Nails
Each finger and toe on the human body has a flat plate commonly called a nail on its uppermost surface. Nails are epithelial cell structures that are composed of a fibrous protein called keratin. The same protein is also the constituent of wool, hair, horn, hoofs, and the quills of feathers.
The chemistry of keratin makes nail very resilient. The keratin protein is made up of large quantities of the amino acid cysteine. Adjacent cysteine molecules are able to link together by virtue of disulfide bonds between sulfur residues that are part of the cysteine structure. The formation of many of these disulfide bonds produces keratin chains that are tightly linked to each other, much like bundles of twigs lashed together. The result is a bundle of great rigidity that is resistant to breakage. When these linked polypeptides are on a flat plane, as in a nail, the plate they form acts like armor.
Each nail is formed from a nail root that is made up of normal layers of skin cells. As the nail grows, a layer of epidermal cells called the nail bed supports it. The nail grows from one end, with the older cells being pushed further along the nail bed. As the end of a nail extends beyond the nail bed, it is removed by filing or trimming, or is worn away.
Nails on the fingers grow at a rate of about 0.004 in. (0.1 mm) per day. At that rate, a nail is completely replaced in about 4-6 months. The nail on the toes grows more slowly and 6-8 months is required to completely replace the toenail. The growth rate of nail can vary with age, the time of year, and with nutrition. A poorly nourished person will exhibit a slower nail growth rate, for example.
As resilient as they are, nails are still subject to damage and disease. Paronychia is a bacterial, viral, or fungal infection of the tissues surrounding the nail. The condition is painful and the nail can begin to form ridges because of underlying pressure. Onychomycosis is a fungal infection of the nail, which can be spread from a site of infection elsewhere in the body. Despite therapy with antifungal agents, it can be difficult to eradicate. Skin conditions like dermatitis and psoriasis can also affect nail. Other ailments of the body's chemistry or function can be manifest as changes in the nail. For example, nail may grow thinner when one has a fever, is in shock, or experiences a myocardial infarction. An exaggerated curve of the nail plate can be a sign of congenital heart disease in children and pulmonary disease in adults.
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