Pigmentation and Hair Growth in Black Rats, as Modified by the Chronic Administration of Thiourea, Phenyl Thiourea and Alpha-Naphthyl Thiourea1

OBSEKVATIONS have often been made of the fact that hair growth is intermittent, so that at any particular time some hairs are growing actively while other neighboring ones are not lengthening at all. In measuring the growth of marked human hairs Trotter (1924) found that the underlying cause of this apparent irregularity is a rhythmic alternation between activity and quiescence in individual hair follicles. In man the distribution of active and inactive follicles would seem to be random, so that of two adjacent hairs one can be growing while the other has reached its maximum length and protrudes from a temporarily inactive follicle. In many laboratory animals, however, there is a certain regional coordination in activity, with the result that at any given time hair is being produced from most if not all follicles in some areas while no growth occurs from those elsewhere. This fact was known, for instance, to Forster (1929) who used cats rather than mice, rats or rabbits to study hair growth stimulants, because the last named animals all had "irregular, islandlike" hair regrowth patterns.