Hair follicle
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 (still) 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 hairs 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 phase. 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 hairs have a much shorter length limit compared to hairs on the head. Growth cycles are controlled by a chemical signal like epidermal growth factor.