On the development of the cerebellum of the trout, Salmo gairdneri. II. Early development.

The early histogenesis of the cerebellum of Salmo gairdneri RICHARDSON, 1836 has been studied in fish ranging in length from about 5 to 14 mm, both with light microscopical and electron microscopical techniques. Structurally, the matrix cells correspond to those of other vertebrates. Mitoses occur predominantly at the ventricular surface, but peripheral mitoses are found as well, particularly in the period of highest mitotic activity. Mantle cell somata can be distinguished from the elongated matrix cells on the basis of their rounded shape. The neurogenetic and gliogenetic periods overlap considerably. Presumably the first mantle cells are all neuroblasts: as soon as the mantle layer starts to form, axonal profiles are found. In a slightly later stage glial differentiation is manifest in the radial processes contacting the meningeal surface. In young stages a distinction between neuroblasts and glioblasts can only be made on the basis of the structure of their processes. Processes of glioblasts can be distinguished from axons and dendrites by their paucity of microtubules. Dendrites, appearing in late-embryonic stages, contain the same organelles as axons, but in larger amounts. The first differentiation of mantle cell somata is an increase of rough endoplasmic reticulum, and that to a lesser degree in glioblasts than in neuroblasts. Neuronal nuclei are rounded and more electronlucent than those of mantle cells. Apart from zonulae adhaerentes between the internal processes of matrix cells, puncta adhaerentia occur frequently in the cerebellar anlage. However, they rarely occur on young neuron;. The possible significance of these junctions is discussed. The present study indicates that growth cones and filopodia are characteristic of most and probably of all types of cells in the early developing cerebellum. Growth cones contain much vesicular and tubular endoplasmic reticulum and in filopodia a fine filamentous network is present. In the somata of mantle cells growth areas were found, i.e. areas under the cell membrane with a similar content as growth cones. It is suggested that these areas anticipate the outgrowth of a new process.