It may come as a surprise to nearly everyone to learn that except for mucus membranes, the palms of the hands and soles of the feet, both men and women, and all children, are completely covered in hair. How is that possible, you ask? It has to do with the differences in types of hair that exist on our bodies.
The fact is the “naked ape” is not naked at all. Per square centimeter, human skin has as many hair follicles as that of other great apes. The difference is not in the number, but in the fineness of the hair that grows from those follicles. These fine human hairs do not seem to be performing any of the functions of their counterparts in more hirsute species (insulation and, through coloration, either signaling or camouflage). So what are they for?
Some scientists feel that fine body hair may serve as an evolutionary alarm system, alerting their owner to creepy crawlies such as bed bugs and other biting insects or arachnids. Compared to our cave dwelling days, modern life has less need for such an alarm system. Nonetheless, we all freak when we feel something tiny and nearly weightless working their way across our skin during the night. The system works well.
Anatomy of hair – the hair follicle
The hair follicle is the mammalian skin organ that produces hair. Within the follicle, stem cells are responsible for hair production and the shape of the hair follicle has an effect on the hair shape and texture of the individual’s hair.
Hair follicle structure includes:
Papilla – a large structure at the base of the hair follicle.[The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non-existent.
Matrix – which surrounds the papilla.
Root sheath – composed of an external and internal root sheath. The external root sheath appears empty but is filled with cuboid cells. The internal root sheath is composed of three layers, Henle’s layer, Huxley’s layer and an internal cuticle that is continuous with
Bulge – located in the outer root sheath at the insertion point of the arrector pili muscle. It houses several types of stem cells, which supply the entire hair follicle with new cells, and take part in healing the epidermis after a wound.
Other structures associated with the hair follicle include the cup in which the follicle grows known as the infundibulum, the arrector pili muscles, the sebaceous glands, and the aprocrine sweat glands. Hair follicle receptors sense the position of the hair. The arrector pili muscle causes the follicle to become more perpendicular and protrude slightly above the surrounding ski. The phenomenon is the cause of “goose bumps”. The sebaceous gland produces the oily or waxy substance sebum. The higher the density of hair, the greater the number of sebaceous glands. During puberty, the production of sebum increases dramatically, sometimes clogging pores that becoming inflamed resulting in an acne pustule.
The structure of hair – the hair shaft
Hair fibers have a structure consisting of several layers, starting from the outside: the cuticle, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles; the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like. It is the source of mechanical strength and water uptake. It also contains melanin which gives hair its color.; the medulla, a disorganized and open area at the fiber’s center.
The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. People with straight hair have round hair fibers. Oval and other shaped fibers are generally wavier or curly. The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water. The diameter of human hair varies from .017 to .18 millimeters (0.00067
Three kinds of hair exist on the human body – lanugo, vellus, and terminal
Lanugo is very fine, soft, usually unpigmented, downy hair found on the body of the fetus and occasionally on newborn babies. It is the first hair produced by fetal hair follicles, and usually appears at about five months of gestation. It is normally shed before birth, around seven or eight months of gestation. It is sometimes present at birth but disappears on its own within a few days or weeks.
Lanugo hair is then replaced on the same surfaces by vellus hair which is finer and more difficult to see. Anyone who closely examines a skin surface where hair is not readily visible will see fine vellus hair protruding from every follicle.
The much more visible hair (e.g. on the head during childhood) that persists into adulthood is called terminal hair. It forms in specific areas and is hormone dependent.
Puberty dramatically increases the amount of terminal hair in both sexes, and from the same follicles that formerly produced vellus hair. The flood of hormones during our teen age years s the cause. Axillary and pubic terminal hair becomes visible as does hair on the extremities and the torso.
Males, because of substantially higher levels of testosterone, generally develop coarser, more visible hair, as well as facial hair. Ethic heritage, however, has great impact in how the terminal hair distribution pattern evolves. Some women are naturally much hairier than others, and there are men with surprisingly little body hair.
For women, excessive hirsutism can also indicate polycystic ovarian syndrome. The Ferriman-Gallwey scoring system (below) is used to evaluate the significance of hair patterns in women. Scores higher than eight are suggestive of abnormally high androgen (testosterone) levels and possible ovarian dysfunction. Notice that scores lower than eight are associated with considerable amounts of terminal hair that is widely distributed.
Phases of hair growth – anagen, catagen, and telogen
There are three distinct and concurrent phases of hair growth. All three occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase. Hair in different parts of the body also have specific characteristics that determine the length of hair.
Anagen phase – Anagen is the active growth phase of hair follicles during which the root of the hair is dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2–7 years and is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
Catagen phase – The catagen phase is a short transition stage that occurs at the end of the anagen phase. It signals the end of the active growth of a hair. This phase lasts for about 2–3 weeks while the hair converts to a club hair. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. This process cuts the hair off from its blood supply and from the cells that produce new hair. When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase.
Telogen phase – The telogen phase is the resting phase of the hair follicle. When the body is subjected to extreme stress, as much as 70 percent of hair can prematurely enter a phase of rest, called the telogen phase. This hair begins to fall, causing a noticeable loss of hair. This condition is called telogen effluvium. The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair. Fifty to one-hundred club hairs are shed daily from an average adult.
Hair growth cycle times – affected by individual, hair color, and follicle shape Scalp:
- anagen phase, 2–8 years (occasionally much longer)
- catagen phase, 2–3 weeks
- telogen phase, around 3 months
- anagen phase, 4–7 months
- catagen phase, 3–4 weeks
- telogen phase, about 9 months
Hair exists in a variety of textures. Three main aspects of hair texture are the curl pattern, volume, and consistency. A chart of the types of textures of hair is below.
The derivations of hair texture are not fully understood. All mammalian hair is composed of keratin, so the make-up of hair follicles is not the source of varying hair patterns. There are a range of theories pertaining to the curl patterns of hair. Scientists have come to believe is that the shape of the hair shaft has an effect on the curliness of the individual’s hair. A very round shaft allows for less disulfide bonds to be present in the hair strand. This means the bonds present are directly in line with one another, resulting in straight hair. The flatter the hair shaft becomes, the curlier hair gets, because the shape allows more cysteines to become compacted together resulting in a bent shape that, with every additional disulfide bond, becomes curlier in form. As the hair follicle shape determines curl pattern, the hair follicle size determines thickness. While the circumference of the hair follicle expands, so does the thickness of the hair follicle. An individual’s hair volume, as a result, can be thin, normal, or thick. The consistency of hair can almost always be grouped into three categories: fine, medium, and coarse. This trait is determined by the hair follicle volume and the condition of the strand. Fine hair has a small circumference in relation to medium and coarse strands; coarse hair having the largest circumference. Coarse hair has a more open cuticle than thin or medium hair causing it to be the most porous.