The Dermal Papilla and Maintenance of Hair Growth

Abstract

The dermal papula is present as an aggregation of mesodermal cells at the earliest stages of follicle development and persists as an anatomical entity throughout the life-time of the hair follicle. It contrasts with the hair-producing epidermal matrix which is a transitory structure appearing only during the anagen phase of the hair growth cycle. Using the adult rat vibrissa follicle as a model system, cell culture, microsurgical and transplantation techniques have provided direct evidence of a crucial role for follicular dermis and, in particular, the dermal papilla in follicle development, the maintenance of follicles and the control of hair growth through subsequent hair cycles. Hair growth does not occur in the absence of the papilla. In situ wounding of papillae can lead to hairs growing up to 30{\%} longer than normal. Following excision of the entire bulb, in both vibrissa and human follicles, new papillae regenerate from cells from the follicular sheath and hair growth is resumed. Implants of isolated papillae, or cultured papilla cells, into follicles induce hair growth and the formation of new, vibrissa-producing follicles when recombined with skin epidermis. In these interactions papilla cells induce the development of an epidermal matrix, heightened epidermal mitotic activity and the differentiation of hair follicle-type keratin products. Cultured vibrissa papilla cells also stimulate keratinocyte proliferation in vitro. These findings suggest that the behaviour of the epidermal component of the follicle is under the control of the follicular dermis which appears to retain embryonic characteristics.