The induction of hair follicle formation in the adult hooded rat by vibrissa dermal papillae.

Hair follicles are essentially composed of two tissues. The inner epidermal component, which gives rise to, among other products, the keratinized hair shaft, is confluent with the surface epidermis and is ensheathed by the dermal component which is confluent with the pars papillaris of the dermis. A specialization of the dermal component is the dermal papilla which, in follicles producing hair, is enclosed by the epidermal matrix of the hair bulb and is connected to the dermal sheath by the papilla stalk. Many authorities have considered that the dermal papilla is an essential component of the hair follicle (reviews: Cohen, 1965; Oliver, 1969). It has been suggested that the dermal papilla may be involved in both the induction of follicle lengthening and hair growth during the proanagen phase (Chase, 1965) of the hair cycle, a concept now justified by direct experimentation in the vibrissa follicle at least (Oliver, 19676), and perhaps also in determining the nature of the hair produced by a follicle. It also seems likely that the developing dermal papilla is an important, if not the primary, source of induction in follicle development during ontogeny (Jacobson, 1966; Kollar, 1966, 1968). Indeed various authorities have considered that the dermal papilla is capable of inducing follicle formation in the adult after wounding of the skin (Billingham, 1958; Billingham & Silvers, 1963). In order to investigate these possibilities directly Cohen (1961) devised techniques for isolating dermal papillae from excised bulbs from the comparatively large rodent vibrissa follicles, a possibility suggested by Billingham (1958). Cohen (1961, 1964, 1965) has reported the results of implanting vibrissa dermal papillae into ear dermis in the adult hooded rat. He has presented persuasive evidence that where the vibrissa papillae contacted ear epidermis, whether of follicular or non-follicular origin, they induced by 14 days the development and growth of ear-type hairs. These papillae underwent a drastic