Skin Anatomy and Physiology

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Skin Structure

· Largest organ consisting of  a number of specialised cells

· The skin has two main structural layers: the epidermis and the dermis

· The third and deepest layer is the sub dermis

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Epidermis

· The epidermis is the outer most layer of skin

·  It can be categorised into 5 horizontal layers  

·  Function: the epidermis acts as a protective shield for the body and totally renews itself approximately every 28 days

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Layers of the epidermis

·          Stratum Basale

·          Stratum spinosum

·          Stratum granulosum

·          Stratum lucidum

·          Stratum corneum

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Stratum Basale

  • The deepest layer of the epidermis and sits on top of the dermis
  • It is  a single layer of cube-shaped cells
  • Keratinocytes are formed in this layer through cell divsion, to replace those shed continuosly from upper layers of the epidermis - this is known as skin cell renewal
  • Melanocytes are responsible for the production of skin pigment (melanin); they transfer melanin to keratinocytes that will eventuall migrate to the surface of the skin
  • Melanin is photoprotective and so protects skin against UV radiation
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Stratum Spinosum

  • This is the second layer, also known as the prickle-cell layer
  • It is composed of 8-10 layers of polygonal keratinocytes
  • In this layer, the keratinocytes start to become flattened
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Stratum Granulosum

  • This is the third layer, also known as the granular layer
  • It is composed of 3-5 layers of flattened kertin
  • Keratin is a tough, fibrous protein that gives skin its protective properties
  • Cells in this layer are too far from dermis to recieve nutrients through diffusion, so they begin to die
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Stratum Lucidum

  • This is the fourth layer, also known as the clear layer
  • It is obnly present in the figertips, palms and soles of the feet
  • It is 3-5 layers of extremely flattened cells
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Stratum Corneum

  • The fifth and outermost layer of the epidermis, also known as the horny layer
  • It is 25-30 layers of flattened, dead keratinocytes and is the real protective layer of the skin
  • Keratinocytes, in this layer, are continuously shed by frictio and replaced by cellls formed in the deeper sections of the epidermis
  • In between the keratinocytes in this layer are peidermal lipids that act as cement between the skin cells
  • this combination forms a waterproof moicture barrier that minimises transepidermal water loss to retain moisture in the skin
  • This moisture barrier also protects against invading microorganisms, chemical irritants and allergens 
  • If the intergirty of the barrier is compromised, then skin will become vulnerabel to itching, redness and other skin care concerns
  • The very outer layers of the mositure barrier has a slightly acidic pH, and these slightly acidic layers are called the acid mantle
  • The acid mantle functions o inhib growth of harmful bacteria and fungi; it also helps maintain the hardness of keratin proteins
  • If the skin surface becomes alkaline then keratin fibres loosen and soften, losing their protective properties
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Dermis

  • The dermis is located between the subdermis and the epidermis
  • It is a fibrous network of tissues
  • Function: It provides structure and resilience to skin
  • The network of the dermis consists of structural proteins (collagen and elastin), blood and lymph vessels, and specialised cells (mast cells and fibroblast)
  • These are surrounded by a gel-like substance called ground substance, composed mostly of glycosaminoglycans
  • Ground substance layers play a critical role in the hydration and moisture levels within the skin
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Structural components of dermis

  • Most common structural component within the dermis is the protein collagen which gives the skin strength and flexibility
  • The glycosaminoglycan (moisture binding molecules) enable collagen fibers to retain water and provide moisture to the epidermis
  • Another protein found throughout the dermis is, elastin (coil-like protein), which gives the skin its ability to return to its original shape after stretching; elastin provides skin with its elasticity
  • Both of these proteins are produced in fibroblasts, located mostly in the upper edge of the dermis, bordering the epidermis
  • Blood vessels, lymph vessels, nerves, and mast cells are interwhined in the dermis
  • Mast cells play an important role in triggering the skin's inflammatory response to invading microorganisms, allergens, and physical injury
  • Blood vessels help in thermoregulation of the body by constricting or dilating to conserve or release heat; they also aid in immune function and provide oxygen and nutrients to the lower layers of the epidermis (these blood vessels do not extend into the epidermis)
  • The junction between the dermis and epidermis is a wave-like border - it provides an increased surface area for the exchange of oxygen and nutrients between the two sections
  • Along this junction are projections called dermal papillae
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Components Common to Dermis & Epidermis

  • There are a number of components that are common to both the dermis and epidermis
  • These components are: pores, hair, sebaceous glands and sweat glands
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Pores

  • Pores are formed by a folding-in of the epidermis into the dermis.
  • The skin cells that line the pore (keratinocytes) are continuously shed, just like the cells of the epidermis at the top of the skin. 
  • The keratinocytes mix with sebum and clog the pore, when they are being shed; this is the precursor to acne. 
  • If oil builds up inside pores, or if tissue surrounding the pore becomes agitated, pores may appear larger.
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Hair

  • Hair grows out of the pores and is composed of dead cells filled with keratin proteins 
  • At the base of each hair is a bulb-like follicle that divides to produce new cells
  • The follicle is nourished by tiny blood vessels and glands. 
  • Hair prevents heat loss and helps protect the epidermis from minor abrasions and exposure to the sun's rays
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Sebaceous Glands

  • Sebaceous glands are usually connected to hair follicles and secrete sebum that protects hair and lubricatse skin 
  • Sebum also contributes to the lipids and fatty acids within the moisture barrier 
  • Oil production within the sebaceous gland is regulated by androgen levels (hormones such as testosterone)
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Sweat Glands

  • Sweat glands are long, coiled, hollow tubes of cells. 
  • The coiled section is where sweat is produced, and the long portion is a duct that connects the gland to the pore opening on the skin's surface. 
  • Perspiration excreted by the sweat glands helps cool the body, hydrate the skin, eliminate some toxins (i.e. salt), and maintain the acid mantle.
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Subdermis

  • This is the deepest sectiob of skin, and is also known as the hypodermis or subcutaneous fat
  • It refers to the fat tissue below the dermis that insulates the body from cold temperatures and provides shock absorption
  • Fat cells of the subdermis also store nutrients and energy
  • It is thickest in the buttocks, palms of the hands, and soles of the feet
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