How does hair grow?
Hair grows in a cycle that has three phases: Anagen, Categen and Telogen.
Anagen is the active growth phase. In healthy hair this phase lasts between
two to six years during which time the hair continues to grow and produce a
visible, terminal hair shaft.
Eventually, once the Anagen phase completes
hair enters the transition phase called Catagen. In this phase the hair shaft
detaches from the dermal papilla cells at the base of the hair shaft and the
hair stops growing. The transition from the Catagen phase to the Telogen phase
takes between one and two weeks.
During the Telogen phase of the hair cycle,
the dermal papilla cells (the cells that produce the visible hair shaft) are
resting or dormant for a period of time of about five to six weeks. The hair
shaft typically sheds sometime during this phase.
Eventually the dermal
papilla produce a new hair matrix, the beginning of a new hair shaft, as the
hair returns to anagen once more. It is common for up to about 100 hairs
to shed daily as a result of the hair cycle.
Androgenetic Alopecia occurs when this cycle starts to produce progressively
smaller and smaller hair shafts. The Anagen phase becomes shorter in duration while the
Telogen phase becomes longer in duration. Eventually the hair becomes so small
that it is no longer visible and it is thought that hair cell aptosis (programmed
cell death) occurs for some of the hair follicles, while others remain dormant
indefinitely, resulting in baldness.
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