A hairy matter

Humans have about 5 million hairs scattered throughout the body except the navel, palms, soles of the feet and mucous membranes. In the head, we have between 90,000 and 146.00 strands. Hair grows in the hair follicles that are located in the dermis. Each follicle has a cycle consisting of the following phases: anagen, during which the hair grows uninterruptedly at the rate of 1 mm every 2.5 days, thanks to the mitotic activity of the stem cells, the catagen phase, during which the hair is taken off from the bulb and stops growing, and the telogen phase, in which the hair falls out. The duration of the life cycle of a hair depends on the area where it grows, in the head it lives from 2 to 5 years, in the beard it lives 1 year and the eyebrows lives 1 month. In the follicle there are also sebaceous glands that lubricate and protect the hair, and the erector muscle, responsible for “goosebumps.” Depending on their type, the hair is classified as straight, wavy, curly and kinky. These types depend on the section of the hair and have a certain distribution in the different continents. Another very important feature of hair is color. Although the range is infinite, they are classified as dark brown-black (75-85% of the world population), brown (11%), blond (3%) and redhead (2-3%). The hair is basically formed by chitin, a colorless protein. The melanocytes of the follicles secrete two types of pigments that combined determine the final coloration. When the melanocytes age, they stop secreting pigments, the hair fades and the gray hair appears. In men with aging also appears alopecia, which can show different degrees. Finally, the most prominent inhabitant of human hair is the louse, a parasitic insect that has co-evolved with us for millions of years, and remains a public health problem despite numerous pharmacological treatments.