Hair follicle and growth phases

The target is the hair follicle. Melanin in the hair shaft and bulb absorbs the light. The heat has to reach the cells that regrow the hair, and only growing follicles respond.

Lasers and devices

Skin to bulb

The parts of the hair follicle

1. Epidermis and skin type

Melanin in the skin's outer layer also absorbs the light and competes with the follicle. Skin is sorted into Fitzpatrick sun-reactive types I to VI. Side effects in a 900-treatment series were concentrated in tanned skin and types III and higher.[1][2][3]

2. Hair shaft and melanin

Pigment in the hair shaft absorbs the laser light and the heat damages the follicle. Plucking before treatment removes this target. Dark hair responds best.[1][2][3]

3. Bulge

The bulge is part of the outer root sheath in the middle of the follicle, where the arrector pili muscle attaches. Cotsarelis and colleagues found the follicle's slow-cycling stem cells there in 1990. It marks the lower end of the permanent part of the follicle.[1]

4. Bulb and dermal papilla

The bulb at the base of the follicle holds the matrix, once thought to hold the follicle stem cells. The first ruby laser study used a handpiece designed to deliver light to the deep (reticular) dermis. A needle epilator destroys the dermal papilla with electric current.[1][2][3]

5. Anagen (growth phase)

Actively growing, pigmented anagen follicles were sensitive to ruby laser pulses in a 1998 study. ASPS says only actively growing follicles are affected, which is why several sessions are needed.[1][2]

6. Catagen and telogen (regression and rest)

Catagen and telogen follicles resisted laser pulses in the same study. After treatment, follicles can enter a prolonged rest (telogen) or shrink into fine, vellus-like hairs, which was seen in the 2-year ruby laser follow-up.[1][2][3]