Biomedical Engineering Reference
In-Depth Information
15.2 Preliminaries
15.2.1 Skin Microanatomy
Figure 15.1 presents a photomicrograph of a human skin section. The het-
erogeneity of the tissue is immediately evident. The main barrier to perme-
ability resides in the highly specialized outermost layer of the epidermis, the
stratum corneum (SC). A major function of this layer is to maintain water
homeostasis, although many other SC functions have been described [7]. This
10-20
m thick layer consists of anucleated keratin-rich corneocytes embed-
ded in a highly ordered lamellar layer comprised of fatty acids, ceramides,
and cholesterol derivatives. The standard “bricks and mortar” representation
of SC structure in which the lipids act as the “mortar” holding the corneo-
cyte “bricks” in the appropriate geometric array is depicted in Fig. 15.2a.
Several other models exist in the dermatology literature. In particular the
detailed “domain mosaic” model of Forslind [8] has gained popularity. As
shown in the atomic force micrograph in Fig. 15.2b, corneocytes are highly
asymmetric cells,
μ
∼
0
.
5
m thick along the “
z
”-direction (perpendicular to the
μ
∼
plane of the skin) and
m in the other two dimensions. The lipid con-
stituents organize themselves into non-covalent supramolecular membranous
sheets which constitute the primary barrier to permeability. Some possible
packing motifs available to the lipids, namely orthorhombic, hexagonal, and
liquid crystalline, are schematically depicted in two ways in Fig. 15.2c. The top
figure depicts standard bilayer structures which become thinner with looser
50
μ
SC
Epidermis
Dermis
Fig. 15.1.
Photomicrograph of a human skin section. The thickness of the stratum
corneum (SC) shown ranges from about 15 to 20 μm. Scale bar = 20 μm
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