Early follicular development and atretic changes in the ovary of the lamb-- fine structure and histochemistry.

The fine structure of follicles was studied in ovaries from lambs up to 32 days old. Some animals received pregnant mare serum gonadotrophin prior to sampling. Follicles examined were in the primordial to early antral stages. The oocyte is described and changes associated with its development and the formation of the zona pellucida are recorded. Characteristics in which the lamb oocyte differed from those of other mammals include a greater prominence of smooth endoplasmic reticulum, the presence of microtubules, fine filaments in the peripheral parts and a large number of almost empty vacuoles. The latter were more numerous in oocytes which appeared to be degenerating, including some of those in primordial follicles. Granulosa cells had characteristics usually associated with protein secretion while some theca cells had large amounts of endoplasmic reticulum bearing few ribosomes. In early atresia changes were evident successively in the oocytes, granulosa and theca. There was a weak positive reaction for 3 beta-hydroxy-delta 5-steroid dehydrogenase in the theca of some vesicular follicles from both control lambs and lambs treated with pregnant mare serum gonadotrophin. Follicles showing sings of atresia were negative for this enzyme.