Skin: Glycosaminoglycans, Water, Lipids

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Glycosaminoglycans

Glycosaminoglycans (GAGs) are composed of specific repeating disaccharide
units. Those attached to a core protein are referred to as proteoglycans
and are found widely distributed throughout the skin. GAGs most often
present in human skin are hyaluronic acid (not attached to a protein core)
and the proteoglycan family of chondroitin sulphates, including dermatan
sulphate [41]. GAGs are especially important in skin because they bind
up to 1,000 times their volume in water. Therefore, skin hydration is
highly related to the content and distribution of dermal GAGs, especially
hyaluronic acid [41].
GAGs increase in photoaged skin compared to young or intrinsically aged
skin [38,41]. This seems paradoxical, because photoaged skin appears
leathery and dry, unlike newborn skin, which also contains high levels of
GAGs. Confocal laser scanning microscopy reveals that GAGs in photodamaged
skin are abnormally deposited on elastoic material, rather than
diffusely scattered as in young skin [41]. This aberrant localization may
interfere with normal water binding by GAGs, despite their increased
number.

Water

In young skin, most water is bound to proteins and, appropriately, is called
bound water [39]. This is important for the structure and mechanical properties
of many proteins and their mutual interactions. Water molecules not
bound to proteins bind to each other, and are called tetrahedron or bulk
water [39].
Intrinsic aging does not appear to alter water structure significantly [38].
However, in photoaged skin, Raman spectroscopy reveals an increase in
total water content. Again, this seems paradoxical, because aged skin
is often dry and weathered. However, structurally, significantly more of
the water in aged skin is in tetrahedron form. Thus, because proteins are
more hydrophobic and folded, and GAGs are clumped on elastoic material,
they interact less with water, and water in aged skin binds to itself
instead. This lack of interaction between water and surrounding molecules
in photoaged skin likely contribute to its characteristically dry and wrinkled
appearance.

Lipids

The “brick and mortar” model is often employed to describe the stratum
corneum’s protein-rich corneocytes embedded in a matrix of ceramides,
cholesterol, and fatty acids, and smaller amounts of cholesterol sulphate, glucosylceramides,
and phospholipids. These lipids form multilamellar sheets
amidst the intercellular spaces of the stratum corneum, and are critical to
the SC’s mechanical and cohesive properties, enabling it to function as an
effective water barrier [42]. Changes in SC lipid content have been linked to
skin conditions such as xerosis and possibly atopic dermatitis [42].
Many authors agree that overall lipid content of human skin decreases with
age [42–44]. Using high performance thin layer chromatography (HPTLC),
Rogers et al. found a 30% decrease in the face, hand, and leg in older subjects,
but the older group only extended to age 50. No significant change was seen
in proportional composition of lipid classes or ceramide species [42].
Schreiner et al. used small angle X-ray diffraction to compare lipid composition
in subjects aged 23-27 years to subjects aged 63-69 years [45]. They did
not see any overall difference in lipid quantity or composition between the
groups. However, the aged group consisted of only four subjects, and again,
included a narrow age range. Saint Leger et al. studied the lower legs of
50 subjects and found that the lipid profile was constant from age 50 upward;
overall, aging was associated with a slight decrease in sterol esters and triglycerides
[44]. Cua et al. noted significant regional variation within individuals
as they studied 11 sites on 29 people, comparing individuals in their
third decade of life to those in their eighth. Interestingly, they, too, found little
relation between skin surface lipid content and age, except on the ankle,
where the elderly demonstrated decreased lipid content [46]. From these conflicting
studies, it is difficult to conclude with certainty whether lipid content
decreases with age. Many confounding factors may hinder such studies,
including seasonal and diurnal variation, general interindividual variation,
and the use of several different methodologies by different researchers.

Author : kaabinet

kaabinet

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