Just like other biological processes, hair growth is in cycle. The growth phase of hair normally continues for 2-6 years. Hair life cycle is divided into three stages such as Anagen phase, Catagen phase and Telogen phase. A ratio of about 85% anagen hair, approximately 1% catagen hair and about 14% telogen hair is considered as normal situation. A hair follicle can produce new hair about 20 to 25 times. Your hair is replaced several times during entire lifetime.
During the anagen phase, there is continuous production of protein and keratin. The hair shaft is produced and pushed in upward direction to its natural length during this phase. This growth phase lasts from 3-5 years and represents the development of about 90% hair. At the end of this phase, blood supply to the hair bulb becomes slow and gradually stops.
The catagen stage is also called as transitional phase. During this stage, structural and chemical changes take place in the hair follicle. The hair growth is stopped and it remains in this stage for about 2-3 weeks. Then, they move to the next stage.
The talogen phase is the last stage in hair life cycle. The development of the hair is stopped. The hair remains attached to the scalp for about 3 months on your head. Then, it is replaced by the budding hair present in the anagen phase. It starts to grow from the same hair follicle. These replaced hairs get thinner and finer with aging. In most cases of baldness, the hair follicles are blocked and there is no replacement for the fallen hair. Normally, any person shed up to 100 hair every day.
Hair life cycle starts even before our birth. Hair growth can be about 1 inch per month. It is faster during summer and slower during the winter. The rate of hair growth becomes slower with aging. Some of the things that can influence the hair growth are hormones, vitamins, nutrition and emotional states.
Hair & Hair-loss All about hair
Hair growth cycle
Each hair follicle lives in a cycle consisting of a long period of growth followed by a relatively short period of rest. During the rest period the hair is still attached to the hair follicle, but it is not growing. After the resting phase the hair is shed and a newer hair begins to grow, thus, starting a new hair growth cycle.
There is a genetic predisposition of each hair follicle to follow a particular pattern of growth and rest. Hair follicles are programmed to stop producing hair and spend more time in the resting stage with the progression of age.
The growth cycle of a hair consists of 3 stages:
- ANAGEN (GROWING) STAGE
This is the name for the growing period of a hair follicle. The anagen stage for the hair follicles in the scalp typically lasts about 3 to 5 years.
- CATAGEN (INTERMEDIATE) STAGE
At the end of the growth period, hair follicles prepare themselves for the resting phase. This transition period of a hair follicle from growth to rest is called the catagen stage. This stage of the hair growth cycle usually lasts about 1 to 2 weeks or so. During the catagen phase the deeper portions of the hair follicles start to collapse.
- TELOGEN (RESTING or SHEDDING) STAGE
This is the resting period of a hair follicle. It is usually 3 to 4 months in length and at the end of this period older hairs that have finished their life will fall out and newer hairs will begin to grow.
The growing (anagen) phase constitutes about 90% (1000 days or more) of the growth cycle of a hair follicle, while intermediate (catagen phase; 10 days) and shedding (telogen phase; 100 days) phases constitute only 10% of it. That is, at a given time, about 10% of hair follicles are in the intermediate and shedding phases; thus, not growing. These hair follicles, however, are randomly distributed over the scalp, so that no bald spots are seen.
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Hair & Hair-loss All about hair
Types of hair
Morphologically, there are 3 types of hair growing on the human body. These are vellus hairs, terminal hairs and intermediate hairs.
- VELLUS HAIRS
Vellus hairs are short hairs of a centimeter or two long. A vellus hair contains little or no pigment, and thus, is colorless. Vellus hair follicles do not have adjacent sebaceous glands. At the same time, their shaft does not have a melanin layer. Vellus hairs are fine and soft, and are not cosmetically important.
- TERMINAL HAIRS
Terminal hairs are long hairs that grow on the scalp and in many people on the body. They are produced by hair follicles with adjacent sebaceous glands. Terminal hairs have large, darkly pigmented hair fibers that have a medulla at the innermost part.
- INTERMEDIATE HAIRS
An intermediate hair shows the characteristics of both vellus hairs and terminal hairs. Intermediate hairs have a medulla and contain a moderate amount of pigment, less than that found in the terminal hair type. During the balding process terminal hair follicles and intermediate hair follicles change in such a way that they no longer produce terminal hairs. In these areas, hair follicles grow vellus hairs rather than terminal hairs.
HAIR STRUCTURE
Hairs are elongated keratinized structures. Keratin is a special protein, which is resistant to wear and tear. It is the protein that also makes up the nails. Like other proteins in the body, keratin is also a large molecule made up of smaller units called amino acids. The amino acids are joined together in a chain, like beads on a string.
The diameter of a single hair fiber varies from person to person; but it is usually around 0.05 to 0.09 millimeters.
The epidermis is the outermost layer of the skin. Each hair arises from an indentation on the epidermis. The hair has two parts: the hair follicle and the hair shaft.
HAIR FOLLICLE
The hair follicle is the point from which the hair grows. It is a tiny cup-shaped pit buried in the fat of the scalp.
The terminal part of the hair follicle seated within the skin is called a hair bulb. The hair bulb is the structure formed by actively growing cells. These cells produce the long, fine and cylindrically shaped hair fibers. Here in the hair bulb, there are some special cells, which produce the pigment that gives the hair its color. This pigment is called melanin and the cells producing it are known as melanocytes. We also know that receptors for the male hormones – androgens, are located on the cells of this structure.
At the base of each hair bulb is the dermal papilla containing a vessel tuft. Thus, it is essential for the nourishment of the growing hairs. Within the skin, internal and external root sheaths cover the hair follicles. The external root sheath of a hair follicle is continuous along with the epidermis. There are also some glands adjacent to the hair follicles. The most important one of these glands is the sebaceous gland, which produces and secretes the natural oils lubricating hairs, namely sebum.
HAIR SHAFT
The part of the hair seen above the skin is called the hair shaft. The hair shaft is made up of dead cells that have turned into keratin and binding material, together with small amounts of water. This structure explains why we do not feel any pain while our hair is being cut.
The hair shaft is formed by three layers. The innermost layer of the hair shaft is named the medulla. It is seen only in large and thick hairs. The middle layer of the hair shaft is called the cortex, made of keratin fibers. The strength, color and texture of a hair fiber are provided by the cortex layer of the hair shaft. The outermost layer of the hair shaft is the cuticle. This thin and colorless layer made up of between six to ten overlapping layers of long cell remnants, serves as a protection to the cortex.