QUANTITATIVE STUDIES OF HAIR GROWTH IN THE ALBINO RAT

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Abstract
SUMMARY: 1. The patterns of the first and second hair growth waves, forming the second and third coats, were followed in albino rats which had been dyed black before the start of hair growth. 2. The length of hair produced during the growth waves varies from region to region of the body. These regional differences are partly the result of differences in rates of growth, but mainly due to differences in the duration of the period of follicular activity. 3. The duration of the growing and resting periods varies in different areas of the body. For example, in the ventral region the resting period between the first and second waves lasts for about 14 days and the growing period of the second wave for only 12 days, whereas in the mid-dorsal region the resting period lasts for about 4 weeks and the growing period about 19 days. 4. As the growth wave spreads over each area every follicle produces a new hair alongside the dead or 'club' hair. Loss of club hairs then begins, and the rate of loss varies from region to region of the body. 5. In female rats the growth wave moves more slowly, the definitive length of hair is shorter, and the loss of club hairs is slower than in the male. The sexual differences in hair length are the result of differing rates of growth, and there is no difference in the duration of follicular activity between the sexes. 6. It is suggested that in assessing the effect on hair growth of any treatment the following aspects should be considered: (a) Time of initiation of the growth wave and its duration. (The length of the resting period in any region will depend on both these factors.) (b) Duration of the growing period. (c) Rate of elongation of the hair during the growing period. (d) Rate of loss of club hairs.
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