Human hair follows a pattern of growth and rest. The hair grows during the ‘growing’ phase of the follicle, and then it is retained as a dead club hair during the subsequent ‘resting’ phase. A new growing phase follows, producing a new hair which may dislodge the old hair or may grow alongside it. This pattern of growth, loss and regeneration is commonly called the hair cycle .
It is through the hair cycle that animals, including humans, are able to adapt their hairs (by replacing them) to suit changes in conditions; for example, a change of hair colour to suit a change in season, or a change of hair type with age, as at puberty in humans.
The hair cycle is a dynamic, continuous process, but for convenience of description and understanding it is divided into four main stages.
These stages are called anagen (the growing phase), catagen (the transition period between growing and resting) and telogen (the resting phase) and exogen(hair shedding).
Anagen phase as ‘the largest part of the growing period, lasting from the initiation of growth until, in coloured hair, pigment ceases to pass upwards from the bulb at the base of the hair-root’.
Catagen Is defined as the phase ‘during which the root is decreasing in size’,
Telogen was when the hair had ‘become a club-hair and [was] not growing further’.
The telogen phase can be considered as the mature phase of hair growth.
Exogen is the Shedding Phase of the Hair Growth Cycle which makes way for the new hair to come out.
Anagen is a period of high metabolic and mitotic activity. The follicle re-forms after its previous resting phase, in a process similar to its initial development except that the follicle bud (hair germ) is already present and the hair canal to the skin surface is established.
Many changes take place in the anagen phase, and to aid description it is helpful to divide anagen into substages.
Chase denoted the substages anagen I to anagen VI, with anagen VI being the stage where the hair has emerged beyond the skin surface.
The substages may also be known by the letters A-F, or the terms proanagen (anagen I-IV), mesanagen (anagen V) and metanagen (anagen VI).
Anagen I is the phase in which the hair germ begins its mitotic activity.
In anagen II the follicle is growing down around the dermal papilla and the first differentiation of germinative cells into inner root sheath begins.
In anagen III, the follicle attains its maximum length of 4–5 mm (about three to six times that of its resting length) and the bulb surrounds the dermal papilla. Melanocytes can be seen in the bulb, and the inner root sheath has formed.
At anagen IV there is high mitotic activity in the cells of the bulb and the formation of the hair cortex and medulla can be seen. Pigment granules are visible. The hair, however, does not yet extend beyond the inner root sheath.
By anagen V the follicle has reached its final characteristic onion shape in which the lower part of the bulb encloses the dermal papilla. The point of the new hair has pushed aside the brush-like attachment of the old hair and surrounding cells and has reached the
epidermis. The process to this stage takes about 3 weeks.
At anagen VI the hair is visible above the skin surface and continues to grow without further changes to the follicle until the catagen phase begins.
Hair plucked at the anagen stage of growth need a strong pull to detach them from the dermal papilla.
The roots will appear fleshy and dark , sometimes with pigmentation. They may be stretched or broken or both. When anagen hair is removed quickly (even by brushing or combing), sheath tissue may be attached.
In fact, anagen hairs plucked with a very quick jerk may come away surrounded by all the elements of the lower portion of the follicle, including the papilla.
Because the root and sheath tissues are metabolically and mitotically active they are
amenable to enzyme typing , sexing by fluorescent chromosome analysis, and DNA
typing by the polymerase chain reaction.
During catagen the follicle undergoes gradual, orderly morphological and functional changes as it enters its regression phase. The hair gradually stops growing.
In human scalp the process is relatively short and probably takes about two to three weeks.
As catagen begins, the inner root sheath begins to disintegrate. Melanin production stops. The dendrites contract and no more pigment is injected into the (still growing) cortex
which thus becomes pale or white at the root end.
Cell division in the bulb decreases and eventually stops. The cells in the upper part of the bulb continue to move up the follicle for some time and to differentiate, but medulla and cuticle formation cease and only the cortex and inner root sheath are formed. Only remnants of the hair bulb remain.
Vacuoles containing hydrolytic enzymes form in the cells of the lower follicle and the esterases and acid phosphatases break down the follicle cells, allowing the area they formerly occupied to be filled by connective tissue cells.
The remains of the follicle cells form an epithelial strand between the dermal papilla and the hair germ. The hair germ forms from the outer root sheath at about the level of the middle of the follicle (at the embryonic bulge, just below the arrector pili muscle attachment). It is from this hair germ that a new follicle will form when growth is initiated.
As a result of these processes the follicle reduces to about one-third of its length. The root of the hair eventually becomes a brush-like club consisting only of cortical cells (the hair club). These cells fill with filaments and form keratinous rootlets which anchor the club root in the follicle. The rootlets are firmly attached to cortical cells of the hair shaft on one side and on the other side they interdigitate the two or three layers of surrounding germ cells which form the epithelial capsule or sac.
Hairs at this stage would normally stay attached to the remains of the follicle, but if they are pulled out the roots will often appear non-pigmented and brush-like, sometimes surrounded by the epithelial sac but with no sheath material adhering.
Telogen is the mature, stable state of hair growth. The hair is anchored in the follicle by the club root as a result of the processes involved in catagen. The telogen follicle is very short—about one-third the length of an anagen follicle—extending to just below the level of the sebaceous gland. The telogen follicle is relatively simple compared with an anagen follicle; the dermal papilla is separated as a ball of cells located below the epithelial capsule or sac.
There are no germinative cells, or cuticle, or inner or outer root sheaths. The cells in the lower region of the follicle are mitotically inert and contain less DNA than the cells of an anagen bulb.
Telogen lasts for 3–4 months for human scalp hair. Contrary to popular belief, the mature hairs do not ‘fall’ out. They remain in place until pulled out (for example, by brushing or washing or other forms of friction). Frequently, though, the mature hairs are displaced at the end of telogen by the emerging new hair of the next anagen phase, although sometimes the new hair will grow alongside the old one.
Hair at telogen stage form the majority of ‘naturally shed’(exogen) hairs found on clothing and the like, and this is expected as, compared with hairs in anagen phase, they require only a small force to dislodge them. Typically they do not have any root sheath attached, no medulla near the root and little or no pigment in the root. Not infrequently cortical fusi will be present in the shaft above the club root.
Telogen finishes when a new anagen phase commences. Each time a new hair is formed it must be preceded by the formation of an almost entirely new follicle in a regeneration akin to the initial development of the follicle.
The first mitotic activity in the production of the new hair occurs in the basal cells of the lower follicle, which grows downwards as a solid column of undifferentiated and dividing cells to surround the dermal papilla and begin the new anagen phase.
The phase of the cycle wherein the signals for shaft release are given and the processes for that release are initiated and successfully executed. Exogen ends when the shaft is liberated.
Approximately 50 to 150 hairs are reported to fall out on a daily basis, representing the exogen phase. The hair growth cycle starts again after the hair shedding observed in the exogen phase.