HAIR GROWTH CYCLE

More than just attractive, hair plays an important role in the protective barrier function of the entire skin system, keeping microbes out and bodily fluids in, healing scratches and wounds, and resisting environmental attack from sun and chemicals. Individual hairs grow in and fall out through a continuous cycle. Normally, a hair follicle cycles at least a dozen times in a person’s life.

Each cycle consists of an anagen growth phase, which may last for several years, a catagen transitional phase, which lasts for two or three weeks, and a telogen resting phase, which lasts for approximately three months. The mean hair-growth rate is 0.3–0.4 mm per day. In the case of androgenic alopecia, the anagen phase shortens, and the telogen phase lengthens. So at any given time, fewer follicles are growing and more are resting. This condition accounts for the reduced density of strands, often called hair thinning, that characterizes most aging scalps.

It is theorized that the chemical signaling required to stimulate anagen becomes increasingly difficult to achieve. For this reason, Revita.COR conditioner is formulated with several of the active compounds that participate in follicle signaling. Each hair consists of a shaft and a bulb anchored deep in the dermis. The shaft grows up through the epidermis and protrudes from the surface. A healthy follicle can produce a shaft several feet long.

Each follicle is divided into three parts:
1) the infundibulum, which is the pore in the epidermal surface,
2) the isthmus, where the sebaceous gland and arrector pili muscle attach to the follicle, and
3) the inferior segment, which manufactures the hair strand.

Growth happens in the proliferative hair-matrix cells of the hair bulb, in the inferior segment. Slow-cycling, multipotent stem cells reside in a bulge near the insertion point of the arrector pili muscle.

Early in each hair cycle, the new follicle becomes encapsulated in a highly proliferative matrix of cells. Stem cells from a permanent bulge, amid what will become the new follicle, migrate downward from their reservoir and proliferate to form a new hair bulb, from which the new hair shaft will grow, and an inner root sheath, which will act as a channel to guide the growing shaft up to the surface.

During the catagen phase of the cycle, normally a few years later, the inferior portion of the follicle disintegrates, called apoptosis. The blood supply that nourished it, called the dermal papilla, then migrates up toward the bulge, which remains in place with a permanent reservoir of stem cells. What remains of the follicle enters the resting phase, called telogen, and waits for chemical signals to begin the cycle all over again.

Male hormones regulate this activity. The hormone testosterone, produced in the testes, migrates through the blood into the dermal papilla cells. Here the enzyme 5α-reductase catalyzes the conversion of testosterone into 5α-dihydrotestosterone, which exerts powerful effects on the follicle, suppressing cell division and hair growth, especially in the condition known as male pattern baldness.