Early embryonic development and ovarian activity during concurrent pregnancy and lactation in the hopping-mouse Notomys alexis

Abstract

In the hopping-mouse, following a postpartum mating, embryos enter the uterus on about day 5. Implantation occurs on about day 7 if there are no suckling pups, but zona-free blastocysts may remain up to day 18 if from four to seven suckling pups are present. Implantation is eccentric and initially involves interdigitation of trophectoderm and uterine epithelial cells, followed by epithelial cell displacement. The orientation is antimesometrial and, during further development, the embryo invaginates into the yolk-sac cavity. In the ovary, corpora lutea develop during the first few days of pregnancy and then remain unchanged in size or cellular morphology until implantation, regardless of the length of its delay. Peripheral progesterone levels likewise show little change during the preimplantation period. After implantation, a similar number of corpora lutea are found but they increase progressively in size due to luteal cell hypertrophy. The cells show all the organelles typical of steroid hormone synthetic activity and there is a corresponding increase in blood progesterone levels at this time. Vesicular follicles are present throughout concurrent pregnancy and lactation and are larger after implantation. There is no evidence of spontaneous ovulation except at the time of parturition.