Development and survival of thoracic motoneurons and hindlimb musculature following transplantation of the thoracic neural tube to the lumbar region in the chick embryo: Functional aspects

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Abstract
Following heterotopic transplantation of the thoracic neural tube to the lumbar region on embryonic day (E) 2, the transplanted cord differentiates normally and establishes neuroanatomical connections with the host central nervous system and hindlimb muscles. Beginning on about E12, however, the neuromuscular system begins to undergo regressive changes resulting in motoneuron degeneration and muscle atrophy (O'Brien and Oppenheim, 1990). In the present paper, we have examined the development of neuromuscular function in thoracic transplant embryos from E6 to the time of hatching on E20–21. The onset of hindlimb movements and reflexes occurred at the same time (E6–E8) in both control and thoracic transplant embryos. Further, both the nature (pattern) and frequency of these movements appeared normal in the thoracic transplats up to E10–E12, after which there was a gradual and marked reduction in the frequency, and an alteration in the pattern, of both spontaneous and reflex‐evoked hindlimb movements. After E16 normal movements were virtually absent in many of the thoracic transplant cases. By contrast, movements of the head, trunk and wings were normal in these embryos throughout the observation period. Hindlimbs innervated partly by the thoracic transplant and partly by remaining host lumbar cord did not exhibit the regressive changes in function after E10 that occurred in hindlimbs innervated exclusively by the thoracic transplant. EMG recordings from specific hindlimb muscles innervated solely by thoracic motoneurons demonstrated that the activation pattern of both flexors and extensors was similar to the repetitive pattern observed in normal thoracically innervated intercostal muscles (i.e., extensorlike). Muscles did not show distinguishable EMG burst patterns with inhibitory periods as do control lumbar innervated muscles. We conclude that the development of the pattern generating circuitry in the transplanted thoracic cord was similar to normal thoracic cord and thus appeared to be uninfluenced by having contacted the foreign hindlimb muscle targets early in development. Activity blockade with curare from E6 to E14 suppressed the loss of motoneurons that occurs in the thoracic transplant after E10. Thus, the abnormal thoracic‐like activation pattern of thoracically innervated hindlimbs may be a critical signal in the initiation of the neuromuscular regression that occurs after E10 in these preparations. Finally, although the innervation and formation of neuromuscular endplates in thoracic transplants appeared normal up to E12, by E14 both the intramuscular nerves and the endplates exhibited signs of degeneration and regression. Thoracic motoneurons are initially able to innervate and functionally activate hindlimb muscles in a manner similar to that of thoracically innervated intercostal muscles. Eventually, however, even this abnormal activation pattern disappears and the entire neuromuscular system breaks down.