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On the skin surface are the sweat gland pores (100-200/cm2)
and the openings of the sebaceous glands (50-100/cm2).
Their secretions ensure skin moisture and oiliness, and thus maintain
the hydrolipid film. The epidermis itself has no blood vessels, so
the nutrients are supplied through the fine blood vessels in the dermal
papillae.
The epidermis consists of up to 90 percent keratinocytes, the actual
epidermal cells, that are held together by what are called desmosomes.
The epidermis is differentiated into five layers:
 Horny layer
(stratum corneum)
Clear layer
(stratum lucidum)
Granular layer
(stratum granulosum)
Prickle-cell layer
(stratum spinosum)
Basal layer
(stratum basale) |
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Schematic diagram of the epidermis: the basal cells change, through
differentiation, into flat horny skin cells that are without nuclei.
1 Horny layer
2 Clear layer
3 Granular layer
4 Prickle-cell layer
5 Basal layer
6 Basal membrane
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The stratum basale (basal = basis, ground/lat.) is the lowest layer
of the epidermis. The basal cells lie directly on the basal membrane
that forms a definite border between the dermis and epidermis. The
basal cells acting as mother-cells, by cell division, provide for
the continuous regeneration of the skin. The daughter-cells are slowly
driven, by the active cell division, into the outer lying layers where
they undergo various development stages. In the basal layer are also
found the melanocytes, which are the pigment producing cells.
The stratum spinosum (spino = thorn, prickle/lat.), the prickle-cell
layer, is above the basal layer. In it are, visible for the first
time, the keratinosomes, membrane-bounded vacuoles (Odland bodies).
They contain the precursors of the epidermal lipids in the form of
disk-like (lamellar) lipid bilayer membranes. |
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Above the prickle-cell layer is the stratum granulosum (granula =
grain/Lat.), where the cornification (keratinization) of the keratinocytes
begins. It gets its name from its appearance, which is due to the
presence of what are known as keratohyaline granules,
a mixture of several smaller protein units.
The stratum lucidium is also called the clear layer as it is highly
refractive. The cells have been extremely flattened and are closely
packed. The cell boundaries are no longer recognizable.
The stratum corneum (cornea = horny skin/Lat.) is the uppermost layer
of the epidermis. Between the cornified cells (corneocytes) lie the
epidermal lipids. The horny layer - especially the bottom third -
forms the permeability barrier, which is the skin's true barrier against
exogeneous factors. |
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Besides keratohyaline,
which is a precursor of keratin, the granules contain filaggrins -
the intercellular cement of the skin structure. |
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Through differentiation, the living, cylindrical basal cells lose
their nuclei and become flattened cornified cells, changing their
shape and composition in the process. The cells pass through the barrier
zone, the border zone between the living epidermal layers and the
horny layer, where the epidermal lipids are released.
The epidermis renews itself every 28 days through continual reproduction,
differentiation / cornification and desquamation (mechanical sloughing-off
of the uppermost horny cell layer). |
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Scanning electron microscope image of scaling
horny skin cells. |
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