Embryologic development of hair follicles is the same for all races, with the important exception of the distribution of melanin along the hair follicle of individuals of African descent.
The shapes of hair follicles and the hair shaft differ based on race.
The amino acid structure of hair is consistent across all racial groups.
Textured hair of individuals of African descent is more susceptible to breakage (lower tensile strength) than Asian or Caucasian hair.
As measured by the number of hair follicles in a 4-mm punch biopsy specimen, Asian and textured African hair is less dense than Caucasian hair.
Textured African hair swells less than either Asian or Caucasian hair when in contact with water and has a decreased moisture content.
Hair serves many useful biologic functions such as protection against physical and chemical damage; insulation against heat loss, desiccation, and overheating; and dispersion of eccrine and apocrine gland secretions. It also has a particular psychosocial importance in our society, especially with regard to sex, class, and racial distinctions. Diseases and disturbances in hair (eg, excessive hair growth or loss or deformity) cause significant morbidity in affected individuals.
ANATOMY OF THE HAIR FOLLICLE
The hair follicle and shaft reveal a complex architecture that is made of many distinct structures [FIGURE 14-1]. There are three main types of hair follicles: lanugo, vellus, and terminal. These follicles each demonstrate similar microanatomic features. The dermal papilla, a mesenchymal derivative, interacts with the hair matrix, which is of ectodermal origin.1,2,3 The interaction between the dermal papillae and the hair matrix controls hair growth and differentiation. The bulb is the lowermost portion of the hair follicle and contains the proliferating matrix cells; these cells produce the hair shaft and all epithelial compartments of the hair follicle and shaft except the outer root sheath.3
Hair follicle cycling and hair anatomy. Sec Grm, secondary germ. (Reproduced with permission from Goldsmith LA, et al. Fitzpatrick’s Dermatology in General Medicine. 8th ed. New York, NY: McGraw-Hill; 2012.)
The hair shaft consists of three cell lineages: the cuticle, cortex, and medulla. These three layers contribute to the appearance of the shaft by regulating its structure. In addition, the structure of hair influences attributes such as light absorption, reflection, and refraction.2 The cuticle, composed of flattened cells, forms the hair surface. It protects the shaft from weathering. Cuticular damage causes the hair shaft to fracture, split, and break. The cortex is the multicelluar compartment and the site of keratinization. It is composed of hair-specific intermediate filaments and associated proteins and is essential for shaft rigidity.1 The medulla is centrally located within the cortex of terminal hair; it is often absent in vellus hair.3