Well-documented evidence suggests that animal sex, sex hormones, and pregnancy influence the structural and material behavior of tendons and ligaments. Not only has growing evidence consistently demonstrated the ability of the hormone relaxin, in conjunction with estrogen, to stimulate the breakdown of collagen (the primary load-bearing protein in tendons and ligaments) during pregnancy, but sex hormones have further been found to directly affect soft tissue outside of the reproductive tract. While evidence does suggest that the hormone-induced changes in soft tissues during pregnancy dissipate with a day of parturition, no research has studied the possibility of long-term, cumulative effects due to multiple gestational cycles. The aim of this study was to determine whether multiple cycles of pregnancy and birth have an effect on the composition, geometry, and structural and material behavior of the mouse Achilles tendon. The experimental model used consisted of twenty male and twenty female C57B1/6 mice, thirty-nine weeks in age, from two groups: retired breeders, meaning that the females had given birth to at least six litters and virgins. The left Achilles tendon was used for mechanical testing and the right for determination of biochemical composition. Mechanical testing was conducted using the Instron 5542 Axial Testing Machine to apply pure tensile loading to the Achilles tendon, while DNA content, glycosaminoglycan content, and hydroxyproline content (indicative of total collagen content) were assessed as well. Results demonstrated that female breeders have significantly lower tendon strength (structural and material) than do female virgins. No such distinction was seen in males. Surprisingly, a thirty-seven percent greater collagen content was seen in female breeders relative to female virgins. While breeder status did not significantly affect either structural or material stiffness in males or females, female breeders, demonstrated a significantly lower extension at failure, energy absorbed at failure, ultimate strain, and strain energy density when compared to female virgins. These data support the hypothesis that multiple gestational cycles cause what appear to be cumulative, long-term effects in tendon strength and thus contribute to growing evidence that sugguests that sex, sex hormones, and pregnancy affect tendon structural and material behavior. Further study is necessary to better understand why female breeders exhibit lower tendon strength in the face of greater collagen content. Possible explanations lie in the ability of relaxin to promote the breakdown of collagen, collagen ultrastructural organization, and collagen cross-linking.
Cloutier, Alison, "The effect of multiple gestational cycles on tendon structural and material behavior in mice" (2008). Honors Project, Smith College, Northampton, MA.
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