Experiments in vitro on the role of movement in the development of joints.

I T has been pointed out by Fell & Canti (1934) as a result of their experiments in vitro concerning the early formation of the avian limb skeleton and kneejoint, that the appearance of the articular rudiment is independent of the bloodand nerve-supply as well as of mechanical influences. These authors believe that the formation of articular surfaces occurs in consequence of the differential growth of the scleroblastema (the 'Anlage' of the limb skeleton, skeletal rudiment) the essential factor in joint formation being the association of undifferentiated tissue with the rapidly growing chondrification centres. They emphasize, however, that only the earlier stages of joint formation can be obtained in vitro', the conditions of cultivation are not adequate for the further development of the joints. The articular rudiment disappears by secondary fusion of the cartilages of the limb skeleton. Fell & Canti mention the possibility that though mechanical movement is not necessary for the appearance of the joint, it may be an important and possibly an essential factor in its subsequent development. Giindisch (1943), examining cultures of chick-limb scleroblastemas, came to the conclusion that the growth observed in vitro is, for the most part, erroneously interpreted as organotypical, since the development in vitro of the organ anlage represents a histotypical proliferation of tissues. According to his work only the cartilage cells continue to differentiate in the explanted limb skeletal blastema, multiplying and producing ground substance. Thus cartilage centres are produced which should be considered as separate cultures of cartilage tissue in an organ culture undergoing dedifferentiation. In consequence of the histotypical growth of these cartilage centres the loose tissue between the articular surfaces becomes chondrified. The more developed the joints are at explantation, that is, the smaller the undifferentiated tissue between the articular ends is, the later the chondrification takes place. Giindisch mentions that movement inhibited to some extent the fusion of the cartilaginous surfaces.