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

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.