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

The importance of mechanical influences during skeletal development has bee n 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 im paired, resulting in fusion of the cartilaginous elements across the presum ptive joint line. Two structures in particular are noticeably absent from l ate-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 joi nt and, in the case of sesamoid structures, to provide a mechanical advanta ge to muscles acting across the joint. We demonstrate in this study that em bryonic immobilization differentially affects the developmental fate of the se two distinct fibrocartilages. The absence of the plantar tarsal sesamoid in late-stage immobilized embryos is due to a failure in the initial forma tion 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 degener ates.