Hair Follicle Anatomy

The hair follicle can be divided into three anatomical compartments: the infundibulum, isthmus, and the inferior segment. The upper follicle is permanent, whereas the lower fol­licle, the inferior segment, regenerates with each hair follicle cycle. The infundibulum extends from the skin surface to the sebaceous duct. The isthmus, the permanent middle portion, extends from the duct of sebaceous gland to the exertion of arrector pilli muscle. The inferior segment consists of the suprabulbar area and the hair bulb. The hair bulb con­sists of extensively proliferating keratinocytes and pigment-producing melanocytes of the hair matrix that surround the pear-shaped dermal papilla, which contains specialised fibro­blast-type cells embedded in an extracellular matrix and separated from the keratinocytes by a basement membrane [22]. The hair matrix keratinocytes move upwards and differenti­ate into the hair shaft, as well as into the inner root sheath; the melanocytes transfer pig­ment into the developing hair keratinocytes to give the hair its colour. The epithelial portion of the hair follicle is separated from the surrounding dermis by the perifollicular connec­tive tissue or dermal sheath. This consists of an inner basement membrane called the hya­line or glassy membrane and an outer connective tissue sheath. The major compartments of the hair follicle from the innermost to the outermost include the hair shaft, the inner root sheath, the outer root sheath, and the connective tissue sheath (Fig. 1.1).