Ultrastructural study of synapses at the time of neuronal migration and early differentiation in the tangential vestibular nucleus of the chick embryo in vivo

The chick tangential nucleus is a primary vestibular nucleus whose two main neuron types migrate and begin to differentiate between 5 and 8 days in the embryo (gestation takes 20‐21 days). Based on rapid Golgi impregnations of developing tangential neurons and growing fibers, we have identified ultrastructural counterparts and characterized interactions in the nucleus from 5 to 8 days. Developing tangential neurons received the earliest synapses at 5 days on their primitive processes and subsequently on their cell bodies by longitudinal fibers of unknown origins. In contrast, the primary vestibular afferents did not form identified synapses on the developing tangential neurons until 7½ days. In conclusion, the earliest synapses in the tangential nucleus are formed by longitudinal fibers, which are probably not primary vestihular afferents. Since a specific class of fibers forms particular synapses on the tangential neuron precursors at predictable times prior to and during neuronal migration and also at the onset of differentiation, the role of these synapses in developmental events should be explored.