All you should know about hair follicles
Hairs are the horny parts of the skin of filamentous and cylindrical shape, emerging at an angle from epidermis’ gulfs, i.e. pilosebaceous follicles or hair follicles.
Each hair consists of keratin proteins, which are only found in mammals. There are two types of hair: terminal hair, which are thicker and long, and vellous hair, which are tiny translucent hair.
Hair exist in all parts of the human body, excluding the soles, the palms, the lateral surfaces of the fingers, the terminal phalanges of fingers, the nipples, the foreskin and semi mucosal areas (lips, clitoris, of the hole anus, inner surface of the labia majora).
It is estimated that at birth, the total number of hair follicles is five million, of which one million is found on our head. The hair follicles of embryonic hairs are formed before the 9th week of embryonic life.
Each hair consist of two parts: the stem and the root. The stem is the visible part of hair and extends from the point of skin where the sebaceous gland emerges (neck of the hair follicle) towards the free end. The number of sebaceous glands varies (one, two or more) and they surround the hair’s root.
The root is located inside the skin and ends to a thicker, pear-like end, which is called the bulb. The bulb’s lower part has a cavity, in which the papilla of the dermis is located, a structure full of vessels and nerves.
The papilla contributes in both the feeding and the growing of hair. In case of defects in the dermis’ papilla the hair is lost. The papilla is surrounded by the basal layer of the bulb, the cells of which reproduce all layers of the hair. The mitotic activity of basal layer’s cells is influenced by receptors sensitive (or insensitive) to circulating androgens.
Melanocytes are among the basal layer’s cells, which have melanin and are responsible for hair colour. Eumelanin is responsible for brown-black hair colour, while phaeomelanin is responsible for the red-yellowish hair colour.
Hair whitening appears to be due to the reduction of melanin production and due to the presence of microscopic air bubbles in the marrow cells, which reflect the light.
Dermis surrounding the hair follicle forms a connective tissue sheath, on which bundles of smooth muscle fibers are attached, originating from the papillary dermis and comprising the arrector pili muscles of hair. A triangle is formed, consisting of the arrector pili muscle, the epidermis and the hair follicle.
The normal hair loss rate is approximately 50-100 hair daily, while the average number of hair on the head is around 100,000. Each hair follicle needs approximately 6 months to replace a lost hair with a new one.
As times goes by and we get older, hair growth rate slows down. The average lifetime of each hair is seven years. During summertime hair production rate is increased, partly due to the higher temperatures. When melanin is reduced, hair start becoming grey or white.
Elasticity is a characteristic of healthy hair.