J. Soc. Cosmet. Chem. 22 741-755 (1971) ¸ 1971 Society of Cosmetic Chemists of Great Britain The dermal development papilla and the and growth of hair R. F. OLIVER* Presented on loth March 1971 in Eastbourne, Sussex, at the Symposium on "Appendages of the Skin", organised by the Society of Cosmetic Chemists of Great Britain. S•,nopsis--Dermal-epidermal interactions in the development and growth of S KIN appendages have been reviewed. Particular emphasis has been placed on the dermal papilla of the HAIR FOLLICLE because, apart from its importance in the ONTOGENETIC development of hair follicles, it has also been shown to induce follicle development from adult EPIDERMIS, and ORAL EPITHELIUM, and to induce hair growth in the adult follicle. It is suggested that the dermal papilla may also be involved in the termination of hair growth at CATAGEN and in the determination of FIBRE characteristics. It is further suggested that the intrinsically determined properties of hair follicles may reside in the dermal component of the follicle. INTRODUCTION The hair follicle presents a number of problems of biological, cos- metic, economic and social interest. Perhaps the most intriguing are the elucidation of the factors which control the intermittent production of hair from a follicle, those which determine the characteristics of the hair, and those which are involved in follicle maintenance. The hair follicle, in common with skin and its other appendages (e.g. feather follicles, teeth and scales), is essentially composed of epidermal and dermal tissues which derive from the outer embryonic ectoderm and the underlying mesoderm. Enveloping the epidermal component of the hair *A.R.C. Institute of Animal Physiology, Babraham, Cambridge. Now at Department of Biological Sciences, The University, Dundee, Scotland, DD1 4HN. 741

742 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS follicle is the glassy membrane and the dermal component consisting of the dermal sheath, which is confluent with the pars papillaris beneath the epidermis, and the dermal papilla at the base of the follicle. In follicles growing hair the dermal papilla is itself enclosed by the epidermal matrix of the hair bulb and is connected to the dermal sheath by the papilla stalk (1). At telogen the papilla persists as a papilla "rest" adjacent to the germ plate of the shortened epidermal component, the collapsed dermal sheath and glassy membrane, which were vacated during catagen, remaining lower in the dermis (2). It is the purpose of this paper, by selectively reviewing work on the development and growth of hair, and also of feathers, to consider how far highly localized interactions between these epidermal and dermal com- ponents, and in particular the dermal papilla, may relate to the problems set out above. I think it will be seen that a proper consideration of the adult situation inescapably demands a knowledge of the ontogenetic events which lead to the development of follicles, for it is at this time that structural and behavioural characteristics of many follicle types are determined. It has also been suggested that processes similar or identical to those involved in follicle development may recur at proanagen, the early morphogenetic phase of the hair cycle (3). SOME FEATURES OF THE DEVELOPING HAIR FOLLICLE The developmental anatomy of the hair follicle has been well described e.g. mouse whisker and pelage hair follicles (4) and human follicles (5). The first histological signs of development are a focal crowding and elongation of epidermal basal cells in association with an underlying aggregation of dermal cells (6, 7). Mitotic activity within the epidermal cells leads to the development of a solid cylindrical peg which grows into the mesoderm, ensheathed by dermal cells which are confluent with the prominent dermal condensation carried before it. The lower end of this peg comes to invest the dermal condensation which is now called the dermal papilla. Wessells and Roessner (6) demonstrated that the dermal condensation initially derives from local mitotic activity and that a period of nonproliferation quickly intervenes and largely persists during the period of hair peg down- growth. They consider that specialized syntheses may occur in these dermal papilla anlagen cells during this period, when interactions are thought to take place (see below). Electron microscope studies show that they change