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Structure and chemical composition of hair

Collaborazione scientifica del prof. Marco Toscani e del dott. Pasquale Fino, Cattedra di Chirurgia Plastica, Ricostruttiva ed Estetica, Policlinico Umberto I – Università “Sapienza” di Roma.

The hair on our bodies has a particular structure and is divided into thin and thick hair. Thin hair, also called lanugo or vellus, is located on all skin surfaces except for the palms of the hands and the soles of the feet. Thick hair, also called terminal hair, is dark and located only in some areas such as the scalp, the armpits, the pubic area, the beard area in the case of men, etc…

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.

38-1-3struttura e chimica del capello

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.
Lipids present in the hair’s chemical structure are made up of triglycerides, waxes, phospholipids, cholesterol, squalene and free fatty acids. Quantifying these is extremely complex given that they are mostly derived from sebum.
Minerals and trace elements of the hair’s chemical structure are an essential component of the protein-enzymatic systems. These are iron, magnesium, zinc, copper and lead.

Protein and/or electrolyte deficiencies in the hair’s chemical structure may create hair problems and will become evident under the microscope due to the presence of a thin shaft associated with small bulbs. If the hair is structurally thin the bulbs will instead appear to be a normal size.
The pigments consist of melanin (colored substances), present in the hair in a diffused or granular form. These are not water soluble but they are soluble in strong acids and the color may be removed with hydrogen peroxide.
Melanocytes, using tyrosine (an amino acid that synthesizes protein) as a precursor, synthesize two main types of melanin: eumelanin, dark and present in black hair, and pheomelanin, lighter and present in golden, blonde and red hair.

The main chemical elements present in hair are composed of carbon (45%), oxygen (28%), nitrogen (15%), hydrogen (6.7%) and sulphur (5.3%).

Moreover, various trace elements are present (these can be found by performing a trace mineral analysis): Ca, Mg, Sr, B, Al, Si, Na, K, Zn, Cu, Mn, Fe, Ag, Au, Hg, As,Pb, Sd, Ti, W, Mo, I, P, Se. It is important to remember that the percentages of trace elements present in hair are subjective and vary in each individual.
Cystine is the main amino acid present in keratin (17.5%), followed by serine (11.7%) and glutamic acid (11.1%). Threonine, arginine and glycine are instead present in smaller percentages (approximately 6%).
80% of the weight of hair is due to the presence of protein (amino acid polymers), among which the main one is keratin, composed of 18 amino acids.

The main amino acids that make up keratin are: cystine (17.5%), serine (11.7%) glutamic acid (11.1%), threonine (6.9%), glycine (6.5%) and lastly arginine (5.6%).

Sebaceous glands

The sebaceous glands are glands that secrete sebum, an oily, acidic substance with a pH of 3.5. They are connected laterally to the hair follicle. Sebaceous glands are found with a density of approximately 100/cm2 throughout all areas of the skin. In the human body, they are located across the entire surface of the skin, except for the palms of the hands and the soles of the feet.

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Seborrheic dermatitis

Seborrheic dermatitis is a very common scalp condition characterized by the presence of yellow and oily flakes on the skin.

It is associated with erythema, small scaly, and intense itching.
Seborrheic dermatitis is a well-known condition but is still not easily identifiable. Many men and women starting from puberty have oily, greasy, shiny and thickened skin with large hair follicles, especially noticeable around the nose and mouth, forehead, torso, scalp (affected by so-called oily dandruff).

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Split ends

The phenomenon of split ends, also called trichoptilosis, is a hair problem that depends on an alteration of the hair shaft with concurrent modification and deterioration of the structure, form and physiology of the hair.

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Hair loss (telogen defluvium)

With the term Telogen Defluvium we mean a modest, not excessive, loss of hair in the telogen phase, but which tends to often be irreversible, with the precise characteristics of hair in decay or involution. The hair that falls out is usually short and fine, with bulbs that are undeveloped and reduced in size.

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Hair follicles and bulbs

A hair follicle is a formation of epidermal origin, anchored in the dermis and consisting of a hair and its sheath.
The formation of hair follicles occurs early. They begin to appear as early as the second month of gestation. These follicles will continue to form for the entire duration of the pregnancy. At birth, the baby will indeed have the entire supply of follicles with which it was genetically equipped and from this moment on, no others will form.

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Dandruff or pityriasis

Dandruff is a scalp condition that usually arises between the ages of 10 and 25. It may improve between the ages of 45 and 55 or may continue during old age.
Its cause is a result of an accelerated turnover of epidermal cells, which following an increase in migration speed, are unable to reach complete maturity before detaching. Whitish-yellow flakes (masses of corneum cells) form and detach, gathering in patches or often spreading evenly across the scalp.

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