Ultrastructural and quantitative dynamics of the granulosa of ovarian follicles of the lizard Gerrhonotus coeruleus (family Anguidae).

The progression of ovarian follicular development in the Northern Alligator Lizard has been documented ultrastructurally and by enumeration of cells, with a focus on changes in the granulosa component of the follicle. The pattern of cellular differentiation of the granulosa entails, as in other lizards, the transformation of a simple, cuboidal epithelium in small follicles into a complex layer consisting of three types of cells. Marked differences in size and ultrastructure of the cell types indicate different functional states: the smallest cells are little differentiated and serve primarily as stem cells to other granulosa cells throughout follicular growth, whereas the larger "intermediate" and "pyriform" cells do not divide and show ultrastructural features indicative of synthetic activity. Contrary to some views that this latter cell type is the final step in cellular differentiation and provides organelles and cytoplasm to the oocyte through an intercellular bridge, the results of this study suggest that only relatively small molecules such as ribosomal RNA might pass between cells. Further, these observations support the interpretation that a heterogeneous granulosa results from the fusion in early follicular stages of some cells that are in surface contact with the oocyte. Several of the cytological features of the larger granulosa cell types are seen in the oocyte and in germ-line cells generally, such as highly dispersed chromatin, large nucleoli, abundant nuclear pores, mitochondrial "rosettes," annulate lamellae, "ribosome bodies," and surface microvilli. This strongly suggests that the cytology of large granulosa cells is induced by the oocyte. The heterogeneous granulosa persists only through previtellogenesis and at the onset of exogenous yolk uptake by the oocyte it becomes a secondarily homogeneous layer. The appearance of the granulosa at this stage is similar to that of reptiles whose granulosa remains a single-cell layer throughout folliculogenesis (e.g., turtles and crocodilians). Thus, although follicular development has been scrutinized in only a few representative genera of reptiles to date, the course of follicular development among lizards is similar in detail and involves the transitory development of a heterogeneous population of cells. This feature appears to be exclusive to the squamate reptiles.