Skin, Hair and Nails

Skin The skin is the outer covering of the body. In humans, it is the largest organ of the integumentary system made up of multiple layers of epithelial tissues, and guards the underlying muscles, bones, ligaments and internal organs.Skin of a different nature exists in amphibians, reptiles, birds. Human skin is not unlike that of most other mammals except that it is not protected by a pelt and appears hairless though in fact nearly all human skin is covered with hair follicles. The adjective cutaneous literally means "of the skin

Because it interfaces with the environment, skin plays a key role in protecting against pathogensand excessive water loss. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B folates. Severely damaged skin will try to heal by forming scar tissue. This is often discolored and depigmented.

In humans, skin pigmentation varies among populations, and skin type can range from dry to oily. Such skin variety provides a rich and diverse habit for bacteria which number roughly a 1000 species from 19 phyla.

HairHair is a protein filament that grows through the epidermis from follicles deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class Although other non-mammals, especially insects, show filamentous outgrowths, these are not considered "hair" in the scientific sense. So-called "hairs" 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 cats, dogs, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life. The main component of hair fiber is keratin.
Transverse section of hair follicle

The hair can be divided into three parts length-wise, the bulb, a swelling at the base which originates from the dermis, the root, which is the hair lying beneath the skin surface, and the shaft, which is the hair above the skin surface. In cross-section, there are also three parts, the medulla, an area in the core which contains loose cells and airspaces the cortex, which contains densely packed keratin andthe cuticle, which is a single layer of cells arranged like roof shingles.

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