The integumentary system

The integumentary system (Etymology - 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 (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, regulate temperature and is the attachment site for sensory receptors to detect pain, sensation, pressure and temperature. In humans the integumentary system additionally 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. 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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Epidermis:
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. When cells are keratinized it means that it is waterproof. 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. Then there are 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.