THE SKIN IN ENGLISH

                                                     

                                                THE SKIN IN ENGLISH

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SKIN-

Ø  The skin, sometimes referred to as the integumentary system, makes a protective covering of our body. It protects the body in a number of ways.

Ø  The skin is the largest organ in the body and has a surface area of about 1.5 to 2 square meter in adults. There are two main layers of the skin:

Ø  The epidermis and

Ø  The dermis.

EPIDERMIS- The epidermis is the most superficial layer of the skin and is composed of stratified keratinised squamous epithelium. There are no blood vessels or nerve endings in the epidermis

Ø  Epidermis contains four key cell types 

Ø  •• keratinocytes

Ø  •• melanocytes

Ø  •• Langerhans cells

Ø  •• Merkel cells.

Ø  • keratinocytes- These cells are organised in four layers. They are responsible for producing a protein called keratin. Keratin is a tough, fibrous protein that provides protective power to the skin

Ø  • Melanocytes- These cells produces a pigment melanin that is responsible for skin color.

Ø  • Langerhans cells- These cells are part of the immune system and arise from the red bone marrow. These cells migrate from the bone marrow to the epidermis.

Ø  • Merkel cells- A Merkel cell has the ability to have contact with a flattened process of a sensory neuron and makes synaptic disc and help in sense of touch.

LAYERS OF EPIDERMIS-

          The superficial and deeper levels of the epidermis are:

          •• the stratum basale

          •• the stratum spinosum

          •• the stratum granulosum

          •• the stratum lucidum

          •• the stratum corneum.

          •• the stratum basale- The stratum basale is the deepest layer of the epidermis. This is made up of a single row of columnar keratinocytes which rests on the basement membrane. This layer provides a definite border between the dermis and epidermis.

          •• the stratum Spinosum- stratum spinosum lies Above the stratum basale .It is composed of several layers. The keratinocytes in this layer have spinelike projections. The keratinocytes are tightly packed here. This tight packing arrangement provides strength and flexibility to the skin.

          •• the stratum granulosum- the stratum granulosum lies above the stratum sponosum. There are  three to five layers of flattened keratinocytes in this layer of the skin. These cells contain granules  that form a water‐resistant lipid, protecting the body from losing excess fluid and at the same time protecting against the entry of microbes

          •• the stratum Lucidum- There are four to five layers of flat dead cells in this layer. this layer is not found on all aspects of the body, only on areas of thick skin; for example, the heels. The cells have no nucleus and are tightly packed, providing a barrier to fluid loss.

          •• the stratum Corneum- This is the outermost layer of the epidermis and is made up of a number (about 25) of scale‐like layers of dead keratinocytes. This layer provides physical barrier to light and heat waves, microorganisms, chemicals and injury. this layer this layer is being continuously rubbed off and new layers comes up from bellow.Complete replacement of epidermis takes place in about 30 days

          DERMIS-The second, deeper part of the skin, the dermis, is composed of a strong connective tissue containing collagen and elastic fibers. This woven interlaced  network of fibers has great tensile strength

          The dermis can be divided into two layers:

          The papillary aspect- the papillary layers connect the dermis to the epidermis. The fingerprints arise from this layer.

          The reticular aspect- which is attached to the subcutaneous layer, consists of dense irregular connective tissue containing fibroblasts, bundles of collagen, and some coarse elastic fibers.

          Rupture of elastic fibres occurs when the skin is overstretched, resulting in permanent striae, or stretch marks, that may be found in pregnancy and obesity.

          The structures lie in the dermis are blood vessels, lymph vessels, sensory (somatic) nerve endings, sweat glands and their ducts, hairs, arrector pili muscles and sebaceous glands.

          ACCESSORIES OF SKIN-

          Sweat glands-  These are widely distributed throughout the skin and are most numerous in the palms of the hands, soles of the feet, axillae and groins. They are formed from epithelial cells. the sweat produced here is a clear, watery fluid important in regulating body temperature.

          Hairs- hairsare present on most skin surfaces except the palms, palmar surfaces of the fingers, the soles, and plantar surfaces of the feet. Each hair is composed of columns of dead, keratinized epidermal cells bonded together by extracellular proteins. The shaft is the superficial portion of the hair, which projects above the surface of the skin

          The root is the portion of the hair deep to the shaft that penetrates into the dermis, Surrounding the root of the hair is the hair follicle, which is made up of an external root sheath and an internal root sheath.

          Sebaceous (oil) glands and a bundle of smooth muscle cells are also associated with hairs.

          Sebaceous glands-  Sebaceous glands consist of secretory epithelial cells derived from the same tissue as the hair follicles. They secrete an oily substance, sebum, into the hair follicles. Sebum keeps the hair soft and pliable and gives it a shiny appearance. On the skin it provides some waterproofing and acts as a bactericidal and fungicidal agent, preventing infection.

          Arrector pili  are little bundles of smooth muscle fibres attached to the hair follicles. Contraction makes the hair stand erect and raises the skin around the hair, causing ‘goose flesh’. The muscles are stimulated by sympathetic nerve fibres in response to fear and cold.

          Nails- Nailsare plates of tightly packed, hard, dead, keratinized epidermal cells that form a clear, solid covering over the dorsal surfaces of the distal portions of the digits. Each nail consists of a nail body, a free edge, and a nail root

          The root of the nail is embedded in the skin and covered by the cuticle, which forms the hemispherical pale area called the lunula. The nail plate is the exposed part that has grown out from the germinative zone of the epidermis called the nail bed.

          FUNCTIONS OF SKIN-

          Skin functions include:

          • sensation

          • thermoregulation

          • protection

          • excretion and absorption

          • synthesis of vitamin D.

          •Sensation-

           There are several receptor sites on the skin that have the ability to sense change in the external environment in respect to temperature and pressure; these receptors  are made up of  nerve endings. The messages picked up in the skin are then  transferred to the brain.

          • Thermoregulation-

          The skin has a role to play in homeostasis through thermoregulation, helping to keep the temperature of the body within narrow ranges. The amount of heat lost from the skin depends largely on blood flow through dermal capillaries. As body temperature rises, the arterioles dilate and more blood enters the capillary network in the skin.

          • Protection-

          There are many ways in which the skin protects the body. The skin forms a relatively waterproof layer,  which protects the deeper and more delicate structures. As an important non-specific defence mechanism it acts as a barrier against invasion by micro-organisms.

          • Excretion and absorption-

          The skin has the ability to excrete substances from the body; sweat is composed of water, sodium, carbon dioxide, ammonia and urea. The skin also has the ability to absorb substances from the environment. A number of fat‐soluble vitamins – A, D, E and  k  and toxins such as lead are  absorbed by the skin.

          • Synthesis of vitamin D-

          The skin is actively involved in the production and synthesis of vitamin D. For vitamin D to synthesise effectively, activation of a precursor molecule (7-dehydrocholesterol is a lipid-based substance) in the skin by ultraviolet rays in the sunlight (ultraviolet radiation) is required.