Studies on the developing cerebellum. II. The ultrastructure of the external granular layer.

Abstract

AbstractThe ultrastructure of the external granular layer (EGL) of the rat cerebellum has been studied with special attention given to the changes occurring during its postnatal existence. Three periods are described: In the first period, lasting until day 5, the EGL is composed of a homogeneous population of germinal cells; the second period, lasting until day 17, is characterized by differentiation of neuroblasts which accumulate under the germinal cells forming the mantle zone. In the third period, the cell proliferation ends and the layer becomes progressively thinner and disappears by postnatal day 20. During these periods, the following cells have been identified: (A) germinal cells, ovoid or polyhedral and usually ciliated, have a comparatively large nucleus, a rim of cytoplasm, and an undetermined number of thin, long processes which persist during mitosis, although their fate during postmitotic differentiation was not established. (B) horizontal neuroblasts, with flattened nucleus and soma, are richer in organoids than the germinal cells, have neurotubules and thick lateral processes. (C) transitional bipolar neuroblasts resemble horizontal cells, except that most organoids, including the cilium migrate into the descending process. (D) A stage of apolar neuroblast has not been observed and there was no evidence of glial formation in the layer. A variety of cellular attachments are present, but synapses were not seen at any stage. At birth, the EGL adjacent to the pia mater is formed by EGL cells and their processes which are later replaced by the end feet of Bergmann glia which during a time exhibit giant mitochondria. Glial processes separate the EGL cells into columns and a neuropile is formed by these processes, and those of the EGL cells. The extension of the extracellular space is discussed as is the spontaneous degeneration of cells.