Morphological and fine structural features of the basilar papilla in ambystomatid salamanders (Amphibia; Caudata)

The morphology and fine structure of the basilar recess and basilar papilla were investigated in four species of salamanders from the family Ambystomatidae. The otic relationships of the recess and papilla to the proximal part of the lagena and saccule are described, and new terminology is suggested for the periotic relationships of the basilar recess to a diverticulum of an intracapsular periotic sac. The basilar papilla consists of supporting cells united laterally by gap junctions, capped by microvilli uniformly arranged around a short, central cilium, and hair cells that typically show several synapses with a single afferent nerve fiber, each marked by a rounded synaptic body surrounded by vesicles. In contrast to anuran basilar papillae, efferent nerve terminals were observed in synapse with hair cells and, rarely, upon afferent fibers. The distal half of the ambystomatid papilla contained hair cells capped by tall ciliary bundles, with kinocilia that show swellings near their tips with delicate attachments to adjacent tall stereocilia. A tectorial body covers only this region of the papilla. Hair cells with shorter stereocilia, situated in the proximal half and at the papillar margins, are related only to filamentous extensions of the tectorial body. The ambystomatid basilar recess and papilla are compared to auditory end-organs in other vertebrates, and it is suggested that a basic distinction can be made between aural neuroepithelia in amniotes versus that in nonamniotic vertebrate ears.