Ultrastructural changes of the corpora lutea in LH-treated cyclic female rats

As LH is capable of stimulating ovarian progesterone secretion at the beginning of 4-day cycles, it was the aim of this work to determine whether corpora lutea formed at the end of a preceding cycle are responsible for the increase in ovarian progesterone secretion. 4-day cyclic female rats were injected with LH on dioestrus I at 8...9 a.m. (64 microgram/100 g/b.w. expressed in terms of NIH-LH-S3). An increase in the peripheral blood progesterone concentration was shown to occur on the morning of dioestrus II in LH-treated females when compared to control females. Concomitantly enlarged mitochondria, numerous membranes of the smooth endoplasmic reticulum and a decrease in the number of lipid droplets were observed in LH-treated females. Inversely signs of involution of the corpora lutea were noted from the morning of dioestrus I to that of dioestrus II in noninjected females. It is concluded that LH-induced progesterone increase at dioestrus II results from the activation of the corpora lutea formed at the end of the preceding cycle.