Autoradiographic Studies on Cell Proliferation and Differentiation in the Human Epidermis in Vivo

Using in vivo local labeling method, cell proliferation and differentiatiation in the human epidermis covering the mammary gland of 41 and 42 year old women were studied by means of 3H-thymidine autoradiography.1. Stratum germinativum or G-zone of the human epidermis in which cells are proliferating corresponds to epibasal and basal layers, but not the basal layer alone. Proliferating cells are predominantly located in the epibasal layer.2. Melanin containing cell in the basal layer is a kind of minor-differentiated cell and proliferates at much slower rate than the non-pigmented basal cell.3. Generation time of the genertive cell is estimated at 4 days and the maximum length of DNA synthesis, at 10 hours. Many differentiating cells stay in the epibasal layer and they amount to 50% of the total cells in the layer. Their transit time in the layer is estimated at an average of 4 days.4. The unequal division or (G, M) division is not a mechanism to maintain the steady state, nor do any types of mitosis play a decisive role in the regulation of cell differentiation. Mitoses yield two equivalent daughter cells which are in the same state as the mother cell.5. Cell differentiation in the human epidermis seems to be determined by the rule of probability in an individual daughter cell during t1 or G1 period, as has been confirmed in various other cell systems.