Background An accurate understanding of the mechanical response of ligaments is important for preventing their damage and rupture. To date, ligament mechanical responses are being primarily evaluated using simulations. However, many mathematical simulations construct models of uniform fibre bundles or sheets using merely collagen fibres and ignore the mechanical properties of other components such as elastin and crosslinkers. Here, we evaluated the effect of elastin-specific mechanical properties and content on the mechanical response of ligaments to stress using a simple mathematical model. Methods Based on multiphoton microscopic images of porcine knee collateral ligaments, we constructed a simple mathematical simulation model that individually includes the mechanical properties of collagen fibres and elastin (fibre model) and compared with another model that considers the ligament as a single sheet (sheet model). We also evaluated the mechanical response of the fibre model as a function of the elastin content, from 0 to 33.5%. Both ends of the ligament were fixed to a bone, and tensile, shear, and rotational stresses were applied to one of the bones to evaluate the magnitude and distribution of the stress applied to the collagen and elastin at each load. Results Uniform stress was applied to the entire ligament in the sheet model, whereas in the fibre model, strong stress was applied at the junction between collagen fibres and elastin. Even in the same fibre model, as the elastin content increased from 0 to 14.4%, the maximum stress and displacement applied to the collagen fibres during shear stress decreased by 65% and 89%, respectively. The slope of the stress–strain relationship at 14.4% elastin was 6.5 times greater under shear stress than that of the model with 0% elastin. A positive correlation was found between the stress required to rotate the bones at both ends of the ligament at the same angle and elastin content. Conclusions The fibre model, which includes the mechanical properties of elastin, can provide a more precise evaluation of the stress distribution and mechanical response. Elastin is responsible for ligament rigidity during shear and rotational stress.

[Background] It is important to accurately understand the mechanical response of ligaments to prevent damage and rupture. Most mathematical simulation studies consider the ligament as a single uniform sheet or focus only on collagen fibers, ignoring the other major component such as elastin. We evaluated how elastin affects the mechanical response of the ligaments under stresses using a simple mathematical model. [Methods] Based on multiphoton microscopic images of porcine knee collateral ligaments, we constructed a simple mathematical simulation model that individually includes the mechanical properties of collagen fibers and elastin (fiber model) and compared with that considers the ligament as a single sheet (sheet model). We also evaluated the difference in mechanical response in the fiber model depending on the elastin content. [Results] Uniform stress was applied to the entire ligament in the sheet model, while strong stress was applied at the junction of collagen fibers and elastin in the fiber model. In the same fiber model, as elastin content increased, the stress and displacement applied to the collagen fibers during tensile and shear stresses decreased and the slope of the stress-strain relationship increased especially under shear stress. The stress required to rotate the bones at both ends of the ligament by the same angle increased with increasing elastin content. [Conclusions] The fiber model, which included the mechanical properties of elastin, could provide us more precise stress distribution and mechanical response. It was shown that elastin is responsible for the rigidity of the ligaments during shear and rotational stresses.

Summary The cause of hybrid sterility and inviability has not been analyzed in the fin-fish hybrid, although large numbers of hybridizations have been carried out. In this study, we produced allo-diploid hybrids by cross-fertilization between female goldfish (Carassius auratus) and male golden venus chub (Hemigrammocypris rasborella). Inviability of these hybrids was due to breakage of the enveloping layer during epiboly or due to malformation with serious cardiac oedema around the hatching stage. Spontaneous allo-triploid hybrids with two sets of the goldfish genome and one set of the golden venus chub genome developed normally and survived beyond the feeding stage. This improved survival was confirmed by generating heat-shock-induced allo-triploid hybrids that possessed an extra goldfish genome. When inviable allo-diploid hybrid cells were transplanted into goldfish host embryos at the blastula stage, these embryos hatched normally, incorporating the allo-diploid cells. These allo-diploid hybrid cells persisted, and were genetically detected in a 6-month-old fish. In contrast, primordial germ cells taken from allo-diploid hybrids and transplanted into goldfish hosts at the blastula stage had disappeared by 10 days post-fertilization, even under chimeric conditions. In allo-triploid hybrid embryos, germ cells proliferated in the gonad, but had disappeared by 10 weeks post-fertilization. These results showed that while hybrid germ cells are inviable even in chimeric conditions, hybrid somatic cells remain viable.

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