Phases of hair growth

Human hair goes through three phases of development, smoothly transitioning from one to another: anagen (growth phase), catagen (regressive changes phase) and telogen (rest phase). The duration of each phase depends on a whole range of features: localization, hair length, gender, age, racial and genetic characteristics. Anagen lasts from 2 to 5 years, the average duration of this phase is given as 1000 days, however, even this is not based on subtle scientific observations. Catagen is a process that takes relatively little time - for hair on the scalp it takes 2-3 weeks. The duration of telogen is about 100 days.

The anagen phase is characterized by the elongation of the follicle, activation of the papilla, cambial elements and melanocytes of the bulb, as well as the growth of the internal sheath and root of the hair. The proliferation and differentiation of the cambial cells of the bulb is impossible without the stimulating effect of the papilla. Various researchers have conducted experiments on transplanting the hair papilla in mammals. It turned out that papilla transplantation induces hair growth even in atypical places (finger pads, mucous membrane, etc.).

The catagen phase is characterized by the cessation of division of the bulb cambial cells, the disappearance of the processes of melanocytes, thickening of the terminal part of the hair with the formation of a "hair flask", destruction of the internal sheath and shortening of the follicle. The catagen phase is characterized by apoptosis processes - a biological mechanism characterizing the death of a cell under normal conditions and the absence of any inflammatory process.

The telogen phase is characterized by the fact that the hair bulb formed in catagen is retained in a shortened follicle, and the proliferation and differentiation of epithelial cells ceases. The hair bulb falls out only at the beginning of anagen. This phenomenon is characterized as an active biological process that can trigger the anagen phase. In general, the identified changes in the expression of antigens on the epithelial cells of the hair follicle, occurring in parallel with changes in the matrix of the hair papilla, may indicate the interaction of various dermal and epidermal factors during the hair growth phase.

Features of the structure of the scalp.

The scalp has a number of features, due to which many dermatoses in this localization proceed in a peculiar way. The structural features of this zone should be taken into account when diagnosing a number of diseases, when prescribing various forms and means of external therapy, as well as a number of cosmetic procedures.

In general, the scalp has a normal structure and consists of three layers: the epidermis, dermis and subcutaneous fat.

One of the features of the epithelium in the scalp is a large number of appendages - hair follicles and associated sebaceous glands. Adults have up to 100 thousand follicles on the head out of approximately 2 million on the body surface. The presence of a large number of sebaceous glands determines the fact that the scalp is one of the areas affected by seborrhea. Given the presence of a large amount of hair, such medicinal forms as powder, chatterbox and paste, which contain indifferent powders, should not be prescribed here. Due to the presence of a large number of hair follicles and sebaceous glands, the use of ointments containing petroleum jelly, naphthalene and tar, which can cause blockage of the mouths of hair follicles, follicular hyperkeratosis and, thus, provoke the development of folliculitis, is also not indicated. The most preferable forms are shampoo, solutions (water and alcohol), cream on a hydrophilic basis, gel, aerosol.

As for the skin appendages, it should be emphasized that hair growth is subject to a wide range of endocrine influences due to the presence of receptors for hormones on the cellular elements, in particular, for testosterone (see the section “Androgenic alopecia”).

The sebaceous glands of the scalp secrete a substance that normally consists of triglycerides (60%), esters of fatty acids and fatty alcohols with a long carbon chain (20-25%), and squalene (15%), a substance characteristic only of humans. The sebum secreted by the sebaceous glands is mixed with lipids secreted from epidermocytes - cholesterol and its esters, as well as glycerides. In principle, the composition of sebum and the number of sebaceous glands (400-900/cm2 ⁾ on the scalp and forehead are the same. The difference lies in the rate of secretion. The rate of secretion on the scalp is significantly lower, which is explained by the fact that the sebaceous-hair "reservoir" is 80% filled with the hair root.

The subcutaneous fat tissue on the scalp is poorly developed. Directly beneath it are the muscles of the cranial vault, characterized by a special structure. Almost the entire cranial vault is covered by a thin epicranial muscle, which has an extensive tendinous part in the form of a tendinous helmet, or epicranial aponeurosis, and a muscular part that breaks down into three separate muscle bellies (frontal, occipital and lateral). Being loosely connected to the periosteum of the skull bones, the epicranial aponeurosis closely grows together with the scalp, so it can move together with it under the influence of contraction of the frontal and occipital bellies. If the epicranial aponeurosis is fixed by the occipital belly of the muscle, contraction of the frontal belly raises the eyebrow upward, making it arched, and forms transverse folds on the forehead. This connection between the muscles of the cranial vault and the facial muscles explains why it is customary to begin a classic scalp massage with a massage in the forehead and eyebrow area.

The blood supply to the scalp comes from the posterior (occipital, posterior auricular arteries) and terminal branches (superficial temporal artery, parietal, frontal and maxillary arteries) of the external carotid artery. There are anastomoses between the above-mentioned arteries, a number of the listed vessels supply blood to the inner and middle ear, the dura mater, the organ of vision and other structures located in close proximity to the scalp. Venous outflow is carried out through the system of the external and internal jugular veins, which also provide outflow from nearby vital organs and structures.

The arteries that directly supply the scalp originate from a plexus located in the subcutaneous fat, parallel to the skin surface. They have a tortuous course and give branches for hair follicles, sweat and sebaceous glands. The reticular layer contains numerous arteriovenous anastomoses, the capillaries are insignificant, they are mainly associated with hair follicles and glands. The superficial capillary network, which is important for the nutrition of the epidermis and thermoregulation, is located under the epidermis, while the capillary plexuses surround the hair follicles, sweat and sebaceous glands.

Lymphatic drainage is carried out through the lymphatic vessels into the occipital, mastoid, parotid, facial, submandibular, and submental lymph nodes. Consequently, any inflammatory and neoplastic processes on the scalp may result in enlarged lymph nodes being detected in the areas listed. Lymphatic drainage disorder and lymphostasis development in the scalp due to compression or blockage of lymphatic vessels may occur in malignant tumors (e.g., lacrimal gland, visual organ, chiasm, etc.). Lymphostasis leads to impaired microcirculation in the dermis and subsequent destruction of elastic fibers, which in turn leads to the development of a focus of aseptic inflammation and the formation of secondary fibrosis, which clinically manifests itself as dermatosclerosis. The result of this process may be the formation of scleroderma-like lesions on the skin of the scalp and face.

Both sensory and motor fibers participate in the innervation of the scalp. The scalp is innervated by various cranial nerves (the 1st branch of the trigeminal nerve, the facial nerve), as well as the spinal roots of the first, second and third thoracic vertebrae, which form the large and small occipital nerves. In addition, the vagus nerve participates in the innervation. These features should always be taken into account when performing various procedures in this localization, since any manipulations can lead to an effect on the vegetative and sensory centers of the brain, which participate in ensuring the trophic functions of both the scalp and the facial skin.

It should also be emphasized that the scalp is an area that is exposed to insolation, especially in the case of baldness. The consequence of massive exposure of the scalp to ultraviolet rays A and B is simple dermatitis and various photoreactions. Repeated minor effects on this area cause the development of not only photoreactions, but also lupus erythematosus, precancerous conditions and diseases (chronic actinic dermatitis, actinic keratosis, etc.), basalioma, squamous cell skin cancer, melanoma with predominant localization on the skin of the forehead, parietal and temporal areas.

In conclusion, it is necessary to recall that the scalp is also an area of constant use of various cosmetic products (dyes, solutions for chemical perms, shampoos, soaps, varnishes, foams, hair styling gels, etc.). The consequence of this may be the development of allergic dermatitis caused by the effects of facultative (chemical) irritants included in the composition of the listed products. With the combined action of an allergen and ultraviolet rays, photodermatitis is also possible. In some cases, failure to comply with the rules for the use of various dyes or components for chemical perms (concentrated acids, alkalis, ammonia, etc.), intensive rubbing of irritating liquids (tincture of red pepper, etc.) may cause the effect of obligatory factors on the skin of the scalp and the development of simple dermatitis. In addition, when using comedogenic hair care products, there is a risk of acne appearing along the hairline (so-called “pomade acne”) due to blockage of the hair follicle openings and the subsequent development of follicular hyperkeratosis.

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