Biomedical Engineering Reference
In-Depth Information
Hair
Stratum
corneum
Meissner's
corpuscle
Pore
Stratum
lucidum
Stratum
granulosum
Epidermis
Dermal papillae
Free nerve
ending
Stratum
spinosum
Stratum
basale
Sebaceous
gland
Papillary layer
Arrector
pili muscle
Sensory
nerve fiber
Hair follicle
Dermis
Reticular layer
Hair root
Hypodermis
(superficial fascia)
Adipose tissue
Pacinian corpuscle
Eccrine sweat gland
Vein Artery
Root hair plexus
Figure 12.1 Schematic cross-section of the skin.
different layers identified according to their role in influencing skin permeability
through various pathways.
12.1.2.1 Stratum Corneum
The thick (approximately10-20 m) surface layer, the SC, is highly hydrophobic and
contains nearby 10-15 layers of dead or dying keratinized cells, corneocytes, which
are constantly shredded and renewed. Its organization can be described by the “brick-
and-mortar” mode, in which extracellular lipid accounts for approximately 10% of
the dry weight of this layer, and 90% is intracellular protein (mainly keratin). Such
“brick-and-mortar” architecture of the outer skin layers effectively prevents transder-
mal drug permeation. The SC lacks phospholipids but is enriched in ceramides and
neutral lipids (cholesterol, fatty acids, cholesteryl esters) that are arranged in a bilayer
format called lipid channels. Interdigitated long-chain -hydroxyceramides provide
cohesion between corneocytes by forming tight lipid envelopes around the corneo-
cyte protein component. The barrier function of the skin is created by lamellar gran-
ules, which are synthesized in the granular layer and later become organized into the
intercellular lipid bilayer domain of the SC. Barrier lipids are tightly packed, and any
impairment to the skin results in active synthetic processes to restore them. The skin's
barrier function appears to depend on the specific ratio of various lipids; among polar
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