A REGIONAL ULTRASTRUCTURAL ANALYSIS OF THE CELLULAR AND SYNAPTIC ARCHITECTURE IN THE CHINCHILLA CRISTAE AMPULLARES

The chinchilla crista ampullaris was studied in 10 samples, each conta ining 32 consecutive ultrathin sections of the entire neuroepithelium. Dissector methods were used to estimate the incidence of various syna ptic features, and results were confirmed in completely reconstructed hair cells. There are large regional variations in cellular and synapt ic architecture. Type I and type II hair cells are shorter, broader, a nd less densely packed in the central zone than in the intermediate an d peripheral zones. Complex calyx endings are most common centrally. O n average, there are 15-20 ribbon synapses and 25-30 calyceal invagina tions in each type I hair cell. Synapses and invaginations are most nu merous centrally. Central type II hair cells receive considerably fewe r afferent boutons than do peripheral type II hair cells, but have sim ilar numbers of ribbon synapses. The numbers are similar because centr al type II hair cells make more synapses with the outer faces of calyx endings and with individual afferent boutons. Most afferent boutons g et one ribbon synapse. Boutons without ribbon synapses were only found peripherally, and boutons getting multiple synapses were most frequen t centrally. Throughout the neuroepithelium, there is an average of th ree to four efferent boutons on each type II hair cell and calyx endin g. Reciprocal synapses are rare. Most synaptic ribbons in type I hair cells are spherules; those in type IT hair cells can be spherical or e longated and are particularly heterogeneous centrally. Consistent with the proposal that the crista is concentrically organized, the interme diate and peripheral zones are each similar in their cellular and syna ptic architecture near the base and near the planum. An especially dif ferentiated subzone may exist in the middle of the central zone. (C) 1 997 Wiley-Liss, Inc.