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Issue
41 — July 2006 |
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Collagen
fibre bundles and their significance in skin aging |
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The skin is the largest, heaviest and functionally most versatile organ
of the human body. The skin has important functions including regulation
of body temperature, protection against environmental insult and moisture
loss as well as various immune functions. The appearance of the skin changes
with increasing age. There is a loss in elasticity, tautness, smoothness
and softness, and a general decrease in the overall functionality of the
skin as well as an increase in dryness and roughness of the skin surface.
The formation of wrinkles, a typical phenomenon of skin aging, is a complex
procedure and is a central topic in cosmetic research and product development.
As they form a scaffold of connective tissue within the dermis, collagen
fibre bundles are responsible for many skin-typical phenomena including
the formation of wrinkles. Fibroblasts, the cells of the dermis that actively
form connective tissue, supply the raw material e.g. collagen, reticular
and elastic fibres for the extracellular as well as for the intercellular
matrix. Inactive fibroblasts are termed fibrocytes, although they may become
active under certain conditions such as regeneration.
Fibre formation primarily takes place extracellularly. Building blocks which
resemble bamboo shafts, the microfibrils, are secreted and attached to the
cell surface of the fibroblasts. This is also the area where polymerisation
of the raw material into fibre-like components takes place. Microfibrils
grow and mature until they reach their typical length. By incorporating
further raw materials present in the surrounding matrix they then aggregate
and form bundles of defined collagen fibrils and fibres.
Contrary to the elastic fibres, which are arranged in a meshwork within
the connective tissue, the elasticity of the collagen fibre is insignificant
(< 5 percent = high module of elasticity). A certain degree of elasticity
may be attributed to the fact that the fibrils cross-link covalently to
each other to form a latticework. The amount of cross-linking correlates
with the required elasticity. The largest possible shift exists at right-angled
fibre crossings. In areas in which a stronger mechanical stress occurs,
the collagen fibre systems are predominantly oriented along the lines of
stress.
Our illustration gives a schematic representation of collagen fibre bundles
formation, the age-dependent decrease of collagen formation, fibroblasts
and the latticework of collagen fibre bundles. Fibroblasts -- which are
shown as spindle-shaped connective tissue cells -- with irregularly formed
cell extensions, produce raw material, while the fibre formation takes place
extracellularly (collagen, elastic, reticular). In regeneration processes,
more collagen than elastic fibres are produced.
The meshwork structure of the connective tissue (see schematic representation
below) provides for a uniform elasticity of the skin.
The age-correlated decrease of the production of collagen as well as the
increase in degradation of the collagen fibre bundles (see two inserts,
enlarged) disturb the optimal function of the cross-linking principle. This
leads to irregularities in skin elasticity and a reduction in the collagen
supply of the skin.
Reference: Otto Bucher, Histologie und mikroskopische Anatomie des Menschen (3rd
Edition, 1962) Medizinischer Verlag: Hans Huber, Berne, Stuttgart
