Anatomy of hair

Hair is an appendage of the skin. Being related structures, they have much in common, from the structure plan to the features of growth and development. The laying of hair follicles begins in 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 hair root is a complex structure that is not yet fully understood. Recently, interest in it has been rekindled by discoveries related to stem cells that are found to be present in it.

The hair root is located mainly in the dermal layer (sometimes reaching the hypodermis) and is surrounded by epithelial root sheaths (together with them it is called a hair follicle). The hair follicle is separated from the dermis by a connective tissue sheath - the root (or vaginal) bursa. A little higher is the so-called bulge region (from English - bag, protrusion, bulge), and the lower part of the root is called the bulb.

The cellular composition of the follicle is diverse and is a mixture of both specialized (mature) and non-specialized cells:

• • specialized cells: melanocytes (produce the pigment melanin), 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 are located in the bulge area, inside the bulb and in the basal layer of the epidermis). The sweat and sebaceous glands, as well as the muscle that raises the hair (arrector pili), are adjacent to the hair follicle. Nutrition and innervation of the entire complex is carried out due to the connection of capillaries and nerve endings with the dermal papilla.

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

The follicle is embedded in a layer of subcutaneous fat. With age, this layer of the scalp becomes thinner. Researchers have noted an accumulation of fat cells (adipocytes) around awake normal follicles actively producing healthy hair, and a relative paucity of them around dormant follicles. This suggests that adipocytes serve to "support" the function of the hair follicle. Factors that inhibit hair growth, such as chemotherapy or starvation, also reduce the subcutaneous fat layer.

Interesting statistics:

• • On average, there are 100 thousand hair follicles on the scalp (maximum - 150 thousand).
  • Average density on the scalp: in a child - 600 follicles per cm2, in an adult - 250-300 follicles per cm2.
  • Over the course of a lifetime, up to 30 hairs can grow from a follicle.
  • Normally, approximately 90% of the hair on the head is in the growth stage, 1% is in the intermediate stage, and 9% is in the resting stage.

Hair shaft

Continuous hair growth occurs due to the division (proliferation) of cells sitting on the hyaline basement membrane, which separates the interior of the follicle from the dermal papilla. Detachment from the basement membrane serves as a signal for the beginning of maturation (differentiation), which ultimately leads to the death of the cell - maturing cells gradually lose their nuclei and are filled with keratin. Due to the continuous division of cells inside the follicle, pressure is created, forcing keratinized cells to move upward at a speed of approximately 0.3-0.4 mm per day - this ensures the growth of the hair, or rather, its shaft.

• The cuticle is the protective shell of the hair shaft.

The hair shaft (like the epidermis) has a layered structure. The outer layer, the cuticle, consists of 6–10 layers of overlapping keratin scales, laid like tiles (Fig. I-1-3). The scales are oblong (0.2–0.4 μm thick, about 0.3 μm wide, up to 100 μm long) and are held 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 by lipids (the lipid barrier of the skin).

• The cuticle is the strongest part of the hair, protecting its interior.

As the hair grows, the cuticle is damaged and gradually destroyed, exposing the cortex. In places where the cuticle is destroyed, increased evaporation of water occurs: the hair shaft begins to lose moisture, becomes brittle, and an electrostatic charge accumulates on it. All this leads to the fact that the aesthetic properties of the hair deteriorate - they tangle, break, become dull, and are difficult to style. A major
role in the destruction of the cuticle is played by aggressive external factors - temperature, UV radiation, combing, chemical compounds (including those that get on the hair as part of hair products).

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

Under the cuticle is a cortex layer, made up of longitudinal rows of keratinized cells. This layer provides the hair shaft with flexibility and strength. The matrix fiber of the cortex is protein in nature and is rich in cysteine, a sulfur-containing amino acid. Disulfide bonds formed during keratinization give the hair shaft its characteristic shape. The number and location of these bonds are determined genetically, so to change the shape of the hair, it is necessary to first destroy the disulfide bonds and then restore them in a new sequence (see Chemical curling and hair straightening).

• Medula - the heat-insulating properties of hair.

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

Hair barrier system

Human hair, like the epidermis, has its own barrier system that protects it from adverse external influences. The 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 lipids of the 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. Covalently binding with the protein matrix, they form a complex of cellular membranes (CMC), which not only ensures the adhesion of cuticular and cortical cells, but also serves as a barrier to the diffusion of various substances into the hair.

The cell membrane complex is located between the cells of the cuticular and cortical layers. Electron diffraction patterns of a hair cross-section show that the cells are 25–30 nm apart, with a denser layer of approximately 15 nm thick clearly visible in the center, with two less dense layers on either side that directly contact the cell boundaries. The combination of the intercellular substance and the outer cell membrane is called the cell membrane complex. The CMC forms an extensive network along the fiber, ensuring the adhesion of the cells to each other.

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

Interesting fact: visible light destroys the CMC much more than UVA and UVB. The pigment eumelanin protects hair lipids from photochemical destruction: the lipids of light hair are destroyed faster than those of black hair.

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