The skin consists of two parts, an outer
epidermis and the
dermis which underlies it.
The epidermis is a
keratinized stratified squamous epithelium consisting
of four distinct cell types and five distinct layers. It thickness varies:
0.07-0.12 mm over most of the body
to 0.8 mm on the palms and 1.4 mm on the soles.
Its surface layer,
consisting of dead cells, is
rich in keratin, a protein which renders it dry and more or
less waterproof, thus resisting surface
evaporation and preventing excessive water loss.
It also serves as a protective barrier against
ultra-violet light, bacteria, many chemicals and abrasion.
The epidermal cells are:
- - 1 - the keratinocytes:
- compose most of the epidermis. They produce a protein: the keratin
that helps waterproof the skin and that protects the skin and the
underlying tissues from heat, microbes, abrasion and chemicals.
- - 2 - the Merkel cells:
- sensory receptors (touch).
They associate with a disclike sensory nerve ending
to form the Merkel disc.
- - 3 - the melanocytes:
- synthesize the melanin pigments
which protect the skin against ultraviolet damage.
- - 4 - the Langerhans cells:
- macrophages used in the defense against
microorganisms.
The epidermis consists of several layers of cells.
From the deepest to the most superficial we have:
- - 1 - the stratum basale (or stratum germinativum):
- is the deepest epidermal layer.
It consists of one row of cuboidal to columnar shaped cells. Those
cells are mostly stem cells that divide rapidly to produce
new keratinocytes which push up toward the surface
and become part of the more superficial layers
(You can understand why this layer is called "germinativum":
it "germinates" cells).
Ten to 25% of cells found here are melanocytes: their long branching
processes extend in between epidermal cells and reach into the more
superficial Stratum spinosum. Melanocytes that synthesize the pigment
melanin. Melanin passes on from the
melanocytes into the keratinocytes and will protect them
from the destructive effect of ultraviolet radiation.
Merkel cells are also found in this layer.
- - 2 - the stratum spinosum:
- is 8-10 layers thick. The keratinocytes contain thick bundles of intermediate filaments
(tonofilaments) made of a tension-resisting protein. Histological preparations
shrink these cells and make them look prickly hence the name of this
layer: spinosum (little spine). Scattered amongst keratinocytes
are the Langerhans'cells that are most abundant in this layer.
- Stratum germinativum and stratum spinosum
- are immediately adjacent to the
dermis and thus, contain the only epidermal cells that
receive adequate nourishment (by
diffusion of nutrients from the dermis).
As the daughter cells are pushed
upward, away from the source of nutrition, they gradually
die and their soft protoplasm
becomes keratinized (hard).
- - 3 - the stratum granulosum:
- here the keratinization process
begins and the cells begin to die. This layer is called granulosum because the
cells contain granules of the precursor of keratine.
- - 4 - the stratum lucidum
- is only found in thickened areas of
the epidermis such as the sole of
the feet. The cells appear clear (hence lucidum in their name: luci means clear)
because of an accumulation of
keratin precursor. The cells begin to degenerate.
- - 5 - the stratum corneum:
- is the outermost layer composed of dead, flat,
keratinized cells which are being sloughed off (they are your dandruff and the flakes that
slouch off dry skin. Corneum means horn: hard dead cells.
An easy trick to remember these 5 epidermal layers
in order from the most superficial to the deepest:
"Can Little Girls Speak German" (given by Lizzy and Jamie - L01 1997)
Can | Corneum |
Little | Lucidum |
GiRl | GRanulosum |
SPeak | SPinosum |
GERMan | GERMinativum |
The average thickness of the dermis is about 1 to 2 mm and varies from
0.6 mm on the eyelids to 3 mm or more on the soles and palms.
It contains lots of nerve fibers, sensory receptors, blood and
lymphatic vessels as well as the hair follicles, and the sebaceous and
sweat glands.
The dermis consists of two layers:
- - 1 - the Papillary layer
- is the outer layer closest to the epidermis.
It is composed of areolar loose connective tissue proper.
Its superior region contains fingerlike projections
called dermal papillae
(also give the layer its name: papillary) that indent the epidermis.
Dermal papillae contain
capillaries, bare nerve endings (pain receptors),
Meisner corpuscles (touch receptors: make us feel light touching) and the disclike sensory nerve
endings of the Merkel disks.
- - 2 - the Reticular layer
- is the deeper layer and the thickest (4/5th of the dermis).
It is made of dense irregular connective
tissue proper containing thick bundles of interlacing collagen
fibers and some coarse elastic fibers that run in several directions (but mostly in directions that are
parallel to the skin surface). The collagen fibers
in the reticular region provides the skin with strength and extensibility
(= ability to stretch) and elastic fibers provide its
elasticity (= ability to return to the original shape after stretching).
The reticular layer is
richly supplied with blood vessels and nerves,
and contains sensory endings for touch
(Pacinian corpuscle for sensing deep pressure such as bumps), pain, heat,
cold, etc. The epidermis projects down into the dermis
to form sweat glands, sebaceous glands and hair follicles.