1 BASIC SC

Apuntes

1 BASIC SCIENCE 1A. HAIR ANATOMY AND HISTOLOGY Paul T Rose, Ronald Shapiro, Michael Morgan, and Nishit S Patel EMBRYOLOGY Hair follicles are derived from an interaction between the embryological ectoderm and the mesoderm beginning around the second month of pregnancy (1,2). The earliest event is the development of regularly spaced epidermal thickenings, called placodes, initiated by molecular signaling from the underlying mesenchyme (3). In response, the overlying epidermis sends molecular signals back inducing thickening of the mesen- chyme. This complex epithelial–mesenchymal metabolic signaling and crosstalk occurs throughout follicular morpho- genesis. The major embryological stages are described below. Pregerm Stage (Fig. 1A-1a) The first signs of a hair follicle are an aggregation of mesen- chymal cells in the superficial level of the dermis and a simultaneous thickening of basal epidermal cells immediately above it ( 4,5 ). Germ Stage (Fig. 1A-1b) At this point, the basal epidermal cells become elongated and start to bulge downward as the “hair germ,” while at the same time the underlying mesenchymal cells begin to replicate to form the rudiment of what will become the derma...