THE ACTIVITY OF THE HAIR FOLLICLE 449 epithelial column lengthens. This transitional stage (catagen) in the human scalp has been described by Kligman (1). After the ascent of the club hair the epithelial strand shortens to form a little nipple called the "secondary germ" this resting stage is called telogen. When the next period of activity starts, the secondary germ elongates by cell division, grows downward to enclose the papilla and gives rise to a new hair bulb. The newly formed hair subsequently emerges beside the old dub, which is then lost. In the human scalp, the daily rate of growth of hairs is about 0.35 mm (2) thus to produce a hair, say, 35 cm long would require 1,000 days of growth, and it is dear that some hairs must have an even longer active period. Kligman (3) has pointed out that if there are about 100,000 hairs in the whole scalpsand blondes may have more--about 100 hairs ought to be lost per day, though actual counts gave a figure nearer 50. Normally, about 13}/o of the follicles are in telogen at any one time on this basis one can calculate that club hairs may be retained for about four months. The first hair to be produced in the human follicle is the long, fine lanugo which is shed by the embryo about one month before birth. Within the first month of life a further shedding of scalp hairs may occur with sufficient synchronization to cause temporary alopecia (3). In the adult, however, the activity of each follicle is independent of that of its neighbours, though a degree of synchronous shedding may result from conditions such as pregnancy, fever or psychological •* .... The skin of the guineapig seems to resemble that of the adult human scalp in having a mosaic pattern of follicular activity (4). Most animals, ß 0000 00000 O0 00000130 00000 000000000000 000000000000 ...,.......o f:oooooooo00000000000000000000 OOOOOOOOOOOO OOOOOOOOOOOO OOOOOOOOOOOO 000000000000 000000000000 000000000000 a b Figure 7 Diagrammatic representation of patterns of hair replacement. (a) Mosaic pattern in human scalp (active follicles shown black). {b) 'Wave pattern in rat. The advancing front of activity is shown by arrows. a: ariagert c = catagen t = telogen.

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS however, have periodic moults, implying that activity of the follicles is more or less synchronized. In white rats, successive waves of replacement of hair start on the belly and move over the flanks to the back (Figure 2) each hair is fully formed in 11 to 18 days and the club hair is retained for a very much longer period (5,6). Experiments on rodents have contributed much towards the understanding of hair growth. THE CONTROL OF HAIR GROWTH In trying to analyse the control of hair growth, it is important to appreciate that different factors may be involved at different phases of the follicular cycle. Of major interest is the problem of the initiation of activity, i.e. what controls the period of the follicle from the start of one anagen to the next. In addition, however, the duration of the active phase, the rate of growth of the hair, and the rate of loss of club hairs could all react independently. In order to simplify rather than to complicate the problem, it is proposed to discuss only those factors the existence of which seems likely from experiment. Three different sets of causes appear to interact in the control of follicular activity: An intrinsic follicular rhythm, transmissible systemic factors, and the influence of environmental changes. The relative importance of these varies from species to species. Intrinsic activity appears to be a major feature of human follicles, but it can be clearly demonstrated in rodents. In animals which breed and moult seasonally, such as the ferret (7,8) and the mountain hare (0), the role of environmental changes in the daily duration of light or in temperature may be demonstrable, and hormonal mechanisms are of obvious importance in these forms. But psychological factors can influence the human scalp (2), and the influence of hormones is clearly demonstrable even in domesticated forms, such as the laboratory rat and man, in which sexual cycles are almost completely disengaged from environmental influences. The evidence will now be briefly reviewed. Intrinsic factors The mosaic pattern of activity in the human scalp, when each follicle is out of phase with its neighbours, can only be explained on the assumption that there is a control mechanism within the follicle. Such follicles could, of course, still be subject to systemic influences, and under abnormal circum- stances a limited degree of synchrony may be achieved. In rodents the existence o[ innate control can be demonstrated by putting follicles out of phase with adjacent ones. This may be achieved by transplantation of skin to new sites or by plucking hairs in the resting phase it also occurs to some extent in grafts which are severed but replaced in their original positions.