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 identif ied by injecting animals with [H-3]thymidine and sacrificing 3 h later . Sensory epithelia (saccules, utricles, and canals) were processed fo r light-level autoradiography. Sections containing thymidine-labeled c ells were re-embedded and re-examined using transmission electron micr oscopy. The results indicate that S-phase cells differ from mature sup port cells only in nuclear position and shape. Otherwise their cytopla smic characteristics are indistinguishable. Both cell types, on the ot her hand, are readily distinguishable from hair cells. These data prov ide ultrastructural evidence for the ability of mature support cells t o enter the cell cycle in postembryonic vertebrates.