Integumentary system is another important system of human body that protects all other system helping them function properly. Skin is the main organ of integumentary system. It is also the largest organ of human body that serve as the protective barrier against all the external agents.
Skin consist of mainly two layers, the outer layer epidermis and the inner dermis. Beside these two layers there is hypodermis as well, which is not exactly the part of skin yet it is very important layer that serves in several way.
Epidermis: it is the superficial layer of skin that covers the body. It is thinner than dermis and is composed of keratinized stratified squamous epithelium. This layer of skin consist of different types of cells that have different functions. 90% of skin cells are keratinocytes that produce the protein keratin. These cells have different function, like protection of underlying tissue from heat, microbes and chemicals. Beside this, lamellar granules are also produced by these cells that checks the loss and entry of water and inhibits any foreign pathogen to enter the skin. Likewise, 8% of cells are melanocytes, that produce melanin. Melanin is completely responsible for skin coloration and also it absorbs UV rays that may damage the skin and its underlying cells or tissues. Langerhans cells and Merkel cells makes rest of the epidermal cells of skin. On one hand langerhans cells fight microbes and pathogens preventing them enter inside the skin on the other hand detection of touch sensation is well accomplished by Merkel cells. In this way different epidermal cells have varied function in accomplishing the target for protection of the body.
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Skin is subdivided into different layers in different body parts. In most the regions of body, epidermis has 4 parts; hence called thin skin and in other regions it has 5 layers hence called thick skin. The different parts of epidermis are:
1. Stratum basale: it is the deepest part and is made of cuboidal or columnar keratinocytes. It is also called stratum germinativum. Some cells in this layer of epidermis are stem cell that undergoes cell division to produce new keratinocytes. Its cytoskeleton include tonofilament i.e. a type of intermediate filament. This tonofilament gets attach to desmosomes that binds stratum basale to each other and also to the neighbouring cells. Melanocytes and Merkel cells are also present here.
2. Stratum spinosum: in this layer eight to ten rows of many sided keratinocytes along with bundles of monofilaments are present. It also includes arm like processes of melanocytes and langerhans cells.
3. Stratum granulosum: it consist of 3 to 5 rows of flattened keratinocytes, in which organelle present undergo apoptosis. A darkly stained protein called keratohylalin is present that converts tonofilament into keratin. A fluid filled membrane enclosed granule called lamellar granule is present in this layer that is rich in lipid and fills the cells of above mentioned layers. This granule prevents loss of water and entry of any foreign material.
4. Stratum lucidum: it is also called thick skin and present in parts of our body like palm. It consist of 3-5 rows of dead, clear and flat kerotinocytes with large amount of keratin.
5. Stratum corneum: it contains 25-30 rows of dead and flat keratinocytes that has large amount of keratins.
Dermis: it is the second layer that lies below the epidermis. It is made of collagen and elastic fibres that has great tensile strength. Fibroblast, adipocytes, and macrophages are all found in this layer of skin. On the basis of tissue layer, it is further divided into two types:
Papillary region: it is made of aerolar connective tissue that has collagen and elastic fibres. It has dermal papillae that has fingerlike projections. Beside this, nerve endings , capillaries and Meissner corpuscles are also present.
Reticular region: it consist of dense irregular connective tissue that consist of collagen, elastic fibres and fibroblasts. This portion has elasticity and extensibility due to the presence of these fibres. Epidermal ridges are also present in this layer as the downward projection from epidermis to dermis. These epidermal ridges varies in every individual thus helping in identity recognisation. Beside this, it also serve as an important factor that provides nutrient for the epidermis.
Hairs: hairs are present on most parts of the body, except palm and sole. In other parts like head, eyebrow, armpits, genetelia hairs are present abundantly whereas in rest of the body little hairs can be seen. The structure and thickness of the hairs varies as per the changes in the hormones. Hairs regulate heat balance of the body and protects it from foreign particles and harmful rays of the sun.
Hairs can be divided into three different parts: follicle, root and shaft. Hair follicle lies deep in the dermis whereas within the follicle are the roots, the portion of hair below the skin and the parts that can be seen above the skin is the shaft. All the portions of hair are made of cuticle, cortex and medulla.
Nails: these are dead, hard keratinized epidermal cells that forms a covering at the distal portion of the finger tip. There are three parts of a nail, root, body and free edge. Nail root means the portion under the skin, body is the visible portion in the finger and free edge is the nails above the tip that has grown to extra length. Nails helps in scratching, holding and manipulation of the objects.
Sebaceous gland: these are branched gland that secrete oily substance called sebum. Sebum is mixture of triglyceraldyhydes, cholesterol, proteins and inorganic salt. Sebaceous gland is excessively distributed in most part of the body whereas on the other parts it is present in small amount. Beside this, it is completely absent in palm and sole. The main function of this gland is to coat the hairs thus, preventing them from drying out and it keeps the skin soft by preserving water.
Sudiferous gland: this is also called sweat gland, and is present in most part of the body. It secrete sweat through pores of the skin and in the hair follicle. These sweat glands can be divided into two different types: eccrine and apocrine. Eccrine gland is present in most of the body parts and its excretory duct projects through the dermis and epidermis whereas apocrine gland is mostly present in axillary and pubic regions and its secretory portion is present in hypodermis and excretory duct open through hair follicle.
Ceruminous gland: these gland produce waxy fluid in the dermis of ear. The fluid protects the ear canal and lubricate the eardrum.
Keratinization: it is also the process by which keratin get accumulated in the keratinocytes.
Temperature homeostasis: skin controls the body temperature by regulating the interaction of body with the environment around.
Vitamin D synthesis: skin absorbs UV light and converts it into vitamin D thus, converting it into useable form.
Protection: skin protects body from harmful external agents thus, preventing it to enter inside the body.
Skin coloration: different pigments present in the skin like melanin, hemoglobin and carotene provides coloration to the skin.
Excretion: most of the waste materials like salts and unusable liquid product are also excreted from the skin.
Therefore, skin is the main gland that covers our entire body. It keeps all the internal organs protected keeping them away from harmful environmental diseases. skin has the capacity to regenerate due to the presence of stem cell, that is way if there is any cut or burn, after some days new skin can be seen in that place. It is because the old skin is replaced with the new ones, sometimes leaving behind a scar or a mark. Skin is very important organ that actually adds up to the beauty of an individual and make a presentable human being. It would be worthless to imagine life without the covering of skin because all the internal organs would not function in any condition if there in no harsh and protective boundary between environment and the body.
It is important to keep skin clean and healthy through washing and bathing because germs will multiply in dirty skin leaving behind the bad smell thus opening up way for many more diseases.