۱۳۸۹ مرداد ۲۵, دوشنبه
۱۳۸۹ مرداد ۲۴, یکشنبه
About Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. The human body, apart from its 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.
Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class.[1] Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods such as insects and spiders are actually insect bristles, composed of a polysaccharide called chitin. There are varieties of dogs, cats, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life.
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 regrow skin after a wound;[2] and 2) the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft can be divided roughly into three zones. Starting from the outside: 1) the cuticle which consists of several layers of flat, thin cells laid out like roof shingles, 2) the cortex which contain 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.
Construction of the root and strand
Hair Follicle Structure
Hair is mainly composed of the protein keratin. Keratin assembles into rope-like intermediate filaments. The structure of these filaments provides strength to the hair shaft.
Hair growth begins under the skin in a 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.[4] The base of the root is called the bulb, which contains the cells that produce the hair shaft.[5] Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the erector pili muscles, which are responsible for causing goose bumps.[6]
Strand of human hair
Each strand of hair is made up of the medulla, cortex, and cuticle.[7] The innermost region, the medulla, is not always present and is an open, unstructured region.[4][8] The highly structured and organized cortex, or middle layer of the hair, is the primary source of mechanical strength, 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.[4] 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.[7]
The diameter of human hair varies from 17 to 180 µm (0.00067 to 0.0071 in), depending on the color. Flaxen hair is the thinnest and black hair is the thickest.[9]
Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class.[1] Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods such as insects and spiders are actually insect bristles, composed of a polysaccharide called chitin. There are varieties of dogs, cats, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life.
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 regrow skin after a wound;[2] and 2) the shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft can be divided roughly into three zones. Starting from the outside: 1) the cuticle which consists of several layers of flat, thin cells laid out like roof shingles, 2) the cortex which contain 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.
Construction of the root and strand
Hair Follicle Structure
Hair is mainly composed of the protein keratin. Keratin assembles into rope-like intermediate filaments. The structure of these filaments provides strength to the hair shaft.
Hair growth begins under the skin in a 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.[4] The base of the root is called the bulb, which contains the cells that produce the hair shaft.[5] Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the erector pili muscles, which are responsible for causing goose bumps.[6]
Strand of human hair
Each strand of hair is made up of the medulla, cortex, and cuticle.[7] The innermost region, the medulla, is not always present and is an open, unstructured region.[4][8] The highly structured and organized cortex, or middle layer of the hair, is the primary source of mechanical strength, 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.[4] 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.[7]
The diameter of human hair varies from 17 to 180 µm (0.00067 to 0.0071 in), depending on the color. Flaxen hair is the thinnest and black hair is the thickest.[9]
اشتراک در:
نظرات (Atom)