During development of the nervous system, neurons in many regions are overproduced by proliferation, after which the excess cells are eliminated by cell death. The survival of only a proportion of neurons during normal development is thought to be regulated by the limited availability of neurotrophic agents. One such putative trophic agent is ciliary neurotrophic factor (CNTF), a polypeptide that promotes the survival of ciliary, sensory, and sympathetic neurons in vitro. In contrast to the results of in vitro studies, however, the daily treatment of chick embryos in vivo with purified human recombinant CNTF failed to rescue any of these cell populations from cell death, whereas CNTF did promote the in vivo survival of spinal motoneurons. Thus, CNTF may not act as a neurotrophic agent in vivo for those embryonic neurons (especially ciliary neurons) on which it acts in vitro. Rather, CNTF may be required for in vivo survival of motoneurons.

The primary aim of this study was to clarify the role of supraspinal, propriospinal, and primary sensory afferents in motoneuron (MN) development in the lateral motor column (LMC) of the lumbar spinal cord of the chick embryo. For this purpose three types of operations were carried out on embryonic day (E) 2. (1) The spinal cord was transected at the cervical (C-gap) or at the thoracic (T-gap) level so as to eliminate supraspinal and/or propriospinal inputs to the lumbar cord. (2) The entire lumbar neural crest was removed (NCR) in order to eliminate primary sensory inputs arising from the dorsal root ganglia (DRG). (3) A combined operation of T-gap and lumbar NCR was performed. The numbers of MNs in the LMC of the lumbar spinal cord were counted in embryos sacrificed between E10 and E18. The number of MNs on E10, when naturally occurring neuron death is almost complete, was not changed following either operation 1 or 2 described above. However, by E16, when naturally occurring neuron death is over, these same deafferented groups had 20 to 25% fewer MNs than did controls. Thus, the removal of either descending or sensory (DRG) afferents results in a significant increased loss of MNs that appears to take place only during the final stages of natural neuronal death or later. By contrast, the removal of both sources of input (T-gap + NCR) results in an additional 37% loss of MNs by E10 compared to controls. Thus, in this group deafferentation significantly increases cell loss during the major period of naturally occurring MN death (E5 to E10). No further loss of MNs occurs in this group after E10. Chronic treatment of deafferented embryos with curare from E6 to E9 or from E10 to E14 prevented the naturally occurring MN loss during these stages but was without effect on the increased cell loss induced by deafferentation. These results imply that the cellular mechanisms involved in target- versus afferent-regulated cell death are different. Collectively, these results indicate that the regulation of MN numbers is more complicated than previously thought. Both targets and afferents appear to be involved in controlling the survival of this population of neurons during the period of naturally occurring MN death.

With only a few exceptions, most investigations of the mechanisms involved in naturally-occurring neuron death have focused on interactions between neurons and their targets, with much less attention having been paid to the possible role of the afferent inputs in this phenomenon. This is true of the avian ciliary ganglion (CG), which is composed of a population of peripheral autonomic neurons that project to smooth and striated musculature in the eye and which receive afferents from a single source, the accessory oculomotor nucleus (AON), which is the avian homolog of the Edinger-Westphal nucleus. Although several lines of evidence strongly support the important role of targets in regulating the death and survival of CG neurons, the role of afferents has not yet been systematically examined. Following the destruction of the AON on embryonic day (E) 4, which is several days before the onset of normal cell death in the CG, we have found that by the end of the normal cell death period (E14-E15), 85–90% of the CG neurons degenerate and die, compared to 50% in controls. This is comparable to the amount of induced cell loss that occurs following removal of the optic vesicle containing the CG targets. The neurons surviving after deafferentation appear to be sustained by some influence from their targets since combined deafferentation and eye removal results in the loss of virtually all neurons in the CG. Following deafferentation of the CG on E4, the ganglion develops normally up to about E10, after which a precipitous loss of cells occurs. Based on several kinds of evidence (e.g., axon counts, silver stain, retrograde labeling of the CG), we conclude that the deafferented neurons project to and innervate their muscular targets in the eye. Therefore, the increased cell death following deafferentation cannot be due to the failure of deafferented neurons to contact their targets. The deafferented neurons undergo a normal sequence of initial ultrastructural differentiation. When they do begin to degenerate, the type of fine structural changes they exhibit appears indistinguishable from the degenerative changes observed in control embryos. Neurons in deafferented ganglia were occasionally observed to receive synaptic contacts, which we attribute to aberrant intraganglionic connections induced by deafferentation. These contacts probably play little, if any, role in the maintenance of neurons since, as noted above, following combined deafferentation and target deletion virtually all neurons degenerate and die.(ABSTRACT TRUNCATED AT 400 WORDS)

Following complete transection of the thoracic spinal cord at various times during embryonic development, chick embryos and posthatched animals exhibited various degrees of anatomical and functional recovery depending upon the age of injury. Transection on embryonic day 2 (E2), when neurogenesis is still occurring and before descending or ascending fiber tracts have formed, produced no noticeable behavioral or anatomical deficits. Embryos hatched on their own and were behaviorally indistinguishable from control hatchlings. Similar results were found following transection on E5, an age when neurogenesis is complete and when ascending and descending fiber tracts have begun to project through the thoracic region. Within 48 h following injury on E5, large numbers of nerve fibers were observed growing across the site of transection. By E8, injections of horse-radish peroxidase (HRP) administered caudal to the lesion, retrogradely labelled rostral spinal and brainstem neurons. Embryos transected on E5 were able to hatch and could stand and locomote posthatching in a manner that was indistinguishable from controls. Following spinal cord transections on E10, anatomical recovery of the spinal cord at the site of injury was not quite as complete as after E5 transection. Nonetheless, anatomical continuity was restored at the site of injury, axons projected across this region, and rostral spinal and brainstem neurons could be retrogradely labelled following HRP injections administered caudal to the lesion. At least part of this anatomical recovery may be mediated by the regeneration or regrowth of lesioned axons. Although none of the embryos transected on E10 that survived to hatching were able to hatch on their own, because several sham-operated embryos were also unable to hatch, we do not attribute this deficit to the spinal transection. When E10-transected embryos were aided in escaping from the shell, they were able to support their own weight, could stand, and locomote, and were generally comparable, behaviorally, to control hatchlings. Repair of the spinal cord following transection on E15 was considerably less complete compared to embryos transected on E2, E5, or E10. However, in some cases, a degree of anatomical continuity was eventually restored and a few spinal neurons rostral to the lesion could be retrogradely labelled with HRP. By contrast, labelled brainstem neurons were never observed following E15 transection. E15 transected embryos were never able to hatch on their own, and when aided in escaping from the shell, the hatchlings were never able to stand, support their own weight or locomote.(ABSTRACT TRUNCATED AT 400 WORDS)