"Over the centuries, hair has always been very important for people, both for aesthetic and psychological reasons."

Though hair, including hair that covers the body, is no longer regarded as a protective means against heat or cold, as it was in the very old times, it is indeed true that people of all cultures have always cared for and looked after hair through history.

What is hair?

A single hair, wherever it may grow on the body, stems from a hair follicle. Hair follicles are found everywhere on the external body except for mucous membranes and glabrous skin, such as that found on the palms of the hands, soles of the feet, and on the lips.

Follicles in the scalp grow the hair on our head - thick and pigmented, which is technically defined as terminal hair, to distinguish it from vellus hair, which is thin and almost invisible and found in other parts of the body.

Anatomy and physiology of hair

Each hair has two distinct structures: the part beneath the skin, called the hair follicleand the shaft, which is the hard filamentous part that extends above the skin surface, hence is visible.

The hair follicle is a tubular invagination of the epidermis and consists of several layers (sheaths). It roots into skin and reaches the dermis with a certain inclination, which is the reason why hair normally is not straight, but slightly inclined.

Each hair follicle has a sebaceous gland and a small muscle. The former makes sebum, an oily secretion that lubricates hair, while the latter - called arrector pili - is what gives us "goose bumps."

The bottom portion of the follicle is slightly enlarged and is called the hair bulb. It includes the germinal matrix and the dermal papilla.

The germinal matrix is the source of hair growth and is involved in transfering melanin to hair to give it pigmentation. During the active hair growth phase, the cells in the matrix grow quickly and differentiate (by the mitosis process of cell division), causing the hair shaft to grow.

The dermal papilla is structure with abundant blood vessels that provides the necessary elements for the activity of the bulb.

the shaft has a cylindrical shape and is formed by three layers called  cuticle, cortex, medulla from the outside to the inside.

Hair colour is given by the pigment produced by melanocytes located in the hair bulb. Melanocytes provide pigment to the cells that are "building" the shaft, hence the shaft takes on its characteristic colour as it grows along the follicle. The pigments are concentrated in the middle layer of the shaft, that is in the cortex.

What is hair made of?

In addition to water, hair is made of: keratin, minerals, pigments and a small amount of lipids.

Keratin is a protein rich in sulfur, because it contains high amounts of sulfur amino acids (cysteine and cystine). It is precisely thanks to the sulfur atoms in between keratin chains that very strong chemical bonds can be formed. These bonds, called disulphide bridges, give strength and resistance to nails, body hair and scalp hair.

Hair keratinisation process is regulated by genetic factors, but also by hormones and vitamins.

The minerals in hair are a very important component. Actually, there is a direct correlation between the amount of minerals in the blood and those in hair.

The main minerals in hair are iron, magnesium, copper and zinc. The latter is especially essential for the proper activity of the germinative cells of the matrix. In fact, when there is zinc deficiency, hair weakens and its growth rate slows down.

The pigments in hair are called melanins. Dark hair has a prevalence of eumelanin (brown-black pigment), while blonde-red hair has more pheomelanin (yellow-orange pigment).

The lipids in hair (triglycerides, waxes, phospholipids, cholesterol, squalene, free fatty acids) are derived from the sebum produced by the sebaceous gland attached to the hair follicle.

Hair growth cycle

Throughout a person's life, hair continuously cycles through growth and resting phases, hence we speak of hair growth cycle. The average growth cycle spans from 2 to 7 years, thus it is estimated that each follicle produces around 20 hair strands in a person's life.

The three stages of hair growth are:

• the ANAGEN phase, or growth phase, is the phase when hair grows longer. It’s the longest phase, with an average duration of several years.
  In men it lasts for 2 -4 years, while in women it is 3-7 years. Hence, in men hair changes at double or triple the speed compared to women. Since the length hair can grow is basically the same in both males and females, hair of men is though to fall out at a theoretical length of about 30-35 cm, while hair in women can reach 100-120 cm - that's why women have longer hair than men usually.
• the CATAGEN phase, or involuting phase, is when vital functions diminish and hair growth stops. This phase lasts for 2-3 weeks.
•  the TELOGEN phase, or resting phase, is when functional activity ceases completely, but the hair is still anchored to the hair follicle, and will fall eventually upon hair being washed or brushed. This phase, which on average lasts for 3-4 months, marks the beginning of a new hair cycle.

In normal conditions, each time a hair in telogen falls out, the hair follicle will grow a new one in anagen, hence hair shedding is a natural event. However, a number of factors can cause excessive hair shedding, which results in hair thinning and increasingly larger hairless scalp areas showing.

<adapted from Bionike.it>