The research presented here examines the progression of aging in skin, and its effects on the structural properties of that tissue. There are few surprises here for those familiar with the SENS vision for rejuvenation therapies. The accumulation of cross-links in the extracellular matrix is thought to be a primary cause of loss of elasticity in tissues, and features prominently in this open access paper. Methods of safely breaking persistent cross-links, perhaps resulting from the work on glucosepane carried out at the Spiegel Lab, funded by the SENS Research Foundation, should help to restore elasticity to skin, and more importantly, to blood vessels.
The most visible effects of aging are observed in skin and have been extensively studied for medical and cosmetic purposes. The three skin layers are affected both structurally and functionally. However, aging primary impacts the mechanical integrity of the dermis. At macroscopic scale, the mechanical behavior of aged dermis shows an increased stiffness and a decreased ability to recoil. At lower scales, a complex multi-parameters process eventually results in a decrease of collagen and elastin contents due to an imbalance between matrix proteins synthesis and degradation by matrix metalloproteinases, an increase of collagen cross-linking, a deterioration of proteoglycans and a subsequent loss of water.
Collagens are the main component of the dermis and other connective tissues. Fibril-forming collagens assemble into striated fibrils, the diameter and three-dimensional organization of which are tissue-specific. They form multiprotein networks with other matrix proteins such as the elastin fibers and non-fibrillar matrix (proteoglycans, glycoaminoglycans...) that determine the mechanical behavior of dermis and other collagen-rich tissues. Collagen fibers are usually heterotypic structures. In dermis, they are made of type I, III and V collagens. Type V collagen is a minor component that acts as a regulatory fibril-forming collagen. As such, it plays an important role in the pathogenesis of the classical Ehlers-Danlos (EDS) syndrome. This rare connective tissue disease illustrates the close link between collagen microstructure and tissue mechanics since it is caused by mutations in collagen V genes. EDS patients show a prematurely aged skin, which illustrates the close link between collagen microstructure and skin aging.
The relationship between collagen hierarchical structure and mechanical behavior has been explored using numerical simulations from the molecular scale and constitutive models have been proposed to explain the skin mechanical behavior. Recently, multiphoton microscopy has been used to monitor the reorganization of collagen microstructure during mechanical assays in skin and in various tissues. This allows us to measure simultaneously the microstructural reorganization of the tissue under mechanical stimulation and the mechanical behavior at macroscopic scale, which provides multiscale experimental data not accessible using other techniques.
This study aims at addressing the role of aging on the mechanical multiscale behavior of skin. This issue is addressed in murine skin because of easier availability of matched groups at different ages. We combined traction assays with multiphoton microscopy in ex vivo skin samples from mice aged 15 to 20 months. We compared these data to our previous results obtained in one-month old mice. We studied both wild type (WT) and genetically-modified mice for which collagen V expression in skin has been modulated, inducing modified biomechanical behavior in young mice.
Age-related microstructural changes in the dermis affect collagen fibers as well as the other components of the extracellular matrix. Notably, there is a progressive decrease in collagen, elastin and proteoglycans content, an increase in glycation cross-linking within and between fibers and the matrix proteins become fragmented with little spatial structure. Accordingly, the dermis is thinner, as reported in the literature for human skin and observed in our data for murine skin. The age-related structural changes of the skin have been reported to be largely similar in human and murine skin. Specifically, it has been reported that the collagen content decreases by about 30% between 2 (young) and 22 (old) months in murine skin.
Microstructural data for WT mice showed that the microstructural reorganization upon imposed stretch is the same for young and old mice. This might be surprising considering the many age-related changes in the skin ultrastructure and the quantitative changes of the mechanical parameters. This absence of variation may be explained by the combination of two effects that compensate each other: the increase of cross-linking in old mice impedes the collagen network reorganization, while the decrease of collagen content facilitates this reorganization. The collagen network has a higher level of organization in old mice. This may be attributed to the cumulated effect of skin stretching during lifespan, which is facilitated by the reduced content of collagen and its increased cross-linking. Further, the overall volume of old skin samples decreased upon stretching compared to young mice. This may be explained by the degradation of proteoglycans in old mice, which results in a decreased efficiency to retain water during mechanical stimulation, and therefore a smaller final volume.
In conclusion, our simultaneous observations of the mechanical behavior at macroscopic scale and of the microstructure of dermis are well explained in the framework of our multiscale interpretation of skin mechanics, which seems applicable to both aged and young murine skin. The two main microstructural changes affecting the mechanical properties appear to be the age-induced cross-linking and the degradation of the proteoglycan non-fibrillar matrix, which let the water flow out more easily. All these considerations emphasize the complex role of the microstructure in the mechanical properties. Our findings open the door to future research on human skin to verify whether the above findings obtained in murine skin fully apply to human skin.