The catagen phase begins when there are no longer mitotic cells
in the matrix. It ends with the loss of the sheaths, when the
bulb reaches the infundibulum.
There is some disagreement about how long the catagen phase lasts.
Perhaps anatomy and histology can provide us with an answer.
Let us observe this follicle and this hair closely. It is the
beginning of the catagen phase. It would be easy to mistake the
hair for one in the anagen phase. The sheaths are still intact
and their colouring is obtained from cinnamaldehydes, which cleaves
to the citrulline of the inner epithelial sheath. The bulb is
detached from the papilla and the matrix. The matrix is beginning
to degenerate and is starting to form the sac. All these various
factors indicate that the anagen phase is over. The presence of
perifollicular adipous tissue shows that this bulb is still embedded
to a depth of approx. 6 - 7 mm (6000 - 7000 micron). This catagen
hair can no longer grow at a rate of 400 microns a day (10 mm
a month), as it did before, since the matrix does not now contain
any mitotic cells. The hair (together with the inner epithelial
sheath to which it is attached) will have to climb back up the
follicle at the same (and much less impressive speed) of ordinary
skin regeneration, i.e. at a rate of approximately 65 microns
a day. To reach the infundibulum, it will have to travel 6,500
microns. Since it will be moving at a speed of 65 microns per
day, this will take at least 90 - 100 days. The catagen phase
of this hair will last at least 3 months.
The length of the catagen phase is proportional to the depth
of the follicle. It will last 2 weeks for miniaturized hair but
over 3 months with deep, terminal hair.
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