Intraepithelial lymphocytes in the bovine uterus during the oestrous cycle and early gestation.

The numbers of lymphocytes occurring in 5 mm of uterine epithelium were counted in cyclic heifers on Days 5, 10, 15 and 20 after oestrus and in pregnant heifers between Days 19 and 27 after insemination. Specimens were taken from the oviducal, middle and body regions of the ipsilateral and contralateral uterine horns of the pregnant and cyclic animals. Mean +/- s.e.m. lymphocyte numbers in cyclic animals were 25.8 +/- 1.5 on Day 5, 19.7 +/- 5.5 on Day 10, 22.0 +/- 0.6 on Day 15 and 28.2 +/- 1.5 on Day 20 after oestrus. There was a significant reduction in lymphocyte numbers as the chorion and uterine epithelium developed into a functional placenta from 24.5 +/- 1.1 on Day 19 to 9.0 +/- 0.9 on Day 27. Qualitative observations suggested an increase in degenerating cells concomitant with the decrease in lymphocytes. Some of these degenerating cells may have been lymphocytes while others were probably uterine epithelial cells. As gestation progressed and the placenta was established, the number of potentially damaging cells at the fetal-maternal interface was reduced.