Quantitative analyses of postembryonic hair cell addition in the otolithic endorgans of the inner ear of the European Hake, Merluccius merluccius (Gadiformes, Teleostei)

Abstract

Bony fishes add sensory hair cells to the saccule and lagena of the ear for at least several years after hatching. However, it is not known whether hair cell proliferation occurs for the whole lifetime of an animal, whether proliferation occurs in all endorgans of the ear, or whether the rate of proliferation is the same in all of the endorgans. To obtain answers to these questions, the extent of postembryonic hair-cell proliferation was determined in the saccule, lagena, and utricle of the ear in the European hake, Merluccius merlaccius, for fish ranging from 7 to 75 cm in total length (6 months to 9 years of age). Results demonstrated that hair-cell addition continued throughout this period in all three otic endorgans, although endorgan size was proportionally greatest in smaller animals. Of the three endorgans, cell addition was greatest in the saccule. Moreover, far more cells were added to the caudal end of the saccule than to the rostral end. Each saccule of the largest hake had over 900,000 hair cells. It is estimated that each saccule adds approximately 110,000 new hair cells each year (or 302 cells/day) over the life span of the fish studied. A significant number of small ciliary bundles, thought to represent newly proliferated hair cells, was found throughout each endorgan, and the number of such bundles declined as the rate of hair cell proliferation decreased. The results demonstrate that extensive proliferation occurs in all three otolithic endorgans of the ears in a fish and that such proliferation continues for virtually the whole life of the animal. The functional significance of this addition is not known