What are lipids?
As the daughter cells move upwards their shape
flattens, and they become joined by spiny processes to make another recognisable layer, known as the spiny
layer. These cells make special fats called sphingolipids.
When the cells reach the stratum corneum these lipids will play
an important part in the retention of moisture in the skin.
As the cells migrate further upwards they
develop characteristic granules; they now form part of the granular
layer. In the upper cells of this layer these granules discharge
and fill up the spaces between the cells with lipids,
The
different forms of the cells of the epidermis: as the cells move
upwards they gradually change shape.
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ultimately creating an appearance of a wall of bricks (cells) and
mortar (lipids).
As the cells rise into the top layer of
the epidermis - the stratum corneum, sometimes called the
horny layer or the cornified layer - they take the
form of flattened discs, tightly packed together. These flattened
cells, now called corneocytes, are effectively dead. The
number of layers of cells in the stratum corneum depends on the
site on the body; on the sole of the foot the stratum corneum is
at its thickest, and is there made up of hundreds of layers of densely
packed cells.
The stratum corneum acts as an outer 'covering'
to the skin, able to resist scrapes and scratches on the outside
and helping to keep water on the inside. In this respect it is rather
like the bark of a tree.
Sections
of the epidermis, compared: (top) aged 18, (bottom) aged 80. The
reasons why the epidermis changes throughout life are discussed
in Chapter 4, 'Skin and aging'.
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