The size and form of the medulla of human scalp hair is regulated by the hair cycle and cross-sectional size of the hair shaft

Hairs were sampled from long-haired Caucasian females, and cross-sectional measurements were performed using a rotating profile method at fixed humidi ty (100%). The effect of the hair cycle on medullation was investigated by examining medulla size and form along the lengths of anagen and telogen ter minal hairs and also from a composite model of the entire medulla, as produ ced from a full cycle's growth, by amalgamating the results from the anagen and telogen hairs. The effect of hair shaft cross-sectional size on medull ation has been investigated by controlling any effect of the hair cycle on terminal hairs and by use of the maximal medulla size in short, fine (vello us) hairs from the same subjects. All terminal hairs were medullated for th e majority of their lengths. The presence and size of the medulla, in termi nal hairs, was profoundly affected by the hair cycle. It was largest-early in anagen, where the medulla minor axis, major axis and cross-sectional are a represented about, 26%, 23% and 7% of the corresponding whole hair shaft parameters (at 100% humidity). The medulla was virtually absent towards the end of anagen. The shape of the medulla cross-section was, less elliptical than that of the whole hair shaft and was close to circular There was no s ignificant change in medulla shape through anagen. The form of the medulla was also affected by the hair cycle; approximately, it was continuous for t he first 50% of anagen, discontinuous for the: next 25% and virtually, abse nt or absent for the final 25%. The maximal size of the medulla, as occurre d in early anagen, was markedly associated with the cross-sectional size of the whole hair shaft, both within terminal hairs and between all scalp hai rs. The medulla was large in terminal hairs and small or absent in very sma ll hairs. The proportion of the whole hair shaft occupied by the medulla in creased with increase in hair size and reached a maximum in terminal hairs, in which the medulla minor axis represented about 30% of the whole hair sh aft minor axis. Furthermore, this proportion was constant in the terminal h airs and was not related to whole hair shaft size. Such maximal proportiona l medullation might represent a defining feature of terminal hairs. Variati on in size of the medulla. is not the cause of the previously reported cycl e-dependent change in cross-sectional size of the whole hair shaft of termi nal hairs.