Hair cell precursors are ultrastructurally indistinguishable from mature support cells in the ear of a postembryonic fish.

The ultrastructure of S-phase cells in the postembryonic fish ear was compared with that of mature support cells. S-phase cells were identified by injecting animals with [3H]thymidine and sacrificing 3 h later. Sensory epithelia (saccules, utricles, and canals) were processed for light-level autoradiography. Sections containing thymidine-labeled cells were re-embedded and re-examined using transmission electron microscopy. The results indicate that S-phase cells differ from mature support cells only in nuclear position and shape. Otherwise their cytoplasmic characteristics are indistinguishable. Both cell types, on the other hand, are readily distinguishable from hair cells. These data provide ultrastructural evidence for the ability of mature support cells to enter the cell cycle in postembryonic vertebrates.