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Sensory hair cells are produced in the ears of birds and mammals only during early development, so that a programmed termination of hair cell proliferation leaves adult birds and mammals susceptible to irreversible deafness and balance disorders. This study reports that this is not an inherent feature of hair cells and is not shared through all the vertebrate classes. In toads (Bufo marinus) hair cells accumulate throughout life, increasing in the sacculus from approximately 400 cells at metamorphosis to more than 1600 in adulthood. In both embryonic and postembryonic ears new hair cells have been identified by scanning electron microscopy and through uptake of radioactively labeled thymidine. In the otic vesicle of postneurulation embryos there is a single primordial sensory epithelium that contains approximately 100 hair cells. Scanning electron microscopy has demonstrated that these newly formed hair cells all have stereocilia bundles that are shorter than 1.5 micron, whereas the majority of hair cells in postembryonic ears have stereocilia bundles that are at least 3 micron long. In the postembryonic sacculus the proliferation of hair cells never appears to cease, since newly produced hair cells identified by their short stereocilia have been found in a distinct peripheral growth zone at the edge of the sensory epithelium even in specimens from the oldest 1% of natural populations. This peripheral growth zone is also the site of the most frequent labeling of newly synthesized DNA in hair cells. It appears that the postembryonic enlargement of the toad sacculus occurs primarily through appositional addition of new hair cells at its edge, with few hair cells added within the existing structure of the epithelium.

Perpetual production of hair cells and maturational changes in hair cell ultrastructure accompany postembryonic growth in an amphibian ear.