Hair Growth Phases

, medical expert
Last reviewed: 20.11.2021

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

The anagen phase is characterized by lengthening of the follicle, activation of the papilla, cambial elements and melanocytes of the bulb, as well as growth of the inner vagina and the root of the hair. Proliferation and differentiation of cambial cells of the bulb is impossible without stimulating influence from the papilla of the hair. Various researchers conducted experiments on transplanting the papilla in mammals. It turned out that transplantation of the papilla induces hair growth even in non-typical places (fingertips, mucous membrane, etc.).

The phase of the catagen is characterized by the cessation of the division of the bulb's cambial cells, the disappearance of the processes in melanocytes, the thickening of the terminal part of the hair with the formation of a "hair bulb", the destruction of the inner vagina and the shortening of the follicle. For the catagen phase apoptosis processes are characteristic, a biological mechanism that characterizes cell death in normal conditions and the absence of any inflammatory process.

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

Features of the skin structure of the scalp.

Skin of the scalp has a number of features, due to which many dermatoses in this localization occur in a peculiar way. The peculiarities of the structure of this zone should be taken into account when diagnosing a number of diseases, with the appointment of 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: epidermis, dermis and subcutaneous fat.

One of the features of the epithelium in the scalp is a large number of appendages - the hair follicles and associated sebaceous glands. In adults, the head counts up to 100,000 follicles out of about 2 million on the surface of the body. The presence of a large number of sebaceous glands determines the fact that the scalp is one of the zones that is affected by seborrhea. Given the presence of a large amount of hair, do not prescribe here such dosage forms as powder, mallet and paste, which have indifferent powders in their composition. 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 are capable of causing blockage of the mouths of the hair follicles, follicular hyperkeratosis, and thereby provoke the development of folliculitis, is not shown. Most preferable are the forms of shampoo, solutions (aqueous and alcoholic), hydrophilic base cream, gel, aerosol.

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

Sebaceous glands of the skin of the scalp secrete a secret that normally consists of triglycerides (60%), fatty acid esters and fatty alcohols having a long carbon chain (20-25%) and squalene (15%), a substance characteristic only for rights. The sebum secreted by the sebaceous glands is mixed with the lipids released from the epidermocytes - cholesterol and its esters, as well as glycerides. In principle, the composition of sebum and the number of sebaceous glands (400-900 / cm 2 ) on the skin of the scalp and forehead are the same. The difference is in the rate of secretion. The rate of secretion on the scalp is much lower, which is explained by the filling of the sebaceous-hair "reservoir" with 80% of the hair root.

Subcutaneous fatty tissue on the scalp is poorly developed. Immediately below it are muscles of the cranial vault, characterized by a special structure. Almost the entire cranial vault is covered with a thin supracranial muscle, which has an extensive tendon part in the form of a tendon helmet, or supracranial aponeurosis, and muscular, disintegrating into three separate muscular abdomen (frontal, occipital and lateral). Being loosely associated with the periosteum of the bones of the skull, the supracranial aponeurosis fuses closely with the scalp, so it can move with it under the influence of contraction of the frontal and occipital abdomens. If the supracranial aponeurosis is fixed by the occipital abdomen of the muscle, the contraction of the frontal abdomen raises the eyebrow upward, making it arcuate, and forms transverse folds on the forehead. This relationship between the muscles of the cranial vault and the facial muscles of the face explains why the classical massage of the scalp is usually started with a massage in the forehead and overhead regions.

Blood supply of the scalp originates from the posterior (occipital, posterior ear artery) and terminal branches (superficial temporal artery, parietal, frontal and maxillary arteries) of the external carotid artery. Between these arteries there are anastomoses, a number of these 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 by the system of external and internal jugular veins, which also provide an outflow from nearby vital organs and structures.

Arteries, directly supplying the skin of the scalp, depart from the plexus located in the subcutaneous fat, parallel to the surface of the skin. They have a tortuous course and give branches for hair follicles, sweat and sebaceous glands. In the mesh layer there are numerous arterio-venous anastomoses, capillaries are insignificant, they are mainly associated with hair follicles and glands. The superficial capillary network, which is important in the nutrition of the epidermis and thermoregulation, is located under the epidermis, with capillary plexuses surrounding the hair follicles, sweat and sebaceous glands.

The lymphatic drainage is carried out through the lymphatic vessels to the occipital, mastoid, parotid, facial, submandibular, and chin lymph nodes. Therefore, with any inflammatory and neoplastic processes on the scalp, it is possible to detect enlarged lymph nodes in the areas listed. Disturbance of lymphopathy and development of lymphostasis in the scalp due to compression or blockage of lymph vessels is possible with malignant tumors (eg, lacrimal gland, organ of vision, chiasma, etc.). The state of lymphostasis leads to a violation of microcirculation in the dermis and subsequent destruction of the elastic fibers, which in turn leads to the development of a foci of aseptic inflammation and the formation of secondary fibrosis, which is clinically manifested as dermatosclerosis. The result of this process can be the formation of scleroderm-like foci on the skin of the scalp and face.

In the innervation of the skin of the scalp, both sensory and motor fibers participate. The skin of the scalp is innervated by various cranial nerves (the first branch of the trigeminal nerve, the facial nerve), as well as the spinal roots of the first, second and third thoracic vertebrae, from which large and small occipital nerves form. In addition, the vagus nerve is involved in the innervation. These features should always be taken into account when carrying out various procedures in this localization, since any manipulation can lead to an impact on the autonomic and sensitive centers of the brain, which take part in providing trophic functions both of the scalp skin and of the facial skin.

It should also be emphasized that the scalp is an area that is exposed to insolation, especially when it is alopecia. The result of a massive effect on the skin of the scalp of ultraviolet rays A and B is simple dermatitis and various photoreactions. Repeated minor effects on this zone cause the development not only of photoreactions, but also of lupus erythematosus, precancerous conditions and diseases (chronic actinic dermatitis, actinic keratosis, etc.), basal cell carcinoma, squamous cell carcinoma, melanoma with predominant localization on the forehead skin, parietal and temporal areas.

In conclusion, it must be recalled that the scalp is also a zone of permanent application of various cosmetic preparations (dyes, solutions for chemical wave, shampoos, soaps, varnishes, foams, hair gels, etc.). A consequence of this may be the development of allergic dermatitis caused by the effects of facultative (chemical) stimuli, which are part of the listed preparations. With the combined action of the allergen and ultraviolet rays, photodermatitis is also possible. In a number of cases, if the rules for the use of various dyes or components for chemical perm are not observed (concentrated acids, alkalis, ammonia, etc.). Intensive rubbing of irritating liquids (tincture of red pepper, etc.), the obligatory factors can affect the skin of the scalp and the appearance of simple dermatitis. In addition, with the use of comedogenic preparations for hair care, there is a risk of acne on the border of hair growth (the so-called "pomade acne") due to occlusion of the mouths of the hair follicles and the subsequent development of follicular hyperkeratosis.

trusted-source[1], [2], [3], [4]

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