Transepidermal water loss (TEWL): a measure of SC
barrier function
Transepidermal water loss which is also known as
TEWL is the amount of water that escapes through
the skin and into the environment. This occurs
because of the differences in the water vapour
pressure between the environment and the skin
(Honari and Maibach, 2014). It is often used as a
measure of how well the barrier of the SC functions
(Barel and Clarys, 1995). Shimada et al., 2008
suggested that when TEWL is high, the barrier
function is usually reduced, meaning that individuals
with lower TEWL have greater barrier function
(Shimada et al., 2008).
The studies produced from previous literature have
shown that there are mixed views about which ethnic
group contains the highest and lowest TEWL values,
for example, Kompaore et al., 1993 proved that
TEWL was higher in Black and Asian ethnic groups.
These findings were further emphasised by Sugino
et al., 1993, who also found that the most significant
amount of TEWL was within Black individuals and in
between were Whites and Hispanics.
However, this study also found that Asians had the
least amount of TEWL, which contradicts Kompaore
et al., 1993 Asian TEWL results. On the contrary,
Berardesca et al., 1991 established that all three
ethnic groups, Hispanics, Blacks, and Whites, have no
variation in TEWL; these findings, therefore, dispute
all the studies mentioned above. The contradictions
amongst the studies consequently show the need for
new studies regarding TEWL; as it can be applied in
important processes such as, in product development
and certain clinical settings. TEWL is also a reasonable
tool in measuring skin irritation, meaning that it can
play a major role in cosmetic and pharmaceutical
transdermal formulations (Honari and Maibach, 2014).
The differences in Corneocytes within different
ethnicities
Corneocytes, as mentioned, plays an essential role in
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maintaining the barrier function of the skin (Wickett
and Visscher, 2006), a study by Corcuff et al., 1991
emphasised on the size of the corneocytes between
Blacks, Asians, and Caucasian individuals. The results
from their study proved that Black-skinned individuals
have larger corneocytes, followed by Asian and
Caucasians. The values obtained were 911μm 2 for
black skin, 909 μm 2 for Asian skin and 899 μm 2 for
Caucasian skin, because the values are closely related
to each other, it was, therefore, suggested that there
is no significant difference in the size of corneocytes
within all three ethnic groups (Corcuff et al., 1991).
In addition, the amount of corneocyte desquamation
has produced conflicting results; desquamation
is a complex biological process of shedding single
corneocytes; meaning shedding of the skin (Pierard et
al., 2000). Warrier et al., 1996 found that Caucasians
had greater desquamation in comparison to Black
individuals; however, Wesley and Maibach, 2003
observed that Black ethnic group have a greater rate
of corneocyte desquamation. This was suggested to
play a role in the high cases of Xerosis also known
as dry or ashy skin in Black skinned individuals in
comparison to other ethnic groups (Warrier et al.,
1996; Wesley and Maibach, 2003).
Pigmentation
Pigmentation plays a crucial role in bringing variation
within different ethnic skin types; it is controlled by
melanocytes. These cells emerge from neural crest
melanoblasts and migrate to areas such as the hair
follicles and the basal layer of the epidermis after
the neural tube is closed (Videira et al., 2013). The
melanocytes secrete an essential pigment known
as melanin, which gives rise to the diversity in the
skin colour of individuals of different ethnic groups
(Yamaguchi and Hearing, 2009).
Melanin undergoes a synthesis process carried out by
membrane-bound organelles known as melanosomes.
Melanosomes contain tyrosinase, which is a melanin
specific enzyme that is involved in the melanin
production process. During this process, dopachrome