Normal head-hair length is correlated with its diameter.

Shed head-hair fibres of young (16-20-year-old), nonalopecic women (n = 25), exhibiting both exogen clubs and anagen tips (EA) were studied. Such fibres are shown, for the first time, to comprise approximately 44% of shed hair and to form a uni-modal, positively skewed distribution with a mean length of 16.7 +/- 4.9 cm, which is also correlated with the length of the haircut. As individual fibres exit the skin in early anagen VI, their major-axis diameters increase rapidly to maxima at about 25% of their total potential length and subsequently decrease to their exogen clubs, at a rate of 1.31% per cm (n = 28). EA diameters are further correlated with their lengths. Maximal and proximal diameters increase by 1.40% per cm and 1.02% per cm increments in fibre lengths, respectively (P < 0.0001 each; n = 14), these changes being also different from each other (P < 0.001). Besides identifying and characterizing a new class of normal hair (EA) which will probably feature prominently in future hair research, this study reveals several other important aspects of hair growth: (i) the classically described concept of hair miniaturization in androgenetic alopecia (AGA) is excessively broad and should therefore be revised; (ii) female AGA need not necessarily require a mechanism for rapid miniaturization as recently proposed; and (iii) the putative large variability of normal hair diameters is significantly overestimated, which further opens the field of hair diameter evaluation as a biological indicator of disease and physiological function.

Scalp hair diameter increases during pregnancy.

The hair diameter major-axis has been shown to decrease normally toward the scalp in individual fibres. In this report sharp increases, rather than decreases, of hair-shaft major-axis diameters during pregnancy are described. Mean major-axis hair diameters of 15-20 fibres from each of 12 pregnant and 13 nonpregnant women were measured from the scalp distally at regular 6-mm intervals. Sample diameters were normalized relative to putative times of conception or a distance from the scalp equivalent to full term pregnancy, for the pregnant and nonpregnant women, respectively, and averaged. Major-axis diameter increases toward the scalp in hair of pregnant women were found to start at the beginning of pregnancy with a slope measuring 0.58%/cm, which is highly significant (r = 0.99, P < 0.0001). Hair diameters of the nonpregnant women, on the other hand, decreased toward the scalp during analogous times to the pregnancy, with a slope measuring -0.66%/cm, also highly significant (r = -0.97, P < 0.0001), and in good agreement with previously published data. To our knowledge this is the first description of hair diameter increases during a normal physiological process.