A morphological study of neurogenesis in the nucleus vestibularis tangentialis of the chick embryo.

Abstract This study is a light microscopic investigation of the normal pattern of morphogenesis in the chick tangential nucleus. Rapid Golgi and reduced silver methods have been used to reconstruct the sequences of morphological changes in the developing neurons and colossal-fiber endings of the vestibular nerve in a staged series of chick embryos. The distinctive morphology of the principal and elongate cells and of the colossal fibers and their spoon endings permits the rigorous identification of these structures from very early stages of development. Three stages are discussed: the cell migration stage from 4½ to 6½ days (Hamburger-Hamilton, Stages 25–29), the young neuron stage from 6½ to 8 days (Stages 30–34), and the stage of spoon formation from 8½ to 13 days (Stages 35–39). In the cell migration stage , although the vestibular nerve fibers have already grown into the medulla, no cell bodies appear within the presumptive tangential nucleus in embryos younger than 6 days (Stage 29), nor are the prospective colossal fibers distinguishable by their axonal diameters. The presumptive tangential nucleus contains the external processes of primitive epithelial cells which extend from the matrix zone to the external limiting layer; these cells in 6–7 day (Stage 30) embryos give rise to the migratory neuroblasts which appear in the mantle zone medial to, and also within, the primordial tangential nucleus. The perikarya of the migratory neuroblasts move through the primitive external processes to the presumptive tangential nucleus, while their primitive internal processes withdraw from the matrix zone and reduce to shorter trailing processes. In the young neuron stage the axon of the principal cell emerges from the primitive internal process at a point far medial to the perikaryon; this outgrowth can occur even before the translocation of the nucleus is completed. The colossal fibers are first distinguishable by their greatly enlarged diameters. During the rest of this stage, the post-migratory neuroblasts lose their primitive processes and begin to form dendrites that continue to develop during the next stage. At the beginning of the stage of spoon formation , each colossal fiber forms a characteristic swelling en passant within the tangential nucleus. By 10 days (Stage 36) a sheet-like outgrowth appears just medial to the swelling, which soon disappears as this outgrowth continues to expand over the principal cell soma to form a young spoon ending. It is concluded that the principal cells undergo a specific, time-locked sequence of structural changes in their development, which proceeds in advance of an equally strict sequence of events in the differentiation of the spoon endings that contact these cells. These findings indicate that the ingrowth of the colossal fibers does not initiate cell migration and differentiation in the tangential nucleus, although the fibers and spoon endings could influence subsequent cell development and maintenance. Perhaps the formation of spoon endings depends on migration and differentiation of the principal cells.