Follicular and luteal development in pregnant mares.

Examination of the ovaries of mares at various stages of pregnancy and after hysterectomy, together with measurement of progesterone concentrations in the peripheral plasma of pregnant and hysterectomized mares and in uterine and ovarian venous plasma of pregnant mares, demonstrated that the primary CL of pregnancy remains functional until at least Day 160 of gestation. The results showed that primary and secondary CL, and the placenta or uterus, all contribute to the total progesterone pool in mares during pregnancy. Similarities and differences in ovarian function observed between pregnant and hysterectomized mares suggested that, while PMSG does not appear to stimulate follicular development, it does prolong the life-span and stimulate the secretory activity of the primary CL, and induces ovulation and/or luteinization of secondary follicles in pregnant mares.