Skin

The skin is the largest organ of the body. In humans, it accounts for about 12 to 15 percent of total body weight and covers 1.5-2m2 of surface area.

The (integument) is composed of at least two major layers of tissue: the  and. (The hypodermis or subcutaneous layer is not part of the skin.) The epidermis is the outermost layer, providing the initial barrier to the external environment.It is separated from the dermis by the basement membrane. The epidermis contains melanocytes and gives color to the skin. The deepest layer of epidermis also contains nerve endings. Beneath this, the dermis comprises two sections, the papillary and reticular layers, and contains connective tissues, vessels, glands, follicles, hair roots, sensory nerve endings, and muscular tissue. The deepest layer, the hypodermis, is primarily made up of. Substantial collagen bundles anchor the dermis to the hypodermis in a way that permits most areas of the skin to move freely over the deeper tissue layers.

Epidermis
The epidermis is the top layer of skin made up of. It does not contain blood vessels. Its main functions are protection, absorption of nutrients, and homeostasis. In structure, it consists of a keratinized stratified ; four types of cells:, , , and. The major cell of the epidermis is the keratinocyte, which produces, a fibrous protein that aids in skin protection. An overwhelming amount of keratin can cause disease by giving rise to eruptions from the skin that will protrude outwards and lead to infection. Keratin is also a waterproofing protein. Millions of dead keratinocytes rub off daily. The majority of the skin on the body is keratinized. The only skin on the body that is non-keratinized is the lining of mucous membranes, such as the inside of the mouth. Non-keratinized cells allow water to "stay" atop the structure.

The protein keratin stiffens epidermal tissue to form s. Nails grow from a thin area called the at an average  of 1 mm per week. The is the crescent-shape area at the base of the nail, lighter in color as it mixes with the matrix cells. Also, the stratum corneum is the top part of the epidermis.

Layers
The epidermis is composed of 4 or 5 layers, depending on the region of skin being considered. Those layers in descending order are:


 * Composed of 10 to 30 layers of polyhedral, anucleated s (final step of keratinocyte ), with the palms and soles having the most layers. Corneocytes contain a envelope (cornified envelope proteins) underneath the plasma membrane, are filled with water-retaining  proteins, attached together through  and surrounded in the  space by stacked layers of . Most of the barrier functions of the epidermis localize to this layer.


 * clear/translucent layer (, only in palms and soles)
 * This narrow layer is found only on the palms and soles. The epidermis of these two areas is known as "thick skin" because with this extra layer, the skin has 5 epidermal layers instead of 4.


 * granular layer ()


 * Keratinocytes lose their and their  appears granular. Lipids, contained into those keratinocytes within, are released into the extracellular space through  to form a lipid barrier. Those polar lipids are then converted into non-polar lipids and arranged parallel to the cell surface. For example  become  and  become.


 * spinous layer ()


 * Keratinocytes become connected through and start produce lamellar bodies, from within the, enriched in polar lipids, , free ,  and catabolic enzymes. Langerhans cells, immunologically active cells, are located in the middle of this layer.


 * basal/germinal layer ().


 * Composed mainly of proliferating and non-proliferating keratinocytes, attached to the by . Melanocytes are present, connected to numerous keratinocytes in this and other strata through . Merkel cells are also found in the  with large numbers in touch-sensitive sites such as the  and s. They are closely associated with cutaneous  and seem to be involved in light touch sensation.

The (stratum malpighi) is both the stratum basale and.

The epidermis is separated from the dermis, its underlying, by a.

Dermis
The dermis is the middle layer of skin, composed of dense irregular connective tissue and such as a collagen with  arranged in a diffusely bundled and woven pattern. The dermis has two layers. One is the papillary layer which is the superficial layer and consists of the areolar connective tissue. The other is the reticular layer which is the deep layer of the dermis and consists of the dense irregular connective tissue. 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 are also stored in this layer, as are the bases of integumental structures such as, , and.