Skin and hair possess many important biologic functions, serving as the bodyâ€™s first line of defense against the outside world. In continuity with the skin, the hair follicle functions to protect the skin from the environmental elements and disperses sweat gland products (eg, pheromones).1 Eyebrow and eyelash hair protect the eyes from sweat and moisture. In addition, the hair follicle transmits sensory information to the nervous system via neuroreceptors that respond to mechanical stimuli above the skin surface. Hair also has essential immunologic functions, as Langerhans cells at the opening of the follicle detect surface pathogens and stimulate the immune system, in conjunction with perifollicular immune cells such as macrophages and mast cells.1
Furthermore, hair holds aesthetic importance, and conditions that result in hair loss (alopecia) or excessive hair growth can have devastating psychosocial effects. Eyebrows are vital for facial expression and, in conjunction with the eyelashes, cheekbones, hairline, and nose, eyebrows frame the eyes, thereby contributing to an individualâ€™s unique facial appearance.
Structure of Hair Follicles
Although hair follicles on the body vary in size and shape, they all share the same basic structure. The lower portion of the hair follicle comprises the hair bulb, which is composed of rapidly proliferating matrix cells that produce the hair shaft. The epithelial component of the hair follicle is composed of at least 8 concentric layers: the outer root sheath (ORS), the companion layer, the inner root sheath (IRS), which is subdivided into Henleâ€™s layer, Huxleyâ€™s layer, and the cuticle of the IRS, and the hair shaft cuticle, the cortex, and the medulla (Figures 1 and 2). These layers are composed of characteristic intermediate filament keratins, enzymes, and adhesion molecules.2 The ORS of the hair follicle is continuous with the epidermal basal layer and contains melanocytes, Langerhans cells (dendritic antigen-presenting cells), and Merkel cells (specialized neurosecretory cells).1
Pigment in the hair shaft is produced by melanocytes located in the hair bulb that transfer melanin to keratinocytes in the developing hair shaft cortex and medulla. As the matrix cells differentiate and move upward, they are compressed by the rigid IRS, whose structure determines the shape of the hair shaft.1 The dermal papilla, which is composed of specialized mesenchymal cells located at the base of the follicle, is thought to control the proliferation of matrix cells and thus the size of the hair shaft.1
The bulge consists of a cluster of biochemically distinct cells located in the ORS, near the insertion of the arrector pili muscle. Bulge cells have the characteristic properties of epithelial stem cells: they are slow-cycling (quiescent) and are thought to persist for the lifetime of the hair follicle.3,4 It is believed that the bulge population contributes to epithelial cells that proliferate and regenerate the new lower follicle during the growing stage of the hair cycle. Epithelial stem cells in the bulge portion of the ORS may also serve as a reservoir for epidermal and sebaceous-gland cells.5,6 The bulge region of the hair follicle is especially rich in nerve endings and Merkel cells.1 Hair follicle bulge cells express CD34 in mice and keratin 15 in humans.3
Morphogenesis of Hair Follicles
Hair morphogenesis is initiated in utero through complex interactions between the epithelium and underlying dermis.