In order to gain a more comprehensive understanding of how laser hair removal procedures work, one must have knowledge of relevant hair microanatomy, physiology, and growth cycles associated with growth of hair on the body.
Hair follicles are found over almost the entire body surface, with the exceptions of the palms of the hands and soles of the feet. Each person has approximately 5 million individual hair follicles. Hair follicles may be straight, wavy, helical, or spiral, and the morphologic features of follicles vary among different anatomic locations and racial backgrounds. White persons typically have thinner hair shafts than persons of Asian or African American backgrounds. The density of hair follicles in a given skin surface area also varies widely among individuals. Hair shape is also highly variable; on cross-section, straight hair tends to be round and wavy or spiral hair is oval. Keep in mind the variations in color, texture, and amount of hair on a given body surface area when deciding upon the need for hair removal procedures.
Types of hair
Adult hair has 2 primary types. Vellus hair can be found over the entire body and appears as soft, fine, short hairs that are nonpigmented or very lightly pigmented and can be all but invisible. Terminal hair is the longer, coarser, and more darkly pigmented hair found on the scalp, underarm area, and groin. Subgroups of terminal hair include those on the scalp, pubic region, and eyebrows. Terminal hair is also found on the androgen-sensitive areas of the body, such as the beard and chest area in males. Although greatly outnumbered by vellus hairs, terminal hairs are more important, as they are responsible for the appearance of hair on the body. They may aid in enhancing a person's image, or they can cause great anguish and distress due to their distribution, length, or texture.
Hair follicle anatomy
Hair follicles are associated with other structures within the skin. As the hair follicles develop, they become associated with developing sebaceous glands, apocrine glands, and erector pili muscles. Together, these structures form the folliculosebaceous-apocrine unit.
Each hair follicle may be divided into 4 histologic divisions, which are (from superficial to deep) the infundibulum, isthmus, stem, and bulb.
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The infundibulum is the most distal portion of the hair in the follicle, extending from the surface of the skin down to the entrance of the apocrine gland into the follicle.
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The isthmus of the hair then extends from the apocrine gland entrance to the connection to the sebaceous gland.
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Still deeper, the stem extends from the sebaceous gland opening to the erector pili muscle attachment. The action of this muscle makes the hair stand erect, causing goose bumps.
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The bulb of the hair follicle is deep to the attachment of the erector pili muscle and is the site of the follicular matrix. This is the principal site responsible for hair growth and development.
Hair phases
Adult hair has 3 distinct phases of development: anagen, catagen, and telogen. As hairs progress through these phases, they are affected differently by treatment with laser-assisted hair removal.
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Anagen is the phase in which active hair growth occurs. Follicular matrix cells are actively producing cells that will make up the growing hair. The growing hair consists of all 4 histologic divisions in anagen.
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In catagen (the regression phase), matrix cells degenerate and the bulb begins to atrophy, causing the hair follicle to shrink.
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Telogen is the final resting phase of the hair follicle. Growth of the hair has stopped, and the follicle becomes inactive. Now the follicle contains only the superficial components, the infundibulum and the isthmus. During telogen, the hair is released from its attachment to the skin and sloughs away. The process begins again in anagen to form a new hair.
The length of time spent in each phase depends upon the location of the hair. On the scalp, hair follicles spend up to 10 years in anagen, but on the trunk, brow, and limbs, anagen lasts no longer than 6 months. Catagen lasts only 2-3 weeks, and telogen lasts from 3-4 months.
Up to 90% of hairs in a given body location can be in the anagen phase at a given time. This fact becomes important because only hairs in the anagen phase of development are susceptible to injury during a session of laser-assisted hair removal. Thus, multiple treatments are necessary to treat all hair follicles on a given body surface area.
Melanin
Melanin is a biologic pigment that is found in many cells in the body and is primarily responsible for the color of the skin and hair. The primary function of melanin in the skin is to protect it from the harmful effects of sunlight. The amount of melanin in hair and skin varies widely between individuals and races and is determined by the concentration of melanin within the skin. This variability produces a great range of hair darkness, skin color, and tone.
Melanin is the target chromophore, or light-absorbing molecule, for laser treatment and has an absorption spectrum of 250–1200 nm, which spans the entire ultraviolet, visible, and infrared light ranges. Melanin absorbs energy in the form of heat from a pulse of laser light. This heat causes thermal injury to the melanin-containing cell and its surroundings. Ideally, the laser energy is absorbed selectively by the melanocytic hair bulb and matrix, thus destroying the hair follicle and its capacity to regrow, while protecting the surrounding tissue where the melanin concentration is minimal. In persons with darker skin, however, the higher levels of melanin in heavily pigmented skin compete as a chromophore for the laser light. This light is converted to heat and can cause skin blistering or changes in skin pigmentation.
