Evolutionary trends in the organization of the vertebrate crista ampullaris

Intraaxonal labeling studies in the toadfish, frog, turtle, and chinchilla suggest broad evolutionary trends in the vertebrate crista ampullaris. The crista of anamniotes (fish, amphibians) contains type II hair cells innervated by bouton afferents and is longitudinally organized. Type I hair cells are first seen in reptiles and birds, where they are confined to a central zone and are innervated by calyx and dimorphic afferents. The central zone is surrounded by a peripheral zone containing only type II hair cells innervated by bouton afferents. Results in the turtle suggest that the peripheral zone in reptiles and birds is organized similarly to the entire anamniote crista. The turtle central zone finds no parallel in anamniotes but resembles the mammalian central zone in its structure and afferent physiology. With the advent of a central zone in reptiles, a concentric organization is superimposed on a linearly organized peripheral zone. The mammalian crista, in contrast, has an entirely concentric organization. This may be related to the extension of the neuroepithelium further down the slopes of the crista in mammals than in other vertebrates and to the distribution of type I hair cells throughout the mammalian neuroepithelium.