Facial Hair Biology

1.4.1 Hair Follicle Structure, Growth Characteristics, and Regulation

Human body hair can be defined as three major types: lanugo, vellus, and terminal hair. Lanugo hair is present on infants and is lost shortly after birth. In contrast, vellus and ter­minal hair remain as the major hair type on the human body. There are clear physical dis­tinctions between these hair types. Vellus hairs are finer hairs, nonpigmented, and range in size from 0.01 cm thick to < 2 cm in length and are present on all body parts with the excep­tion of plantar skin (palms and soles). In contrast, terminal hairs are coarser, pigmented, and average in thickness, and 0.05 cm and >2 cm in length. In adults, terminal hair is found on the scalp, limbs, axilla, and male face [14]. The latter three regions represent areas from which consumers remove hair on a nearly daily basis.

The hair follicle is a dynamic skin structure that is the product of numerous cumulative biological processes that permit constrained rapid cell proliferation similar to that found in the epithelial lining in the intestinal system, and which at the same time gives rise to the highly differentiated keratinized hair fiber. The growth of hair is cyclical and includes an active growing stage (anagen) followed by a transitional stage (catagen), and a resting stage (telogen). At any given time, about 90% of terminal facial hair is in anagen in males, which may last up to several years. In contrast, the majority of vellus hair on the face is in telogen. In the upper lip and chin regions of women, terminal hair may be androgen-sensitive similar to the male beard and contribute to the hirsute phenotype, whereas dark vellus type hairs on the cheek may be a function of genetic predisposition, and independent of androgens. Table 6.1 compares hair growth rates and diameters across body sites (P&G internal data).

Anatomically, the hair follicle has been segmented into four major regions: the infundibu­lum, the sebaceous gland region, the isthmus, and the bulb [14]. The infundibulum is

contiguous with the surface of the skin and extends to the insertion site of the sebaceous duct. This region of the hair follicle is where the naked hair fiber emerges from the follicle as well as the path of sebum secretion from the sebaceous gland to the surface of the skin. The sebaceous gland region contains the lobular sebaceous gland full of cells filled with lipid to be secreted into the infundibulum. The isthmus is the middle region of the follicle from the sebaceous gland to the bulge area that is near the insertion point for the arrector pili muscle. It is in this region that the inner root sheath disappears. The highly metabolic bulb region includes the epithelial and germinative matrix cells that envelope the mesen­chyme-derived dermal papilla cells. In the anagen phase of the hair cycle, the rapidly pro­liferating matrix cells are in close physical contact with the dermal papilla. This contact is lost as the hair cycle progresses through catagen and into the resting telogen stage. The hair follicle bulb also includes melanocytes which provide melanin for incorporation into the hair fiber. Coupled with the rapid proliferation rate of the hair follicle germinative cells are keratinization processes that give rise to the highly differentiated hair fiber and the inner and outer root sheaths. The size of the hair fiber has been suggested to be a function of the volume of the dermal papilla compartment [15].

The hair growth cycle is characterized by marked changes in metabolic activity as well as morphology, and includes an active growth phase (anagen) in which the hair bulb sur­rounds the dermal papilla and has a high mitotic index. In humans, anagen hair follicles descend to the level of the hypodermis in such a way that their bulb regions are highly vas­cularized and immersed in fat—potentially to support the high energy requirements required by the growing follicle. Following a period of active growth that varies by body site and age—among other factors—the follicle moves through a transitional stage (catagen) where the follicle regresses in a way that the dermal papilla and matrix cells become separated and disengaged. The process is characterized by a high level of proteolytic and apoptotic activity. Telogen is the resting phase of the hair cycle, and represents a quiescent hair fol­licle showing little or no cell proliferation or metabolic activity. It remains unclear what controls the cycle, although numerous studies have elegantly demonstrated the contribution of a variety of cytokines in its regulation [16-18].