We have now confirmed that ovulations detected in the ewes of the present study during MAP treatment (Figs. 3 and 5) were not preceded by an increase in gonadotropin secretion. The underlying mechanism of these ovulations that occurred during administration of the synthetic progestogen remains to be elucidated.
None of the ovulations above were followed by the formation of CL, only transient CH (Table 1), as evidenced by both ultrasonographic examinations (experiment 2; Fig. 3) and radioimmunoassay of serum progesterone (experiment 1, Fig. 2; and experiment 2). The failure of normal luteogenesis has previously been reported in ewes induced to ovulate during the luteal phase. This may be attributed to low LH concentrations during an established luteal phase or to the presence of a fresh MAP sponge (present study).
Histological micrographs of the dissected CH (Fig. 5, B and D) revealed their relatively small sizes, suggesting limited hypertrophy of granulosa cells. In addition, the pattern of staining of the thecal and granulosa lutein cells as seen in the forming ovine CL (Fig. 5E) was not observed in the present study (experiment 3).
Impaired luteogenesis of some ovulated follicles was observed in both treated and control ewes after the final ovulation in this study. Luteal inadequacy in ewes may be due to a lack of luteinization of some ovulated follicles or premature regression of CL. A report by Mann and Lamming showed that in cattle, low serum concentrations of estradiol around the time of ovulation were associated with an earlier increase in an oxytocin receptor level and augmented PGF2a response to oxytocin stimulation (i.e., stronger luteolytic signal).