DERMAL PAPILLA AND THE DEVELOPMENT AND GROWTH OF HAIR 751 component is essential for hair growth. On removal of the whisker papilla from the follicle, hair growth ceased and was only renewed when a papilla spontaneously regenerated or, failing this, when one was implanted. Induction of hair growth where the bulbar region was removed and a dermal papilla implanted has several implications. It confirmed the deduction of Montagna and Chase (48), from the results of exposing scalp to X-rays, that the source of the epidermal germ for the next hair growth need not be con- sidered in terms of a specialized population of cells surviving from the previous matrix. Extension of the follicle associated with the initiation and growth of hair, reminiscent of proanagen in pelage follicles, is highly suggestive that the dermal papilla is primarily involved in the naturally occurring initiation and development of proanagen. Furthermore, regard- less of the obvious anatomical differences which initially exist, the morpho- genetic changes and presumed interactions which occur at proanagen appear, in essence, to be identical to those which occur in ontogeny following establishment of the follicle plug. The fact that the adult dermal papilla can induce follicle development gives added weight to this supposition. It is also not unreasonable to suspect papilla complicity in the termin- ation of anagen, perhaps even exercising some control over the orderly regression of the follicle during catagen (2). Evidence is flimsy on this point. While the matrix disappeared, hair growth ceased and the hair was lost from the follicle after removal of the dermal papilla, it could equally be argued that these sequelae were a wounding response. Nevertheless simple cessation of cell division in the matrix, following cancer drug therapy, did not of itself precipitate the follicle into catagen (49). While systemic factors have been shown to influence the growth of hair, their effect seems to be one of modification of an intrinsically determined behaviour acquired by each follicle during its development (50-53). Current results strongly suggest that it is the dermal component of hair follicles which carries this behavioural information. However explanations for the cyclic nature of hair growth include the suggestion that inhibitors accum- ulate somehow and somewhere in the epidermal component and, on attain- ment of a threshold level, cause cessation of mitosis in the matrix (29, 54). A leaching out or loss of inhibitor would then allow growth to be resumed. While such factors may exist and be of importance the stimulatory influence of the dermal papilla should certainly not be forgotten. Indeed, even apart from morphological changes, the cyclic activities which occur within the papilla itself, including the known ones of presence of acid mucopolysaccharides and alkaline phosphatases during anagen and

752 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS their absence during telogen (55, 56), may reflect an underlying intrapapilla synthetic cycle, determined during papilla development, which is causally related to subsequent changes in the epidermal component. (It may be relevant here that feather development in culture was suppressed on in- hibition of alkaline phosphatases (57)). Similarly, observations on the various geometrical correlations which exist between the dermal papilla, the matrix of the hair bulb, and the characteristics of the hair grown from a follicle have invited speculation that the papilla may be exercising ultimate control in some unknown way (58-60). Papilla/matrix interactions would also seem to be involved in the "cellular shunt" system which operates to produce complementary thicken- ing of the Huxley layer of the inner root sheath in follicles producing hairs of varying cross-sectional shapes and sizes (61-63). The reduplication of the basal lamella within the bulb, as observed in follicles of man (64) and the guinea pig (65), may also in some manner relate to papilla/matrix inter- actions. Finally it must not be forgotten that the dermal papilla is structurally confluent with the dermal sheath with which it develops during ontogeny. This sheath is presumably involved both in the vertical integration of the follicle (29) and its maintenance. It was seen to be the source of papillae, where they regenerated, and may have played a role in the preservation of the superficial region of the whisker follicles which did not regenerate papillae. It is also possible that inductive stimuli from the dermal sheath, perhaps involving the glassy membrane, help establish and maintain that peculiar follicular epidermal variant, the outer root sheath. Certainly it is likely that the dermal component of the follicle, by virtue of an overriding specific influence, resists the known influence of the extrafollicular dermis in controlling the behaviour of adjacent epidermal cells. These and other interactive considerations are discussed by Straile (46), Cohen (44), and Oliver (66). (Received.' 24th February 1971) REFERENCES (1) Montagna, W. and Van Scott, E. J. in Montagna, W. and Ellis, R. A. The biology of hair growth 39 (1958) (Academic Press, New York). (2) Straile, W. E., Chase, H. B. and Arsenault, C. T. Growth and differentiation of hair follicles between periods of activity and quiescence. J. Exptl. Zool. 148 205 (1961). (3) Chase, H. B. in Lyne, A. G. and Short, B. F. Biology of the skin and hair growth 461 (1965) (Angus and Robertson, Sydney). (4) Hardy, M. H. The development of pelage hairs and vibrissae from skin in tissue culture. Ann. N.Y. Acad. Sci. 53 546 (1951). (5) Pinkus, H. in Montagna, W. and Ellis, R. A. The biology of hair growth 1 (1958) (Academic Press, New York).