The re-formation of the epidermis of embryonic Rana pipiens following treatment with ultrasound

Abstract

The hypothesis is tested that mitotic activity is responsible for epidermal stratification by increasing the density of cells in a cell layer and thereby promoting cell migration out of this layer. The stratification process was observed in the epidermis of Rana pipiens embryos following treatment with ultrasound which causes the shedding of the periderm; the single layer of residual cells gives rise to two cell layers by the end of the first day after this treatment. The density of epidermal cells was observed in whole mounts of tail epidermis. The epidermal cell density of sonicated animals decreased during the first day so that the cells in both layers were only half as densely packed as those in the control epidermis. Changes in the cell density, therefore, could not be responsible for peridermal re-formation. Equations for the total number of epidermal cells on one side of the tail as a function of time were derived by multiplying corresponding functions for cell density by those for the surface area of one side of the tail. The rates at which cells were added to the epidermal cell population by mitosis were calculated from the mitotic index divided by the duration of mitosis and were used to estimate the percentages of cells added by mitosis during the first day. Only one-third of the tail epidermal cells of sonicated animals divided during the first day. Since there were about as many cells in the periderm as in the basal layer at that time, at least 33% of the new peridermal cells had migrated out of the basal layer without having been replaced there by cell division. Cell division, therefore, could not be responsible for peridermal re-formation. The hypothesis is thus rejected. Epidermal cells evidently migrated into two layers from the original single layer of residual cells irrespective of the cell density and of cell division.