Hair Growth Cycle
Hair growth begins with the activation of the passive hair follicles by the action of hormones and physical stimuli. The hair that starts to form grows for a while. Then, the growth stops when it is exposed to a stimulant. When the growth stops, the inner sheath breaks down and the hair is pulled towards the surface. Then, its connection with the root decreases and shedding occurs. After a while, hair growth starts again and the same process takes place. This process is called the ‘hair growth cycle’. Almond Hair
Hair growth periods are successive. The hair cycle consists of 3 phases: anagen, catagen and telogen. First, the active growth period (anagen phase) occurs and it is followed by a short regression period (catagen phase) and then, the resting period (telogen phase). Almond Hair
The hair that has reached the end of its life is shed and new healthy hair starts to form. As a result of this hair cycle, an average of 100 hair loss per day should be considered normal.
- Hair
Hair develops as lifeless extensions from the follicle occurring as a result of the curved growth of mesenchymal cells towards dermis or subcutaneous tissue during embryogenesis.
Hair follicles are present at different depths in our bodies (e.g. 2–2.5 mm on the eyebrows, 3 mm on the arm, 3–3.5 mm on the armpit, 3–4 mm on the groin, and 2–3 mm on the leg) and consist of 3 main parts: the lower segment, the middle segment, and the upper segment. Almond Hair
The lower segment, which is the most active part of the follicle, is where the hair begins to form and it consists of ‘dermal papilla’, ‘matrix’, and ‘inner and outer root sheaths’.
- Dermal Papilla
Egg-like papillae protruding into the bulbous and surrounded by matrix cells is the place where the first signals that initiate hair growth are sent. These signals, which are carried by cytokines through the Golgi Apparatus, bind to receptors in the respective follicle cells. Cytokines are proteins that positively or negatively affect the division according to the type of target cell. For example, cytokines delivered by the action of androgens bind to the respective receptors causing hair formation. The uptake of receptors by other substances affects the function of the cytokines and thus the relevant cells. Almond Hair
- Matrix
The matrix cells surrounding the papillae, which have a large number of ribosomes and mitochondria in their cytoplasm, divide rapidly and play an important role in hair formation.
- Inner root sheath
The melanin-free and non-keratinized ‘inner root sheath’ cuticle cells consist of the Huxley and Henle Layers.
- Outer root sheath
The function of the outer root sheath is not fully known. It consists of several rows of cells and wraps the follicle like a glove by extending from the lower level of the bulbous to the entrance of the sebaceous canal. The outer root sheath divides and prolongs in the early stages of the anagen phase, but the division stops in the later period.
Layers of hair
- Medulla
Medulla cells, which have an amorphous appearance because they are partially keratinized, develop from the matrix cells surrounding the papillae. There is no medulla in vellus hair and lanugo hair and even sometimes in terminal hairs. Almond Hair
- Cortex
The cortex consists of cells that are formed by matrix cells and that are keratinized as they migrate toward the skin surface. Cortex cells made up of macrofibrils that are tightly coalesced are spindle-shaped cells that are aligned along the hair axis and connected to each other. Macrofibrils with different amounts of melanin granules contribute to the mechanical properties of the hair.
- Cuticle
Some cells in the area above the bulbous extending in a way to cover each other, like fish scales, form a 6-8-row structure consisting of keratinized flat cells, which extends from the tip of the hair to the root, located on the outermost part of the hair – i.e. the cuticle.
Distribution of hairs in the body
There are 5 million hairs in the human body, of which 150,000 are in the head. These hairs are spread all over the body, except palm, inward faces of fingers, foot soles, lips, and sex organs, and are found in different densities in different regions: Almond Hair
Proteins
Proteins in the structure of hair are insoluble in water and resistant to proteolytic enzymes. Keratin is the main structure of the hair. It is formed by keratinocytes, which are composed of 18 amino acids and are located at the base of the dermal papilla. It is also found in nails and skin.
Its structure is rich in cysteine – a sulfurous amino acid that strengthens hair. Keratin, which is insoluble in water, resistant to proteolysis and stable, has disulfide bonds in its structure. Almond Hair
- Hydrogen bonds:
Although very weak, hydrogen bonds are important for the macromolecular structure of keratin.
- Disulfide bonds:
The bonding of two sulfur-containing cysteines with strong sulfur-sulfur bonds during the keratinization process of the hair ensures that the hair is resistant to enzymes and chemical effects. Almond Hair
Disulfide bonds are broken down only by:
* Ultraviolet light
* Boiling for a long time
* Oxidizing/reducing agents
* Strong acidic/basic agents
- Salt bonds:
Strong bonds result from the electrostatic force of attraction and such bonds are affected by acidic and basic substances.
Water
The water content of the hair is affected by physical and cosmetic factors. When the hair is moistened, its weight increases by 12-18%. Humidity of above 80% adversely affects hair health. Almond Hair
Lipids
The lipid content of the hair is of little importance. After puberty, the amount of lipids in the hair increases in both sexes and decreases with age; this decrease is more pronounced in women. Almond Hair
Trace elements can interact with hair in both exogenous and endogenous ways.
As endogenous sources, matrix, connective tissue papillae, sebaceous, eccrine, apocrine glands and surface epidermis are important.
Exogenous sources are environmental factors such as industrial materials and hair cosmetics.
Arsenic, cadmium, chromium, copper, mercury and zinc content of the hair is used to assess the environmental pollution. Several organizations have explained that there are human-specific hair photon activation analyses, like fingerprints. Almond Hair
Elasticity
One of the most important properties of hair is its elasticity. Thanks to this property, it can take different shapes and can be restored after a short time without any damage.
It is the long keratin strands in the structure of the cortex that allow the hair to stretch, change and curl.
The length of the hair can be increased by up to 30% when it is wet. However, hair breakage will occur if more tension is applied.
Permanent chemical applications, such as bleaching and permanent wave, or natural or artificial light sources that interact with chemical substances in the hair damage the cortex and causes the hair to lose its elasticity. Hair that loses its flexibility does not curl.
Static Electricity
You can observe that the hair strands push each other in dry and hot weather as a result of static electricity and the hair lifts up. In order to prevent this, moisturizers are used or additives are added to shampoos to reduce the formation of static electricity and ensure smooth hair. Almond Hair
Humidity Level
The water content of the hair increases in moist environments, the cortex swells and the hair surface temporarily loses its smoothness. Therefore, it takes longer to comb wet hair. Almond Hair
Porosity
Porosity is a term used to describe the measure of moisture retention of hair. This is rated as ‘low’, ‘normal’ and ‘high’ depending on the nature of the cuticle.
- Hair Cycle
Hair growth begins with the activation of the passive hair follicles by the action of hormones and physical stimuli. The hair that starts to form grows for a while. Then, the growth stops when it is exposed to a stimulant. When the growth stops, the inner sheath breaks down and the hair is pulled towards the surface. Then, its connection with the root decreases and shedding occurs. After a while, hair growth starts again and the same process takes place. This process is called the ‘hair growth cycle’.
Hair Cycle Phases:
* Active growth phase (Anagen): It is the period of active growth of the hair. The interim period until the full maturation of the hair is called the proganagen whereas the period in which the hair reaches full maturity, that is, when it is seen on the surface of the skin, is called methanogen.
* Transition phase (Catagen): This is the period when growth and nutrition stop.
* Resting phase (Telogen): It is the period before the hair is shed, it can be called ‘resting phase’. This period can be divided into 2 according to some researchers:
– Shedding phase (exogenous)
– The period when the follicle remains empty (Kenogen) Almond Hair
