The Skin - Epidermis

Thursday, October 28, 2010 Posted by Matt

The human skin is composed of three main layers. These layers have their very own physiology:
  • Epidermis
  • Dermis
  • Hypodermis / Subcutaneous Layer


This layer is composed of 5 sublayers:

  • Stratum Corneum (Horny Layer) -
                       This sublayer also has 2 sublayer's:  Stratum Compactum & Stratum Disjunctum

  • Stratum Lucidum (Clear Layer) - (Only present on palms of the hand, and soles of the feet).
  • Stratum Granulosum (Granular Layer)
  • Stratum Spinosum (Prickle Cell layer)
  • Stratum Basale / Stratum Germinativum (Germinating layer)

Cells of the Epidermis start in the Germinating Layer. The cells present here are Stem cells, Merkel cells and Melanocyte cells.

The Stem cells (Basal cells) constandly divide themselves through a process called  Mitsosis, resluting in one daughter cell that will change form, and the other that will continue as a Stem cell and divide again.  At this stage the cells have a nucleus (the brain of the cell).

Once the cells have divided, they start to push forwards and upwards. At this stage the cells are block like and robust.

The cells have now worked up to the Prickle Cell Layer.  This layer has a high amount of protective Langerhan Cells, which are essential for fighting infection in the body.

There are many Desmosomes in this layer which give the prickly/spiney appearence. Desmosomes are crtical for attatching cells together.

Cells become flatter as they push up and  reach the top of this layer. This layer is the biggest layer and approximately 3-6 layers thick

When the cells reach the Granular Layer, they start their dying process. The Nucleus of the cell begins to fail and the cell becomes much flatter. A protein called Keratin is now present in this layer and it mixes with the cells in a process called Keratinisation.

The Clear layer is only present on the palms of the hands and soles of the feet and is made up of clear cells which light can pass through.  These area's of the body are known as 'Thick Skin' whereas the areas of the body which don't have the Clear layer present are classed as 'Thin Skin'.

The cells begin their final days in the Horny layer. Here they have become completely flat and scale like. They are often referred to as Cornfield Cells in this layer.
This layer is the major skin barrier, and protects us from enviromental assaults penetrating and from the loss of water and moisture escaping.

The SC cells have completely lost their nucleus now and are classed as dead cells.

The cells are joined together by a mortar like consistency called Lipid, this is a waterproof substance which regulates the amount of water in the top layer (to 10%).  It stops the loss of excess water from the skin (Transepidermal Water loss - TEWL).

The Horny layer is divided into two sublayers; The Stratum Compactum where the cells are more densley packed and the Stratum Disjunctum where the cells finally shed from the body.  The cells shed gradually and sporadically.

The whole process takes roughly 28 days in a younger person, but as we get older it slows down and can take upto 50 days.
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