A. Hair is a keratin structure of the epidermis and a specialized skin appendage. The physiologic functions include insulation, esthetic and cosmetic display, tactile perception, thermoregulation, and protection from UV light.
B. Embryonic development
1. First follicles form at about 9 weeks of gestation, mainly in the eyebrows, upper lip, and chin. Remainder develop at 4 to 5 months of gestation.
2. Epidermal cells progressively penetrate downward into the maturing dermis, passing through “germ,” “peg,” and “bulbous peg” stages of development.
3. No hair follicles form after birth.
The hair shaft is organized into seven longitudinal regions, beginning from the epidermis (Figure 22-1
a. Hair canal region—present only during fetal development and extends from the skin surface to the level of the epidermal-dermal junction
b. Infundibulum—funnel-shaped top of follicular canal. Extends to opening of the sebaceous duct
c. Sebaceous gland area
d. Isthmus—located between the entry of sebaceous duct and insertion of arrector pili muscle
e. Area of the bulge—site of the insertion of the arrector pili muscle
f. Lower follicle—extends from the area of the bulge to the top of the hair bulb
g. Hair bulb—is the deepest part of the hair follicle and surrounds the dermal papilla
Cross section of hair follicle reveals a series of cellular compartments (Figure 22-2
a. A cellular basement membrane (“glassy membrane”). Surrounds entire follicle
b. Outer root sheath—most peripheral of cellular components
c. Inner root sheath—comprised of three separate layers. From outermost to innermost:
(1) Henle layer
(2) Huxley layer
d. Hair shaft—also comprised of three separate layers. From outermost to innermost:
(1) Cuticle—the outside portion. Consists of overlapping cell layers arranged like shingles, which protect the hair shaft.
(2) Cortex—the bulk of the hair shaft. “Cigar-shaped” cells, which synthesize and accumulate proteins while in the lower regions of the hair shaft.
FIGURE 22-1. Schematic drawing showing anatomy of the hair follicle. (From Schalock, P. C., et al. (2010). Lippincott’s primary care dermatology. Philadelphia, PA: Wolters Kluwer. Asset provided by Anatomical Chart Co.)
(3) Medulla—present only in large terminal hairs. May be discontinuous.
D. Hair growth
Hair growth is cyclical, and follicles grow in a repeated three-part cycle. Each follicle functions as an independent unit (Figure 22-3
a. Anagen—actively growing hair. Can be subdivided into six stages. For scalp hair, lasts 2 to 7 years.
FIGURE 22-2. Cross-section of a hair follicle. (From Anatomical Chart Company, 2004.)
b. Catagen—metabolic processes associated with hair growth gradually decreasing. The hair follicle regresses. Can be subdivided into eight stages. Lasts approximately 2 to 3 weeks regardless of site and follicle type.
c. Telogen—the resting phase. Existing hair will never grow longer. Lasts 3 months for scalp hairs. Hairs have a club-shaped proximal end, which will be shed from the follicle during the telogen-to-anagen transition called exogen.
2. Length of growth cycle varies according to body site with a general relationship of increasing hair length to a longer growth cycle.
3. Genetic programs determine normal growth parameters for hair on different anatomical locations; for example, eyelash hairs remain short while scalp hair grows much longer.
4. Hormones influence hair growth. Androgens influence pubic, axillary, beard, trunk, and extremity hair as well as lead to hair loss in susceptible individuals. Estrogens tend to prolong the telogen phase and delay the anagen phase.
E. Distribution of hair
1. Covers entire body except palms, soles, interspaces of the digits, and portions of the genitalia.
FIGURE 22-3. The hair growth cycle. (From Anatomical Chart Company, 2004.)
2. Greatest density on scalp, numbering 100,000 in people with brown/black hair. Is about 20% greater in natural blondes and 20% less in redheads.
3. At any given time, 85% to 90% of hairs are in anagen, 10% to 15% are in telogen, and less than 1% are in catagen.
4. Normal hair shedding ranges from 50 to 100 scalp hairs per day.
F. Types of hair
1. Hairs are classified into four groups according to texture and length.
a. Lanugo hairs—soft, fine, lightly pigmented hair. Found in utero on fetal skin. These are shed between 32nd and 36th weeks of gestation.
b. Vellus hairs—fine, short hair with little pigment. Replace lanugo hairs. Found on all parts of body except palms, soles, parts of genitalia, and periungual areas.
c. Indeterminate hairs—fall between vellus and terminal hairs in size.
d. Terminal hairs—thicker, longer, coarser hair. Frequently has a central medulla and is pigmented. Found only on scalp, eyebrows, and eyelashes in children. Androgen production at puberty stimulates conversion from small to large terminal hairs in scalp, beard (males), axillary, and pubic regions.
2. Any one hair follicle can give rise to different types of hair within its lifetime; for example, during puberty, former vellus hairs become terminal hairs in the beard, scalp, pubic, and axillary regions.
G. Hair pigmentation
1. Melanocytes located in the hair bulb of the follicle produce pigment.
2. Pigment is most prominent in the cortex of the hair shaft.
3. Any follicle can produce two types of pigment, although usually only one type at any time is found.
a. Eumelanin—pigment found in brown or black hairs
b. Pheomelanin—pigment found in red or blonde hairs
4. Generally, pigment is not found in the hair shaft during the early stages of new hair formation and in the proximal portion of telogen hairs.
5. Color intensity is generally proportional to the amount of pigment in the fiber.
6. Graying hair is a result of a decreased number of melanocytes in the hair bulb.
H. Morphology of hair
1. Four categories: straight, spiral, helical, and wavy.
2. Caucasians are the hairiest and may contain any of these categories.
3. Asian hair is straight due to straight follicles with lower portions oriented vertically to the skin surface.
4. Hair in blacks is spiraled due to curved follicles with lower portions almost horizontal to the skin surface.
A. History and physical examination
1. Patient history—cause may be associated with a variety of other diseases, which significantly affect treatment decisions. Pertinent history may include the following:
a. Hair change duration, age of onset, and extent of involvement including:
(1) Distinguishing between thinning and shedding
(2) Loss or gain, diffuse or focal
(3) Areas of hair loss, scarring or nonscarring
(4) Increase or decrease in amount of hair lost per day
(5) Change in color or texture
(6) Distribution of hair, normal or abnormal
(7) Gradual or sudden onset
(8) Symmetric or asymmetric hair loss
b. Associated symptoms may include pruritus, pain, skin lesions, fever, pregnancy, psychologic or physiologic stress, or presence of systemic disease.
c. Current medications and supplements
d. Exposure to environmental or occupational toxins or chemicals
e. Nutritional status
f. Current and past treatment and response
2. Physical examination—inspect hair for texture, color, quality, and distribution.
B. Diagnostic tests: when the diagnosis is not clinically evident, the following tests may be indicated.
1. Hair pull test
2. Microscopic examination of the hair
3. Hair counts
5. KOH preparation
6. Fungal culture
7. Immune, endocrine, and other laboratory studies, if indicated