Developmental origins of dermal papilla cells - The precursor of the hair follicle is a local thickening, also known as placode, of the embryonic epidermis, which is detectable at embryonic day 14.5 (E14.5) of mouse development. Soon after, a local condensation (dermal condensate) of fibroblasts forms beneath the placode. Reciprocal signalling between the condensate and the placode leads to proliferation of the overlying epithelium and downward extension of the new follicle into the dermis (Millar, 2002; Schneider et al., 2009; Ohyama et al., 2010; Yang and Cotsarelis, 2010). After the initial downward growth, the epithelial cells envelope the dermal condensate, thereby forming the mature DP. The DP then instructs the surrounding epithelial cells, now called matrix cells, to proliferate, move upward and differentiate into the multiple layers of the outgrowing hair shaft and the channel surrounding the hair shaft, called the inner root sheath (Millar, 2002; Schneider et al., 2009).
The role of the DP in the hair growth cycle - In postnatal life the hair follicles undergo cyclical growth. The resting phase is known as telogen, the growth phase as anagen and the regression phase as catagen (Müller-Röver et al., 2001; Schmidt-Ullrich and Paus, 2005; Ohyama et al., 2010; Yang and Cotsarelis, 2010). During catagen, the epithelial cells at the base of the follicle undergo apoptosis, but the DP remains intact and is pulled or migrates upwards, until it comes to rest next to the stem cells of the hair follicle bulge. This situation persists during telogen. In anagen, cells at the base of the follicle start to proliferate, which results in downward growth of the follicle and envelopment of the DP. DP cells themselves are thought to not divide. However, the number of cells in the DP increases during anagen, possibly as a result of replenishment from neighbouring cells of the dermal sheath (Tobin et al., 2003; Chi et al., 2010).