How Ground Substance Contribute To The Manifestations of Aging Skin

Posted on by
Reply

The ground substance occupies the space between the cells and fibers of dermal connective tissues. It is an amorphous viscoelastic gel in which the other components (cells and fibers) are held in place . Ground substance consists largely of proteoglycans and glycosaminoglycan (GAG) and large amount of water that are synthesized and secreted by fibroblast. Water can make up sixty to seventy percent of the ground substances, and it is retained there because of the glycosaminoglycan (GAGs). Proteoglycans are large macromolecules consisting of a core protein to which many glycosaminoglycan (GAG) molecules are covalently attached. Glycosaminoglycan (GAG) are long-chained polysaccharides made up of repeating disaccharide units. One of the sugars in each disaccharide unit is a hexosamine (also called glycosamine), hence the name GAG. Many of the sugars in GAGs have sulfate and carboxyl groups, which makes them highly negatively charged. The high density of negative charges make proteoglycan and GAGs hydrophilic that are capable to bind to 1000 times its volume of water, forming a hydrated gel with viscous property. This gel permits the rapid diffusion of water-soluble molecules but inhibits the movement of large molecules. There are seven distinct GAGs identified based on differences in the specific sugar residues, the nature of the linkages and the degree of sulfation. Four types of GAGs also comprise of GAG components of proteoglycans: chondroitin sulfate (CS), dermatan sulfate (DS), heparan sulfate (HS), keratan sulfate (KS). Sometimes, nonfibrous glycoproteins also considered as part of the composition of the ground substance as well. Nonfibrous glycoproteins synthesized and secreted by skin fibroblast is not as well defined as compared to that of proteoglycan.

Hyaluronic acid (HA or Hyaluronan ) is the most important and abundant GAG in the dermal connective tissue. It differs in several aspects from typical GAGs. It is extremely long and rigid, consisting of a chain of several thousand sugars, as opposed to several hundred or less in other GAGs. Proteoglycans can indirectly bind to HA via linker proteins, to form giant macromolecules. Various researchers have estimated that HA can bind one thousand to eight thousand times its volume of water. Another estimate suggests each HA protein in the extracellular matrix has fifteen thousand molecules of water associated with it!

The water binding capacity of ground substance give skin its plump characteristics. Loss of water or dehydration is one of the reason skin become wrinkled and laxed. The hydrated gel and semi-liquid property of the dermal connective tissue contribute to skin’s biomechanical property such as viscoelasticity as well as the ability to resist compression (incompressibility) of the skin. Ground substance contribute largely to the viscous component of skin’s nonlinear viscoelasticity when collagen fibers slide and realign in response to tensile (stretching) stress. However, a cyclic loading and unloading of the tissue is important to maintaining health.

Beside its strong water binding capacity, the protein core of the proteoglycans are capable to interact with other components (cells and fibers) of the matrix, thereby is important in maintaining and organizing the matrix structure, influencing fibroblast proliferation, differentiation and migration, and regulating collagen fibrillogenesis. Two of the most abundant proteoglycans in the skin is decorin and versican. Decorin can bind to collagen type I, III whereas versican can bind to elastic fibers.

When the extracellular matrix is well hydrated, cells, nutrients, and other components of the matrix can move about freely. Waste products can migrate out of the matrix into the blood or lymphatic system to be removed from the body. The ground substances are also helpful in resisting the spread of infection and are a part of immune system barrier.

During the aging process, the ability of the body (fibroblast) to create HA, other GAGs and proteoglycan diminishes. Fewer fibroblasts are available in skin and amount of senescent fibroblast increases with aging. As a consequence, the extracellular matrix becomes filled more and more with disorganized and damaged fibers that are cross-linked. As a result, skin’s biomechanical properties changes, causing loss of elasticity, tensile strength, and incompressibility and less open to the flow and migration of the other components such as immune cells in response to infection in the dermis.

Hyaluronic acid (HA) anti-aging dermal filler are widely used to treat wrinkles and facial lines that are dehydrated. Hyaluronic acid is also a common anti-wrinkle ingredients in the formulation of topical anti-aging creams combined with other anti-aging ingredients.

The Role of Fibronectin In Skin Aging

Posted on by
Reply

Fibronectin is a glycoprotein of the extracellular matrix that binds to extracellular matrix components such as collagen, fibrin, heparan sulfate proteoglycans (e.g. syndecans) and membrane-spanning receptor proteins called integrins. Fibronectin exists as a protein dimer, consisting of two nearly identical monomers linked by a pair of disulfide bonds. There are two forms of fibronectin – the soluble and insoluble form. Soluble form of fibronectin is a plasma protein. Insoluble cellular fibronectin is a major component of the extracellular matrix called ECM fibronectin. It is secreted by various cells, primarily fibroblasts, as a soluble protein dimer and is then assembled into an insoluble matrix in a complex cell-mediated process. Fibronectin plays a major role in cell adhesion, growth, migration, differentiation, wound healing and matrix assembly. Altered fibronectin expression, degradation, and organization has been associated with a number of diseases. In the skin, fibronectin can function as a general cell adhesion molecule by anchoring cells to collagen or proteoglycan. FN also can organize cellular interaction with the ECM by binding to different components of the extracellular matrix and to membrane-bound FN receptors (i.e. the adhesion molecule integrin) on cell surfaces . There are three functional domains in fibronectin termed as FN type I, II, III. Twelve type I modules make up the amino-terminal and carboxyl-terminal region of the molecule, and are involved mainly in collagen and fibrin binding. Collagen binding site is FN type I6–9. Only two type II modules are found in FN. They are instrumental in binding collagen. The most abundant module in fibronectin is Type III, which contains the RGD (integrin binding tripeptide) FN receptor recognition sequence along with binding sites for other integrins and heparin. There are four fibronectin-binding domains, allowing fibronectin to associate with other fibronectin molecules one of which is called assembly domain located at FN type I5, and is required for the initiation of fibronectin matrix assembly.

ECM Fibronectin is assembled into an insoluble fibrillar matrix in a complex cell-mediated process. Fibronectin matrix assembly begins when soluble, compact fibronectin dimers that are secreted from cells, often fibroblasts. These soluble dimers bind to integrin receptors on the cell surface and the clustering of the integrins occur next. The local concentration of integrin-bound fibronectin thereby increases, allowing bound fibronectin molecules to more readily interact with one another. Short fibronectin fibrils then begin to form between adjacent cells. As matrix assembly proceeds, the soluble fibrils are converted into larger insoluble fibrils that comprise the extracellular matrix. Fibronectin’s shift from soluble to insoluble fibrils proceeds when fibronectin-binding domains are exposed along the length of a bound fibronectin molecules. Cells are believed to stretch fibronectin by pulling on their fibronectin-bound integrin receptors. This force partially unfolds the fibronectin-binding domain and allowing nearby fibronectin molecules to associate. This fibronectin-fibronectin interaction enables the soluble, cell-associated fibrils to branch and stabilize into an insoluble fibronectin matrix. fibronectin-binding domain (fibrillogenesis) is a critical regulator of extracellular matrix organization and stability. Fibronectin matrix assembly is also involved in the healing mechanism in response to tissue injury.

The age-related changes in fibronectin synthesis and degradation in human fibroblasts were studied in vitro. The amount of cell surface fibronectin and released fibronectin (extracellular fibronectin) in early and late passaged (senescent) human skin fibroblasts were measured. Cell surface fibronectin decreased dramatically in senescent fibroblast in vitro. A progressive increase in the rate of fibronectin synthesis per cell was observed by late passaged (senescent) human fibroblasts with no difference in the turnover of fibronectin, suggesting the rate of synthesis and degradation of fibronectin are both increased in senescent fibroblast in vitro. The increased fibronectin synthesis in senescent fibroblasts appeared to correlate with the general increase in rate of protein synthesis/cell. There are age-related defect in the biological activity of human fibroblast fibronectin regarding its activity in Interaction with collagen and in mediating cell-ECM component adhesion . In comparison to fibronectin isolated from early-passage cells, fibronectin from late-passage cells (in vitro aged) bound poorly to native collagen types I and II. This defective binding to native collagen may account for some aspects of the aged skin phenotype. There are reduced cell-ECM adhesion in aged fibroblast and this decreased cell-ECM component adhesion was due to a defect in fibronectin. Analysis of fibronectins purified from early and late passage (aged) fibroblast indicates that there are striking differences in their abilities to promote cell adhesion. In addition, normal fibroblast morphology were changed in the presence of the fibronectin isolated from aged cell. This defective cell-matrix adhesion function may also account for the overall stability and strength of the ECM in the skin. However, there seems no direct evidence linking the defective fibronectin or fibronectin fibrillogenesis on or released from the aged fibroblast to the clinical manifestations of an aging skin appearance such as wrinkles and skin laxity.

There are some anti-wrinkle ingredients that are claimed to be able to stimulate the production of fibronectin (and collagen) such as the widely used peptide ingredient Matrixyl 3000. ChroNOline™ is an commercial anti-wrinkle ingredient with proven clinical efficacy developed by Atrium that are able to boost the production of key components of the dermal-epidermal junction -collagen VII, laminin-5, and fibronectin. ChroNOline™ can induce/increase fibronectin production by 60% compared to the cells without adding ChroNOline™ in an in vitro assay and thus firmly anchors cells to the matrix for optimal cell functions. Deliner™ is a corn extract that smoothes out wrinkles by activating healing mechanisms in the skin. Deliner™ reduces the depth of wrinkles and smoothes the skin by its specific action on fibronectin and its activation of cell migration and multiplication.

Decreased Expression of Tissue Inhibitor of Metalloproteinases (TIMP) In Chronologic Aged Skin

Posted on by
Reply

Tissue inhibitor of metalloproteinases (TIMP) is a group of endogenous enzymes that inhibit the activity of matrix metalloproteinases (MMPs), which is a group of peptidases involved in degradation of the extracellular matrix. . There are four different TIMP identified – TIMP1, TIMP2, TIMP3, TIMP4. TIMP1 is a glycoprotein that can inhibit the activity of the most of the known MMPs. In addition, TIMP1 is able to promote cell proliferation in a wide range of cell types including keratinocyte and dermal fibroblast, and may also have an anti-apoptotic function. Beside as a natural inhibitor of MMPs, TIMP2 has a unique role among TIMP family members in its ability to directly suppress the proliferation of endothelial cells. TIMP3 is induced in response to mitogenic stimulation and the protein contain a netrin domain and is localized to the extracellular matrix.

The matrix metalloproteinases (MMPs) in the skin play a key role in the maintenance and normal turnover of dermal extracellular matrix. Matrix metalloproteinases (MMPs) are capable of degrading essentially every component of the dermal extracellular matrix including collagen. Excess MMPs activities in the aged skin is associated with the excess degradation and damage of dermal extra cellular matrix (ECM), and with the clinical signs of aging skin. An important natural and internal mechanism for the regulation of the activity of MMPs is via binding to a family of homologous proteins referred to as the tissue inhibitors of metalloproteinases (TIMP-1 to TIMP-4). MMP activity is regulated by the highly specific, endogenous tissue inhibitors of metalloproteinases (TIMPs). MMP and TIMPs can form complexes. Members of TIMPs have been found to exhibit several biochemical and physiological/biological functions including inhibition of active MMPs, proMMP activation, cell growth promotion, matrix binding, inhibition of angiogenesis and the induction of apoptosis.

The regulation of relative amount of and balance of MMP and TIMPs is critical in maintaining skin’s matrix stability and homeostasis in dermal connective tissue. Overexpression of MMP and down regulation of TIMP will result in excess degradation and damage of the extra cellular matrix, particularly the collagen fiber networks, thereby causing wrinkles and skin laxity. Deficiency in MMP level and excess TIMP will also adversely affect the turnover and recycling of the extra cellular matrix. Aged skin is characterized by a strongly increased MMP activity and by a concomitant down regulation of tissue inhibitor of matrix metalloprotease (TIMP) and the resulting dramatic loss and damage of collagen and matrix as well as impaired cell growth and survival. TIMP is decreased with fibroblast senescence, both ex vivo and in vivo, thus contributing to increased catabolic activity within dermis. Although cumulative sun exposure (photo-aging) lead to an immediate and strong increase in MMP activity, the concentrations of the native tissue inhibitors of metalloproteinases (TIMPs) remain unchanged. It is very well established that UV radiation induces MMPs without affecting the amount or activity of TIMPs.

Some anti-aging ingredients were found to be about to regulate the TIMP amount and activity in the skin. DHEA significantly decreased the basal expression of MMP-1 mRNA and protein, but increased the expression of TIMP-1 protein in aged skin. PEPHA®-TIMP is a commercial patented anti-wrinkle ingredient, utilizing the bioengineered TIMP2 protein. The addition of PEPHA®-TIMP attenuates the signs of aging by reducing the degenerative effects of excessive MMP activity. Due to the versatile specificity of TIMP-2 on the most important MMPs regarding skin aging, PEPHA®-TIMP supports the attenuation of the degenerative processes of photo-aging and intrinsic aging based on in vitro and in vivo studies.

Structural Change In Decorin With Skin Aging

Posted on by
Reply

Decorin is an extracellular matrix proteoglycan belonging to the small leucine-rich proteoglycan (SLRP) family with a protein core containing leucine repeats with a glycosaminoglycan (GAG) chain of either chondroitin sulfate (CS) or dermatan sulfate (DS). Decorin, together with versican (bind to elastic fibers) are the most abundant proteoglycan found in the connective tissue of dermis. Decorin binds to type I collagen fibrils – the dominant collagen type (another collagen in skin dermis is type III collagen) forming the network of collagen fibers in the skin dermis. Biological functions of decorin in the skin is to regulate matrix assembly and collagen fibrillogenesis – the development of fine fibrils (fine fiber of ~ 1 nm in diameter) normally present in collagen fibers of connective tissue. Collagen fibrils (1 nm) are formed via cross-linking of its precursor procollagen or tropocollagen. Procollagen or tropocollagen is a triple helix protein with 3 peptide chains and is synthesized by fibroblast and then secreted. The formation of collagen fibril (the polymer of tropocollagen) from tropocollagen or procollagen is known as collagen fibrillogenesis. Because collagen fibers and elastin fibers form the structural bone of skin dermis layer, providing skin its strength and elasticity. The disrupt and dysfunction of collagen fibrillogenesis and extracellular matrix assembly will adversely affect the overall health of skin. Decorin were discovered in the epidermal cells (keratinocytes) suggesting that this molecule plays an important role in the regulation of skin’s homeostasis as well.

Studies have shown that decorin level decreases in photo aged skin which may account for a progressive disorganization of collagen fibers in aged skin. The decorin alteration in chronological aged skin appear to be contradictory to that of photodamaged skin. Decorin amount was increased by 50% secreted from the senescent fibroblast culture in vitro. In a study of skin samples extracted from matured and aged skin showed an significant increase of decorin with a concomitant decrease of versican in senescent skin. Not only the amount of decorin changes with skin aging, the structure of the GAG chain of decorin changes with skin aging as well. Decorin is distributed along collagen fibrils with the core protein and the decorin GAG chain controls the distance between the collagen fibrils. Reducing the length of the decorin GAG chain reduces the distance between the collagen fibrils. Age-related changes in decorin are apparent in the length of GAG chain and sulfate location but not in the core protein. Structural changes in the decorin GAG chain may be related to changes in collagen matrix assembly during the aging process. The increase of decorin and decrease of collagen Type I, III and the decreased of collagen to decorin ratio in aged skin may be responsible for the decreased collagen fiber bundle diameters in aged skin which might affect their tensile strength.

In addition, the study showed the presence of a truncated form of decorin in its core protein component in aged skin which is a catabolic fragment of decorin known as decorunt that is absent in fetal skin. Analysis of collagen binding by surface plasmon resonance indicates that the affinity of decorunt for type I collagen is 100-fold less than that of decorin. This observation correlates with the structural analysis of decorunt, in that it lacks regions of decorin previously shown to be important for interaction with type I collagen. The detection of a catabolic fragment of decorin suggests the existence of a specific catabolic pathway for this proteoglycan. However, the role of this catabolic pathway and decorunt in regulating collagen fibrillogenesis and matrix assembly is not clear. In addition, decorunt also lack the domain for binding to epidermal growth factor receptor (EGFR) and transforming growth factor-beta (TGF-beta) which may induce a defect of skin cell stimulation and renewal.

One peptide anti-aging ingredient used in cosmetic products has been shown to increase decorin – the Copper TriPeptide GHK-Cu. This peptide has been scientifically proven to tighten collagen in aging skin, tighten loose skin, reduce wrinkles and improve skin elasticity.

The Phenotypes of Aging Facial Skin

Posted on by
Reply

The clinical signs of aging facial is commonly described and treated based on three general anatomical locations. The upper face has many interrelated components, including the hair, forehead, glabellar area (the space between the eyebrows), the temple, eyebrows and eyes. Midfacial aging is directly related to changes that occur in the periorbital region and the cheek. Areas of lower facial aging include the perioral region and chin. Rhytidosis is wrinkling of the face to a degree disproportionate to age. Ptosis is the droopiness of a body part.

  1. thinning hair and receding hairline
  2. forehead rhytids (forehead creases)
  3. glabellar rhytidosis (glabellar lines i.e. frown lines); Glabellar is the smooth area between the eyebrows just above the nose.
  4. brow ptosis (drooping eyebrows);
  5. eyelid ptosis (drooping or falling of the upper or lower eyelid);
  6. temple rhytidosis and ptosis (temple creases and sagging);
  7. upper eyelid redundancy (dermatochalasis) and ptosis; Dermatochalasis is a medical condition defined as an excess of skin in the upper or lower eyelid. Redundant and lax eyelid skin is known as dermatochalasis.
  8. lateral canthal rhytidosis (crow’s feet); canthus is either corner of the eye where the upper and lower eyelids meet
  9. lower eyelid redundancy and rhytidosis
  10. nasal root rhytidosis; nasal root is the top of the nose, the indentation at the suture where the nasal bones meet the frontal bone.
  11. lower eyelid bag (puffy eye)
  12. malar bag (cheek bag) formation; malar bag is the swelling over their cheekbones
  13. cheek rhytidosis;
  14. preauricular rhytidosis; preauricular is in front of the auricle of the ear
  15. nasal tip ptosis and dependency;
  16. cheek sagging and fat atrophy changes; Fat atrophy is the loss of fatty tissue in a localized area of the body. Also known as lipoatrophy or lipodystropohy
  17. deepening nasolabial crease (nasolabial fold, smile lines); nasolabial folds are the two skin folds that run from each side of the nose to the corners of the mouth.
  18. facial (cheek) rhytidosis and sagging;
  19. perioral rhytidosis;
  20. upper lip flattening and lengthening;
  21. thinning and atrophy of vermillion
  22. chin pad ptosis and retraction;
  23. jowl formation;  sagging of the lower part of cheek (drooping jawline). Jowls are the areas of excess tissue around jawline.
  24. cervical rhytidosis;
  25. platysmal banding; The platysma covers the superficial fascia of the neck and is closely connected to the skin. It draws the lower jaw and the corners of the mouth down, expands the skin of the neck, and extends the skin in vertical lines. Platysmal bands occur due to diastasis of the midline platysmal muscle and loss of submental fat.
  26. rhytidosis and midneck hollowing;
  27. submaxillary gland ptosis. submaxillary gland (submandibular gland) is the salivary glands situated on each side behind the lower jaw

Phenotypes of various facial skin aging can be treated with anti-aging plastic surgery or nonsurgical skin procedures most commonly the anti-aging dermal filler. Dermal fillers are the nonsurgical procedures to fill the areas of wrinkles and lax skin with water, collagen and etc. to treat the wrinkles and laxity caused by loss of volume.

Sirtuins And Skin Aging

Posted on by
Reply

Sirtuin (SIRT1 -SIRT7) are a group of enzymes with varying intracellular location that either function as histone deacetylase or as mono-ribosyltransferase. Sirtuins are classified according to intracellular location and enzymatic activities and function. SIRT1, 2, 3. 6 are deacetylase and SIRT4, 6 are ADP-ribosyl transferase.

Sirtuins are involved in a diverse biological functions including cell development, metabolism, gene silencing, DNA repair, cell cycle progression, apoptosis, heterochromatin formation and especially longevity. Sirtuins regulates transcription, apoptosis and energy efficiency especially in response to stress and calorie restriction. Sirtuins regulates the activity of the many genes that are responsible for the metabolism, cell defense, reproduction and other bodily functions. Sirtuins are known to play a role in the aging process and life extension. The role of Sirtuins in aging is based on the fact that:

1) Sirtuins with histone deacetylase activity mediate the histone acetylation/deacetylation modification of DNA which in turn either switch on or off the genes during aging process. Histone deacetylation (gene silencing) pattern have been known to be associated with the programmed mechanism of aging. Sirtuins are NAD dependent enzymes that the acetylation/deacetylation reaction require NADH hydrolysis. The dependence of sirtuins on NAD links their enzymatic activity directly to the energy status of the cell via the cellular NAD:NADH ratio. Mitochondria Sirtuins with deacetylase activities may be associated with cellular energy regulation.

2) Sirtuins with mono-ADP-ribosyltransferase activity mediate DNA repair function. Body’s internal DNA repair mechanism is an indispensable component of the protective mechanisms that functions to repair DNA damages (DNA mutation) induced by external stressor such as free radical (oxidative stress) and UV radiation and other mutagens. This process provide a protective mechanism for slowing down the aging process.

The pattern and the level of sirtuins from skin samples with varying age were studied and compared. Comparative studies of skin samples aged 30 to 55 did not reveal any significant age-related difference in SIRT1 level without external stimuli. Sirtuins levels are increased, however, upon UVB irradiation in a dose dependent pattern suggesting the anti-aging role of sirtuins in response to stress induced aging process. With aging, the extent of the increase of sirtuin proteins in response to stress is decreased, indicating that this adaptive anti-aging function to stress is less effective in aged skin cells. The addition of STAC (selective sirtuin activating compound) to the aged skin cell samples in vitro restores this adaptive ability which result in extension of the lifespan of the aged skin cells.

Sirtuins can be the anti-aging enzyme to be targeted on in the skin care industry. Sirtuins may prolong the life of the fibroblasts – the cell synthesize important skin proteins collagen and elastin and other ECM components – of skin. Various studies i vitro and ex vivo using sirtuins stimulators (resveratrol or other sirtuin activator) has revealed that the significant increase of SIRT1 level in normal human dermal skin fibroblasts in vitro (+172%) and in epidermal cells of healthy human skin ex vivo induced by SIRT1 activator can result in decreased cell senescence and DNA fragmentation induced by ultraviolet-B (UVB) stress. In vivo studies indicate that facial improvements could be seen on fine lines and wrinkle, complexion radiance, firmness, complexion homogeneity, and texture was significant immediately after the first application of SIRT1 activator.

One of the most effective known methods of life extension proposed is caloric restriction which works in part by increasing the activity of sirtuins. The calorie restriction approach emphasize a diet strategy with low calorie intake without compromising the essential nutrients such as vitamins and others etc. The theory that caloric restriction may extend life span is base on the fact that the mitochondria energy metabolism is accompanied with byproduct free radicals (ROS, the reactive oxygen species) which can damage cells and biomacromolecules. Through suppressing the energy metabolism to an appropriate level, the effect of free radical damage may be minimized. Sirtuins are the class of enzymes that are induced and increased in response to caloric restriction.

The fact that Sirtuins are partially responsible for the life span and effects on health of caloric restriction has lead to the search for ingredients that will stimulate sirtuin activity in the body. An effective sirtuin activator is resveratrol – an anti-aging ingredients in topical skin care products and other life extension products. Sirtuins, found in Pro-SirtuinSX in Avon Anew Ultimate Age Repair Elixir Serum and Night Cream, prolong the life of the fibroblasts by turning off unnecessary gene expression (i.e., when the fibroblasts aren’t expending more energy than they need to on unnecessary tasks, they will last longer). There are companies that are claiming to produce creams that contain sirtuins instead of sirtuins stimulators, this would presumably by-pass the need for a sirtuin activator. However sirtuins are fairly unstable and a large protein that may not be effective if the ingredient can not be absorbed into the dermis layer of the skin efficiently, the same issue as using collagen as the ingredient instead of collagen stimulators.

Age-Related Changes in The Proteoglycans of Skin

Posted on by
Reply

The dermis of skin is a connective tissue that contains an extensive extracellular matrix (ECM) whose biophysical properties are determined primarily by this matrix. Although collagen and elastin are the major extracellular matrix molecules of the dermis that provide skin its structural strength and elasticity, other molecular components such as proteoglycans also contribute to the overall mechanical properties of skin. Proteoglycans is one of the major ground substances in the extracellular matrix of the dermal connective tissue synthesized and secreted by dermal fibroblast.

Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a “core protein” with one or more covalently attached sulfated carbohydrate (glycosaminoglycan (GAG)) chains. The major biological function of proteoglycans derives from the physicochemical characteristics of the glycosaminoglycan component of the molecule, which provides hydration (due to its strong water binding capacity) and swelling pressure to the tissue enabling it to withstand compressional forces (skin’s incompressibility) as well as the viscous component of skin’s viscoelasticity when collagen fibers slide and realign in response to tensile (stretching) stress. In addition, Proteoglycans act as connective tissue organizers, influence fibroblast proliferation, differentiation and migration, and regulate collagen fibrillogenesis.

Proteoglycans can be categorized depending on the nature of their glycosaminoglycan chains. There are four types of glycosaminoglycan (GAG) found in proteoglycans: chondroitin sulfate (CS), dermatan sulfate (DS), heparan sulfate (HS), keratan sulfate (KS). Proteoglycans can also be categorized by size. Large proteoglycans include versican, perlecan, neurocan, aggrecan; Small proteoglycans include: decorin, biglycan, testican, fibromodulin, lumican. The most abundant proteoglycans in skin are decorin or decorunt (a catabolic fragment of decorin) and versican. Versican interact with elastic fibers in skin whereas decorin interacts with collagen by binding to type I collagen fibrils, and plays a role in matrix assembly and collagen fibrillogenesis thereby is crucial for robust tensile strength of skin through the establishment of a sound network of collagen fibers . Decorin also interacts with fibronectin, thrombospondin, the complement component C1q, epidermal growth factor receptor (EGFR) and transforming growth factor-beta (TGF-beta) which suggest a role in angiogenesis and skin cell renewal in response to tissue injury. Versican also have a role in cell adhesion, migration, and proliferation.

The contribution of age-related changes in skin proteoglycans to the age-related changes in the mechanical properties of skin were studied. Proteoglycans from human skin of various ages were extracted and analyzed. Samples were obtained from fetal, mature, and senescent skin. A decrease in versican and a concomitant increase in decorin was observed as a function of age. There are age-related differences in the size and polydispersity of decorin and the core proteins of decorin. The most pronounced change is the appearance in mature skin of decorunt (a catabolic fragment of decorin). Because of the known ability of decorin to influence collagen fibrillogenesis and fibril diameter, the appearance of decorunt may have a significant effect on skin elasticity and extensibility. The observation that proteoglycans in skin show dramatic age-related differences suggests that these changes may be involved in the age-related changes in the biophysical properties of skin.

Based on the immunochemical studies using monoclonal antibodies to the distinct epitope of GAG chains, an age-related common distribution pattern of GAGs in proteoglycans existed in skin. The relative amounts of specific types of glycosaminoglycans (GAG) chains in proteoglycan varied in an age- and layer-dependent manner. In the epidermis there was a notable increase in keratan sulfate beginning at age 50. Chondroitin 6-sulfate, found principally in the basal lamina, decreased after age 60. In the papillary dermis, the amount of dermatan sulfate increased after age 50, whereas the amount of a chondroitin sulfate epitope decreased with age. Thus, age-related changes in proteoglycan distribution exist and correlate with morphologic and functional changes that occur in the intrinsic process of skin aging although this study can not reveal which specific core protein (proteoglycan) is associated with the various domains of GAG chains detected.

The Structural Basis of Skin’s Biomechanical Properties

Posted on by
Reply

The mechanical properties of the skin are of importance for various cosmetic and clinical applications. The mechanical properties of the skin are age-, gender-, and race- and region dependent and are influenced by the use of different skin care products and by numerous other parameters and different physiologic conditions as well as skin disorders. Therefore, monitoring alteration in the mechanical properties of the skin as reflected in changes in its viscoelasticity help to diagnose skin lesions or diseases as well as evaluate cosmetic treatment results.

A number of techniques have so far been introduced in experimental and clinical medicine for quantitative in situ evaluation of the mechanical properties of skin. These assays, based on different physical principles, include indentometry, uniaxial tensiometry, torsion measurements, skin compliance to suction, and measurements of speed of elastic shear wave propagation in the skin. The measurement of the speed of propagation of elastic shear waves in viscoelastic materials may be the most accurate noninvasive assay method for the evaluation of their mechanical properties. The portable and user-friendly viscoelasticity skin analyzer (VESA) is the advanced device based on this principle.

Skin is a heterogeneous anisotropic (directional variations in viscoelasticity), non-linear viscoelastic material due to the nature and organization of skin structure. Skin has both viscous and elastic characteristics when undergoing deformation. Elasticity is the ability to retract back to the original shape after deformation. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. The viscous component of skin’s viscoelasticity is due to the viscous resistance fibers experience while moving through the ground substance which account for the time dependent behavior (hysteresis) of dermis. The viscous component is associated with energy dissipation and the elastic component is associated with energy storage.

Skin is multilayered (epidermis and dermis) and has different mechanical properties in each layer. Generally, the nonlinear anisotropic viscoelasticity of skin depend mainly on the structural organization of the dermal collagen and elastic fibers network, and water, ground substances in the extracellular matrix with less importance, and with less or neglected contribution by epidermis layer. Although the epidermis is stiffer than the dermis, usually the contribution of the epidermis to the mechanical properties of full thickness skin is neglected. Collagen fibers (type I and type III): and elastic fibers arrangement give out different material properties. Applying mechanical stress to skin changes collagen fibers and elastic fibers arrangement and thereby producing skin’s unique stress-strain curve – nonlinear, hysteresis, creep. Skin has non-linear viscoelastic properties and exhibit hysteresis loop effect with energy loss when deformation occurs. Purely elastic materials do not dissipate energy when a load (stress) is applied, then removed. However, a viscoelastic substance loses energy when a load is applied, then removed. Hysteresis is observed in the stress-strain curve, with the area of the loop being equal to the energy lost during the loading cycle. Viscoelastic biomaterials show a time-dependent elastic behavior – that is – the stress-strain curve is not the same for loading and unloading, leading to hysteresis. Creep is a skin mechanical failure-the result of water molecules displacement from collagen fibers network. Creep is the tendency of a solid material to deform permanently under the influence of stresses.

In order to understand the mechanical behavior of the skin, the mechanical behavior of the dermal components are described followed by the illustration of the stress-strain curve of skin:

1) Collagen fibers – the main constituent of the dermis – form an irregular network of wavy coiled fibers that run almost parallel with the skin surface. The fibers are separate from each other along most of their length and held together by the ground substance. Collagen is characterized by high tensile strength of 1.5-3.5 MPa, high stiffness. The width of the bundles is 1-40 μm. Upon stretching, collagen fibers straightens and realign parallel to one another. Collagen types self-assembly, i.e. tilt angle of collagens (orientation), fiber length, volume fraction of the fibers, collagen molecular stretching contribute to the biomechanical behavior of skin in response to stress (tensile or compressive loading). Energy applied to skin is partially dissipated through viscous sliding of collagen fibrils during alignment with the direction of the stress. Changes in the collagen fibril orientation during deformation of the dermis are critical to maintaining the large extensibility of the skin.

2) Elastin fibers are the second main component of the dermis. They are less stiff than collagen and show reversible strains of more than 100%. The fiber width is 0.5-8 μm. The elastin fibers are responsible for the elastic mechanism (shape recovery after deformation) when stress or deformation is released.

3) The ground substance is responsible for the viscoelastic behavior of the dermis and they do not contribute to the tensile strength of the dermis. Fibers experience viscous resistance while moving through the ground substance.

The relationship between the stress and strain that a particular material displays is known as that material’s Stress-Strain curve. It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading (stress). The stress-strain curve of skin for uniaxial tension (uniaxial tensiometry) is nonlinear due to the non-uniformity of its structure, as can be seen in the figure. Four stages of deformation (stretching) can be identified:

I. stage I is the initial stretch: the effect of undulated collagen fiber on the strain (extension) is neglected, elastin is responsible for the skin stretching, and the stress-strain relation is approximately linear. The extension is immediate and reversible when tension is applied, and is therefore called an elastic extension – The elastic component dominates the deformation.

II. Stage II is a gradual straightening of an increasing fraction of the collagen fibers causes an increasing stiffness.

III Stage III , In the third phase all collagen fibers are straight and the stress-strain relation becomes linear again. is the subsequent continuous extension while tension is maintained and is gradual and irreversible and has therefore been termed viscous extension, viscous slip, or creep.

IV Stage IV, is where yielding and rupture of the fibers occur beyond the III stage. Tensile strength can be used to measure at which stress point yielding and rupture occurs.

Tensile strength measures the force required to pull (stretch) something. Tensile strength is the opposite of compressive strength and the values can be quite different. Ultimate Tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen’s cross-section starts to significantly contract. Typical human skin’s ultimate tensile strength (UTS) is ~ 20 MPa

The yield strength or yield point of a material is the stress at which a material begins to deform plastically. Plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. Typical human skin’s yield strength is ~ 15 MPa

For strains corresponding to the initial large deformation part of the stress-strain curve (phase I), skin shows elastic behavior. For stage II and III skin shows visco-elastic behavior. Skin exhibits all three features of a viscoelastic material. If the dermis is subjected to consecutively applied load cycles, slightly different curves are obtained and significant hysteresis can be observed. Furthermore skin shows stress-relaxation under constant strain and creep under constant stress. The dermal thickness in vivo is less thick than the excised dermis indicating that skin is in a state of pre-tension (prestress). The extent of prestress varies with age, body location and direction relative to that of the fibers. The network of elastic fibers is believed to be mainly responsible for the prestress.

Age-Related Changes in Facial Skin Contours and Rheology

Posted on by
Reply

Topography of the skin surface is a mirror of the functional skin status. Surface roughness is an important criterion for assessing the health status of the skin. Changes in roughness occur, among other things, in the case of congenital keratinization disturbances, environment- and job-related skin irritations, infectious skin diseases and age-related defects. Such changes to the skin cannot be objectively evaluated by classical examination methods such as palpation or visual assessment. The biomechanical properties of skin and skin surface contours reflect the structural organization of aging tissues. Therefore, its evaluation is of great interest for dermatological research. As a result, rheological and profilometric measurements permit a simple and noninvasive characterization of the overall aging process of facial skin. Usually, non-invasive methods are used to evaluate the skin surface topography. Most of these methods are based on the preparation of skin replica or reprints. The replica are measured using mechanical or optical profilometry techniques and surface image analysis. Skin contours (the effect of wrinkling on the topography of the skin surface) can be assessed and quantified using optical profilometry and computerized image analysis, although the diversity of measurement methods available with wide variation in experimental conditions has created discrepancies.

Research data showed a significant increase in skin extensibility and a significant decline in elasticity with aging. The loss of tonicity is accompanied by a progressive deepening of facial creases. Age-related rheological alterations was evident with a significant increase in MD (maximum deformation), DD (differential distension), and HY (Hysteresis). Conversely, values of BE (Biological elasticity) declined significantly.

Several skin surface parameters or profilometric variables are used to characterize skin contours -roughness (Ra), depth of roughness (Rz), maximum roughness (Rm, Rz max), waviness (Wt). These parameters indicate different properties of the network of the furrows. The age-related skin contours were measured. The research data show a significant dependence of the skin surface topography on the age the body site measured. Particularly the waviness and the furrow profiles reflect such dependencies. Data obtained from facial samples revealed that Ra showed a non-significant trend in increase over time while Rz increased significantly with age. Rz max or Rm showed by far the greatest relative changes with a value of 42 ± 5 mum in the youngest age and almost doubled every decade. The deepening of natural expression lines was also evidenced by the progressive increase in the number of moderate (150-250 mum) and deep (> 250 mum) furrows, reaching about 15% and 30% per decade, respectively. Significant correlations ( P < .01) were found between values of Rz max and both the decrease in BE and the increase in HY, suggesting the correlation between the changes in biomechanical properties and the skin contours with age.

A disadvantage of the replica technique is that the quality of the skin replica could be compromised such as the formation of air bubbles. Therefore, in vivo methods without direct skin contact may be used for more accurate results. Micro-mirrors is based on the stripe projection onto the skin surface, and measuring its reflection. One such optical system is the PRIMOS optical 3D in vivo skin measurement device.

In order to assess the success of a therapy or of the application of an anti-aging product, the depth, variation and distribution of the furrows and wrinkles need to be measured prior and after application of the pharmaceutical or cosmetic product. The quantification and measurement of skin contours can be used to assess the efficacy of anti-aging anti-wrinkle treatments and procedures such as laser skin rejuvenation techniques by comparing the difference in rheological parameters and skin roughness parameters with that of the skin before applying the treatment.

Scientists Identified Genes Involved In Various Skin Aging Mechanisms/Processes

Posted on by
Reply

Using scientific advances that have recently emerged from the Human Genome Project, skin scientists are beginning to gain a deeper understanding of the genes involved in the skin aging process. Gene chip technology can measure the signals sent out by thousands of genes at a time. Using gene chips, skin scientists are in the process of identifying the genes that play a role in maintaining healthy skin as well as how the signals or messages sent out by genes change as skin begins to age. Gene chip technology allowed scientists to identify exact pathways that were overly active in the older skin such as (chronic) inflammation.

Using data generated by the human genome project, out of the 20,000 to 25,000 known human genes, researchers have found around 1,500 genes that play a key role in aging skin. ‘The human genome project has made it possible to analyze aging from hundreds of genetic changes that occur in the skin during the aging process. Skin ages due to intrinsic (e.g. free radical, inflammaging, mitochondria damage) or extrinsic causes (e.g. UV radiation). Scientist have found that groups of genes involved in the various intrinsic and/or extrinsic aging process are switched on/off. About 400 genes involved in inflammation become more active associated with aging process. 40 genes involved in the collagen degradation are found to be more active with aging. About 200 genes are found to affect or weaken the activity of natural neutralizer of free radicals – the natural molecule in the body that are antioxidants – with age. Another group of genes influence how the skin reacts to sunlight.

One of the most important factors in skin aging is hydration – the way that skin collects and retains its moisture, using natural water-binding molecules in the extracellular matrix of the dermis connective tissue known as ground substance such as proteoglycan. As skin gets older, the genes that control this process (moisture retention capacity) become less active and skin can retain less moisture, leading to wrinkles. Scientist has found that up to 700 genes could be involved in this process

By narrowing down the genes involved with skin aging, researchers hope to create drugs and creams which can stimulate some genes and suppress others to restore youthful looks and prevent skin aging. By understanding how specific genes are modulated by the skin aging process, scientists have been able to tailor treatments to inhibit those processes that are overactive in the aging skin and stimulate those pathways with reduced activity in aging skin.