Mitotic activity of prolactin cells in the pituitary glands of male and female rats of different ages.

The anterior pituitary of colchicine-pretreated male and female rats from 20 days to 12 months of age was stained immunohistochemically with anti-rat prolactin serum. Immunoreactive mitotic cells were identified in all groups of rats. In adult female rats the mitotic index of prolactin cells was higher at oestrus than at other stages of the oestrus cycle and than that in male rats of comparable ages. If adult female rats were ovariectomized on the second day of dioestrus or on the day of proestrus, the mitotic indices at presumptive oestrus were less than those in sham-operated controls at oestrus. Estrogen administration to ovariectomized rats significantly elevated the mitotic index of prolactin cells at 48 h after the treatment. The mitotic indices of prolactin cells in female rats reached a peak at 60 days of age, and then decreased with age. In male rats the mitotic indices showed a steady decrease from the value at 20 days of age. A sex difference in the mitotic indices of prolactin cells was noted from 60 days to 12 months of age. The present results clearly demonstrate that differentiated prolactin cells can undergo mitosis and that a sex difference in the mitotic activity of prolactin cells is present during adult life.