Hair Growth

Hair is a filamentous biomaterial that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important biomaterial primarily composed of protein, notably keratin. Attitudes towards hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person's personal beliefs or social position, such as their age, gender, or religion.

The word "hair" often refers to two distinct structures:

1.       The part beneath the skin called the hair follicle or when pulled from the skin, called the bulb. This organ is located in the dermis and maintains stem cells which not only re-grow the hair after it falls out, but also are recruited to re grow skin after a wound;

2.       The shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones.

HAIR STRUCTURE

 

Hair fibers have a structure consisting of several layers. Starting from the outside:

1.       The cuticle which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles,

2.       The cortex, which contains the keratin bundles in cell structures that remain roughly rod-like; and in some cases,

3.       The medulla, a disorganized and open area at the fiber's center.

DIFFERENT LAYERS OF HAIR

 

Description......

Each strand of hair is made up of the medulla, cortex, and cuticle. The innermost region, the medulla, is not always present and is an open, unstructured region. The highly structural and organized cortex, or middle layer of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. Asian hair typically has a round fiber and is quite straight. Oval and irregularly shaped fibers are generally more wavy or even curly. The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water. The diameter of human hair varies from 17 to 180 micrometers (0.00067 to 0.0071 in). There are two million small, tubular glands and sweat glands that produce watery fluids that cool the body by evaporation. The glands at the opening of the hair produce a fatty secretion that lubricates the hair and prevents it from dying.

Hair growth begins inside the hair follicle. The only "living" portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered "dead". The base of the root is called the bulb, which contains the cells that produce the hair shaft. Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the arrector pili muscles, which are responsible for causing hairs to stand up. In humans with little body hair, the effect results in goose bumps.

Hair Natural color:

All natural hair colors are the result of two types of hair pigment. Both of these pigments are melanin types, produced inside the hair follicle and packed into granules found in the fibers. Eumelanin is the dominant pigment in dark-blond, brown hair, and black hair, while pheomelanin is dominant in red hair. Blond hair is the result of having little pigmentation in the hair strand. Gray hair occurs when melanin production decreases or stops, while poliosis, typically in spots is hair (and often the skin to which the hair is attached to) that never possessed melanin at all in the first place, or ceased for natural genetic reasons, generally in the first years of life.

Human hair growth:

HAIR GROWTH CYCLE

STAGES IN HAIR GROWTH DEVELOPMENT

Hair grows everywhere on the external body except for mucus membranes and glabrous skin, suchas that found on the palms of the hands, soles of the feet, and on the lips.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each has specific characteristics that determine the length of the hair. All three occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.

The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection.

Human hair types:

There are different types of hair seen in human beings but biologically, there are only 3 main types; that are naturally formed.

HAIR TYPES

1.       Lanugo hair: Many times we see that new born babies are covered with soft, downy hair, but mostly it is shed within few days of birth, this type of hair is called lanugo, its purpose is to keep baby warm while developing in womb. Lanugo hair is like fur that covers the human baby completely inside the womb to keep the baby’s skin healthy, hydrated and warm. but it is replaced by vellus hair, after the fetus is born, or it reduces with the age. Because in the womb, fetuses have distinct lack of fat to keep themselves warm, so they grow lanugo hair to help them staying warm. So everyone has lanugo hair- atleast until they are born. This hair usually disappears within few days or weeks after the baby is born, and is replaced with vellus hair. Not only humans, but also mammals like elephants and seals too, are born with a coating of such lanugo hair.

2.       Vellus hair: When we pay close attention to our outer skin, we find some colorless fine hair less than few centimeter long that shine in the light. It is called vallus hair that is found all over body since birth. Distinctly not so visible because Vellus hair is short, fine, colorless hair. Finding all over the body, this type of hair can convert into terminal hair after puberty. It grows mostly all over the surfaces of human body, and is mostly light colored, but also can be dark or darker depending on one’s complexion. Vallus hair is mostly found on the face and the back of the neck. This type of natural hair, associated with women and young children, is fairly unnoticeable. After puberty, men tend to develop more intermediate or terminal hair that obscure the vallus hair. Because men’s thick body hair hide the vallus hair, so it is more obvious among children and women. Vallus hair is also called “peach fuzz” and normally is few millimeters long. Its function is not as the protective layer but to regulate the temperature of body.

3.       Terminal hair: Terminal hair is the type of hair that we mostly refer in our everyday life as hair, that grows on head and pubic regions, And almost on some other parts of human body as well, but mostly in men, like face, legs, arms. This is the hair, we are probably most concerned with, A hormone called androgen controls this type of body hair, so it is also known as androgenic hair. It is longer, thicker and coarser than vellus or lanugo hair, and is connected to sebaceous glands that produce oil. In children terminal hair is found only on scalp because it does not grow on other body of children till they reach puberty age. But even though, terminal hair is most found on the scalp but this term covers nearly everything, we think of as body hair, as such armpit, pubic, facial or other body hair also. it grows on the dermis and is visible on the various body parts, and color, texture and thickness of such hair mostly depends on the ethnicities.