ABSTRACT
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 mesenchyme. This complex epithelial-mesenchymal metabolic signaling and crosstalk occurs throughout follicular morphogenesis. The major embryological stages are described below.
