Alteration of Oocytes and Follicles by Delayed Ovulation1

Changes in the preovulatory oocyte and follicle were studied during delayed ovulation in the normal 4-day cycling rat. Ovulation was delayed 48 hr by pentobarbital sodium injection (30 mg/kg body weight, ip). Untreated control groups were sacrificed at 3-hr intervals starting at 12 noon on day 3 (proestrus) and continuing to 6 AM on day 4 (estrus). Groups of treated animals (delayed ovulations) were sacrificed at the same intervals from 6 PM on day 3 to 6 AM on day 6 (day of ovulation). In each rat the progress of oocyte maturation and follicular growth was evaluated histologically. In the control group, oocytes remained in meiotic arrest only until 6 PM on day 3, while with delayed ovulation, the oocytes apparently remained in this stage an additional 48 hr. After arrest, meiotic development was more rapid in the treated animals than in the controls. Mean follicular diameters increased in the controls from 572 μ at 12 noon on the day of proestrus to 695 μ at 3 AM on the day of estrus. On the other hand, during the 2-day delay period, follicular growth continued to a maximum of 809 μ at 12 midnight on day 5. The data demonstrate that a 48-hr delay in ovulation causes a 48-hr delay in meiosis while follicular growth continues.