Ultrastructural study of calycine synaptic endings of colossal vestibular fibers in the cristae ampullares of the developing chick.

Bipolar vestibular ganglion cells give rise to the colossal vestibular fibers in the chicken. These fibers form the largest calycine endings in the cristae ampullares and also the spoon endings in the tangential vestibular nucleus of the medulla oblongata. Because these synaptic endings are two of the largest and most distinctive in the vertebrate nervous system, they are especially suitable for comparisons of the development of synapses and synaptic endings of a specific cell type. An ultrastructural study of the spoon endings and quantitative data on their synapses were available from material of 15-day-old chick embryos, hatchlings, and 3-yr-old chickens. Here we provide similar data on the large calyces. Briefly, large calyces exhibited no ultrastructural changes corresponding to the changes in the spoon endings apparent when they retract from their target cell surfaces around hatching time. However, the concentration of the ribbon synapses at the large calyces decreased around hatching, when the concentration of the chemical synapses at the spoon endings declined. Moreover, the concentration of the ribbon synapses at the large calyces corresponded closely to the concentration of the chemical synapses at the spoon endings at the same age. Thus at the developmental ages studied, there were similar concentrations in the peripheral and central synapses formed at two different synaptic endings, both derived from one cell type and participating in the same neural pathway. These findings raise the issue of how synapses are regulated locally, but also suggest the possibility for central-peripheral interactions to produce correlative changes in parallel.