The neuronal architecture and topography of the nucleus vestibularis tangentialis in the late chick embryo

Abstract The tangential nucleus is a group of secondary vestibular neurons embedded among the fibers of the vestibular nerve root as it first enters the lateral margin of the medulla oblongata. This paper describes the chief morphological features of the neurons and the endings made by afferent axons in rapid Golgi, reduced silver, and Nissl preparations of the tangential nucleus of the chicken. The topography of the tangential nucleus is presented in a series of transverse sections from a reduced silver preparation. The morphological features and topography of the neuronal architecture are well defined in the 18–19 day chick embryo (Hamburger-Hamilton Stages 44–45), in which they provide a useful guide for both embryological investigations and studies of the neuronal architecture in this nucleus. Three types of neurons are defined: the principal cell, comprising 80% of the neurons, the elongate cell, about 20%, and the giant cell, less than 1% of the neuronal population. The principal cell is characterized by an oval body, eccentric nucleus, horizontal dendrites elongated parallel to the incoming vestibular fibers, and short vertical dendrites. Because of their arrangement with respect to the incoming vestibular fibers, the principal cells are in a position to establish connections with select groups of ganglion cells which innervate discrete portions of the vestibular end-organ. This arrangement could preserve the topographical organization of the vestibular ganglion within the brain. Such a pattern is best demonstrated by the connections of the principal cells with the colossal fibers. These fibers form a distinct population consisting of the thickest vestibular nerve axons, which arise from cells in the vestibular ganglion that peripherally innervate small groups of hair cells by means of large calyces in the cristae ampullares of the semicircular canals. Within the tangential nucleus, the colossal fibers form large ‘spoon’ endings en passant. These endings are spoon-shaped enlargements with digitiform appendages that make axosomatic synapses on the principal cells only. Each colossal fiber forms only one spoon ending, which contacts only one cell body; each principal cell body receives only one spoon. This degree of specificity and the morphology of the colossal fiber-principal cell connection provide the basis for a highly specialized, powerful synapse that may subserve important reflex adjustments. The elongate cell is characterized by a smaller, pyramidal body, a central nucleus, and vertical dendrites which are greatly elongated so as to intercept a wide sector of the vestibular nerve root and which consequently can establish connections with many fine fibers innervating widespread regions of the vestibular end-organ. The giant cell is a large, multipolar neuron with very thick, long, branching dendrites and elongated somatic and dendritic appendages. Fine afferent axons of vestibular and nonvestibular origins probably end in relation to all three types of neurons. The neuronal architecture and topography of the chick tangential nucleus offer some unusual opportunities for morphological and experimental studies of neurogcnesis.