How Hair Grows

Provided Courtesy of: Richard Lee, MD


Hair follicles develop during gestation in response to communications between the ectoderm and the underlying mesoderm. In the human fetus, the first primordial hair follicles form at approximately 9 weeks’ gestation and are distributed mainly in the areas of the eyebrows, upper lip and chin. The bulk of the remaining follicles begin to develop at approximately 4 to 5 months gestation in a cephalad to caudad direction.
First, a patch of the mesoderm signals the overlying ectoderm to make an appendage. In response, the ectoderm cells organize, proliferate and invade the mesoderm by forming an elongated structure called a hair germ (or follicle germ). Then the hair germ directs the underlying mesoderm cells to cluster together to form the dermal papilla of the hair follicle. The dermal papilla then becomes a kind of ‘command central’. It instructs the germ cells to multiply further and develop into a full-fledged hair follicle. The final diameter and length of the hair fiber are directly related to the volume of the dermal papilla. In the end, the upper, permanent section of the follicle contains an oil-producing sebaceous gland and a bulge—a swelling just recently recognized to be the home of most or all of the stem cells that replenish the hair, the sebaceous glands and the epidermis throughout life. The lower segment of the follicle becomes the hair-producing region and is the part that cycles through different stages after embryonic development is complete.

The bottom section arises as cells from the hair germ spread downward, like a growing root, deep into the dermis of the skin, where they form a bulb-shaped matrix of cells surrounding the dermal papilla. The melanocytes responsible for the pigmentation of the hair are dispersed among these matrix cells. The dermal papilla prods the matrix cells into dividing and as the matrix cells get pushed upward and lose their contact with the dermal papilla, they stop dividing and mature, producing the fibrous keratins characteristic of hair.

At the end of gestation, a newborn human enters the world with five to six million hair follicles distributed in a genetically determined pattern over the body. Only the palms of the hands and the soles of the feet do not have hair follicles in the skin. No new hair follicles are created after birth.

Normal adult hair density is in the range of 300 to 500 follicles/


The follicle lies at an angle relative to the skin surface. When the arrector pili muscle contracts, as it does as a response to cold temperatures, the hair stands ‘on end’ and creates ‘goose bumps’. The transient portion of the follicle begins just below the bulge/muscle complex and extends to the deepest levels of the follicle.

The bulk of the mitotic activity that gives rise to the hair shaft and the inner root sheath occurs below the widest diameter of the bulb.

Follicles begin cycling within two or three years after birth. The hair growth cycle has three main stages:

Almost all of the hair that you see on your head is in the anagen pahse. The anagen phase is the growing phase and, on the scalp, the anagen follicles will grow a hair shaft that measures ~1/2 inch/month. And it can keep up this rate of production for 2 to 5 or so years. A newly growing anagen hair will have a tapered distal tip, which may also lack melanin. But soon, the matrix kerotinocytes in the bulb region proliferate very rapidly. The cellular activity is comparable to the bone narrow and the lining of the gastrointestinal tract, which explains why patients on chemotherapy have nausea, fatique and hair loss. The anagen follicle penetrates deepest into the skin, typically to the level of the subcutaneous fat. The length of the anagen phase determines how long a single hair can grow. At any given time in a typical 20 year-old, ~90% of the scalp follicles will be in anagen.

The catagen phase is the transitional period of 2 to 4 weeks, when the productivity of the follicle is shutting down. The reduced production of matrix and melanin accounts for the short segment of under-pigmented, narrowed hair shaft just above the bulb of shed hairs. The cells below the bulge essentially commit suicide (apoptosis), leaving behind only the dermal papilla and the basement membrane that formerly encased this dying region. As the cells die, this membrane contracts and draws the dermal papilla up to the bulge.

When the dermal papilla reaches the region of the bulge, follicles enter the telogen or resting phase. Telogen lasts approximately 100 days (3 to 4 months). At the end of the telogen phase, the hair shaft sheds, but it can persist into the early next anagen phase, in which case you can see two hair shafts emerging from one canal. The telogen hair is shed with a club-shaped proximal end surrounded by friable gray debris, which represents the remnants of the inner sheath.

Unlike most mammals, humans have a mosaic pattern of hair cycling. There is a marked variation in the length of anagen depending on the body site. On the legs anagen is only 19 to 26 weeks and only 4 to 14 weeks on the upper lip (mustasche). There is one animal, the Merino sheep, that has hair in permanent anagen, lasting the animals lifetime. If we could learn the secret of the Merino sheep, we wouldn’t have a problem with MPB.

Each follicle on the scalp goes through the hair growth cycle 10 to 20 times in a lifetime. It’s important to realize that during your lifetime, the same follicles can intermittently produce vellus or terminal hairs.

Beside the genetic factors, systemic factors can also influence the hair growth cycle. Hamilton (whom you may know from the Hamilton-Norwood scale) recognized the importance of androgens in human hair growth when he noted in 1942 that men castrated before the age of puberty neither became bald nor grew beards unless they were given testosterone.

A useful way to conceptualize the internal organization of the hair follicle is as a series of concentric epithelial cellular components bounded by an acellular basement (glassy) membrane. The outer root sheath is the most peripheral of the cellular components. Next is the inner root sheath. Innermost is the hair shaft, which has three components: the cuticle, the cortex and the medulla. In furry animals, the medulla traps air, which aids in insulation.

Hairs can be classified according to their texture and length. There is lanugo, vellus, intermediate and terminal hair. Lanugo hair is the soft, fine hair that covers much of the fetus and is usually shed before birth. The classification is not exact and basically relates to the final length if the hairs: vellus hairs never grow more than 1 cm; indeterminate hairs may grow to several centimeters and terminal hairs grow beyond several centimeters