Hair-follicle Cycling
Hair grows in cycles of various phases: anagen is the growth phase; catagen is the involuting or regressing phase; and telogen, the resting or quiescent phase. Each phase has several morphologically and histologically distinguishable sub-phases. Prior to the start of cycling is a phase of follicular morphogenesis (formation of the follicle). There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one of several hair that might arise from a single follicle exits. Normally up to 90% of the hair follicles are in anagen phase while, 10–14% are in telogen and 1–2% in catagen. The cycle's length varies on different parts of the body. For eyebrows, the cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish; this is the reason eyebrow hair have a much shorter length limit compared to hair on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor.
Read more about this topic: Hair Follicle
Famous quotes containing the word cycling:
“I shall not bring an automobile with me. These inventions infest France almost as much as Bloomer cycling costumes, but they make a horrid racket, and are particularly objectionable. So are the Bloomers. Nothing more abominable has ever been invented. Perhaps the automobile tricycles may succeed better, but I abjure all these works of the devil.”
—Henry Brooks Adams (1838–1918)