The cross-sectional size and shape of human terminal scalp hair.

Hutchinson PE, Thompson JR.

Department of Dermatology, Leicester Royal Infirmary, UK.

Change in size of the hair shaft with distance form the scalp has been investigated, using a rotatory profile method of diameter measurement, in terminal human scalp hair of long-haired young Caucasian women. As the whole length of hairs having completed anagen are rarely found intact, two types of hair were investigated: those including segments produced at the onset of anagen ('anagen hairs'), and those including segments produced at the end of anagen ('telogen hairs'). In addition, a method of determining the cause of any size variation has been described and employed. Changes were found in the major axis of the hair cross-section, cross-sectional area and ellipticity with distance from the scalp, while the minor cross-sectional axis remained constant. It was established that these changes were the result of intrafollicular rather than extrafollicular mechanisms. Finally, a composite picture of the cross-sectional size and shape of the 'average' whole anagen hair of the study has been constructed. From the distal tip towards the scalp for approximately 6-8 cm, there was an abrupt increase in size, representing a starting-up phase of early anagen. Following this, the hair was at its greatest cross-sectional size and ellipticity which then progressively decreased through anagen (20% decrease for cross-sectional area and 13% for ellipticity). In contrast, the minor axis of the hair cross-section, remained constant throughout anagen. The hair was not therefore a uniformly sized cylinder. It was approximately spear-shaped, being broadened out in one plane distally where it was more elliptical. Subsequently as anagen progressed the hair shaft became smaller and more circular.