RATE OF STRATUM-CORNEUM FORMATION IN THE PERINATAL RAT

During late gestation, the fetal rat exhibits marked hyperplasia of the interfollicular epidermis and accelerated cornification in preparation for birth. In this study, we utilized simple morphometric techniques to provide quantitative estimates of the rate of stratum corneum (SC) formation during the perinatal period. Cryostat sections of dorsal epidermis from rat pups between - 48 and + 72 h of age were expanded under alkaline conditions and the number of corneocyte interfaces counted from photomicrographs. This method yielded the following regression lines: prenatal, y = 0.19x + 13.07, r = 0.93; postnatal, y = 0.13x + 13.00, r = 0.93, where y = number of SC layers and x = age in hours. Lack of desquamation was assured by the postnatal persistence of the granular periderm. Adhesive stripping of the epidermis followed by phase-contrast microscopy revealed intact monolayer sheets of SC. Quantitation using a computerized image analysis system gave an average corneocyte surface area in the newborn rat of 1908 +/- 36 mu2 (mean +/- SEM). Treatment of neonatal rats with epidermal growth factor (500 ng/g BW) increased the number of SC layers over the first 24 h of life (p < 0.05) and resulted in marked hyperkeratosis by one week of age. These results allow the following conclusions: 1) in the prenatal rat, SC forms at a rate of one layer every 5 h; 2) postnatally, SC formation slows to one layer every 8 h; 3) to support normal corneum synthesis at birth, dorsal keratinocytes must enter transition at a rate of approximately 3 cells/second/cm2 of body surface; and 4) treatment with exogenous EGF augments the rate of terminal differentiation of perinatal rat epidermis.