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Exogen
in the hair follicle cycle
The study of hair follicle has gained importance
due to its implicit relation to the study of major biological
phenomena. Half a century ago Chase in his review defined the
dynamics of hair growth and the cycling of the follicle. Today
the biological and psychosocial importance of hair growth is realized
even more. In humans, the hair shaft plays a major role in influencing
social intercourse. Fortunately, with the availability of various
analytical tools, the medical world today believes that with proper
therapeutic intervention, hair growth can be controlled to a considerable
extent.
It is important to understand a little of hair biology. The hair
follicles differ according to their location in the body, but
their main function is to grow a hair shaft, which has the same
basic structure. The hair follicle grows from the embryonic epidermis.
The central cylinder forms the shaft, the outer cylinder forms
the outer root sheath and the middle cylinder is the inner root
sheath. The shaft grows because of the rapidly multiplying matrix
cells in the hair bulb. As these cells move upwards, they acquire
shape by being compressed by the inner root sheath. The dermal
papilla controls the number of matrix cells thus determining the
size of the hair.
All mature hair follicles undergo a cyclical process consisting
of four phases - anagen (growth), catagen (regression), telogen
(rest) and exogen (shedding). This is a regenerative system and
is continued over the lifetime of a mammal. Exogen is perhaps
of most significance from a patient’s perspective, but medically
very little attention has been given to it. The control mechanism
for exogen is quite independent of catagen and telogen because
it is not unusual for humans to retain the telogen hair for more
than one follicular cycle.
Mechanism
of exogen
As hair follicle cycling is an evolutionary process, each hair
follicle has its own rhythm of anagen, catagen, telogen and exogen.
On any given day, human beings lose about 50-150 scalp hairs due
to exogen. About 5 to 15 percent of scalp-hairs are at the telogen
stage at any given time. So shedding of this hair may be considered
normal. Shedding in excess of this may be due to an increase in
the follicles of scalp hair in the telogen stage and should be
addressed to contain hair loss.
The how and why of hair follicle cycling remains largely unanswered
and several theories have been propounded by biologists. The main
focus of hair research remains to determine the molecular signals
that trigger the change from one phase to the other. As protease
seems to play an important role in the attachment of the hair
club, people being treated with protease inhibitors for immunodeficiency
virus suffer from excessive hair shedding. The exogen phase may
also sometimes be altered in the case of certain diseases. A delayed
exogen occurs in the case of trichostasis spinulosa where hair
shaft from previous cycles are retained and the infundibulum of
the hair follicle becomes dilated. In androgenetic alopecia, the
hair shaft sheds before the onset of anagen so no shaft fills
the pilary canal. The exogen process often starts earlier in such
cases.
Although exogen deals with the shaft base and not directly with
the hair follicle, exogen and anagen influence one another. The
details of this mechanism may be yet unknown but the therapeutic
management of one disorder is certainly dependent upon the control
of the other process.
The controls of exogen may be understood a little by studying
the environmental factors like light, temperature, and nutrition
affecting molt in sheep. But there is also innate local control
of exogen as suggested by the fact that each fiber grows to a
specific length for a specific time before they are shed. This
local control must involve genetics.
Little is known about how the resting hair shaft comes to the
base of the telogen follicle or how it is attached there. As the
catagen phase ends, the hair shaft moves upwards with a shortening
of the inner root sheath. A trichilemmal keratinization starts
and provides an adherent base to the lower keratinizing shaft.
The telogen shaft base is anchored into this trichilemmal base
of the upper outer root sheath. The cells surrounding the attachment
site are rich in desmosomes and keratin.
Studies with transgenic mouse have shown an active role of DG3
and K14 in the mooring of the club hair. Mooring defect was also
observed in case of mice with an inadequate lysosomal protease.
Thus, though lack of DG3 may not indicate the process of exogen
but it establishes the importance of a tight grip of the surrounding
epithelial cells for mooring of hair. Recent studies have also
shown, secretions of a chymotyptic enzyme by the sebaceous gland,
and that a plasminogen activator inhibitor manifests in the cells
of the attachment site. This has led to the conclusion that proteolytic
pathways play a critical role in process of exogen on hair shedding.
Exogen
in the hair follicle cycle references
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