Hair – simple or complicated?

Simple, but only at first glance. When it comes down to it, however, it is quite complicated.

For example, it is no secret that hairs live or die. They grow, and then fall out, and in their place (under normal conditions, or course) grow similar, new hairs. Why is there a mini-construction plant for the creation of hair – the hair follicle in which phases cyclically replace one another? First comes the intensive synthesis of cells, then this process slows and completely terminates. Why did nature create such a highly organized, energy expending system? Unfortunately, to this day scientists are not able to provide a definite answer to these questions, although many hypotheses have already been formed. Maybe these questions will be answered in the near future, since a majority of contemporary theoretical works in the fields of dermatology and trichology are dedicated specifically to the study of biological clocks within the hair follicle and the process of cell differentiation.

Today, scientific studies have reached such heights that it is possible to unlock the genetic code responsible for baldness. There are developments concerning the recoding of genetic information responsible for the amount of hair on one’s head.

However, to understand all this, it is necessary to begin with at least a minimal knowledge of the building, constitution, and phases of hair growth.

You have probably already heard that hair is a nonliving substance, capable of self-regeneration. Living cells, which multiply with great speed, exist in the root of the hair, which is buried deep in the dermis.

The hair follicle is the root of the hair, with its surrounding tissues, which form formations such as exterior and interior root sheets, and the hair-gland complex (sebaceous and apocrine sweat glands, the arrector pili muscle lifting the hair, blood vessels, and nerve endings). We are born into this world with a defined number of such follicles. The size of which is genetically programmed, and unfortunately unalterable, although such advances may become possible in the near future. Scientists will be able to “reprogram” this hereditary information; the primary steps have already been taken.

At the base of the follicle in the dermis exists the follicular papilla – a connective tissue formation containing blood vessels. This provides nutrition and growth activity for the hair follicle.

Each hair follicle has its own nerve system and musculature. Due to muscles and nerve endings, the hair follicle acquires tactile sensitivity, which allows it to complete barely perceptible movements. When a corresponding muscle, the muscle lifting the hair, is contracted from fear or the influence of cold, the hair is lifted and the skin is tightened, forming “goose bumps.” Blood vessels surrounding the hair follicle and the follicular papilla provide them with all nutrients necessary for cell multiplication and hair growth. Another remarkable hair characteristic: the speed of hair cell division in the human body is second only to the speed of cell proliferation in bone marrow.

Each hair follicle is an independent formation, with its own growth cycle, and these cycles are not synchronized between different follicles. Otherwise, we would lose all our hair at the same time, while in reality this process takes place gradually and unnoticeably.

Hair is comprised mostly of keratin, a protein composed of amino acids. Some of these amino acids (cistine, meteonin) contain Sulfur atoms (to these facts we will return when we examine the composition of hair).

Approximate chemical composition of a healthy hair is as follows:

15% water

6% lipid

1% pigment

78% protein

If hair has been exposed to chemical or physical treatments, or if hair diseases have been found, its constitution may be altered. For example, with frequent coloring or chemical perms, improperly chosen hair care products, or overuse of thermal hair styling tools, hair can lose large amounts of moisture.

In this case, it is necessary to choose quality treatments for the care of hair which replenish a normal level of moisture.

Each hair consists of a root – the part of the hair which is located deep within the skin, and shaft – the part visible on the surface (that which we call our “hair”).

In the hair shaft can be found three concentric layers:

1. Extermal layer, or the cuticle. It is also called outer or scale.
2. The cortex.
3. The medullar layer – the central, inner substance.

The medullar substance of hair

This is the central part of the hair shaft, which is not found in all types of human hair. For example, in baby hair the medulla is absent. The medullar substance is replaced by air bubbles, due to which the hair attains some degree of thermal conduction. The medulla plays no role in changing the chemical or physical qualities of the hair.

The hair cuticle

The hair cuticle performs a protective, barrier function. It forms in 6-10 overlapping layers of transparent keratin plates, interconnected by many cross bonds and lipid layers. The cuticle interfaces mechanical and chemical influences on the hair. An undamaged cuticle reflects light well. The hair shines and does not break.

The constitution of hair:

Cortex

Medulla

Cuticle

Cuticle of the follicular papilla

Internal root papilla

External root papilla

Matrix

The papilla of the hair follicle

Melanocytes

Hyaline membrane

Connective tissue

Blood vessels

Core substance (cortex)

Cortex – the main substance of hair (constitutes 70-85% or hair volume) which is made up of thousands of keratin strands. These fibers are twisted between themselves and connected by strong cross links.

A large number of interconnected amino acids forms a polypeptide chain. The structure of the hair stem is similar to a rope or an electrical cable. Polypeptide chains bond between themselves, forming threads which in turn intertwine with each other to make a super-spiral structure – protofibers – and next microfibers of hair. Joined microfibers form the biggest fibers – macrofibers. Intertwining around each other, macrofibers form the major fibers of the core layer.

Single polypeptide chain

7-9 polypeptide chains twisted into a spiral form a protofiber

7-9 protofibers = 1 microfiber

7-9 microfiber = 1 macrofiber

7-9 macrofiber = 1 cortical fiber

Cross links

Long polypeptide chains located parallel to each other in the surface layer of the hair bond with each other, forming cross bridges. If it weren’t for these covalent bonds between certain amino acid remains of neighboring chains, these chains would separate and the fiber would break apart. It is specifically these cross bonds that give keratin its unique properties: firmness and elasticity.

There are three main types of cross bonds:

Disulfite bonds

These covalent chemical bonds are the most sturdy. They occur between Sulfur-containing amino acids, cictine and meteonine, which are located on neighboring polypeptide chains (Two free neighboring sulfur-hydril groups, oxidizing form a disulfite bridge: -SH + HS- = -S-S- + H2). Disulfite bonds are primarily responsible for the natural strength of hair. The principle of chemical hair perming is based upon the breakage and later reformation of these bonds.

Hydrogen bonds (or H444H - bonds)

These bonds are much weaker than disulfte bonds, but are much greater in number. They form due to the mutual attraction between hydrogen atoms which exist on neighboring polypeptide chains. These bonds play an important role in providing the elasticity of hair.

Salt bonds

These are fairly strong bonds which occur between oppositely positioned, positively (acidic) and negatively (basic) charged free groups of amino acid remains which make up the constitution of polypeptide chains.

Structure and growth of hair

Hair grows approximately 1-2 cm per month, and the growth of new hair begins from the follicular papilla, which is located at the base of the hair bulb. Cells divide and multiply within the core substance (which comprises the middle part of the bulb) – this zone, which lies next to the papilla, is called the matrix. As they move loser to the scalp’s skin, follicular keratocytes gradually flatten and coarsen, filling with firm keratin (cells keratinize) turning into anuclear structures. Among cells in the hair bulb also exist melanocytes which comprise the natural color of hair. At the beginning of the hair follicle opens a sebaceous gland duct, containing skin oil, a greasy substance which is secreted onto the surface of the skin on the hairy part of the head. Skin oil, along with shed corneocytes of the epidermis and norma flora, serve as the primary defense mechanism of the skin. Moreover, skin oils hydrate the hair, giving it elasticity, smoothness, and to some degree, water resistance.

The life cycle of hair

The life cycle of hair consists of three stages

The length of the life cycle of an individual hair varies from 2 to 5 years

Each hair follicle is genetically programmed to produce about 25-27 hairs

Each hair grows according to its own “individual plan,” and therefore individual hairs are at varying stages of their life cycles at different times, specifically:

85% of hair is the active growth phase (anagen), 1% in the “rest” phase (catagen), and 14% in the hair loss phase (telogen).

Anagen phase – the continuous division of cells in the hair follicle’s matrix, as a result of which new cells are pushed to the surface of the skin layer of the hairy part lof the head. This period of active growth continues for 2-5 years.

Catagen phase – the division of cells in the matrix is slowed and stopped. The hair follicle enters “sleep,” and the hair bulb gradually detaches from the follicular papilla. This phase is very short – about 3-4 weeks.

Telogen phase – cell renewal is stopped for about 3 months (time during which the bond between a newly-synthesized hair bulb and the follicular papilla is restored and the new hair enters the anagen phase). Completely separated from the dermal papilla, the telogen bulb attains a lengthened shape and starts to move toward the surface of the skin layer on the hairy part of the head. During the telagen phase, the new hair begins to grow, and the old hair falls out.

On the hairy part of the head of every person are distributed on average from 100 to 150 thousand hair follicles, in which hairs are formed, grow, and from which they eventually fall out. Knowing the percentile proportions of hair in different phases, it is possible to calculate the extent of normal hair loss. Normally, during the course of a day we lose on average 70-100 hairs.