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Section
1a. Anatomy
(Bilayer Organ)
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| EPIDERMIS –
The outer thinner layer known as the epidermis
is composed mainly of epithelial cells.
The outermost cells contain the tough
protein keratin and are known as
keratinocytes. The basal or deepest epidermal cells are anchored to the
basement membrane by adhesion molecules (or
glue); namely fibronectin.
These immature cells are continually
dividing and migrating toward the surface,
to replace lost surface cells; e.g.,
after an injury.
The same types of regenerating
epidermal cells are found in hair follicles
and other skin appendages, which are anchored
in the dermis.
As the cells mature and migrate to the
surface, they form keratin, which becomes an
effective barrier to environmental hazards
such as infection and excess water
evaporation. |
| The stratum corneum is the “outermost layer” of the epidermis
consisting of several flattened layers of dead
keratinocytes as well as keratin.
This layer protects against entry of
bacteria and toxins.
Replacement
of the epidermal layer by this regenerative
process takes 2-3 weeks.
However, regeneration depends on an
optimal tissue-healing environment, which
includes contact cues for cell replication and
migration as well as chemical stimuli known as
growth factors.
These cues and stimuli at the wound
surface are necessary to direct proper
orientation and mitotic response of the
epidermal cells.
Many of the cues come from dermal
elements, especially the matrix protein
fibronectin, and matrix compounds such as
hyaluronic acid. |
| DERMIS –
The dermis is divided into a thin, superficial
layer known as the papillary dermis,
containing the anchoring epidermal rete pegs
and the thicker, deeper portion known as the
reticular dermis. The papillary dermis is the
most bioactive portion of the dermis.
The primary cell type is the
fibroblast, which produces the key structural
extra-cellular matrix proteins, collagen and
elastin, as well as matrix or ground
substance.
In addition, these cells produce the
key adhesion proteins used to attach epidermal
cells to the basement membrane and are used
for epidermal cell migration and replication.
Fibronectin is a key fibroblast derived
signal protein for orchestration of healing.
The ground substance or matrix is made
up of complex polysaccharide-protein complexes
known as protein, glycosaminoglycans or the
GAG component, as well as hyaluronic acid. The matrix is a semi-fluid that allows cell and connective
tissue orientation, nutrient diffusion to the
cells and also provides a scaffolding for cell
migration. |
| BASEMENT MEMBRANE – The interface between the layers of the
dermal-epidermal junction is the basement
membrane, which anchors the epidermal cells
from above and the dermis from below. |
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THICKNESS – Average thickness of the bilayer is 1-2 mm
and is considerably thinner in infants and the
elderly, especially the dermis that is
underdeveloped in infants and atrophic in the
elderly.
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| Fig.1
Anatomy of Skin |
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| Normal
skin is shown with the key epidermal and dermal
elements.
The epidermis contains the dividing epithelial
cells at the basement membrane migrating to the
surface forming keratinocytes and maintaining the
surface barrier.
The basement membrane, rich in adherence
proteins, anchors the epithelial cell layer.
The dermis is made of components produced by
fibroblasts, including collagen (mostly type I),
glycosaminoglycans, and other matrix proteins. |
| Fig.2
Anatomy of Normal Skin |
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| This
cross section of normal skin demonstrates the key
components.
Keratin, a product of keratinocytes, resides on
the outer most surface.
Keratinocytes (epithelial cells) make up the
first or outer layer of skin, i.e. the epidermis.
Note that keratinocytes can also be seen in the
cross sections of hair follicles found in the dermis.
The dermal layer is composed of collagen seen
as swirls of pink material.
The lighter pink homogenous material is made up
of glycosaminoglycans and other compounds produced by
the fibroblasts (elongated dark cells) such as
fibronectin and hyaluronic acid. |
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