The Biology OF Healthy And Aging Skin


the importance of understanding the ethnic differences in the properties of
skin increases. Not only do the differences in skin properties influence the
presentation of skin disorders, but cultural practices alter presentation as
well. Moreover, there has been a recent surge of information available
about the properties of ethnic skin. This chapter covers the differences in
the structure and function of ethnic skin.
2.2 Epidermis
The epidermal layer of skin is made up of five different layers: stratum
basale, stratum spinosum, stratum granulosum, stratum lucidum, and
stratum corneum. The stratum basale (also called the basal layer) is the
germinative layer of the epidermis. The time required for a cell to transition
from the basal layer through the other epidermal layers to the stratum
corneum is 24-40 days. The morphology and structure of the epidermis is
very similar among different races, though a few differences do exist.
Additional differences are explained below and listed in Table 2.1 [I].
The stratum corneum, the most superficial layer, is the layer responsible
for preventing water loss and providing mechanical protection. The cells
of the stratum corneum, the corneocytes, are flat cells measuring 50 urn
across and I urn thick. The corneocytes are arranged in layers; the number
of layers varies with anatomic site and race. There is no difference
between races in corneocyte surface area, which has a mean size of
900 ~m2 [2]. The stratum corneum of black skin is more compact than
that of white skin. Though the mean thickness of the stratum corneum is
the same among blacks and whites, black skin contains 20 cell layers
whereas white skin contains 16. This may reflect greater intercellular
adhesion in black skin [3].
Various studies with contradictory results evaluated spontaneous desquamation
of corneocytes in various races. Spontaneous desquamation can
reflect stratum corneum cohesiveness. In one study, the investigators
evaluated spontaneous desquamation on the upper outer arm in AfricanAmericans,
Caucasians, and Chinese-Americans [2]. Spontaneous desquamation,
measured as corneocyte count, was 2.5 times higher in blacks than
in whites and Asians. These authors acknowledged that these findings
were not consistent with previous studies that concluded that black skin
had increased cellular adhesion and transepidermal water loss. That is,
increased desquamation should correlate with decreased cellular adhesion.


Another study found spontaneous desquamation to be increased on the
cheeks and foreheads of whites compared to blacks [4]. Finally, another
study concluded that age and anatomic site but not race influenced spontaneous
desquamation [5]. Therefore, the answer to whether or not there are
racial differences in spontaneous desquamation is inconclusive.
Transepidermal water loss (TEWL) is the amount of water vapor loss from
the skin, excluding sweat. TEWL increases with the temperature of the
skin. Studies investigating differences in TEWL among races yield conflicting
results as well. Some studies conclude there is no difference in
TEWL among ethnic groups [4,6]. Others state that TEWL is greater in
black skin [7]. One study found Asian skin to have the greatest TEWL
among Asians, blacks, and whites [8], whereas another found Asian skin
to have the least amount ofTEWL [7]. This same study [7] found the skin
of blacks to have the greatest TEWL, followed by that of whites, then
Hispanics, and then Asians. Another study [6] found whites and Hispanics
to have similar levels of TEWL. Finally, Reed et al. [9Jcompared TEWL
of skin based on the degree of pigmentation, opposed to race alone. In a
small sample size of seven, this study found that Fitzpatrick skin types V
and VI had a more compact stratum corneum, increased intercellular adhesion,
and increased barrier strength. Warrier et al. [4] compared the TEWL
of skin at different anatomical sites suggesting that the anatomical sites
used to measure TEWL in previous studies may have been a confounding
factor. In this study, TEWL was lower on the cheeks and legs of black skin
compared to white skin. In whites, posterior auricular and forehead stratum
corneum had a higher degree of TEWL than that of the forearm,
medial thigh, and back. In general, studies that measure TEWL of the
forearm, medial thigh, and back concluded that black skin had a higher
rate of TEWL than whites while another measuring TEWL of the checks
and legs concluded black skin to have a lower rate of TEWL. In summary,
concrete evidence regarding the difference in TEWL between different
races has yet to be established.
Aside from TEWL, hydration is also a characteristic of skin. One of the
ways to measure hydration, or water content, is conductance. Conductance,
the opposite of resistance, is increased in hydrated skin because hydrated
skin is more sensitive to the electrical field [10J. Skin conductance is higher
in blacks and Hispanics than whites [10]. Lipid content in black skin
is higher that that of white skin [11]. However, black skin is more prone
to dryness, suggesting that a difference in lipid content plays a role.
This includes the ratio of ceramide/cholesterollfatty acids, the type of

ceramides, and the type of sphingosine backbone. One study suggests
that the degree of pigmentation influences lipid differences [9]. A more
resistant barrier was demonstrated in darker skin.

Author : kaabinet



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