Viscoelastic properties of human skin and processed dermis

Background/aims: The purpose of this work is to attempt to determine the elastic spring constant for collagen and elastic fibers (elastin) in skin and to detemine if the values of these elastic constants are similar to those reported for other tissues.
Methods: We studied the viscoelastic mechanical properties of human skin and dermis by measuring the incremental stress-strain behavior. Elastic stress-strain curves were used to obtain the elastic spring constant of elastin and collagen while the collagen fibril length was obtained from the slope of viscous stress-strain curves.
Results: Our results suggest that the elastic spring constant for elastin is about 4.0 MPa while that for collagen is about 4.4 GPa. The former value is similar to that calculated for ligamentum nuchae while the latter value is about 70% of the value found for tendon and self-assembled type I collagen fibers. The differences between the elastic constants for collagen molecules in tendon and skin is hypothesized to reflect the higher molecular tilt angle and lower D period found in skin compared to tendon as well as a shorter fibril length.
Conclusion: The differences in the collagen types present in skin and tendon may influence collagen self-assembly and the resulting viscoelastic properties.     

A 31P NMR study of extracted phospholipid composition of human skin: full thickness skin,dermis, epidermis and granulation tissue

Background/aims: Phospholipids are the most important component of the plasma membrane in skin cells. Until recently, skin phospholipids have only been studied by chromatography and those results need confirmation by other methods such as ³¹PNMR spectroscopy. Methods: Phospholipid profiles were analyzed from human skin and granulation tissue using ³¹PNMR spectroscopy. Skin samples were taken from lower extremity amputation specimens and from one healthy volunteer. The granulation tissue was obtained from the wound bed of venous stasis ulcers, pressure ulcers and post-op non-healing surgical wounds. Phospholipid extractions were performed on full thickness skin samples (excluding the subcutaneous layer), epidermis, dermis, and granulation tissue biopsies. Results: Twelve phospholipid species were detected in each of the groups studied: PA, CL, DHSM, Eplas, PE, PS, SM, LPC, PI, AAPC, and PC. Statistically significant differences were found between epidermis and full thickness skin in relation to the quantity of PA, CL, DHSM, LPC, AAPC, and PC. The phospholipid ratio [(PE+Eplas+PI+PA)/PC] was calculated for each group: the epidermis value was 1.30, the dermis value was 0.62 and full thickness skin value was 0.49. Conclusions: These phospholipid profiles present a very unique distribution, which correlates with the physiological activity of the individual layer. The study of the phospholipid profile of granulation tissue opens a new line of research, evaluating changes in these profiles during the various phases of wound healing.     

Characterization of the mechanical properties of a dermal equivalent compared with human skin in vivo by indentation and static friction tests

Background/aims: The study of changes in skin structure with age is becoming all the more important with the increase in life. The atrophy that occurs during aging is accompanied by more profound changes, with a loss of organization within the elastic collagen network and alterations in the basal elements. The aim of this study is to present a method to determine the mechanical properties of total human skin in vivo compared with dermal equivalents (DEs) using indentation and static friction tests.
Methods: A new bio-tribometer working at a low contact pressure for the characterization the mechanical properties of the skin has been developed. This device, based on indentation and static friction tests, also allows to characterize the skin in vivo and reconstructed DEs in a wide range of light contact forces, stress and strain.
Results: This original bio-tribometer shows the ability to assess the skin elasticity and friction force in a wide range of light normal load (0.5–2 g) and low contact pressure (0.5–2 kPa). The results obtained by this approach show identical values of the Young's modulus E ^* and the shear modulus G ^* of six DEs obtained from a 62-year-old subject ( E ^*=8.5±1.74 kPa and G ^*=3.3±0.46 kPa) and in vivo total skin of 20 subjects aged 55 to 70 years ( E ^*=8.3±2.1 kPa, G ^*=2.8±0.8 kpa).     

Relationships between changes in mechanical properties of the skin, wrinkling, and destruction of dermal collagen fiber bundles caused by photoaging

BACKGROUND/PURPOSE: Long-term exposure to ultraviolet (UV) radiation induces various cutaneous changes that differ from those because of physiological aging, including structural destruction of dermal collagen fiber bundles (DCFBs), which comprise the major component of the dermis. Wrinkling, a representative change in skin surface associated with photoaging, is often seen at the corners of the eyes and in the space between the eyebrows. These are locations where the skin contracts repeatedly and routinely. Lowered resiliency to skin contraction induced by marked structural changes in DCFBs may represent one cause of photo-induced wrinkles. Using animal models of photoaging, changes in mechanical properties of the skin caused by UV irradiation were measured, and relationships between UV-induced changes were analyzed. METHODS: Animal models of photoaging were prepared by irradiating hairless mice with UVB light. Dorsal skin surface replicas of animals were taken using silicon rubber, and volume of wrinkles was calculated using an image analyzer. Stress of the skin against horizontal contraction was measured using a new device called the Resiliometer. Three-dimensional organization of dermal collagen structures in skin samples collected from the back of each animal was observed under scanning electron microscopy, and compactness of DCFBs was assessed from electron micrography. RESULTS: With time and therefore increasing UV dose, deep wrinkles formed on the backs of mice. Volume of wrinkles peaked at 8 weeks. All parameters obtained from Resiliometer measurements were increased by irradiation. DCFB structure was degraded in a radiation dose-dependent manner. DCFB grading was significantly correlated with each Resiliometer parameter. Significant correlations were also observed between each Resiliometer parameter and volume of wrinkles. CONCLUSION: Stress of the skin against horizontal contraction obtained using the Resiliometer changes following UV irradiation, correlating with photo-induced wrinkling and destruction of DCFBs. These results support the hypothesis that changed force of restitution to skin contraction induced by marked structural changes in DCFBs represents one cause of photo-induced wrinkles. The resiliometric parameter may offer a good indicator for monitoring the condition of DCFB structure, as changes in these would induce failure in restitution to skin contraction, leading to wrinkling.     

Effect of age, breast size, menopausal and hormonal status on mammographic skin thickness

Background/purpose: The localized or generalized skin thickness detected on mammography may reflect an underlying pathology of breast or a systemic disease involving the skin. The aim of this report is to describe the range of normal breast skin thickness in women using a film-screen mammographic technique.
Methods: Measurement of the mammographic skin thickness over different parts of the breast was performed in 144 women who had normal findings in a combined mammographic and ultrasonographic examination. Patients were grouped as premenopause, postmenopause and surgical menopause who were under continuous oestrogen treatment. The skin thickness in four regions (superior, inferior, medial, lateral) of both breasts was compared, and their relations with age, breast size, menopausal and hormonal status were investigated. The interobserver reliability was tested in a small subgroup of patients.
Results: Interobserver agreement was good for all measurements. The range of normal breast skin thickness was between 0.50 and 3.10 mm. There were no differences in skin thickness between the corresponding regions of the breasts, with significant differences between the regions in the same breast. While breast size increased with age, skin thickness decreased in all regions.
Conclusion: The breast size, age, regional variations and hormonal status of the patients should be considered when defining the normal range of skin thickness in mammographic examinations. We assume that upper limit of mammographic skin thickness should be set as 3.0 mm, regardless of the focal spot size and film-focus distance.     

Influence of body water distribution on skin thickness: measurements using high-frequency ultrasound

BACKGROUND: Although it is known that the skin acts as a water reservoir and participates in the fluid content of the whole body, no method has been established to quantify the fluid shifts in superficial tissue. OBJECTIVES: The aim of this study was to investigate changes in dermal and subcutis thickness and echodensity at the forehead and lower leg by high-frequency (20 MHz) ultrasound under various physiological conditions influencing water balance. METHODS: These parameters were measured in the skin of 20 healthy male volunteers at baseline and successively at 30 min after lying down, in a head-down position, after physical activity and after infusion of 10 mL kg-1 body weight of Ringer's solution. RESULTS: Dermal thickness at the forehead showed a significant increase from baseline to a horizontal position and a further increase in the head-down position. Physical activity did not lead to further changes, whereas after fluid infusion the dermal thickness also increased markedly. The echodensity showed inverse changes, with decreasing values. The thickness of the subcutis increased slightly from baseline to a lying position and decreased in the head-down position and after fluid infusion. At the lower leg, skin thickness decreased slightly in the head-down position with elevated legs, and increased after fluid infusion. CONCLUSIONS: Our results show that slight changes in the water distribution of the body influence the thickness and the echodensity of the dermis. Changes are more pronounced at the forehead than on the lower legs. Further, the fluid storage takes place mainly in the dermis and not in the subcutis. High-frequency ultrasound is able to quantify these effects and is a sensitive method for measuring fluid intake and balance during anaesthesia and therapy.     

Intrinsic aging vs. photoaging: a comparative histopathological,immunohistochemical, and ultrastructural study of skin

Cutaneous aging is a complex biological phenomenon affecting the different constituents of the skin. To compare the effects of intrinsic and extrinsic aging processes, a total of 83 biopsies were collected from sun-exposed and protected skin of healthy volunteers representing decades from the 1st to the 9th (6-84 years of age). Routine histopathology coupled with computer-assisted image analysis was used to assess epidermal changes. Immunoperoxidase techniques with antibodies against type I and type III collagens and elastin were used to quantitatively evaluate changes in collagen and elastic fibers and their ultrastructure was examined by transmission electron microscopy. Epidermal thickness was found to be constant in different decades in both sun-exposed and protected skin; however, it was significantly greater in sun-exposed skin (P = 0.0001). In protected skin, type I and III collagen staining was altered only after the 8th decade, while in sun-exposed skin the relative staining intensity significantly decreased from 82.5% and 80.4% in the 1st decade to 53.2% and 44.1% in the 9th decade, respectively (P = 0.0004 and 0.0008). In facial skin the collagen fiber architecture appeared disorganized after the 4th decade. The staining intensity of elastin in protected skin significantly decreased from 49.2% in the 1st decade to 30.4% in the 9th decade (P = 0.05), whereas in sun-exposed skin the intensity gradually increased from 56.5% in the 1st decade to 75.2% in the 9th decade (P = 0.001). The accumulated elastin in facial skin was morphologically abnormal and appeared to occupy the areas of lost collagen. Collectively, the aging processes, whether intrinsic or extrinsic, have both quantitative and qualitative effects on collagen and elastic fibers in the skin.     

Ultrasonographic subepidermal low-echogenic band, dependence of age and body site

Background/purpose: The presence of an age-dependent subepidermal low-echogenic band (SLEB) is well established at specific body sites. Several studies have also proposed it to be a marker of photoaging, but only few studies have included children in their material, and UVR exposure has predominantly been evaluated as differences between body sites with different UVR exposure. The aim of this study was to further investigate the existence of SLEB and exploring its relation to both age and body site/UVR exposure.
Methods: In all 137 healthy volunteers from 4 to 68 years of age were included. In vivo high-frequency ultrasonography (B-mode, 20 MHz) was obtained from sun-exposed and sun-protected skin in a period with negligible environmental exposure to solar UVR. The SLEB was evaluated both visually and quantitatively.
Results: We found a significant correlation between visual scoring of SLEB and dermal echogenicity, which supports the usefulness of SLEB scoring.
We found that the grade (existence and development) of SLEB increased significantly with age at both the forehead and at the dorsal aspect of the forearm, where a well-defined SLEB was absent in children and present in over 50% of adults above the age of 40 years. No such age dependence was found either at the buttock or at the shoulder, and remarkably a well-defined SLEB was present in 37% of the adolescents at the buttock.
Conclusion: These results indicate that the age-related increase in SLEB grade at highly sun-exposed body sites might be attributed to UVR exposure. The presence of SLEB in both the young and at the sun-protected buttock points to a multifactorial etiology, where anatomical differences may play an important role as well as water content. SLEB might not be a very sensitive and specific marker of photoaging.