The basic function of the skin is the physical protection, until the regularity of the skin is not ruptured, nothing can enter from the side of external environment. Skin makes the barrier against bacteria. Open injury is vulnerable to infections. Keratin, protein synthesized by the keratinocytes of the epidermis is one type of structural protein with Sulphur as an essential constituent, functions for developing resistance against infections.

Protection from UV rays is an important function of the skin. Melanin, a protein synthesized by the melanocytes located in the basal layer of the epidermis, functions to absorb the UV rays, so that the underlying cells are secured. Skin also functions for the removal of wastes fromthe body, a like kidney. It removes urea, ammonia and CO2 from the body to maintain the homeostasis.

Skin also play important role in keeping acidand base in balance, through the excretion of urea CO2 and other acidotic factors it restorates the body with normal pH. Skin is one type of sensoryorgan, it functions as a touch sensory to identify the physicalchanges in the external environment e.g., pressure, temperature etc. Skin also functions for the synthesisof vitamin D. Thermoregulation is an important function of skin. During winter and in cold environment, erection of hairs and reduction of surface area are the two changes that are seen in the skin to protect to protect body from cold. By the process of sweating and increase of surface area skin protects body from hot climate. Sweating leads into diffusion of heat from the body keeping uncomfortable.

Basic Layers of the Skin

Skin is made from three basic layers.

1 - Epidermallayer
2 - Dermallayer
3 - Hypodermallayer

1 - Epidermis

This is also called epidermis. It is the most outer layer, derived from ectoderm. This provides an over coat type of arrangement to the body, so that the body is physically protected. This is made from various layers of keratinized cells. In all this have two types of cells, keratinized and non- keratinized cells. Keratinised cells synthesize the keratin protein, Keratin is a structural protein which a part from physical protection it deals with many ways to protect the body. Nails, hairs and wool are also products of keratin. Keratinised cells from five layers. From the level of base (inner side) to surface the layers are given as under-

a) Stratum basale
b) Stratum spinosum
c) Stratum granulosum
d) Stratum lucidum
e) Stratum corneum

a) Stratum basale

This is basic layer lying at the deepest level in the epidermis. This is made from the single lining of newly formed youngest keratinised cells. The cells are cubidal or columnar epithelial cells. Cells contain the basophilic cytoplasma. Since cells undergo a rapid mitotic cell division, therefore stratum basale layer is also called germinating layer. New skin cannot regenerate if a large area of the stratum basale and it&rsquos stem cells are destroyed. Skin wound of this magnitude requires skin graft to heal. This layer being basic layer functions for mitosis and proliferation, so as to provide stolk of the kertinocytes to next layer which is very active and known as stratum spinosum. Laying within this layer are10-25 % cells of melanocytes which synthesize melanin protein pigments, as well as the precursor of vitamin D. Langerhan&rsquoscells and occasional merkel&rsquos cells are also located within stratum basale.

b) Stratum spinosum

This layer has many cell sized thickness. Cells are Irregular and spiny in shape and are closely fit together. They are filled with pre-keratin protein filaments. Scattered among cells are melanin protein molecules located at the sunny side (outer side) of the cellnucleus, forming a pigment shield, so as to absorb the UV rays coming from outer side, to protect nucleus from damaging effects of UV rays. Scattered among keratinocytes are Langerhan&rsquoscells which are most abundant in this layer.

c) Stratum granulosum

The thin layer of keratinocytes kake this layer. Cells are limited tofive cells and are flattened. Cells accumulatethe keratinohyaline granules precursor of keratin for upper layers, and lamellated granules. Lamellated granules contain water proofingglycolipids that is spewed into extracellular space and is a major factor in checking the water loss across the epidermis. Nuclei and organneles begin to disintegrate at this level. Keratinocytes beginto develop hardening, due to poor blood supply, which is away in the dermis. Cell death is completely a normal sequence of events.

d) Stratum lucidum

This is a clear layer so is called stratum lucidum. It forms thintranslucent band just above stratum granulosum. It consists of fewrows of clear, flat, dead keratinocytes with indistinct boundaries. Here and in the stratum corneum above, the gummy substance of the keratinohyaline granules clings to the keratin filaments in thecells, causing them to aggregate in parallel arrays. Stratum lucidumis visible only in thick skin.

e) Stratum corneum

It is also called horney layer being hard and toughened in structure. It is made from cornfield cells. Keratinocytes reaching to this levelare dead and hardend. They are familiar to every one because of dandruff shedding from scalp. Dead keratinocyte cells make durable overco at for the body. Stratum corneum functions to make skin strong to protect from external injuries. It protects from water loss, biological, chemical and physical assaults. It is amazing that a layer of dead cells also function many roles.