Functions of the skin
Being the largest human organ the skin covers an area almost equivalent to 2 square meters and can weigh up to 10 kg. It serves as a protective barrier against the external environment while maintaining homeostasis internally.
Social importance: appearance and identification of a person
Protection: protective barrier against the external environment such as chemical or mechanical injuries or ultraviolet radiation and maintenance of a homeostasis internally
Sensation: sensitive to pain, touch, pressure and temperature
Thermoregulation: thermoregulation through dilatation and constriction of vessels and sweating
Metabolism: synthesis of vitamin D(calcium and phosphate metabolism) in the presence of sunlight
Anatomy of the skin
The skin is made up of two major layers, the epidermis and the dermis which rest on a fatty layer called hypodermis (subcutaneous tissue). Both epidermis and dermis are in turn composed of sub-layers. The area that anchors the epidermis to the dermis is called the dermal-epidermal junction. It is responsible for the exchange of oxygen, nutrients and waste products between the vascularised dermis and the avascular epidermis.
The epidermis is a multi-layered horny squamous epithelium that is usually between 0.03 and 0.05 millimetres thick and consists primarily of keratinocytes in progressive stages of differentiation from deeper to more superficial layers. As keratinocytes divide, they move from the deeper layer to the more superficial layers. Once they reach the stratum corneum (the outermost layer) they are shed inthe process of epidermal turnover, e.g. after bathing or scratching. This process is called «desquamation». A complete epidermal turnover («skin renewal») takes approximately 2 months.
Compared to the thinner epidermis, the dermis is a complex network comprising cellular and acellular components. It contains blood vessels, nerves, hair roots and sweat glands. Structurally, the dermis consists of two sub-layers, the superficial papillary dermis and the deeper reticular dermis. The vascular loops of the papillary dermis supply the epidermis with nutrients and oxygen. Collagen – mainly found in the reticular dermis - is the major structural protein in the dermis providing the skin with its durability. Elastin, another important structural protein in the dermis, gives the skin its elasticity.
Fibroblasts are the principal cells of the dermis and are responsible for synthesis and degradation of the dermal proteins (collagen and elastin). Other cells found in the dermis are macrophages and lymphocytes. These cells are part of the skin immune system.
Hypodermis (subcutaneous tissue)
The hypodermis forms the base for epidermis and dermis and consists primarily of loose connective tissue and lobules of fat. It contains larger blood vessels and nerves than those found in the dermis. The hypodermis serves mainly as an insulator and provides an energy storage.