Dermal fibroblastic cells are associated with hair follicles
Hair follicles are skin appendages formed predominantly by cells of epidermal origin. Mesenchymal cells of the dermis play a vital role in their formation in fetal skin and an equally significant role in regulating their cyclic growth, rest and regression phases in adults (Kulessa et al., 2000; Botchkarev, 2003). In fetal skin, mutual inductive events between localized dermal and epidermal cells proceed in a stringent spatiotemporal manner. First, an as-yet-undefined signal emanating from the dermis induces the formation of thickened epidermal placodes (Holbrook and Minami, 1991; Hardy, 1992; Millar, 2002; Botchkarev et al., 1999; Botchkarev et al., 2002). Differentiated epidermal cells provide a second signal that induces localized mesenchymal cells to condense and form a defined pellet of cells immediately beneath the epidermal placodes (Holbrook and Minami, 1991; Hardy, 1992). These cells stimulate the proliferation of epidermal cells in the placode, which drives the production of hair follicles deep into the dermal matrix (Hardy, 1992; Millar, 2002; Botchkarev, 2003). Simultaneously, condensed mesenchymal cells produce proteases that clear a path for this ingrowth (Karelina et al., 1993; Karelina et al., 1994; Karelina et al., 2000). Once elongation is complete, keratinocytes in the matrix region at the base of the follicle envelop the dermal papilla cells and leave a narrow opening through which the vasculature and nerves penetrate (Hardy, 1992; Millar, 2002; Botchkarev, 2003). Condensed mesenchymal cells also give rise to a second population of dermal cells during the period in which follicles actively invade the dermal matrix. These dermal cells form a thin connective tissue sheath along the shaft of the follicle (Jahoda and Reynolds, 2000).