Ultrastructural studies on the cerebellar histogenesis. I. Differentiation of granule cells and development of glomeruli in the chick embryo.

Abstract

A continuum of transitional forms between the cells in the external (transitory) granular layer of the cerebellar cortex and the granule cells in the internal (definitive) granular layer has been identified with the electron microscope in chick embryos 17–20 day old. This confirms that at least a number of the granule neurons is derived from the cells of the external granular layer, a concept based on studies with Golgi and autoradiographic methods. The differentiating granules in the molecular layer have a vertical orientation (vertical bipolar cells) and are shaped as flattened spindles, with the larger horizontal diameter in the direction of the parallel fibres. The cytoplasmic organelles in the vertical bipolar cells show a distinct polarization. As these cells are encountered in deeper positions in the cortex they have an increasing number of ergastoplasmic cisternae, and their mitochondria lose their dense granules and DNA-filaments. Near the internal granular layer these cells become rounded. Their cytoplasmic membrane systems appear more prominent, extending even into the axon hillock, and thin processes (primitive dendrites) are emitted from the cell body. Mossy fibre knobs are found to make synapse-like contacts not only with the dendritic tips of granule neurons, as usual in adult animals, but also with their dendritic trunks and perikarya. The knobs contain tubular profiles of the smooth endoplasmic reticulum, the number of which in many instances seem to be inversely proportional to that of the synaptic vesicles. In the developing glomeruli dendro-dendritic attachment plaques (desmosomes) are absent. Astrocytes and mitotic astroblasts have been observed at all levels of the cerebellar cortex except in the outer portion of the molecular layer and in the external granular layer.