COCHLEAR CYTOGENESIS VISUALIZED THROUGH PULSE LABELING OF CHICK-EMBRYOS IN CULTURE

Cytogenesis in the basilar papilla sensory epithelium of the chicken w as investigated through pulse labeling of proliferative cells. Tritiat ed-thymidine was injected intravenously in chick embryos cultured in p etri dishes. All embryos received the injection on the seventh day of incubation (E7), when the progenitors of hair cells and supporting cel ls are replicating deoxyribonucleic acid (DNA). Cells that were in the synthesis phase of the cell cycle, either at the time of the H-3-thym idine pulse or within 2 hours, incorporated detectable levels of the r adioactive DNA precursor. Labeled cells were identified in cochleae fr om embryos fixed at 0.5, 1, 2, 4, 6, 12 hours, 6 and 8 days after the pulse. One hour after the injection the majority of labeled nuclei wer e in the basal and middle strata of the sensory epithelium. Four to 6 hours after the injection, a greater number of labeled cells appeared in the lumenal stratum. The patterns of labeled cells in embryos fixed immediately after the injection of H-3-thymidine and in others fixed 6 to 8 days after the injection were unchanged, suggesting that the pr ogenitor cells divide and their progeny differentiate in the sensory e pithelium without appreciable transverse migration. Mitotic figures we re usually observed only in the lumenal stratum. Analysis of DNA conte nt in the populations of Feulgen-stained nuclei at three levels of dep th through the epithelium also produced results consistent with the co nclusion that vertical nuclear migration occurs during development of the cells in this sensory epithelium.