A study of the structure of the papilla neglecta in the lizard, Anolis carolinensis.

Scanning and transmission electron microscopy have been used to investigate the structure of the papilla neglecta in the lizard, Anolis carolinensis. Situated in the posterior part of the utricle, the receptor is innervated by a branch of the posterior ampullary nerve and is covered by an extracellular membrane that exhibits a tapering extension into the orifice of the utriculo-saccular duct. The neuroepithelium contains two populations of sensory cells within a matrix of sustentacular cells; one population (non-calyceal hair cells) is supplied primarily by boutons and clublike terminals, while the other (calyceal hair cells) is served by calycine terminals that invest from one to five sensory cells. The apices of calyceal hair cells bulge into the utricular lumen and have larger diameters and ciliary counts than non-calyceal cells, but other differences are not marked. Although the bases of calyceal hair cells are deeply indented by calyceal processes, both types show similar afferent synaptic structures confined to the nuclear and infranuclear regions. Efferent terminals synapse on afferent endings and hair cells in both cases; synapses directly on calyceal hair cells lie above the calyx, just beneath the apicolateral junctional complexes. Those complexes are basically similar throughout the receptor, except that a hemispheroidal gap junction between a supporting cell and adjacent hair cell was noted in several instances. Arrays of endoplasmic reticulum and specialized mitochondria occur adjacent to aggregations of afferent synaptic vesicles in both types of sensory cell; it is suggested that such arrays may participate in the production of the vesicles in a process involving membrane recycling similar to that at the neuromuscular junction. Differences between calycine units and the type I hair cell of mammals are discussed, and evidence that suggests an equilibratory function for the lacertilian papilla neglecta is presented.