Human scalp hair follicle development from birth to adulthood: statistical study with special regard to putative stem cells in the bulge and proliferating cells in the matrix.

The depths of hair follicle compartments, and in particular of the bulge, the putative site of hair follicle stem cells, have not yet been determined in human scalp skin from infants, children or adolescents. This information is necessary in order to use the scalp safely as a donor site for skin grafts. We therefore investigated the development of the infundibulum, the bulge, Adamson's fringe, the B-fringe and the matrix by measuring the depths of these five follicular compartments in parietal scalp biopsy specimens from 100 patients ranging in age from 2 weeks to 21 years. The thickness of the epidermis and the dermis were also assessed. The correlations of these measurements with age were determined by regression analysis. The regression equation for the bulge was found to be b (microns) = 683.3 + 30.8y (r = 0.73; SEM = 145.5) where y is the age in years, and for the matrix it was m (microns) = 1616.2 + 90.4y (r = 0.76; SEM = 406.5); P < 0.0001 for the null hypothesis. The growth of the inferior portion below the bulge was not parallel but proportional to that of the superior portion. The relative position of the bulge in the dermis was stable, whereas the inferior portion moved progressively more deeply into the subcutis. These findings provide evidence for the postulated biologically advantageous localization of the bulge, and thus is a further argument in favour of the bulge as the site of follicular stem cells.