Fine Structure of the Embryonal Granular Cells of the Chick Cerebellum

Abstract

The fine structure, differentiation and destination of embryonal granular cells of the chick cerebellum were studied by means of electron microscopy. The embryonal granular layer (EGL) was divided into the external layer, the internal layer, and the transitional zone between the internal and molecular layers. Cells of the external layer showed active mitosis and appeared most primitive in shape and in development of cell organellae. Cells of the internal layer possessed long bipolar cytoplasmic processes parallel to the cerebellar surface as viewed in the frontal section. These processes were of two types: In some cells they were electron dense, with microtubules; in others, they were electron light, with microfilaments, and have some deeply folded rumples running longitudinally along the processes. In the transitional zone, the former cells (dense processes) obtained the characteristics of nerve cells, with developed rER and immature synapses; the latter cells (clear processes) the characteristics of glia cells. Those external layer cells with cytoplasmic end-feet in contact with the pia limiting membrane developed into Bergmann's astrocytes. Few degenerated cells were observed in the EGL, their number being far less than that of mitotic cells. Dense cored vesicles about 1, 000 Å in diameter were observed in the EGL, but their nature is not known in the present study.