The optic nerve head as a biomechanical structure: a new paradigm for understanding the role of IOP-related stress and strain in the pathophysiology of glaucomatous optic nerve head damage.

Biomechanical properties of keratoconus and normal corneas

Evolutionary modification has produced a spectrum of animal defence traits to escape predation, including the ability to autotomize body parts to elude capture. After autotomy, the missing part is either replaced through regeneration (for example, in urodeles, lizards, arthropods and crustaceans) or permanently lost (such as in mammals). Although most autotomy involves the loss of appendages (legs, chelipeds, antennae or tails, for example), skin autotomy can occur in certain taxa of scincid and gekkonid lizards. Here we report the first demonstration of skin autotomy in Mammalia (African spiny mice, Acomys). Mechanical testing showed a propensity for skin to tear under very low tension and the absence of a fracture plane. After skin loss, rapid wound contraction was followed by hair follicle regeneration in dorsal skin wounds. Notably, we found that regenerative capacity in Acomys was extended to ear holes, where the mice exhibited complete regeneration of hair follicles, sebaceous glands, dermis and cartilage. Salamanders capable of limb regeneration form a blastema (a mass of lineage-restricted progenitor cells) after limb loss, and our findings suggest that ear tissue regeneration in Acomys may proceed through the assembly of a similar structure. This study underscores the importance of investigating regenerative phenomena outside of conventional model organisms, and suggests that mammals may retain a higher capacity for regeneration than was previously believed. As re-emergent interest in regenerative medicine seeks to isolate molecular pathways controlling tissue regeneration in mammals, Acomys may prove useful in identifying mechanisms to promote regeneration in lieu of fibrosis and scarring.

Skin shedding and tissue regeneration in African spiny mice ( Acomys )

Using a recently developed noninvasive, in vivo suction device for measuring skin elasticity, we evaluated age, sex, and regional differences in the viscoelastic properties of skin. A total of 33 volunteers participated in the study consisting of (a) 8 young females, (b) 9 old females, (c) 8 young males and (d) 8 old males. Measurements were performed on 11 anatomical regions; three different loads were applied: 100,200, and 500 mbar. The parameters used were: immediate distension (Ue); delayed distension (Uv); immediate retraction (Ur); and, final deformation (Uf). To compare between subjects and anatomical regions, relative parameters independent of skin thickness were calculated: Uv/Ue, the ratio between the viscoelastic properties of skin and immediate distension, and Ur/Uf, which measures the ability of the skin to regain its initial position after deformation. Generally, Uv/Ue increased while Ur/Uf decreased with aging. Responses were variable with respect to load applied. Variability within anatomical regions was also noted. However, differences between the sexes were not statistically significant for most regions. These findings are in congruence with earlier studies suggesting the differences are mainly attributable to alterations in the elastic fiber network. This procedure provides a simple, quantitative assessment of elastic properties of the skin. Its application may help in future investigations of other connective tissue disorders.

Elastic properties of human skin: relation to age, sex, and anatomical region

Background and Objective
Low energy laser photostimulation at certain wavelengths can enhance tissue repair by releasing growth factors from fibroblasts and stimulate the healing process. This study was designed to evaluate the influence of laser photostimulation on collagen production in experimentally tenotomized and repaired rabbit Achilles tendons.

Study Design/Materials and Methods: A total of 24 male New Zealand rabbits, ages 10–12 weeks, were used. Following tenotomy and repair, the surgical hind limbs of the rabbits were immobilized in customized polyurethane casts. The experimental animals were treated with a 632.8 nm He:Ne laser daily at 1.0 J cm−2 for 14 days. Control animals were sham treated with the laser head. On the fifth day after repair, the casts were removed to allow the animals to bear weight on the lower extremity. The animals were euthanized on the 15th postoperative day, then, the Achilles tendons were excised, processed and analyzed.

Results
Biochemical analyses of the tendons revealed a 26% increase in collagen concentration with laser photostimulation indicating a more rapid healing process in treated tendons compared to controls. Sequential extractions of collagen from regenerating tissues revealed that the laser photostimulated tendons had 32% and 33% greater concentrations of neutral salt soluble collagen and insoluble collagen, respectively, than control tendons suggesting an accelerated production of collagen with laser photostimulation. A significant decrease (9%) in pepsin soluble collagen was observed in laser-treated tendons compared to controls. There were no statistically significant differences recorded in the concentrations of hydroxypyridinium crosslinks and acid soluble collagen between treated and control tendons.

Conclusion
This study of laser photostimulation on tendon healing in rabbits suggests that such therapy facilitates collagen production in a manner that enhances tendon healing. Lasers Surg. Med. 22:281–287, 1998. © 1998 Wiley-Liss, Inc.

Laser photostimulation of collagen production in healing rabbit achilles tendons

Changes in skin during maturation and ageing as well as after treatment with cortisol acetate were studied in Sprague-Dawley rats. Mechanical parameters (ultimate load and tensile strength) were compared with biochemical parameters, i.e., collagen content and collagen fractions, as well as glycosaminoglycan content and glycosaminoglycan fractions (hyaluronic acid, chondroitin sulfates and heparin sulfate). The values of ultimate load and tensile strength as well as the total collagen and insoluble collagen content showed a biphasic curve during the life span, with a maximum at 4 months or 1 year. Using these parameters, maturation and ageing processes can be distinguished, whereas the other parameters changed only in one direction so that no differentiation between maturation and ageing can be made. Short term treatment with cortisol acetate induced a rise in ultimate load and tensile strength, whereas long term treatment resulted in a decrease in ultimate load and skin thickness and an Increase in tensil...

Vogel Hg

Papers

Correlation between tensile strength and collagen content in rat skin. Effect of age and cortisol treatment.