Healthy Hair Growth and Development

February 4, 2020

 

Healthy Follicles for Healthy Hair

Hair follicles are among the most active and fastest growing structures in the body. Human hairs are produced by follicles – specialized organs that cover the entire body, with the exception of the palms and soles of the feet. Follicles are composed of three major structures – the dermal papilla, hair matrix, and hair shaft. (Fig. 1)

The dermal papilla, a pear-shaped structure located at the base of the hair follicle, is comprised of connective tissues and a capillary loop containing tiny blood vessels that provide the nutrients required for hair growth.

Fig. 2. Three stages of human hair growth: Anagen (growth) phase, lasts from two to six years; Catagen (transitional) phase, lasts from one to two weeks; Telogen (resting) phase, lasts five to six weeks.

The papilla is surrounded by the “matrix” – a bulb-shaped collection of specialized epithelial cells that produce a protein, keratin, that actually forms the hair fiber. The matrix also contains melanocytes, specialized pigment cells that infuse the growing hair shaft with melanin, the compound that gives otherwise transparent hair its natural coloring.

Different types of melanin produce different colors – yellow, rust, brown, and black – that combine to produce a wide variety of hair colors, ranging from blond to brown to pitch-black. Red hair is the result of another type of coloring agent, the iron-containing pigment called trichosiderin.

 

Three Stages of Hair Growth

Mature human hair growth occurs in three distinct phases – growth, transition and rest – that are collectively referred to as the hair cycle (Fig. 2).

  1. Growth Phase (Anagen): Each hair cycle begins with a growth (anagen) phase. This is marked by rapid cell division in the hair matrix that builds up layers of keratin to form a single hair shaft that grows out from the follicle at a rate of up to one inch per month. This growth phase typically lasts two to six years. At any given time, about ninety percent of all scalp hairs are in the active growing phase.
  2. Transitional Phase (Catagen): At the end of the anagen phase, hair growth ceases and the follicle enters the transition (or catagen) phase. During this phase hair matrix cells begin to die off as the follicle base and bulb begin to shrink. Over time, this shrinkage (miniaturization) pinches off the lower hair bulb, cutting off access to the blood vessels and circulation necessary for hair growth. At any given time, one to two percent of all hair follicles are in this transition phase, which lasts one to two weeks.
  3. Resting Phase (Telogen): The final stage of the hair cycle is the resting (or telogen) phase. During this phase, the mature hairs reside in the follicle for five to six weeks before falling out. Approximately 10 percent of all hairs are in this transition period at any given time, accounting for normal shedding of between thirty and fifty hairs each day.

After shedding the hair shaft, the follicle begins a new cycle of hair growth, marked by the swelling of the dermal papilla and the formation of a new capillary loop. As blood vessels reattach to the papilla, they grow and expand four-fold to provide the hair matrix with the nutrients necessary for growing new hair.