Anatomy of the hair

Hair is the appendage of the skin. As related structures, they have much in common, from the building plan to the characteristics of growth and development. The laying of the hair follicles begins on the 4th month of embryonic development and is determined by the interaction of the dermal and epidermal components of the fetal skin.

• Hair root and hair follicle

The root of the hair is a complex structure, which has not yet been fully understood. Recently interest to it has stirred up with new force thanks to the discoveries, connected with stem cells, which, as it turned out, are present in it.

The root of the hair is located mainly in the dermal layer (sometimes reaches the hypodermis) and is surrounded by epithelial root vaginas (together with them it is called the hair follicle). The hair follicle is separated from the dermis by a connective tissue membrane - the root (or vaginal) bag. A little higher is the so-called bulge area (from the English - a bag, ledge, bulge), and the lower part of the root is called an onion.

Cellular composition of the follicle is diverse and is a mixture of cells both specialized (mature) and unspecialized:

• • specialized cells: melanocytes (produce melanin pigment), fibroblasts (synthesize extracellular matrix proteins - collagen, fibronectin), keratinocytes (synthesize keratin), glandular cells-sebocytes (secrete sebum);
  • unspecialized: stem cells and progenitor cells (are in the early stages of maturation and located in the bulge, inside the bulb and in the basal layer of the epidermis). To the hair follicle adjoin the sweat and sebaceous glands, as well as the muscle lifting the hair (arrector pili). Nutrition and innervation of the whole complex is due to the connection of capillaries and nerve endings with the dermal papilla.

Hair color is determined by the ratio of two pigments - eumelanin, giving chestnut or black color to hair, and pheomelanin - greyish and yellow shades. Both pigments are produced by melanocytes in the anagen phase.

The follicle is immersed in a layer of subcutaneous fat. With age, this layer on the scalp skin becomes thinner. Researchers noted the accumulation of fat cells (adipocytes) around the awake normal follicles actively producing healthy hair, and their relative deficiency around sleeping follicles. From this it was concluded that adipocytes serve to "support" the function of the hair follicle. Factors that inhibit hair growth, such as chemotherapy or hunger, also reduce the layer of subcutaneous fat.

Interesting statistics:

• • On the average on the scalp there are 100 thousand hair follicles (maximum - 150 thousand).
  • The average density on the scalp: the child has 600 follicles per cm2, the adult has 250-300 follicles per cm2.
  • Throughout life from the follicle can grow up to 30 hairs.
  • Normally, approximately 90% of the hair on the head is in the growth stage, 1% - in the intermediate stage, 9% - in the stage of rest.

Hair shaft

Continuous hair growth occurs due to the division (proliferation) of cells sitting on the hyaline basement membrane, delimiting the inner part of the follicle from the dermal papilla. Separation from the basal membrane serves as a signal for the onset of maturation (differentiation), leading ultimately to cell death - maturing cells gradually lose nuclei and fill with keratin. Owing to the continuous division of cells within the follicle, a pressure is created that causes the keratinized cells to move upward at a rate of about 0.3-0.4 mm per day - thus ensuring the growth of the hair, more precisely, of its stem.

• Cuticle - the protective shell of the rod.

The hair shaft (like the epidermis) has a layered structure. The outer layer - the cuticle - consists of 6-10 layers of overlapping keratinous scales, laid like tiles (Figures I-1-3). Scales have an elongated shape (thickness - 0.2-0.4 μm, width - about 0.3 μm, length - up to 100 μm) and are fastened together by a lipid layer. The structure of the cuticle resembles the stratum corneum, which is also made up of keratin scales (although they have a different shape - hexagonal), glued together with lipids (lipid barrier of the skin).

• Cuticle - the most durable part of the hair, protecting its interior.

As the hair grows, the cuticle is damaged and gradually destroyed, revealing the cortex. In the places of destruction of the cuticle, the evaporation of water occurs: the hair shaft begins to lose moisture, becomes brittle, an electrostatic charge accumulates on it. All this leads to the fact that the aesthetic properties of hair deteriorate - they become tangled, broken, fade, poorly laid. An important
Role in the destruction of the cuticle is played by aggressive external factors - temperature, UV radiation, combing, chemical compounds (including those that hit the hair as part of hair products).

• Cortex is the layer responsible for the mechanical properties of the hair.

Under the cuticle is located a cortical layer - a cortex composed of longitudinal rows of keratinized cells. This layer provides the rod with flexibility and strength. The matrix fiber of the cortex has a protein nature and is rich in cis-theine, a sulfur-containing amino acid. The disulfide bonds formed during the keratinization process give the rod a characteristic shape. The number and location of these bonds is determined genetically, therefore, in order to change the shape of the hair, it is first necessary to disrupt the disulfide bonds, and then restore them in a new sequence (see Perm and hair straightening).

• Medula - heat-insulating properties of hair.

Under the cortex in some hair is found the medulla layer - a medula that has many voids. In animals, the cerebral layer is well developed - the presence of air inside the hair shaft reduces its thermal conductivity: such hair serves as a good thermal insulation and protects the body against changes in ambient temperature. In humans, the medulla layer is present in hard hair (especially gray hair).

Barrier system of hair

Human hair, like the epidermis, has its own barrier system, which protects it from adverse external influences. Barrier structures of the epidermis and hair are similar. The main similarity is that the main functional load in them is performed by lipids.

In addition to the lipid secretion of sebaceous glands, which form a protective film on the surface of the hair and are removed during washing, there are so-called integral (or structural) lipids inside the hair. By covalently binding to the protein matrix, they form a complex of cell membranes (CMC), which not only ensures the adhesion of cuticular and cortical cells, but also serves as a barrier for the diffusion of various substances into the interior of the hair.

A complex of cell membranes is located between the cells of the cuticular and cortical layers. Electronograms of the transverse section of the hair show that the cells are 25-30 nm away from each other, with a more dense layer approximately 15 nm thick, clearly visible on both sides of which are two less dense layers directly in contact with the cell boundaries. The totality of the intercellular substance and the outer shell of cells was called a complex of cell membranes. The CMC forms an extensive network along the fiber, allowing the cells to co-exist with each other.

It was found out that the lipids of the cuticular CMC are mobile and sensitive to the action of external factors. Perm, solar radiation and radiation can lead to strong changes in the lipid composition of the hair, up to the complete disappearance of CMC from the cuticle.

An interesting fact: visible light destroys the CMC much more than UFA and UVB. Pigment eumelanin protects hair lipids from photochemical destruction: light hair lypides are destroyed faster than black hair.

[1], [2], [3]