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Hair
follicle cycle
The hair follicle is an integral component of the
skin, and each hair is a keratinized product of the follicle.
Each and every
hair follicle undergoes a cycle of activity - The hair grows to
a maximum length, then hair growth ceases and the hair is shed
and replaced. This characteristic three-part cyclical growth pattern
of hair is the result of follicular activity, and is not obvious
by casual inspection. The phases of the hair growth cycle have
been described as:
- Anagen, a long period of growth
- Catagen, the transitional
period from growing to resting lasting 2 to 4 weeks
- Telogen,
a period of inactivity lasting 2-4 months
The relative duration of these phases varies with the individual's
age, nutritional status, hormonal factors, and other physiologic
and pathologic factors. The duration of anagen determines the
final length of the hair and thus varies according to body site;
catagen and telogen duration vary to a lesser extent depending
on site.
In some animal species, such as rats and mice, all hairs are
apparently in the same state of activity, and all cyclic changes
are synchronized. Unlike other mammals, hair growth and loss in
humans is mostly random and the seasons have relatively limited
impact on the hair follicle cycle.
Determining
the molecular signals that orchestrate the follicle’s transit
between the hair follicle cycle stages is one of the key challenges
of hair research.
Numerous growth factors and growth factor receptors are critical
for normal hair follicle development and cycling. Although no
single growth factor appears to exert complete control over the
process, two secreted molecules that have important roles in hair
follicle development and cycling are insulin-like growth factor-1
and fibroblast growth factor-7. Both are produced by the dermal
papilla, and their receptors are found predominantly in the overlying
matrix cells.
Hair follicles in different areas of the body produce hairs of
different lengths, with the length proportional to the duration
of the anagen cycle. For example, scalp hair follicles stay in
the anagen stage for two to eight years and produce long hairs,
whereas eyebrow hair follicles do so for only two to three months
and produce short hairs. Normally this cycle of hair production
and inactivity continues for the duration of the individual's
life, but external factors can influence and inhibit hair production
and in some cases lead to physical destruction of the hair follicle.
Anagen
in the hair follicle cycle
Anagen is the active phase of the hair, and extends from the
termination of the inactive phase, telogen, to the beginning of
the regressing phase, catagen. Although we speak of stages in
the development of a hair follicle, it should be made clear that
these are actually dynamic, flowing processes and the setting
up of stages is purely for our understanding of the human hair
follicle cycle.
Anagen involves the complete re-growth or regeneration of the
lower, cycling portion of the follicle, i.e., the hair shaft factory.
The epidermal cells surrounding the dermal papilla form the germinal
matrix or root of the hair. These cells are constantly dividing,
and as new cells are formed they push the older ones upwards and
eventually out. During this phase, the hair grows about 1 cm every
28 days. Because there is a limit to the time a follicle stays
in anagen, there is also a limit to the length of its product,
the hair shaft.
Within the anagen phase, six sub-stages have been further defined
by Chase et al. Although the sub-stages were initially described
for the mouse, the same sequence of phenomena can be observed
in other animals and in humans. There is generally little variation
in the duration of each stage between species, except for anagen
VI. This is the period during which the hair is produced at its
maximum rate. The developmental processes, which must occur before
a hair is produced, are presumably similar in all mammals.
The 6 stages Anagen may be divided into are:
- Stage I -growth of the dermal papilla and on-set of mitotic
activity in the germ-like overlying epithelium
- Stage II -bulb
matrix cells envelop the dermal papilla and begin differentiation,
evolving bulb begins descent along the fibrous streamer
- Stage III-bulb
matrix cells show differentiation into all follicular components
- Stage
IV-matrix melanocytes reactivate
- Stage V-hair shaft emerges and
dislodges telogen hair
- Stage VI-new hair shaft emerges from skin
surface
Anagen I is the period when the cells of the dermal papilla increase
in size and show increased RNA synthesis. At the same time, germ
cells at the base of the sac undergo vigorous mitotic activity.
In Anagen II, the lower part of the follicle grows down into
the dermis and partially encloses the dermal papilla. In the matrix
ring that surrounds the dermal papilla, differentiation of cells
commences and represents the various layers of the hair and the
inner root sheath.
Anagen III is marked by continued mitotic activity in the external
sheath and particularly the ‘germ’ region, and proliferation
of the hair follicular melanocytes. At this time, the follicle
attains its maximum length, which is about three times the length
that it has in the resting condition. The bulb is now completely
formed and the papilla cavity is constricted at its base. The
melanocytes (epidermal cells capable of synthesizing melanin)
become aligned along the papilla cavity and each develops melanin
granules and numerous dendritic processes. The internal sheath
is now an elongated cone, which extends up to the capsule and
club of the old hair.
In stage four of anagen, the melanocytes that line the papilla
develop dendrites and begin to form melanin (pigment). Although
the hair has formed, it is still within the cone of the internal
root sheath, which now extends upward to about the level of the
sebaceous gland. The keratogenous zone becomes established just
below the level of the sebaceous duct. The cuticles of the hair
and of the internal sheath are clearly visible. In the upper part
of the bulb, a cone of cells, which will become cortex and medulla
of the hair, contains pigment granules. The papilla cavity is
long and narrow.
By Anagen V, the tip of the hair has broken through the tip of
the internal sheath, through the intersection of the capsule with
the external sheath, and has grown to about the level of the epidermis.
The bulb attains its final characteristic shape, which in some
hairs is somewhat laterally compressed, in others rounded and
symmetrical.
Anagen VI begins as soon as the hair emerges at the skin surface
and continues until the onset of catagen. In this stage, the hair
emerges from the cone of the external root sheath and forces its
way to the surface along the original hair shaft, which gets pushed
aside, and eventually the clubbed hair is discharged. In the mouse,
anagen VI lasts for about 8 days and the hair will grow at the
rate of nearly I mm/day. In human beings, a follicle on the scalp
may remain in this stage for two or more years, producing a hair
at the rate of about one-half mm/day.
Thus, the developmental or ‘embryonic’ phase of hair
follicles is from sub-stage I to IV, the substantial growth of
the hair takes place in sub-stage VI. As soon as the growth phase
is complete, degeneration begins. Fully formed anagen follicles
produce hair that is firmly fixed within the follicle. Except
for the last sub-phase, anagen VI (the duration of which dictates
the shaft length), and the length of the anagen sub-phases I–V
do not differ substantially between follicles from different regions
of the body.
Human scalp hair stays in the active anagen phase of growth for
2-6 years, as compared to other sites like on the leg (which stays
in the anagen phase for 19 to 26 weeks), on the arm (from 6 to
12 weeks), and in the mustache area (from 4 to 14 weeks). At any
given time, it has been estimated that approximately 86 percent
of scalp hairs are in anagen, 13 percent in telogen, and 1 percent
in catagen. Hair on the scalp grows at a rate of 0.35-0.43 mm/day
or approximately 1 cm/month depending on the location of the hair
follicle and the age and sex of the individual.
Anagen
in the hair follicle cycle references
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