Hair growth in the rat occurs in a series of waves, which start ventrally and pass over the flanks to the back (Dry, 1926; Butcher, 1934; Johnson, 1958a). The activity of the hair follicle is cyclic; when the hair has been fully formed there is a period of quiescence during which the dead hair is retained as a ‘club’. The duration of the complete cycle varies with site and age, ranging from 24 to about 100 days (Ebling & Johnson, 1964).

When hair follicles are translocated, they continue to maintain the periodicity characteristic of their sites of origin (Ebling & Johnson, 1959). On the other hand, when skin is exchanged between rats of different ages and thus with their hair growth waves out of phase, follicular activity in the graft skin in some circumstances comes into line with the activity of the host (Ebling & Johnson, 1961).

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