Hair is located in the hair follicle, a particular skin structure with a sac-like form, tilted by approximately 75° with respect to the skin's surface. The lower part of the hair follicle is located in the deeper part of the dermis but its depth level varies from hair to hair (in the case of hair on the head, the average depth is around 0.6 -1 cm).
The sebaceous gland is attached to the upper third of the hair follicle. The hair follicle and sebaceous gland together are called the pilosebaceous unit. The arrector pili muscles of the hair are located on andanchored to the sebaceous glands, on the outer wall of the follicle. Inside the hair follicle we can note the following: it emerges from the skin's surface through an opening called an ostium; there is a narrowing in the upper third called the shaft; it also features a portion between the ostium and the shaft, called the infundibulum, a portion located between the shaft and lower part of the arrector pili muscles, called the bulge and finally a part underneath the bulge called the hair root.
In the center of the root, at the base of the follicle, is a loop facing upward and containing connective tissue full of blood vessels, lymphatic vessels and nerves, called the papilla, which is responsible for support, nourishment and endocrine control of the cells in the hair matrix.
Inside the follicle structure, proceeding from the outer layer inwards, we can note: the connective tissue sheath (full of nerve endings and consisting of concentric and longitudinal layers of collagen fibers with numerous fibroblasts); the vitreous membrane (a direct continuation of the cutaneous basement membrane); the external epithelial root sheath (direct continuation of the deeper layers of the epidermis that deepen as it follows the vitreous membrane); the internal epithelial root sheath (made up of three cellular layers, in direct contact with the outermost layer of the hair shaft).
The hair structure of an adult is divided into three parts: shaft, root and bulb. The shaft is the visible, outer part of the follicle, with an average thickness of 65-78 microns in an adult. The root, a part that is usually not visible, within the follicle itself, is located in the skin between the ostium of the follicle and the insertion point of the arrector pili muscles. The bulb, even deeper than the root and located at the base of the root of the hair follicle, contains two-three layers of cells undergoing fast reproduction in its lower part: these constitute the matrix. The matrix cells are the only germinal cells. As they reproduce, the matrix cells push those born previously outward. As they emerge, the cells process the keratin going towards the "keratinization process", thereby becoming progressively more rigid. Immediately above the matrix cells are some melanocytes that are responsible for "coloring", pumping melanin into the cells that will become the hair "cortex".
By horizontally dissecting hair you can appreciate its structure, divided into three parts: the cuticle, the cortex and the medulla.
On the outside we find the cuticle consisting of a single layer of transparent thin cells, with a thickness between 0.2-0.5 microns, layered vertically at the level of the root and obliquely (similarly to scales) at the level of the shaft. Given its position, the cuticle is the first part to be damaged when hair is mistreated (via use of inadequate shampoo, perms, brushing, etc.).
The cortex is located at the intermediate level, and is the thickest part. It consists of thicker, spindle-shaped cells, 90 microns long and 5 microns wide, layered vertically in parallel rows. These contain a colored pigment called melanin, which tends to diminuish with age, thus making hair become "white" (process of going grey).
The medulla is located inside and consists of round cells, layered in columns, generally separated by air pockets.
In addition to water, the elements that compose the chemical composition of hair are: keratin, lipids, minerals and pigments.
Keratin is a protein found in the cortex. Keratin is composed of 18 amino acids. The most abundant amino acids are: Cysteine, cystine, serine, glutamic acid, glycine, threonine, arginine, valine, leucine and isoleucine.
Alpha keratin, fibrous and with a low sulphur content, is the protein we find in the greatest quantity in hair. It has a molecular weight of around 45,000 and is insoluble in water. Keratin may be deformed with water vapor ("styling").
The hair keratinization process is regulated by various elements (hormones, vitamins, genetic factors and metabolism) and is connected to the metabolism of cholesterol and its esterification with fatty acids synthesized by the epidermis. Dietary deficiencies and/or enzyme defects due to cholesterol and fatty acid synthesis may lead to irregular keratinization which results in structural defects in the hair shaft.