ABSTRACT
The natural (normal or physiological) changes in the connective tissue, designated by the general term “aging”, are currently characterized by the following features (Freemont & Hoylan, 2007): • a reduction in tissue mass due to a disbalance between the synthesis and degradation of the extracellular matrix;
• a decrease in cells’ biosynthetic activity; • a reduction in the number of active progenitor cells; • a change in the molecular structure of the matrix, caused by changes in the expression of genes and posttranslational modifications, especially pronounced in fibrillar proteins such as collagens and elastin, but also affecting other numerous macromolecular components;
• accumulation of degraded macromolecules in the matrix;
• a decrease in the functional efficiency of all tissue elements;
• changes in the levels of the regulatory factors (hormones and local-action signaling molecules), as well as in cells’ ability to respond to these factors and
• changes in how the tissue responds to the applied loads.
These are the signs through which aging is manifested in all types of connective tissue. Of course, in each of its types, aging has its peculiarities depending on the cellular and molecular structure of the tissue and its properties. The aging process actually develops throughout ontogenesis and is primarily governed by the action of internal (intrinsic, endogenous) factors. These factors, regulating the process at the molecular level, are subdivided into two groups: genetic and epigenetic. Genetic factors entail the influences of genes that define the program for overall cell activity and control its execution. Epigenetic (from Greek epi – over, after) factors include the influence of gene-encoded products and modulating of the functioning through various mechanisms, and also act in the extracellular matrix after getting out of direct control of genes. During its lifetime, the endogenous factors of connective tissue aging are accompanied by exogenous factors, i.e. the environmental influences, the physiological and pathogenic effects of which are difficult to demarcate. Of all exogenous influences, the effect of light (or, more precisely, of the ultraviolet spectral region), which plays an important role in the aging of the dermal layer of exposed skin areas, should be mentioned in the first place. This role is reflected in special terms such as the “light aging”, “actinic aging” or “photoaging” of skin. The molecular mechanism of light aging, superimposed upon the effects of natural (chronological) aging and accelerating its development, includes, as its main component, the formation, under the influence of ultra-violet radiation, of reactive oxygen species (ROS), which destroys the tissue’s antioxidative protective systems. The
