Skin

Tigers die and leave their skins; people die and leave their names

Japanese Proverb

The skin is the largest organ in the body — both in weight and in surface area — and separates the body's internal environment from the external environment. It protects the body from water loss; uses specialized pigment cells, called melanocytes, to protect the body from ultraviolet radiation; contributes to the body's supply of vitamin D; and helps regulate body temperature and metabolism.

Skin is composed of an outermost epidermis (Gr. epi; on, derma; skin) followed by the dermis and then the subcutaneous layer containing fat cells. Thickness varies on different parts of the body; palms and soles (1.2 mm to 4.7 mm) have the thickest skin while the thinnest is found on the eyelids and lips (~0.05 mm). Males generally have thicker skin than females and it is the Rhinoceros that has the thickest skin of any terrestrial mammal with some areas as thick as 2.5cm.

The epidermis is composed mainly of keratinocyte (Gr. cyte; hollow - nowadays translates as a cell) and melanocyte cells, while the dermis contains blood vessels, nerves, hair follicles, muscle, glands and lymphatic tissue. Each square cm of human skin can consist of up to four million cells, 24 hairs, 35 oil glands, 6.1 meters of blood vessels, 246 sweat glands, 7,480 sensory cells, 23,622 pigment cells, and more than 393 nerve endings.

Keratinocytes produce strong structural proteins called keratins (Gr. keras; horn), that enable the cell to keep a robust rigid structure, and are also the main component of nails, hair, feathers and horns (this is seen as the red filaments in the above photo of a single cell). There are over 30 different types of keratin proteins defects in which can lead to skin either becoming to hard (Ichthyosis) or too delicate (epidermolysis bullosa). As the keratinocytes develop, they move up the skin layers producing different types of these keratin proteins forming rigid scaffolds in the cells. As the cells reach the layers of the stratum corneum they die forming the dead cell layer of our outermost skin. An increase in this process occurs in psoriasis.

Skin forms from the ectoderm during early development. Defects in this can lead to an absence of associated structures such as hair, nails, teeth and sweat glands (ectodermal dysplasia), nerve cells (congenital insensitivity to pain) or can lead to the development of lesions and tumours (neurofibromatosis, tuberous sclerosis and proteus syndrome).

Skin colour is chiefly the result of differing amounts of the pigment melanin (Gr. melas; black). This forms in little packets called melanosomes in a special cell called the melanocyte. These cells are found in the skin, eye and hair follicles and contain many branches which are used to pass the melanosome pigment packets they produce on to keratinocyte cells. These keratinocytes then store the melanosomes over their nucleus where the pigment serves to absorb harmful ultraviolet radiation preventing it reaching the DNA of the cell and causing mutations.

Pigment disorders can be caused by mutations in a variety of genes leading to defects in either: 1, the production of melanin (oculocutaneous albinism); 2, formation of melanosomes (Hermansky-Pudlak syndrome and Waardenburg syndrome); or 3, the development of melanocytes (piebaldism) or the loss of melanocytes in later life (virtiligo).