterns of these parameters were superficially similar between pregnant and nonpregnant animals (mated or unmated). However, statistically significant differences were detected between pregnant and unmated nonpregnant animals in every parameter examined at nearly every time point except day 13. Comparison of these results with known data on embryonic stages, corpus luteum development and plasma progesterone concentrations revealed that the gravid uterus underwent histological changes co-incident with changes in both progesterone concentration and developmental delay or embryonic arrest. It was concluded that the uterus mediates progesterone-driven changes in embryonic developmental rate. Although determination of number of lymphocytes provided inconclusive evidence of cellular immunity against embryos, the possibility that embryonic signalling to the uterus occurs is discussed.