Of all mammals, humans have the least amount of hair. The hair they have still mostly covers the body, but it is fine and cannot really be seen unless you are looking closely.
The average human head will have around 100,000 hair follicles and each follicle is able to grow about 20 individual hairs during the body’s lifetime. People lose about 100 strands of hair a day on average. A human eyebrow has around 250 hairs, but if it has never been plucked, could have as many as 1100 hairs. People generally have around 150 - 200 eyelash hairs on the top lid and 75 - 100 hairs on the bottom lid. The amount of hair people have can change over their life. Due to hormonal changes, women can gain more facial hair around the chin area, and the hair can thin on their head, while men can develop male pattern baldness, a condition where the thick, coloured hair on the top of the head is gradually replaced by shorter, thinner hairs which are not so easily seen.
Hair consists of keratin, a tough protein. It grows from a hair follicle, which is rooted in the epidermis layer of the skin. The living follicle has a continuous growth and rest cycle which is regulated by hormones, ageing and nutrition as well as where the hair is growing. The base of the follicle is the hair bulb where cells grow and divide to enable the hair shaft to grow. The hair is supplied by blood vessels for nourishment and hormones. The hair bulb is made up of fibrous connective tissue, glassy membrane, external root sheath and an internal root sheath which is made up of epithelium stratum or Henle’s layer and granular stratum or Huxley’s layer. The middle segment is the isthmus and upper segment of the follicle is the infundibulum, extending to the surface of the epidermis from the opening of the sebaceous gland.
The cycle of hair growth includes anagen or the growth phase. Depending on where it is growing, hair can last for several months or years. The catagen or transitional phase means that the hair growth is slowing down and hair follicles shrink. During the telegenic or resting phase, the hair growth stops and the old hair is detached from the follicle and falls out. A new hair will begin growing which helps to push the old hair away. Hair growth rates vary from person to person, but an average growth rate is about 2.5 cm a month.
Most of the hair on the body is tiny and colourless, known as vellus hairs. Only a few areas of the body have no hair at all, including the palms of the hands, soles of the feet, lips and eternal genitals. The thicker, pigmented hair that grows on eyebrows, eyelashes and the scalp, are terminal hairs. These are present at birth. During puberty, some vellus hairs are affected by hormones to change to terminal hairs on the face, underarm and genital area. These hairs become more pigmented, thicker and longer. Gonadotropin triggers puberty which enables the pituitary gland to release Follicle Stimulating Hormone and Luteinizing Hormone.
Just along the hair shaft, there is an arrector pili muscle which is between the hair bulge and the derma-epidermal junction. This muscle is attached to the hair follicle base and dermal tissue and enables the hair to generate heat by standing up straight on the skin. Above this muscle, there may be sebaceous glands or sometimes apocrine or sweat glands which open into the follicle. These can especially be found under the armpits, but can also be on breast nipples, eyelids, the ear canal, nostril hair, the perianal area and parts of the external genitals. The hair bulge supplies the hair follicle with new stem cells and helps to heal the epidermis if there has been a wound.
The hair shaft has three layers: the medulla or core of the hair, which is only seen in the thick, large hairs, the cortex which gives strength, texture and colour to the hair strand and the cuticle, which is thin and colourless, the outer layer of the hair.
Functions of hair include helping to regulate the body temperature by helping to evaporate perspiration, protecting the skin and as sense organs. Humans also consider their hair as an important part of social function and convention can sometimes dictate the removal of some body hair as well as hair styles and the styling of beard and moustache hair for men.
Erdogan, B., Anatomy and Physiology of Hair, 2016, Hair and Scalp Disorders, open access article