Contribution of the Ovary Versus Hypothalamus-Pituitary to Termination of Estrous Cycles in Aging Rats

Vaginal smear patterns were monitored for 80 days after orthotopic ovarian transplants (OvTr) between young and old rats to examine the importance of the ovary and the hypothalamus-pituitary in the termination of estrous cyclic activity. At the end of this period, the rats were sacrificed at 2200 h, blood was collected for measurement of ovarian steroids and gonadotropins, and ovaries were prepared for histological study. A second experiment examined the effect of a reduction in ovarian tissue by unilateral ovariectomy of old rats on estrous cyclicity during the first 16 days following surgery. During the 80 days of observation, 25 % of the old and 0 % of the young sham-operated rats showed irregular estrous cycles. This decrease in cyclicity followed a decrease in the total number of oocytes in the ovaries. Rats with OvTr did not cycle as regularly as young or old intact rats; and 75 % of both young and old recipients with OvTr were acydic by Day 80. Ovarian factors were implicated in the termination of cyclicity since: 1) old ovaries in either old or young recipients were found to have a limited ability to maintain cyclicity compared to young or prepuberal ovaries; 2) rats with OvTr which continued to cycle had more ovarian tissue and oocytes than rat