Overview
According to the medical text, “Fitzpatrick’s Dermatology,” there are approximately five million hair follicles on the body. One hundred thousand of these hair follicles are on the scalp. Though we do not acquire new hair follicles after birth, the size of the follicles changes as we age. Each follicle continually proceeds through a life cycle that consists of three phases: anagen (growth phase), catagen (involution phase) and telogen (resting phase).
Anagen
Dr. Thomas P. Habif, author of “Clinical Dermatology,” describes the anagen phase of the hair follicle cycle as the growth phase. Follicles of different parts of the body have differing lengths of time spent in the anagen phase. The amount of time spent in the anagen phase is directly proportional to the length of hair. For instance, scalp follicles are in the growth phase for two to eight years, resulting in long hairs. But eyebrow follicles spend approximately two months in the anagen phase, resulting in shorter hairs. At any given point in time, 90 percent of the scalp hairs are in the anagen phase.
Catagen
According to “Clinical Dermatology,” the catagen phase is characterized by involution of the hairs. Less than 1 percent of scalp hairs are in this two- or three-week phase at any time. During this phase, production of new cells within the hair matrix ceases. The outer root sheath degenerates and withdraws around the lower hair shaft, forming a club hair and marking the end of catagen.
Telogen
In “Clinical Dermatology,” the telogen phase is described as a period in which all activity ceases and the hair follicle rests. In the scalp, this phase lasts for two to three months then the follicles reenter the anagen phase. The amount of follicles in the telogen stage varies according to the body region. The telogen phase is longer in eyebrow, eyelash, trunk, arm and leg hair than scalp hair. About half of follicles on the trunk are in the telogen phase. Telogen hairs have a white node at the end attached to the scalp, and the white color is due to a lack of pigment. The telogen hair is eventually extruded and replaced by a new anagen hair. The normal individual sheds 25 to 100 telogen hairs per day.
Hair Types
“Fitzpatrick’s Dermatology” says that the two primary types of hairs vary in size and distribution on the body. Terminal hairs are typically greater than 60 μm in diameter and can grow up to 100 cm in length. Vellus hairs, on the other hand, are generally less than 30 μm in diameter and less than two cm in length. Terminal hairs are found on the scalp, eyebrows, and eyelashes at birth. Vellus hairs are found everywhere else. At puberty, vellus hair follicles in the genital region, underarms, trunk, and beard area (in men) become terminal hair follicles under the influence of androgens.
Abnormal Hair Loss
According to “Current Diagnosis and Treatment in Otolaryngology,” there are both reversible and irreversible causes of hair loss. Reversible causes are due to disruption of the hair follicle growth cycle. The most common reversible cause is male-pattern baldness/female pattern hair loss (also known as androgenetic alopecia). Irreversible hair loss is generally caused by trauma or scarring.
About this Author
Marie Bell has earned a Bachelor of Science in sports medicine and is currently working toward a doctoral degree (M.D.). She has a passion for health and wellness and shares her knowledge in her writing. Bell has written articles for eHow and Answerbag.
