Abstract
During renewal of the lizard epidermis, the rate of new cell production varies cyclically from a low level (resting stage) to a high level (renewal stage). Renewal involves the formation and shedding of an βepidermal generationβ in which cells of the outermost layers synthesize beta keratin whereas cells of the inner layers synthesize alpha keratin. The cells are derived sequentially from a morphologically homogeneous basal layer. Tokay geckos were injected with a single pulse of tritiated thymidine at the end of the resting stage, 24 hours prior to the onset of epidermal renewal. Sequential biopsies were taken and analyzed by radioautography. Twentyβfour hours after the pulse, labeled nuclei were found only in the basal layer. During the subsequent four to five days, both labeled and unlabeled mitoses were present in the basal layer. Although at least seven new layers of cells were added above the basal layer during this same four to five day period, none of their nuclei were labeled. Most of these cells would normally subsequently synthesize beta keratin. The first cells with labeled nuclei emerged from the basal layer after this four to five day delay. These cells would normally go on to synthesize alpha keratin. The labeling index in the basal layer, which was less than 10% at 24 hours after injection, rose to about 50% and fell coincident with emergence of cells destined to form alpha keratin. Thus, these cells must have divided more than once before leaving the basal layer. The studies indicate that the pattern of migration of cells from the basal layer plays an important role in the emergence of a new pattern of differentiation.