Skin is the soft outer covering of vertebrates. Other
animal coverings such as the
arthropod exoskeleton or the
seashell have different
developmental origin, structure and chemical composition. The adjective
cutaneous means "of the skin". In
mammals, the skin is the largest
organ of the
integumentary system made up of multiple layers of
ectodermal tissue, and guards the underlying
muscles,
bones,
ligaments and
internal organs.
Skin of a different nature exists in
amphibians,
reptiles, and
birds.
All mammals have some hair on their skin, even
marine mammals which appear to be hairless. The skin is one of the most important parts of the body because it interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the
body against
pathogens and excessive water loss.
Its other functions are
insulation,
temperature regulation, sensation, and the production of
vitamin D folates. Severely damaged skin may heal by forming
scar tissue. This is often discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans for example, the skin located under the eyes and around the eyelids is the thinnest skin in the body at 0.5 mm thick, and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is 4 mm thick and the thickest skin in the body.
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