Development of the diarthrodial joints in the rat embryo.

The development of appendicular synovial joints of both legs was studied with histological and histochemical techniques in 43 rat embryos aged 12 to 21 days. From this and previous studies, it appears that joints develop by a sequence of cellular events leading to a full expression of the phenotypic characteristics. The classically described stages: cell condensation, three layered mesenchyme, vascular invasion and joint clefting, were chronologically recorded in all joints. The observations relevant to the intra-articular structures, such as joint capsule, menisci and ligaments, were also presented. Previously unreported, cellular aspects were described during joint morphogenesis and their biological significance was discussed. Among these cellular aspects, of particular interest are: a. an early wave of cell necrosis occurring immediately after differentiation of the interzone. Disappearance of necrotic cells is thought to prevent chondrification of this tissue by clearing up the cells with chondroblastic potentialities; and b. a morphologically peculiar type of cells that differentiate alongside, and by the time of, clefting and seem to be related to this process. Thus, the joint clefting appears also to result from a cell-tissue related phenomenon, acting in conjunction with the joint motion, the importance of which has been previously demonstrated.