The development of synapses and "spoon" synaptic terminal space in the tangential vestibular nucleus: a quantitative electron microscope study.

The developmental study focuses on the quantitation of synapses and synaptic terminal space belonging to one identifiable population of synaptic endings, the spoon endings. Spoon endings contact only one cell type, the principal cells of the chick tangential nucleus. The tangential nucleus in an interstitial nucleus of the vestibular nerve, comprising part of the avian lateral vestibular complex. In addition to the spoon endings formed by the vestibular fibers, the principal cell bodies receive synaptic and contacts from small endings of vestibular and non-vestibular inputs. The spoon ending's synaptic space on the target cell diminishes significantly at or around the time of hatching. This diminution is expressed in quantitative terms. The spoon ending covers 34.8% of the somatic linear surface in the embryo, 10.9% in the hatching, and 9.8% in the 3 year old. Although the spoon ending's vesicular synapses become rare after hatching, the spoon-ending interface exhibits a significant increase in gap-junction density during development. Neither the spoon endings nor their target cells exhibit a constancy in synaptic junction covering during development, although the spoon endings show a constant covering of attachment plaques. In the light of the present findings, the role of very large endings in the formation of synaptic connections and the control of developmental changes in junctional complexes is discussed.