Postembryonic production and aging of inner ear hair cells in sharks

TLDR

Inner‑ear hair cells in many animals vary ultrastructurally within epithelia, a variation that has been linked to cell function and age. This study examined growth‑related changes in hair cell populations in the macula neglecta of juvenile and adult sharks. Tritiated thymidine autoradiography and histological analysis revealed that new sensory cells are produced in growth zones at the edges of the epithelia. Scanning electron microscopy showed that more than 80 % of the ~200,000 hair cells in adult macula neglecta are produced postembryonically, with growth‑zone cells exhibiting small, heterogeneous cilia and sparse innervation similar to embryonic hair cells, indicating a persistent germinal zone and suggesting that latent growth zones may persist in the ears of other anamniotes and amniotes.

Abstract

Abstract In many animals the sensory hair cells of the inner ear are ultrastructurally variable within individual epithelia. This variation has been hypothetically related to both the function and the age of the individual cells. In this study, growth‐related changes in hair cell populations were examined in the macula neglecta sensory epithelia of juvenile and adult sharks. Scanning electron microscopy demonstrated that more than 80% of the 200,000 hair cells in the adult's macula neglecta are produced postembryonically. Tritiated thymidine autoradiography and histological descriptions of the hair cells in this sound detector indicate that new sensory cells are produced in growth zones at the edges of the epithelia. The hair cells in those zones have small cell bodies, small and heterogeneous cilia complexes, and associations with small numbers of particularly thin nerve terminals. Their cytological features and their sparse innervation contrast with the features of the more numerous central cells in each epithelium, but appear to resemble the published descriptions of embryonically developing hair cells. Thus, a germinal zone at the leading edge of sensory epithelium growth appears to persist into adult life in sharks. Published reports reinterpreted in light of this evidence suggest that such hair cell population growth may be expected in other anamniotes and that latent growth zones might persist in the ears of amniotes.