THE SKIN AS A COMMUNITY OF STRUCTURES E. W. POWELL, M.A., A.R.I.C.* The first part of a study of the sirin as a balanced complex of inter-related structures. Tissue culture research is reviewed to show the extent to which skin structure development has been found to take place in absence of the whole animal. INTRODUCTION MANY COMMONLY observable phenomena relating to the human skin and its. various component structures, the hair, sebaceous glands, sweat õlands and pigment-producing cells, give rise to interesting thoughts upon the inter- dependence of these structures. The changes in the condition of the hair which can occur with variations in general health emphasise the relation to the body as a whole, a relationship which is studied in one aspect by investigations of the effects of various natural hormones. Upon the same scalp, white and dark hairs can õrow simultaneously for many years, indicat- ing that the conditions which permit growth of hair can result in cessation of the production of pigment even when pigment-producing cells have been present. A scalp which bears but a thin growth of white hair may acquire a tan in sunlight and the ability to produce piõment is still present in the epidermis although lacking in the hair bulb. Such observations, which point to the skin and its appendages as a system of organs dependent upon the whole organism but, in addition, with each structure having some degree of individuality, prompt further enquiry into the extent of this dependence and into the differences in response of the structures to changes of conditions. The evolution of the human body, for example, is frequently represented as the development of more precise control of the environment in which the constituent cells of the body can live. Associated with this control is the specialisation of function with the cells grouped to form organs making particular contributions to the metabolism of the body as a whole. Such organs as the liver, kidneys, intestines and lungs function with a certain degree of independence, but rely upon the blood stream, the nervous system and the hormonal system for their normal activity. Similarly, the particular structures of the skin such as the epidermis, hair follicles, sebaceous glands and pigment-producing cells have their particular functions to perform which involve the production of the keratin proteins of the epidermis and the hair, the formation of the oil, sebum, and of the pigment, melanin, respectively. In the whole animal, they have their particular relations to the blood, nervous and hormonal systems. The various appendages of the skin have been described individually in a number of excellent reviews. It is the object of this paper, however, to regard the skin as a balanced and complex * County Laboratories, Ltd., Honeypot Lane, Stanmore, Middlesex. 118

THE SKIN AS A COMMUNITY OF STRUCTURES community of these structures and to bring together some of the information which illustrates this point of view. The paper is in three parts, the first of which considers the extent to which the development and growth of the skin structures have been shown to take place in the absence of the whole animal. It is an account of some of the work carried out by tissue-culture techniques upon tissue from embryos and upon adult mammalian skin. The second part of the paper will consider some of the characteristics of the growth of hair and the production of pig- ment in order to demonstrate some of the similarities and differences in the activities of the hair follicles and the pigment-producing cells. It will attempt to show what requirements these structures have in common and in what circumstances the functioning of one structure is markedly affected without apparent effect on the other structure. Finally, these considerations will be extended to the relation between the hair follicles and the sebaceous glands with the additional object of weighing evidence for competition and a balance of activity between them. It is obvious that the discussion of such questions is relevant in any consideration of the possibility of treatments affecting only certain of the skin structures, particularly by local application of such treatment. Part I THE DEVELOPMENT OF SKIN STRUCTURES INDEPENDENT OF THE MAMMALIAN BODY When the responses of an organ or a tissue to a certain treatment are studied while that organ or tissue remains part of the intact animal, it is very difficult to distinguish between effects which are produced by direct action and those which result through the mediation of some other organ in the animal. Thus, a change in the activity of an endocrine gland as a result of the treatment can affect a number of organs, including, perhaps, the organ which is being studied. In order to separate the direct from the indirect effects and so obtain information about the intrinsic potentialities of the tissue, many studies have been made of small pieces of tissue growing in isolation from the whole animal. The tissues can be cultivated as organs, that is, organised assemblies of cells, and it is such studies relating to the skin which are considered below. ¾.MBRYO TISSUES The main development of the mammalian epidermis and its associated structures takes place in the embryo. In the mouse, for example, the hair follicles are formed before birth and for some time shortly afterwards, but in the human most follicles are formed before birth. Studies of the develop- 119