Skin is a resilient protective layer of cells protecting the muscles and organs of animals. Skin is formed in several layers. Living skin is a material of exceptional quality, with good temperature tolerance and insulating ability. As well as these functions, skin has roles of vitamin D production, sensation and excretion (through sweat).
Skin on creatures regularly subjected to sunlight have pigmentation which absorbs the potentially dangerous radiation contained therein. Human skin pigmentation varies across populations in a visually dramatic manner. This has sometimes lead to the classification of people(s) on the basis of skin color. See the article on skin color">human skin color for more information.
Mammalian skin often contains hairs, which in sufficient density is called fur. The hair mainly serves to augment the insulation the skin provides. On some animals the skin is very hard and thick, and can be processed to create leather.
The skin is often known as "the largest organ in the human body": this does not only (obviously) apply with regard to surface area, but also with regard to weight, it weighs more than any single internal organ. It contains different tissues that combine and work together.
Skin is composed of the epidermis and the dermis. Below these layers lies the hypodermis, which is not usually classified as a layer of skin. The outermost epidermis is made up of stratified squamous epithelium with an underlying basement membrane. It contains no blood vessels, and is nourished by diffusion from the dermis. The main type of cells which make up the epidermis are keratinocytes, with melanocytes and Langerhans Cells also present. The epidermis can be further subdivided in to the following strata (beginning with the outermost layer): corneum, lucidum, granulosum, spinosum, basale. Cells are formed through mitosis at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with keratin. They eventually reach the corneum and become sloughed off. This process is called keratinization and takes place within days (check).
The dermis lies below the epidermis and contains a number of structures including blood vessels, nerves, hair follicles, smooth muscle, glands and lymphatic tissue. It is made up of dense connective tissue - collagen, elastin and reticular fibres are present. The main cell types are fibroblasts, adipocytes (fat storage) and macrophages. The dermis can be split in to the papillary and reticular layers. The papillary layer is outermost and extends in to the dermis to supply it with vessels. It is composed of loosely arranged fibres. Papillary ridges make up the lines of the hands. The reticular layer is more dense and is continuous with the hypodermis. It contains the bulk of the structures (such as sweat glands). The reticular layer is composed of irregularly arranged fibres and resists stretching.
The hypodermis is not part of the skin, and lies below the dermis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It is made up of loose connective tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (the hypodermis contains 50% of body fat). Fat serves as padding and insulation for the body.
Skin can be dividided in to thick and thin types. Thick skin is present on the soles of the feet and the palms of the skin. It has a larger stratum corneum with a higher keratin content. Thick skin does not grow hair. Its purpose is to help grip. Thin skin is present on the bulk of the body and has a smaller stratum corneum and fewer papillae ridges. It has hair and is softer and more elastic.
As skin ages is becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself. Skin sagging is caused by the fall in elasticity. Skin also receives less blood flow and lower gland activity.
The skin is subject to constant attack from without, and so can be afflicted by numerous ailments, such as: