GDF-5 deficiency in mice alters the ultrastructure, mechanical properties and composition of the Achilles tendon.

Acromesomelic dysplasia of the Hunter-Thompson and Grebe types are rare human disorders based on growth/differentiation factor (GDF)-5/CDMP-1 genetic mutations. Numerous skeletal abnormalities are present in these individuals, including shortened limb bones and severe dislocations of the knee. In the GDF-5 deficient brachypodism mouse, similar, although less severe, phenotypes are observed. It is unknown whether the joint dislocations observed in these disorders are due to a defect in the original formation of joints such as the knee, or to abnormalities in the tendons and ligaments themselves. We hypothesized that tendons from GDF-5 deficient mice would exhibit altered composition, mechanical properties, and ultrastructure when compared with heterozygous control littermates. GDF-5 deficient Achilles tendons were structurally weaker than controls, and structural strength differences appeared to be caused by compromised material properties: after normalizing by collagen per unit length, mutant tendons were still 50% weaker (P < 0.0001) and 50% more compliant (P < 0.001) than controls. Despite comparable levels of skeletal maturity in the two cohorts, the majority of mutant tendon failures occurred in the mid-substance of the tendon (64% of all failures), whereas the majority of control failures occurred via avulsion (92% of all failures). Mutant Achilles tendons contained 40% less collagen per microgram of DNA when compared to controls (P = 0.004). No significant difference in glycosaminoglycan (GAG)/DNA was detected. Ultrastructural analyses indicated a slight trend toward increased frequency of small diameter (30-100 nm) collagen fibrils in the mutant Achilles. Our findings suggest that increased tendon and ligament laxity may be the cause of the joint dislocations seen in patients with Hunter-Thompson and Grebe type dysplasia, rather than developmental abnormalities in the joints themselves.

Matrix and cell injury due to sub-impact loading of adult bovine articular cartilage explants: effects of strain rate and peak stress.

Mechanical overloading of cartilage has been implicated in the initiation and progression of osteoarthrosis. Our objectives were to identify threshold levels of strain rate and peak stress at which sub-impact loads could induce cartilage matrix damage and chondrocyte injury in bovine osteochondral explants and to explore relationships between matrix damage, spatial patterns of cell injury, and applied loads. Single sub-impact loads characterized by a constant strain rate between 3 x 10(-5) and 0.7 s(-1) to a peak stress between 3.5 and 14 MPa were applied, after which explants were maintained in culture for four days. At the higher strain rates, matrix mechanical failure (tissue cracks) and cell deactivation were most severe near the cartilage superficial zone and were associated with sustained increased release of proteoglycan from explants. In contrast, low strain rate loading was associated with cell deactivation in the absence of visible matrix damage. Furthermore, cell activity and proteoglycan synthesis were suppressed throughout the cartilage depth, but in a radially dependent manner with the most severe effects at the center of cylindrical explants. Results highlight spatial patterns of matrix damage and cell injury which depend upon the nature of injurious loading applied. These patterns of injury may also differ in terms of their long-term implications for progression of degradative disease and possibilities for cartilage repair.

GDF-5 deficiency in mice leads to disruption of tail tendon form and function.

Although the biological factors which regulate tendon homeostasis are poorly understood, recent evidence suggests that Growth and Differentiation Factor-5 (GDF-5) may play a role in this important process. The purpose of this study was to investigate the effect of GDF-5 deficiency on mouse tail tendon using the brachypodism mouse model. We hypothesized that GDF-5 deficient tail tendon would exhibit altered composition, ultrastructure, and biomechanical behavior when compared to heterozygous control littermates. Mutant tail tendons did not display any compositional differences in sulfated glycosaminoglycans (GAG/DNA), collagen (hydroxyproline/DNA), or levels of fibromodulin, decorin, or lumican. However, GDF-5 deficiency did result in a 17% increase in the proportion of medium diameter (100-225 nm) collagen fibrils in tail tendon (at the expense of larger fibrils) when compared to controls (p < 0.05). Also, mutants exhibited a trend toward an increase in irregularly-shaped polymorphic fibrils (33% more, p > 0.05). While GDF-5 deficient tendon fascicles did not demonstrate any significant differences in quasistatic biomechanical properties, mutant fascicles relaxed 11% more slowly than control tendons during time-dependent stress-relaxation tests (p < 0.05). We hypothesize that this subtle alteration in time-dependent mechanical behavior is most-likely due to the increased prevalence of irregularly shaped type I collagen fibrils in the mutant tail tendons. These findings provide additional evidence to support the conclusion that GDF-5 may play a role in tendon homeostasis in mice.

Differential effects of embryonic immobilization on the development of fibrocartilaginous skeletal elements.

The importance of mechanical influences during skeletal development has been well established in both experimental studies and computer models. Under conditions of embryonic immobilization, it has been observed that the early stages of joint formation proceed normally (up to and including interzone formation), but the later stages of joint cavitation and maintenance are impaired, resulting in fusion of the cartilaginous elements across the presumptive joint line. Two structures in particular are noticeably absent from late-stage synovial joints in immobilized chick embryos: the menisci of the tibiofemoral joint and the plantar tarsal sesamoid of the tibiotarsal joint. Both of these fibrocartilaginous structures are known to serve mechanical functions in postnatal animals, helping to distribute loads within the joint and, in the case of sesamoid structures, to provide a mechanical advantage to muscles acting across the joint. We demonstrate in this study that embryonic immobilization differentially affects the developmental fate of these two distinct fibrocartilages. The absence of the plantar tarsal sesamoid in late-stage immobilized embryos is due to a failure in the initial formation of this structure. In contrast, the early stages of meniscus formation proceed normally. Without the normal mechanical stimuli of skeletal muscle contractions, however, the meniscus fails to mature and ultimately degenerates.