Anatomy and physiology- The skin

Also known as the integument, the skin is the largest organ of the body. It has two layers, the epidermis, and the dermis. Beneath these is the subcutaneous layer.

The functions of the skin include:

  1. Sensation
  2. Heat regulation
  3. Absorption
  4. Protection
  5. Excretion
  6. Secretion
  7. Production of vitamin D

 

The epidermis

The epidermis is stratified squamous epithelium tissue. There are no blood vessels; nourishment comes from interstitial fluid.

The epidermis is the outer covering of the skin and consists of the following five layers:

  1. The stratum corneum/horny layer
  2. The stratum lucidum/clear layer
  3. The stratum granulosum/granular layer
  4. The stratum spinosum/prickle layer
  5. The stratum germinative/basal layer

 

The stratum corneum

This is the surface layer of skin, consisting of 25 to 30 cells deep of flattened irregular dead keratinised cells. This layer of skin is a waterproof protective covering, which prevents excessive dehydration of the tissues, hence the term ‘horny layer’.

 

The stratum lucidum

This layer lies below the stratum corneum and is four cells deep. It is a narrow transparent layer consisting of flattened and closely packed cells, with an indistinct outline and no nuclei, hence the term ‘clear layer’. This layer is more pronounced in thick hairless skin, e.g., on the palms of the hands and soles of the feet.

 

The stratum granulosum

This is the third layer of the epidermis and is between one and three cells deep of cells, consisting of flattened spindle shaped cells. The nucleus begins to break down here and granules of keratin appear here, hence the term ‘granular layer’. This layer forms the main part of all horny tissue.

 

The stratum spinosum

This layer is eight cells deep, with round nucleated cells that vary in size and shape. They produce fibrils, which interconnect them with neighbouring cells, hence the term ‘prickle cell layer’.

 

The stratum germinative

Within this layer cells are one cell deep, rounded and clearly defined, with a nucleus, and rest on the papillae of the dermis. These epithelial cells divide and reproduce themselves by a process called mitosis, hence the term ‘germinative layer’. As these cells form, they push themselves up towards the skin surface. Within this layer there are also melanocyte cells, which produce the pigment melanin. This gives both the hair and skin its colour density. Langerhans cells are also found within this layer and absorb and remove foreign bodies that enter the skin.

 

The dermis

The largest layer of the skin is also known as the ‘true skin’. It is composed of areolar connective tissue and contains the protein collagen, elastin fibres, fibroblasts, macrophages, mast cells and an enzyme, numerous blood vessels, lymph vessels, nerves, sweat glands, hair follicles, and arrector pili muscles, and is bathed in water.

The dermis is composed of two layers called the papillary and reticular layers.

 

The papillary layer

This lies directly under the germinative layer and nourishes the epidermis. It contains small cone-shaped projections called papillae, which contain looped blood capillaries. Others contain tactile corpuscles and nerve endings.

 

The reticular layer

This is the larger of the two layers containing cells called fibroblasts, which produce the yellow elastin fibres. These make up 4 per cent of connective tissue and give the skin its flexibility and ability to recoil. White protein collagen fibres give the skin its strength and make up 75 per cent of connective tissue. Reticular fibres run between the dermis fibres and structures helping to support them and hold them in situ.

 

There are also specialised cells within this layer – macrophages and mast cells. Macrophages destroy bacteria and tissue debris, whilst mast cells secrete histamine, a substance that enlarges small blood vessels and heparin, an anti-coagulant that stops the blood clotting.

 

The skin contains sensory nerve endings that run through the dermis, terminating in nerve endings of various types. Branched nerve endings are sensory and found in the hair root and papillary layer. They are sensitive to temperature changes.

 

Also within the dermis are sweat glands, sebaceous glands, hair follicles, arrector pili muscles etc.

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