Structure and Function of Hair Follicles
Howard P. Baden, M.D.
The hair follicle is a complex skin appendage that results from epidermal-dermal interaction and modeling, which begins in the first trimester. No new hair follicles are formed after birth, although the size of the hair, its cyclic behavior and color change throughout life. The highest density of follicles is present on the head, which is the site of longest hairs. The term vellus is used to denote the fine, short hairs that are present on the cutaneous surface, whereas the term terminal is used to describe thicker, pigmented hairs. Hair growth is cyclic with alternating periods of growth (anagen), which lasts for years, arrest (catagen), which lasts for days, and resting (telogen), which lasts for months. The differences in the length of hairs is more related to the length of anagen phase than growth rate. About 90 percent of the more than 100,000 scalp hairs are growing (about 6 inches per year). Newly emerging hairs have a tapered end and then continue to grow with a constant diameter. The remaining 10 percent of scalp hairs are either in a transition or a resting phase. Normally about 50 to 100 hairs in a resting stage are shed daily.
The hair follicle is composed of multiple layers with the outermost continuous with the epidermis and the innermost being the cortex surrounded by its cuticle, which gives rise to the shaft that extends above the cutaneous surface (Figure 1). The hair follicle is positioned at an angle with its base in the subcutaneous fat. A muscle is attached to the side of the follicle and runs to the upper dermis, forming an obtuse angle. When the muscle contracts, the hair rises, resulting in goose bumps. The sebaceous gland, located above the attachment of the muscle, connects to the pilar canal through a duct.
The growth of hair occurs in the bulb at the base of the follicle, which consists of rapidly dividing cells surrounding the dermal papilla. Just prior to telogen the lower third of the follicle is reabsorbed, leaving the papilla separated from epithelial cells. It is thought that the upward movement of the papilla and its interaction with the stem cells located near the insertion of the muscle is responsible for initiating a new hair cycle. A large number of growth factors and their receptors are present in the follicle and several of them have been shown to play a role in hair growth and differentiation. The so-called hairless mouse results from a mutation in the hairless gene, and the phenotype and mutation now have been identified in humans.
Some hairs, such as eyebrows and eyelashes, are not regulated by androgens. Androgens stimulate axillary, pubic, and beard hairs to grow and some scalp hairs to undergo miniaturization. The major circulating androgen is testosterone, which is enzymatically converted by 5 a -reductase to dihydrotestosterone in the hair follicle. Androgenetic alopecia is the most common hair disorder. It might start at an early age and is observed in men and women. It is believed to result from the interaction of androgens and genetic factors. The drug finasteride has been shown to be effective in preventing the progression of balding in males.
The hair shaft is a very porous structure and softens rapidly with wetting. The hair is most brittle when dry, a time when it is most susceptible to mechanical injury. Fibrous proteins fill the cortical cells and are surrounded by a matrix protein, and these are locked together by disulfide bonds. The hair shafts can be damaged by sunlight, especially lightly colored hair, and by chemicals used to bleach or color the hair. Reduction of disulfide bonds is commonly done to straighten or permanently curl hair. This results in cumulative injury and must be done with care to avoid breakage.
References
- Olsen E.A., editor. Disorders of Hair Growth: Diagnosis and Treatment. New York: McGraw-Hill, Inc., 1994.
- Stenn KS, Messenger AG, and Baden HP, (eds). The Molecular and Structural Biology of Hair. Ann NY Acad Sci; New York: New York Academy of Sciences, 1991.
- Stenn KS, Paus R. Controls of hair follicle cycling. Physiol Rev. 81:449-494, 2001.
Figure Legends
Figure 1. A schematic diagram of the hair follicle. The hair follicle is composed of multiple layers with the outermost layer continuous with the epidermis.
Questions: Hair Follicles
1. The growth of hair results from:
A) cellular division in the papillae.
B) enhanced blood flow.
C) nervous stimulation.
D) cellular division in the bulb.
E) contraction of the arector muscle.
2. The stem cells of the hair follicle are:
A) in the papilla.
B) in the bulb.
C) near the insertion of the muscle.
D) derived from the epidermis.
E) derived from the dermis.
3. Hair shafts:
A) grow continously on the scalp.
B) are most fragile when wet.
C) have a protective cuticle layer.
D) are not susceptible to photodamage.
E) on the scalp grow 12 inches per year.
4. Androgenetic Alopecia:
A) is related to genetic factors.
B) has a hormonal component.
C) occurs in men and women.
D) is amenable to treatment.
E) all of the above.
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