Skin Physiology





Before we begin to examine injuries to, and the healing of skin, we first must take a look at the organ itself. 
A person’s skin is the largest organ, and part of the integumentary system,  the system  responsible for outside coverings of the body, such as hair, nails, sweat glands and the like.  As the largest organ of the body, it accounts for about 15% of body weight, and on average has a surface area of 1.5-2 square meters.

The skin has several major functions, such as:

Temperature regulation:  When a person is subject to strenuous exercise, or high environmental temperatures, the skin produces sweat from sweat glands.  When the sweat evaporates, it cools the skin, helping to lower the body temperature.  When the body needs to be kept warm, blood vessels to the skin constrict, the avoid the loss of heat to the outside.  There is the erection of body hair to increase insulation, via the pilomotor reflex, more commonly known as goosebumps. 

Protection:  Skin provides a physical barrier that protects underlying tissues from abrasion, bacterial infection, dehydration and UV radiation. 

Sensation:  Skin contains over a thousand nerve endings per square inch, and allows the body  to sense the world around it.  This is important primarily in the case of pain, which tells the brain that the body is being damaged, and should stop doing what it is doing (such as leaning on the stove)

Excretion: Sweat is also a vehicle of organic compounds and salts.  Eccrine sweat glands, located all over the body, but primarily the palms, soles of feet, and forehead have sweat that is mostly water and salt, while sweat from apocrine sweat glands contains fatty tissues that are then broken down by bacteria, and cause an odor.  These glands are located in the armpits and genital area. 

Immunity:  In addition to providing a physical barrier to foreign bodies, the epidermis contains cells called Langerhan’s cells that are active in immune responses.  Upon infection of an area of skin, Langerhan cells in close proximity will capture, uptake and process antigens.

Vitamin synthesis: The epidermis can absorb UV radiation, through melanin, which begins the production of Vitamin D.

The skin is divided into several layers, and those layers into sublayers:


Layers of Skin


Epidermis – The outermost layer of the skin is the epidermis, which is comprised mostly (~90%) of keratinocytes,  with the rest consisting of melanocytes and the previously mentioned Langerhan cells.  Keratinocytes originate in the basal layer, and migrate up to the surface to become squamous cells, which forms the barrier portion of the skin.  Melanocytes exist at about 1000-2000 per square mm of skin, and exist in the basal layer.  These cells are responsible for producing melanin.  The epidermis has no blood flow of it’s own, and is nourished by the dermis.
         Cell Types

      Stratum corneum – This layer of skin is comprised mostly of dead cells, which continuously slough off and are replaced.  An average human forearm will shed cells at about 1300 cells/cm2/hr.  The cells of the corneum contain keratin, which is a protein that aids in water retention and absorption.  The thickness of this layer varies across the body, with the thickest being in the hands and feet.  
       Stratum lucidum – This layer of skin is thin and clear, and rests right below the corneum.  The keratinocytes in this layer are filled with eleiden, an intermediate of keratin.  The cells also contain an oily substance, the result of lysosome disintegration, which gives the skin its waterproof properties. 
        Stratum granulosum -  This layer is the outmost layer of living cells.  It typically contains several layers of squamous cells, with small basophilic granules in their cytoplasm.  These granules are a step in the synthesis of keratin. 
        Stratum sponosum – This layer of skin consists of several layers of cuboidal cells.  Cells are joined by desmosomes, as structure for cell-to-cell adhesion.  These desmosomes give the cells their spiny appearance.  Cells of the stratum ganulosum also produce filaments called cytokeratins, which provide structural support, and help resist abrasion.  The nuclei of the cells are darkened, as the cells are beginning to die, having been cut off from their nutrient supply.  
        Stratum basale – The stratum basale is the layer of keratinocytes directly above the dermis.  The layer consists of a single row of tall columnar cells which lie on a basement membrane, and  undergo rapid mitosis to replenish the cells that are lost on the surface.  All of the cells of the upper layer originate here, and migrate upwards.  A quarter of the cells in this layer are melanocytes, which produce melanin, which gives skin its pigmentation. 

Dermis – The layer beneath the epidermis, the dermis consists of connective tissues, and cushions the body from strain.  This is the layer which contains the nerve endings, hair follicles, blood vessels, and sweat glands.  The blood vessels provice nourishment to not only the dermis, but the stratum basale as well.  The dermis is split into two regions, the papillary region and the reticular region.
            Papillary region - This region is made of areolar connective tissue, which a mesh like tissue with a fluid matrix that functions as a cushion.  It contains fingerlike papillae that extend towards the epidermis and provides the surface that integrates with the epidermis, strengthening their connection and providing nourishment.  In some areas, such as hands and feet, the papillae projects in such a way to form contours in the skin’s surface, called friction ridges, which allow better grasping.  These ridges occur in patterns, such as fingerprints. 
            Reticular region – the region below the papillary region, it is much thicker and composed of dense irregular connective tissues.  It is composed of collagenous elastic and reticular fibers, which give the dermis is strength, extensibility and elasticity.  This region contains hair roots, sweat glands, receptors, nails, and blood vessels.  As a side note, this is also where tattoo ink and stretch marks are located. 

Cutaneous Tissue- this lies below the dermis, and attaches the skin to bone and muscle, along with supplying blood vessels and nerves.  It consists of loose connective tissue and elastin, and on average, 50% of the body’s fat, which serves as padding and insulation.  The cells types found here are fibroblasts and adipocytes. 


Scars – Scar tissue is the tissue that results from the healing of all but the most minor of lesions.  It is not identical to normal skin tissue, and usually of inferior quality.  They are less resistant to UV radiation, and do not contain sweat glands and hair follicles.  When the dermis is damaged, the body lays down new collagen fibers, which results in a fortuna scar, which is flush to the skin.  When the damage extends to underlying fat or muscle, however, scars can have a sunken or pitted appearance.  There are other types of scars, that form when something goes wrong.
            Hypertrophic scars– these appear as a red raised lump on the skin, but stay within the boundaries of the original wound.


Keloid scars– these scars carry on growing into large benign tumors, consisting of inert masses of collagen. 

Keloid in a piercing