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The color of the hair is determined by the amount of melanin in the hardened cells. This can vary a lot from person to person, and it changes over the course of a lifetime. The amount of melanin typically decreases as people get older, and more air gets trapped inside the hair – it then loses its color and turns white.
The Growth Cycle
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". In humans with little body hair, the effect results in goose bumps. Hair is a keratinous filament growing out of the epidermis. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle.
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Root of the hair
They also play a key role in non-verbal communication by displaying emotions such as sadness, anger, surprise and excitement. In many other mammals, they contain much longer, whisker-like hairs that act as tactile sensors. There are various systems that people use to classify their curl patterns.
Hair Growth
The rate of hair loss may increase noticeably if the hair roots are damaged during the growth phase or if a lot of hairs go into the resting phase at the same time. If no new hair grows and replaces the hair, that part of the skin becomes bald. This type of hair loss is referred to as alopecia – regardless of how large the bald spot is or whether it affects the scalp or body hair. But baldness can also be permanent – one typical example is gradual hair loss in men (male pattern hair loss). Hair serves a variety of functions, including protection, sensory input, thermoregulation, and communication. For example, hair on the head protects the skull from the sun.
This feature is also useful for the detection of the presence of insects or other potentially damaging substances on the skin surface. This is visible in humans as goose bumps and even more obvious in animals, such as when a frightened cat raises its fur. Of course, this is much more obvious in organisms with a heavier coat than most humans, such as dogs and cats. Just as the basal layer of the epidermis forms the layers of epidermis that get pushed to the surface as the dead skin on the surface sheds, the basal cells of the hair bulb divide and push cells outward in the hair root and shaft as the hair grows. The medulla forms the central core of the hair, which is surrounded by the cortex, a layer of compressed, keratinized cells that is covered by an outer layer of very hard, keratinized cells known as the cuticle. These layers are depicted in a longitudinal cross-section of the hair follicle (Figure 2), although not all hair has a medullary layer.
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It is surrounded by the hair follicle (a sheath of skin and connective tissue), which is also connected to a sebaceous gland. Anagen growth is the active phase in which the hair follicle takes on its onion-like shape and works to produce the hair fiber. The anagen phase can be further broken down into proanagen and metanagen phases. Proanagen sees the follicle proliferating hair progenitor cells and begins the differentiation process. The new hair shaft appears on the skin's surface to mark the metanagen phase.
The shaft—the part of the hair we see—is made up of dead cells and consists of three different layers. As a piece of hair grows, it goes through three phases before it sheds and a new one grows. At the end of the growth phase, the hair root separates from the papilla. Then a transitional phase called the catagen phase starts, lasting about two to four weeks.
The Biology, Structure, and Function of Hair
About 10 to 15% of all hairs on the body are in this resting phase at any given time and can remain in this state for a variable amount of time, depending on the location of the hair - from a few weeks for eyelashes to nearly one year for scalp hair. The exact mechanism that controls passage from one phase into the next is not fully known. Hair that is darker, thicker, and more visible to the human eye is called terminal hair.
Texture
Each hair follicle is attached to a tiny muscle (arrector pili) that can make the hair stand up. These nerves sense hair movement and are sensitive to even the slightest draft. Your hair’s appearance is determined partly by the shape of your hair. If your hair has a circular circumference, it will be straight. It explains how it grows, what it’s made of, and how it changes. In the time of Confucius (5th century BCE), the Chinese grew out their hair and often tied it, as a symbol of filial piety.
The glassy membrane is a thick, clear connective tissue sheath covering the hair root, connecting it to the tissue of the dermis. The full strand of hair develops from this group of hardened hair cells. Because new hardened cells keep on attaching to the hair from below, it is gradually pushed up out of the skin. In this way, a single hair on your head grows at a rate of about 1 cm per month. Facial hair, and especially eyelashes, eyebrows and body hair grows at a slower pace.
Eating nutritious foods is one way to improve hair from the inside out. Check with your doctor if you have questions on your hair’s growth and how it might be impacted by your health. As you get older, your hair can change color, texture, and thickness. It can even change some of its location, with too much in some areas and too little in others. The eyelash grows at the edges of the eyelid and protects the eye from dirt. The eyelash is to humans, camels, horses, ostriches etc., what whiskers are to cats; they are used to sense when dirt, dust, or any other potentially harmful object is too close to the eye.[32] The eye reflexively closes as a result of this sensation.
Human hair grows at a rate of about 0.5 inch (13 mm) per month. While humans have developed clothing and other means of keeping warm, the hair found on the head serves primarily as a source of heat insulation and cooling (when sweat evaporates from soaked hair) as well as protection from ultra-violet radiation exposure. Hats and coats are still required while doing outdoor activities in cold weather to prevent frostbite and hypothermia, but the hair on the human body does help to keep the internal temperature regulated. When the body is too cold, the arrector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. These hairs then form a heat-trapping layer above the epidermis. The opposite actions occur when the body is too warm; the arrector muscles make the hair lie flat on the skin which allows heat to leave.
Whether it is straight or curly will depend on the cross-sectional shape of hair. The more oval-shaped the cross-section is, the curlier the hair will be. The most important function of hair in mammals is that of insulating against cold by conserving body heat.
The follicle also contains the germinal matrix, where cells produce new hairs. By week 22 of pregnancy, babies already have all of their hair follicles, or openings in the skin where hair grows. This totals about five million hair follicles, with about one million on the head and 100,000 on the scalp. This is the largest number of hair follicles you will ever have. New hair cells then start to multiply at the base of the “empty” hair follicle to form a new hair, and the growth phase of the hair growth cycle starts all over again.
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