The ultrastructural details of mouse oögonia, oöcytes and follicular cells from fetal day 12 through postnatal day 3 are presented.
In fetal oögonia and oöcytes cytodifferentiation proceeds slowly. The most notable change involves an increase in the number of mitochondria and in the size of the Golgi complex. Oögonial itoses, nuclear synaptinemal complexes, true intercellular bridges and multinucleated cells were observed.
After the formation of unilaminar follicles with growing oöcytes, cytoplasmic alterations are striking. The Golgi apparatus, previously predominately of vesicular elements, now becomes lamellar in form and occasionally develops complex anastomosing tubules. There may be important functional significance in the relationship established between the Golgi elements and long endoplasmic reticulum cisternae. The membranes of the endoplasmic reticulum cisternae facing the Golgi complex lack ribosomes and exhibit bud-like evaginations in varying stages of abstriction. Ribosomes are attached to the opposite membrane of the cisternae. Profiles of endoplasmic reticulum are intimately associated with the numerous large mitochondria. Numerous multivesicular and “dense” bodies are present in some oöcytes. Zona pellucida formation is initiated in some of the larger unilaminar follicles.
Cytodiferentiation of follicular cells follows a course different from that of the germinal cells in that their cytoplasmic organelles are well differentiated very early in the fetal period. Similarly they send forth the most delicate cytoplasmic extensions which separate adjacent germinal cells. Postnatally, in unilaminar follicles these same cells constitute a flattened epithelial layer containing only a few cytoplasmic organelles. In contrast when these cells become cuboidal and columnar the numerous organelles, as seen in fetal follicle cells, reappear once again. In addition, when the zona pellucida is formed cell processes from these follicle cells extend into it.