The role of movement in the development of joints and related structures: the head and neck in the chick embryo.

Skeletal muscle contractions are necessary during embryonic life for the normal development of joints. The general features of joint development in immobile limbs were first studied with the techniques of grafting and organ culture. (Murray, 1926; Murray & Selby, 1930; Fell, 1925; Fell & Canti, 1934; Hamburger & Waugh, 1940; Lelkes, 1958). However, these methods of necessity entail a drastic alteration in the environment of the developing articulations, which may result in gross distortions of the skeletal structures themselves. More recently, neuromuscular blocking agents have been used to produce paralysis of chick embryos in ovo. When administered intravenously, these pharmacological substances produce profound paralysis, which may be continued for prolonged periods during embryonic development (Drachman & Coulombre, 1962a, b). Drachman & Sokoloff (1966) have analyzed the primary development of the knee, ankle and toe joints of the chick embryo by the use of these methods. In these relatively simple hinge-type joints, paralysis prevents the initial formation of cavities, although morphogenesis of the joint regions proceeds up to the point when cavitation should occur. Eventually the articular surfaces become united by fibrous bonds or by cartilaginous 'fusion'. The form of the articular surfaces and of the accessory structures is altered in the paralyzed limbs. In a separate series of experiments Murray & Smiles (1965) found that pharmacologically produced paralysis prevents the development of' adventitious cartilage' which normally appears at the articular surfaces of certain membrane bones. Both of these studies were concerned with the role of movement in the initial formation of only a few types of articular structures. The present investigation was undertaken to determine the effects of prolonged paralysis on the morphogenesis of a greater variety of articulo-skeletal structures. For this purpose the joints, cartilages and skeleton of the head and neck were