The hair bulb encloses the follicular dermal papilla, which comprises a group of mesenchyme-derived cells, the dermal papilla cells, mucopolysaccharide-rich stroma, nerve fibres, and a single capillary loop. The follicular papilla is believed to be one of the most important drivers to instruct the hair follicle to grow and form a particularly sized and pigmented hair shaft. Several experiments have shown that the dermal papilla has powerful inductive properties. Dermal papilla cells transplanted into non-hair-bearing epidermis are able to induce the formation of new hair follicles [38,39]. The dermal papilla is an essential source of paracrine factors critical for hair growth and melanogenesis; it is believed to be the interpreter of circulating signals such as hormones to the follicle (discussed in Section 1.7). Specific examples of factors produced by the dermal papilla that influence hair growth include noggin, which exerts a hair growth-inducing effect by antagonising bone morphogenetic protein (BMP) signalling and activation of the BMP receptor IA expressed in the follicular epithelium [40 ] . Keratinocyte growth factor (KGF) is also produced by the anagen dermal papilla, and its receptor, FGFR2, is found predominantly in the matrix kera – tinocytes. The activation of this pathway by injections of KGF into nude mice induces hair growth at the site of injection [41]. Dermal papilla cells also express hepatocyte growth factor (HGF) [42]. Transgenic mice overexpressing HGF display accelerated hair follicle development [42]. Insulin-like growth factor-I (IGF-I) found in the dermal papilla also serves as an important morphogen in the hair follicle [43]. In addition, stem cell factor (SCF) produced by the dermal papilla [44] is essential for proliferation, differentiation, and melanin production by follicular melanocytes expressing its receptor c-kit [26]. The dermal papilla also displays unusually strong alkaline phosphatase activity during the entire hair cycle [45]. Although a role for alkaline phosphatase remains obscure, hair growth is reduced when inhibitors of alkaline phosphatase are applied [46].
Interestingly, recent studies suggested that follicle dermal papilla and connective (or dermal) sheath cells may act as stem cells for both follicular and interfollicular dermis. Moreover, the stem cell potential of follicle dermal cells extends beyond the skin. Jahoda and colleagues have demonstrated that rodent hair follicle dermal cells have haematopoietic stem cell activity [47] and can also be directed towards adipocyte and osteocyte phenotypes (reviewed in [48]).