To understand why we lose hair we first need to understand the biology of the hair and hair follicle.
Biology of the hair follicle
Our skin acts as a barrier and is the primary organ through which we interact with the external world. Hairs (or pili; pilus in the singular) are characteristic of mammals. The hair consists of two parts – the follicle which is beneath the skin and hair shaft which is the part above the skin. The hair follicle is a narrow tubular depression in mammalian skin containing the root of a hair. It is lined with epidermal cells and extends down through the epidermis and dermis to its base in the subcutaneous tissue. The hair follicle is one of the important appendages of the skin. In fact the hair follicle with its sebaceous gland and the arrector pili muscle is called the pilosebaceous unit (Pilosebaceous unit - The structure consisting of hair, hair follicle, arrector pili muscle and sebaceous gland).
The shaft of a hair consists of three layers - the cuticle, the cortex of hard-keratin surrounding, and the soft-keratin medulla (this is absent in some hairs (vellus)) (fig.) Pigmented hairs contain melanin in the cortex and medulla, but pigment is absent from the surrounding sheaths. The color of hair depends mainly on the shade and the amount of pigment in the cortex and, to a lesser extent, on air spaces in the hair. In white hairs pigment is absent from the cortex, and the contained air is responsible for the whiteness; "gray hair" is generally a mixture of white and colored hairs.
The root of a hair is situated in an epidermal tube known as the hair follicle, sunken into either the dermis or the subcutaneous tissue. The follicle is dilated at its base to form the bulb (matrix).
In the obtuse angle between the root of a hair and the surface of the skin, a bundle of smooth muscle fibers, known as an arrector pili muscle, is usually found. It extends from the deep part of the hair follicle to the papillary layer of the dermis. On contraction it makes the hair erect. The arrectorespilorum are innervated by sympathetic fibers and contract in response to emotion or cold. This results in an unevenness of the surface called "goose pimples" or "goose skin."
Why do we need hair?
In all mammals hair works as a temperature regulator in association with muscles in the skin. If the outside temperature is cold, the arrector pili muscles pull the hair strands upright, creating pockets that trap air. This trapped air provides a warm, insulating layer next to the skin. If the temperature outside is warm, the muscles relax and the hair becomes flattened against the body, releasing the trapped air. In humans, body hair is mostly reduced; it does not play as large a role in temperature regulation as it does in other animals.
In humans, hair has no vital function; the main function of the hair shaft is its role as an important facet of appearance. Its psychological functions are extremely important, as any clinical hair transplant surgeon/dermatologist or cosmetician can readily attest from routine daily practice. Across cultures for centuries, the decoration and styling of scalp hair has been a means of social communication and display of social identity or status. Hair is so important in our society that hair loss (in men and women), as well as the overgrowth of terminal hair on the body or face, in excess of the culturally accepted norm (especially in women), has deleterious effects on self-esteem.
Moreover, hair serves other purposes: in particular, it is concerned with sexual and social communication by constructing adornments such as the mane of the lion or the beard of the human male, or assisting in the dispersal of scents secreted by complexes of sebaceous or apocrine glands.
Hair follicle formation, number and distribution
Historically there have been many explanations of how human hair follicles form. Philosopher Aristotle in the fourth century suggested that hair formed from sooty vapors exhaled through the body’s pores that hardened on contact with air. Albertus Magnus a eleventh century scientist suggested that hair had excretory function and was a product of gross humor and phlegm divested from brain. Only with careful study of skin and hair follicles by anatomists of the nineteenth century were formation and hair follicle structure fully defined and mechanism of hair growth correctly extrapolated.
Human hair follicles appear ﬁrst in the regions of the eyebrows, upper lip and chin at about 9 weeks of embryonic development,and in other regions in the fourth month. A full complement of the hair follicles is ready by the 22nd week of gestation. It is generally believed that no “new” hair follicles develop after birth.
Hair follicle formation is the result of a complex signaling mechanism between the dermal layer and the overlying epidermis. The hair follicle formation events can be divided into at least eight distinct developmental stages. When humans are born, they have about 5 million hair follicles, only 2 percent (100,000-150,000) of which are on the head. Hair is present over the entire surface of the body except the palms, soles, glans penis and labia minora. This is the largest number of hair follicles a human will ever have. Considering the distribution of human hair in different areas of the body surface, it is possible to note that human hair growth is reduced with tiny and virtually colorless hair on most of the body surface (vellus hairs), whereas hair is longer, thicker, and heavily pigmented in other areas, such as the scalp, eyelashes, and eyebrows (terminal hairs).
As a human ages, the density of hair decreases. The mean hair follicle density depends on the skin area, because hair follicles are built in the early fetal period. After birth, the body proportions change and the hair follicles move apart according to the growth of body and skin. Because of the relatively lower growth of the head compared to the extremities, hair follicles are much more numerous on the scalp and in the face than on arms and legs
Types of hair
Hair can vary in shape, length, diameter, texture, and color. The cross-section of the hair may be circular, triangular, irregular, or flattened, influencing the curl of the hair. The texture of hair can be coarse as it is in whiskers or fine as it is in younger children. Hair color Hair types are classified on the basis of structure/size, color, ethnicity, anatomical regions etc.
The human hair follicle is capable of producing different types of hair under different circumstances the type of hair produced in any particular follicle can change with age or under the inﬂuence of hormones. In humans, a prenatal coat of ﬁne, soft,unmedullated and usually unpigmented hair, known as lanugo (looks like fur), is normally shed in utero in the eighth to ninth month of gestation.
Postnatal hair may be divided at the extreme into two kinds: vellus, which is soft, unmedullated, occasionally pigmented and seldom more than 2 cm long (frequently referred to as “peach fuzz,”); and terminal hair, which is longer, coarser and often medullated and pigmented.This type of natural hair, associated with women and young children, is fairly unnoticeable. However, there is a range of intermediate kinds. Terminal hair is the type of natural hair to which most people refer in their everyday lives.Beforepuberty, terminal hair is normally limited to the scalp, eyebrows and eyelashes. After puberty, secondarysexual ‘terminal’ hair is developed from vellus hair in response to androgens.
Ethnic variations of hair
While no two heads of hair are the same, there are three main classifications of hair type based on ethnic/geographical/racial basisinto caucasoid, negroid, and mongoloid. These terms are now obsolete as they are scientiﬁcally inaccurate and geopolitically incorrect. Better and more appropriate alternatives are - African or Equatorial-African, European or Caucasoid or Indo-European (IE), and Asian.
In spite of these variations the basic composition remains the same.
The European/Caucasoid hair is generally straight or wavy with small pigment granules which are evenlydistributed. It is oval or round on cross-section, ofmoderate diameterwith minimal variation. Color may beblond, red, brown,or black
The Asian hair is straight and has dense distribution of pigment granules. The cross-section is round with a fairly large diameter. The shaft tends to straight and coarse.
The African hair on the other hand is kinky, curly or coiled with variable sized pigment granules which are densely distributed. On cross-section the African hair is flattened or sometimes ribbon like with significant variation in diameter.
What are the different parts of the hair follicle?
The follicles are slanted in epidermis and an oblique muscle the arrector pili runs from the middle of the follicle wall to the dermo epidermal junction, additionally one or more sebaceous glands open into the follicle.The follicle may be divided into vertical components: Infundibulum, Isthumus, Suprabulbar region and Bulb.
What is the composition of hair?
The hair as we see it is a compact structure made up of dead cells. Individual hairs are composed chiefly of the horny scleroprotein known as keratin and contain neither blood vessels nor nerves. They usually contain pigment (except in the case of albinos). The shaft of the hair consists of modified epithelial cells arranged in columns (cortex) surrounding a central medulla (or core) and covered with thin, flat scales (cuticle).
How does hair grow?
The hair follicle bulb is like a minute factory which manufactures cells continuously. As new cells are formed the older ones are pushed into the follicle towards the surface. The cells on reaching a particular point undergo cell death and are converted into a scleroprotein called keratin, which is then compacted to form the hair. This continuous activity is responsible for the hair growth.
During anagen the follicle actively grows hair.
During catagen the follicle is almost entirely degraded.
During telogen the follicle rests prior to re-initiation of an anagen phase and the growth of a new hair shaft.
On any given day, human beings lose about 50-100 scalp hairs due to exogen, these are the hairs we find in our comb, brush and shower drain. 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.
At any given time approximately 90% of our scalp hair are in the growing phase and about 10% in the resting phase, if the scalp is healthy and not affected by any condition that causes hair loss.
A fourth phase known as the Kenogen or the lag phase has been described. This is the period after the hair has been shed and before the new hair formation begins. It is this phase that is prolonged in patients of Pattern Hair Loss.
The normal growth rate of scalp hair is 0.3 - 0.4 mm per day or roughly 1cm per month. Hair growth has a cyclical pattern that can be affected by a number of genetic, disease, medication or other factors to cause hair loss.
Why do some people have very long hair? How long can your scalp hair grow?
If you do not cut your hair then you can grow it quite long, but there is an upper limit to the maximum length that you can grow your scalp hair. There are various papers that examine the time duration for scalp hair follicles are actively producing a hair fiber. The most frequently quoted time period is 1000 days (2-6 YEARS), but this figure is a rough approximation. The time duration of anagen growth, plus knowing what length of hair fiber a scalp hair follicle can produce per day, will enable a calculation of the average maximum length that scalp hair can grow to. Research on humans suggests that active hair fiber growth production from scalp hair follicles may last from 500 to 1800 days.
The scalp hair fiber growth rate is between 0.3 to 0.4 mm a day. For the most part, the studies were done with male volunteers, but a few other studies indicate similar results for women too.
If you put it all together in a calculation, the research would suggest that the maximum length scalp hair can grow to, is between 20 and 60 centimeters. However, from the picture below you can see that some people are able to grow their hair much longer. The upper limit to scalp hair length is determined by our genes. Some people have genes for short hair, others grow hair much longer.
How is hair growth regulated?
The hormones called androgens are important control factors in hair growth and in inherited male and female patterns of hair loss. The androgen hormone testosterone and its metabolite dihydrotestosterone (DHT) are the key control factors:
Testosterone is a key control factor in the growth of beard, underarm and pubic hair.
Scalp hair loss is associated with presence of DHT in male and female pattern hair loss. DHT plus the presence and activity of hair loss gene(s) are the key factors underlying male and female pattern hair loss.
The androgens mediate their effect by binding to the androgen receptors present in the follicle. The testosterone is converted to DHT by the action of the enzyme 5-Alpha reductase. The DHT then binds to the receptor and initiates its action.