Intrinsic versus extrinsic factors associated with embryonic nerve-muscle specificity

Abstract

<p>Previous analyses showed that following heterotopic transplantation of a thoracic neural tube segment into the region of the extirpated brachial neural tube in experimental (Thor-Br) chick embryos at embryonic day -2 (day -2E) the wings of hatched Thor-Br chicks are immotile. It was not known, however, if functional nerve-muscle interaction occurs during embryogenesis of Thor-Br embryos, although structural nerve-muscle connections do form in wing muscles during initial development of this experimental model. From day 8.5-9E onward, however, thoracic nerve-brachial muscle unions progressively uncouple in individual wing muscles. The present study explored the nature of initial nerve-muscle contacts and employed a well documented functional parameter, wing motility, to monitor daily the development of functional nerve-muscle interactions in wings of individual Thor-Br embryos. Control embryos were either unoperated (UC) or received a homotopic brachial neural tube transplant (Br-Br). The results demonstrated that Thor-Br embryos exhibited normal frequencies of wing movement up to day -8E; but the frequency of wing movement became greatly reduced from that of controls following this initial developmental period. The loss of wing motility in Thor-Br embryos coincided temporally with the withdrawal of intramuscular axons from individual wing muscles reported previously. Thus, foreign thoracic nerves did establish initial functional contacts with wing muscles, however, these connections were subsequently deemed inappropriate and nerve-muscle unions progressively uncoupled. To investigate factors responsible for the nerve-muscle uncoupling phenomenon observed in experimental (Thor-Br) embryos the development of heterotopically transplanted thoracic neural tubes was compared to that of neural tubes in control (Br-Br and UC) embryos. Parameters analysed include the pattern of peripheral nerve outgrowth, neural tube histogenesis, the source of motor innervation to individual wing muscles and the pattern of motoneuron death, growth and differentiation. The results showed that while the pattern of nerve outgrowth was controlled by local environmental signals, developmental events within the neural tube was governed autonomously, independent of the periphery. Thus, heterotopically transplanted thoracic neural tubes developed according to their site of origin. In addition to the heterotopic neural tube (Thor-Br) transplantation experiments a second series of surgically manipulated chick embryos (Thor-Br/som) was employed to determine if nerve-muscle incompatibility is limited to a thoracic nerve-brachial muscle combination. Experimental (Thor-Br/som) embryos were produced by transplanting thoracic somitic mesoderm into the site of extirpated brachial somites at day -2E. Daily wing motility analyses were performed to determine the extent of functional nerve-muscle interaction between in situ brachial nerves and wing muscles derived from heterotopically transplanted thoracic somitic mesoderm. The results demonstrated that functional nerve-muscle interaction did occur in wings of Thor-Br/som embryos and, in contrast to experimental (Thor-Br) embryos, wing motility was maintained in Thor-Br/som embryos throughout the developmental period analysed (day -6E through day -16E). Nerve-muscle uncoupling, therefore, dd not occur in heterotopic somitic mesoderm transplantation experiments. It was concluded that eventual nerve-muscle incompatibility observed following heterotopic neural tube (Thor-Br) transplantation is related to the rigidity of developmental processes within the spinal cord. Whereas heterotopically transplanted thoracic somitic mesoderm exhibited a high degree of plasticity and conformed to peripheral signals derived from the brachial environment, transplated thoracic neural tubes were unable to respond to environmental signals were eventually deemed inappropriate.</p>