It's our largest organ.  It's always visible.  We rub it, scrape it, cut it, burn it, expose it to chemicals and yet it sticks to us--virtually unscathed!  This amazing organ is our skin, technically referred to as our integumentary system.

Structure
Our skin is made of two distinct regions each composed of sublayers.  The superficial region is called the epidermis which literally means "upon the dermis".  This superficial region varies in thickness across the body's surface.  Thick skin covers the palms of the hands, soles of the feet and the fingertips.  Thin skin covers the remainder of the body's surface.  Why this difference in thickness?  Just think of all the rough treatment you give these thick skinned areas!  The deeper layer of the skin is called the dermis.  It contains two loosely organized regions known as the papillary area and reticular regions.  These regions resist tearing and provide the skin with elasticity so that our wrinkles are minimized.  Thank goodness for this wrinkle resistant region!  (For more wrinkle information, just click the link!)  Deep to the dermis is a portion called the hypodermis or subcutaneous layer.  This is actually a layer of connective tissue and is richly supplied with blood vessels.  Although it is not technically part of the skin, it shares in some of our skin's protective features.  In addition, it accumulates adipose tissue (fat) which serves as an insulator and a cushion.  This is the fat layer that many of us complain about--the layer that we often try to deplete with diet and exercise.

The Epidermis
The epidermis is made of clearly defined layers of cells, each showing specific characteristics. These layer include the following which are listed from deep to superficial:

• The stratum basale-the deepest of all epidermal layers.  This layer is made of one single layer of keratinocytes that are healthy and actively dividing (technically called "mitotically active cells").
• The stratum spinosum-is actually several layers of cells containing an abundant system of intermediate filaments (remember these from the cytology unit?).  As a result, cells in this layer appear to have spines connecting them to one another.  Functionally, these spiny projections help this layer to resist tension.
• The stratum granulosum-is made of three to five layes of keratinocytes that gradually flatten as they approach the surface.  In addition, they accumulate keratohyaline granules and lamellated granules which ultimately help form keratin to help waterproof the skin.  These layers of cells are so far from underlying blood vessels that they are unable to aquire nutrients efficiently and as a result, they die.
• The stratum lucidum-is made of a few rows of clear, flat, dead keratinocytes.  As a result, it appears as a clear band when seen using a light microscope.  This layer is present only in thick skin.
• The stratum corneum-is the most superficial layer of the epidermis.  The cells in this layer are flat and scale-like and are shed by abrasion or when bathing.  If fact, over an average lifespan we loose approximately 40 pounds of these dead cells!