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

DERMAL PAPILLA AND THE DEVELOPMENT AND GROWTH OF HAIR 743 from typical long and spindle shaped fibroblasts to rounded cells containing organelles associated with protein synthesis (7, 8). Division products in the developing epidermal matrix region, which invests the dermal papilla, give rise to the various cylindrical elements which comprise the inner root sheath, the first to differentiate, and the hair itself. ECTODERMAL--MESODERMAL INTERACTIONS IN THE DEVELOPMENT OF SKIN APPENDAGES Tissue separation and recombination studies, using ectoderm and mesoderm of the same and differing embryonic ages and from the same and differing anatomical sites, have demonstrated that interactions occur between these elements to ensure the development of skin and appendages in mammals and birds (9-18). Intensive studies of avian skin have established that the mesoderm initially dictates the developmental fate of the overlying ectoderm, while itself showing regional variations in its specificity and the timing and "strength" of its influence (9-11). However in certain instances committed and differentiating ectoderm may then not only become increasingly or totally resistant to the influence of "foreign" mesoderm, but may also be capable of organizing such mesoderm to comply with the developmental course the ectoderm had already embarked upon. Thus recombinants of prospective feather ectoderm and scale mesoderm, from the leg, either developed feathers and scales or feathers alone (11, 12). Dhouailly (13, 14) has further shown that the gross features character- istic of a feather type are determined by the dermis while the fine structure of the barbules is determined by the epidermis. Moreover the mesoderm also largely determines the time of appearance of the teleoptiles (juvenile feathers) as well as the mode of replacement of the neoptiles {"embryo" feathers). In general, so far as the less extensive experimental appraisal allows, an essentially similar situation seems to be true of mammalian skin in relation to the development of hairs (and teeth). Jacobson (15) presented evidence for two interactive crises in the development of whisker follicles in the mouse one at the time of follicle site determination and a later one between the dermal papilla and the follicle epidermis. Recombinants of mesoderm from the lip region, in which whisker follicles normally develop, with foot plate ectoderm or with tooth ectoderm developed whiskers (16,