The ultrastructure of unilaminar follicles of the hamster ovary

Unilaminar follicles consisting of primary oocytes surrounded by a single granulosal layer, composed either of flattened, cuboidal or columnar cells, have been studied with the electron microscope in normal prepubertal hamsters. The most striking feature of the oocyte cytoplasm is the occurrence of the mitochondria in small groups with a dense intermitochondrial substance lying between the individual mitochondria. This substance is composed mainly of very small vesicles with a dense profile and a light center. Some of the mitochondria do not appear to have a limiting membrane, their matrices being seemingly continuous with, and similar in structure to, the intermitochondrial substance. In the younger oocytes the Golgi complex is diffuse and has numerous small vesicles. The nucleoli of the young oocytes are large and complex. They consist of a lighter area, predominantly of small particulate elements, and of darker regions, mainly with fine fibrillar components. The initial steps in the formation of the zona pellucida take place in follicles with a single layer of cuboidal granulosa cells. By the time the oocyte is surrounded by a single layer of columnar granulosa cells a continuous zona pellucida within which lies oocyte microvilli and granulosa cell processes is present.