ELSEVIER Hearing Research 100 (1996) 10-20 Hair cell precursors are ultrastructurally indistinguishable from mature support cells in the ear of a postembryonic fish Joelle C. Presson, Pamela J. Lanford *, Arthur N. Popper Department ~f Zoology, University of Maryland, College Park, MD 20742, USA Received 4 November 1995; revised 27 April 1996; accepted 6 May 1996 Abstract 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. Keywords: Support cell; Hair cell; Cell proliferation; S-phase; Postembryonic development; Saccule; Ultrastructure I. Introduction The developmental process of hair cell production in vertebrates is not restricted to embryonic stages. Post- embryonic hair cell production has been demonstrated in ears of most non-mammalian vertebrate classes (Cor- win, 1981, 1983, 1985; Popper and Hoxter, 1984, 1990; Corwin and Cotanche, 1988; Ryals and Rubel, 1988) and may even occur in mammals (Forge et al., 1993; LeFebvre et al., 1993; Forge, 1994). The majority of researchers who study postembryonic hair cell produc- tion have concluded that mature support cells enter the cell cycle and divide to produce the new hair cells (Co- tanche, 1987; Balak et al., 1990; Raphael, 1992; Stone and Cotanche, 1994; Raphael et al., 1994; Tsue et al., 1994; Presson et al., 1995). This finding is intriguing, since in some tissues (Gordon and Hermiston, 1994; Lassar et al., 1994) the cellular state of mature differ- entiation apparently is not compatible with cell cycle competence. It is premature, however, to conclude that mature * Corresponding author. Tel. : (301 ) 405-6903; Fax: (301 ) 314-9358; E-mail: p144@umail.umd.edu support cells depart from this principle. Although the literature suggests the equivalence of support cells and S-phase cells in the vertebrate ear, all of the available studies have limitations. Presson and colleagues (Pres- son and Popper, 1990a; Presson, 1994) found that sup- port cells and hair cell precursors in fish are cytochemi- cally similar, but that conclusion rests on only two cellular markers. Light microscopic (LM) studies in other species (Balak et al., 1990; Tsue et al., 1994) also suggest a fundamental similarity between support cells and hair cell precursors, but are limited by the lack of detail provided by light microscopy. Raphael et al. (1994) concluded that dividing cells in the regenerating basilar papilla have ultrastructural similarities to sup- port cells. They did not, however, directly compare S- phase cells with support cells, since the animals were sacrificed for study at least 12 h after injection of the S-phase marker. Thus, one still cannot assert that the cells that enter S-phase are indeed identical in structure to support cells not in S-phase. In light of the fundamental importance of this ques- tion, the present study directly compares the ultrastruc- ture of S-phase cells (presumably hair cell precursors) with other (presumably mature) support cells. If the 0378-5955 / 96 / $15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PH S0378-5955(96)00 1 09-8