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Hair growth

Hair grows and is eventually shed and replaced by new hair. . The first stage of this cycle is called the growing phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. In scalp hair. the length of this phase is measured in years, typically from 2 to 7 years. Finally, during the resting phase, the hair follicle is at rest and no new growth occurs. At the end of this phase, which lasts about 2 to 4 months, another growth phase begins. The cells in the hair matrix then produce a new hair, which pushes the old hair out as the growth cycle repeats itself. Hair typically grows at the rate of 0.3 mm per day during the growing phase. On average, 50 hairs are lost and replaced per day. Hair loss occurs if there is more hair shed than what is replaced and can happen due to hormonal or dietary changes. Shorter hairs such as axillary (armpit) hair have shorter growing cycles, thus the hair is pushed out after only a short growth phase, limiting the time the hair can grow before it is replaced. Bald people have such a short hair growth cycle that the hair is shed even before it is visible at the skin's surface. A compulsive habit called trichotillomania compels a person to pull out their hair to relieve stress can cause the appearance of hair loss .Hair loss can also result from the aging process, or the influence of hormones.

Hair color and texture

Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. Different hair color results from differences in the type of melanin, which is genetically determined. As a person ages, the melanin production decreases, and hair tends to lose its color and becomes gray and/or white. "Our hair texture can range anywhere from pin-straight to extremely curly. Follicles that are round in cross-section give rise to straight hair. Those out of which curly hair grows are oval. Very tightly coiled hair is due to the nearly flat, ribbon-like structure of the follicles. This hair texture is very common in people of African ancestry." (http://genetics.thetech.org/)

Nails

The nail bed is a specialized structure of the epidermis that is found at the tips of our fingers and toes. The nail body    (which we refer to as the nail)is formed on the nail bed    , and protects the tips of our fingers and toes as they are the farthest extremities and the parts of the body that experience the maximum mechanical stress ( [link] ). In addition, the nail body forms a back-support for picking up small objects with the fingers. The nail body is composed of densely packed dead keratinocytes. The epidermis in this part of the body has evolved a specialized structure upon which nails can form. The nail body forms at the nail root    , which has a matrix of dividing cells from the stratum basale that enables the nail to grow continuously. The lateral nail fold    overlaps the nail on the sides, helping to anchor the nail body. The nail fold that meets the proximal end of the nail body forms the nail cuticle    , also called the eponychium    . The nail bed is rich in blood vessels, making it appear pink, except at the base, where a thick layer of epithelium over the nail matrix forms a crescent-shaped region called the lunula    (the “little moon”). The area beneath the free edge of the nail, furthest from the cuticle, is called the hyponychium    . It consists of a thickened layer of stratum corneum.

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Source:  OpenStax, Integumentary system. OpenStax CNX. Mar 19, 2015 Download for free at http://legacy.cnx.org/content/col11770/1.1
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