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Timing of activation of primordial follicles in mature rats is only slightly affected by fetal stage at meiotic arrest.

The objective of this experiment was to test the production-line hypothesis by determining whether timing of activation of primordial follicles at different ages throughout the life of female rats was associated with embryonic age at meiotic arrest of their oogonia. Meiotic arrest of oogonia can occur at any time during mid to late gestation in rats. Pregnant rats received injections of 5-bromo-2'-deoxyuridine (BrdU) on either e17 (treatment 1) or on e19 (treatment 2) (e1 = embryonic day 1 [sperm-positive day 1]). Female offspring (pupping = Day 0) were killed at 30 (6/group), 100 (8/group), 180 (8/group) and 350 (7/group) days of age. Immunohistochemistry was used to determine which primordial follicles contained oocytes and pregranulosa cells that were undergoing DNA synthesis at the time of BrdU injection. The percentage of primordial and primary follicles labeled with BrdU was higher (p < 0.05) at all ages with e17 than with e19 injections. There was a significant (p < 0.001) treatment-by-age interaction in the number of BrdU-containing growing follicles. Growing follicles, however, were found at all ages from both e17 and e19 injections. It is concluded that gestational day of entry into meiotic arrest does influence age of activation of primordial follicles, but it is a minor effect.