NORMAL STRUCTURE AND FUNCTION OF THE HAIR FOLLICLE APPARATUS
The pilosebaceous unit is a complex apparatus that comprises a hair shaft and its follicle, sebaceous glands, arrector pili muscle, and, in some regions of the body, apocrine glands. Hair is a complex structure that is made of many different keratin proteins linked by disulfide bonds between neighboring cysteine amino acid molecules. The keratin molecules come in acidic and basic forms. An acidic keratin fiber localizes with a basic keratin fiber and cross-links via disulfide bonds.
The exact function of hair is unknown, but it is theorized to act as an insulator for heat retention and has been postulated to be important to attract a mate. No matter what the function, humans can live a normal life without the presence of hair with no ill effects.
Hair comes in a variety of colors. The amount of melanin or pheomelanin in the hair shaft determines the exact color of the hair. With time, the production of hair pigment decreases, and the hair becomes dull gray or white. This process is unpredictable in a given individual, and even those within the same family may show striking differences in hair color change. As people age, the scalp hair usually tends to thin. This is considered to be a normal physiological process.
There are two main types of hair in the adult. Terminal hair is thick hair that is present on the scalp, axilla, and groin and in the beard region in men. Vellus hair is the fine, thin, lightly pigmented hair that can be found in most areas of the body where terminal hair is not present. No hair is present on the lips, palms, soles, glans, or labia minora. Lanugo hair is present during fetal development and is predominantly seen in premature infants. This type of hair is shed in utero and replaced with vellus hair before delivery. Reversion of vellus hair and terminal hair back to lanugo hair is a sign of anorexia nervosa. Lanugo hair has a soft, fluffy white appearance.
The hair cycle is an extremely complex and highly coordinated process. The anagen phase is the growth phase. The anagen phase of the typical adult scalp hair lasts approximately 2 years. This growth phase is followed by the catagen phase, which is a short (2 week) transition period during which the hair follicle trans-forms from a growing, functioning hair into a club hair. This is followed by the telogen phase, which lasts approximately 2 months and ends with shedding of the club hair. Anagen hairs have a floppy, pigmented end that is easily distinguished from the telogen hair. Telogen hair is termed club hair because of its depigmented bulb at the proximal end. Catagen hairs are almost impossible to identify because they appear somewhere in the spectrum between anagen and telogen hair. The length of the anagen phase is responsible for the overall length of the hair: The longer the anagen phase, the longer the hair can grow. This process is preprogrammed and is different for all hair types on the body. The normal scalp can shed up to 100 hairs per day. The hair follicle is remarkably capable of regeneration after the hair has entered into telogen phase. An unknown signal causes the hair follicle stem cells, which are located in the bulge region, to differentiate and begin producing another hair, restarting the anagen phase. The bulge region is an area found in approximation to the inser ion of the arrector pili muscle into the hair follicle.
Histological examination of a cross section of a terminal hair shaft reveals a complex architecture. The hair is made up of various concentric layers. The inner-most layer is the medulla, which is pigmented. The next layer is the cortex, followed by the cuticle, the inner root sheath (Huxley’s and Henley’s layers), and the outer root sheath. The outer root sheath seamlessly blends into the epidermis. The hair follicle undergoes trichohyalin keratinization, which is different from the keratohyalin keratinization of the epidermis.