| Integumentary system | |
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| Latin | integumentum commune |
The integumentary system (From Latin integumentum, from integere 'to cover'; from in- + tegere 'to cover'[1]) is the organ system that protects the body from damage, comprising the skin and its appendages[2][3] (including hair, scales, feathers, and nails). The integumentary system has a variety of functions; it may serve to waterproof, cushion, and protect the deeper tissues, excrete wastes, and regulate temperature, and is the attachment site for sensory receptors to detect pain, sensation, pressure, and temperature. In humans the integumentary system also provides vitamin D synthesis.
The integumentary system is the largest organ system. In humans, this system accounts for about 16 percent of total body weight and covers 1.5-2m2 of surface area.[4] It distinguishes, separates, protects and informs the animal with regard to its surroundings. Small-bodied invertebrates of aquatic or continually moist habitats respire using the outer layer (integument). This gas exchange system, where gases simply diffuse into and out of the interstitial fluid, is called integumentary exchange.
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This is the top layer of skin made up of epithelial cells. It does not contain blood vessels. Its main function is protection, absorption of nutrients, and homeostasis. In structure, it consists of a keratinized stratified squamous epithelium comprising four types of cells: keratinocytes, melanocytes, Merkel cells, and Langerhans' cells. The major cell of the epidermis is the keratinocyte, which produces keratin. Keratin is a fibrous protein that aids in protection. Millions of dead keratinocytes rub off daily. The majority of the skin on the body is keratinized, meaning waterproofed. The only skin on the body that is non-keratinized is the lining of skin on the inside of the mouth. Non-keratinized cells allow water to "sit" atop the structure.
The epidermis contains different types of cells: The most common are squamous cells, which are flat, scaly cells on the surface of the skin; basal cells, which are round cells; and melanocytes, which give the skin its color. The epidermis also contains Langerhan's cells, which are formed in the bone marrow and then migrate to the epidermis. They work in conjunction with other cells to fight foreign bodies as part of the body's immune defense system. Granstein cells play a similar role. Melanocytes create melanin, the substance that gives skin its color. These cells are found deep in the epidermis layer. Accumulations of melanin are packaged in melanosomes (membrane-bound granules). These granules form a pigment shield against UV radiation for the keratinocyte nuclei.
The epidermis itself is made up of four to five layers. From the lower to upper epidermis, the layers are named: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum (the extra layer that occurs in places such as palms and soles of the feet), and the stratum corneum.
The stratum basale is the only layer capable of cell division, pushing up cells to replenish the outer layer in a process called terminal differentiation. The stratum corneum is the most superficial layer and is made up of dead cells, proteins, and glycolipids.
The protein keratin stiffens epidermal tissue to form fingernails. Nails grow from thin area called the nail matrix; growth of nails is 1 mm per week on average. The lunula is the crescent-shape area at the base of the nail, this is a lighter colour as it mixes with the matrix cells.
The dermis is the middle layer of skin, composed of loose connective tissues such as collagen with elastin arranged in a diffusely bundled and woven pattern. These layers serve to give elasticity to the integument, allowing stretching and conferring flexibility, while also resisting distortions, wrinkling, and sagging. The dermal layer provides a site for the endings of blood vessels and nerves. Many chromatophores are also stored in this layer, as are the bases of integumental structures such as hair, feathers, and glands.
Although technically not part of the integumentary system, the subdermis is the layer of tissue directly underneath the dermis. It is composed mainly of connective and adipose tissue or fatty tissue. Its physiological functions include insulation, the storage of energy, and aiding in the anchoring of the skin.
The integumentary system has multiple roles in homeostasis. All body systems work in an interconnected manner to maintain the internal conditions essential to the function of the body. The skin has an important job of protecting the body and acts as the body’s first line of defense against infection, temperature change, and other challenges to homeostasis. Functions include:
Possible diseases and injuries to the human integumentary system include:
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