Onset and development of intersegmental projections in the chick embryo spinal cord

The ontogeny of intersegmental (propriospinal) projections was studied in the chick embryo spinal cord between embryonic day 2.5 and day 6. Our goals were 1) to determine the earliest projections of intersegmental interneurons between specific spinal regions and to establish the cell types involved; and 2) to follow the ontogeny of these projections during the early formative stages of spinal cord development. Studies were carried out in vitro by using an isolated spinal cord/brainstem preparation. Horseradish peroxidase injections were made either uni- or bilaterally at various levels of the spinal cord along the rostrocaudal axis of the embryo. HRP histochemistry was done on Vibratome sections with diaminobenzidine as the chromogen. Following unilateral injections at day 2.5, labelled commissural interneurons were found contralaterally and were confined to the injected segment. Subsequently, labelled cells were found progressively further away from the injected segment. By day 4.5 reciprocal projections extended between lumbar and brachial regions. Interneurons with intersegmental axonal projections were often undifferentiated, consisting of primitive unipolar or bipolar cells with little, if any, dendritic development. In some cases migrating interneurons could be retrogradely labelled from two or three segments away from the location of their translocating cell body. Anterograde Golgi-like labelling of early undifferentiated cells revealed growing axons, axonal terminals, and growth cones. Five or six reasonably distinct classes of intersegmental interneurons were identified based on their location, axonal projections, and morphology of dendritic arbors. These appeared to be segmentally and bilaterally arranged along the rostrocaudal axis of the spinal cord. The axons of some of these types of interneurons exhibited preferences in their longitudinal projections within the ventral and ventrolateral marginal zone at the very onset of pathway formation. From the present observations it can be concluded that intersegmental connectivity precedes the development of ascending and descending supraspinal, as well as primary afferent connections in the chick embryo spinal cord.

Reduction of naturally occurring motoneuron death in vivo by a target-derived neurotrophic factor

Treatment of chick embryos in ovo with crude and partially purified extracts from embryonic hindlimbs (days 8 to 9) during the normal cell death period (days 5 to 10) rescues a significant number of motoneurons from degeneration. The survival activity of partially purified extract was dose-dependent and developmentally regulated. The survival of sensory, sympathetic, parasympathetic, and a population of cholinergic sympathetic preganglionic neurons was unaffected by treatment with hindlimb extract. The massive motoneuron death that occurs after early target (hindlimb) removal was partially ameliorated by daily treatment with the hindlimb extract. These results indicate that a target-derived neurotrophic factor is involved in the regulation of motoneuron survival in vivo.

Enhancement of naturally occurring cell death in the sympathetic and parasympathetic ganglia of the chicken embryo following blockade of ganglionic transmission

Both presynaptic and postsynaptic blockade of ganglionic transmission during the period of naturally occurring ganglion cell death reduced the number of surviving neurons in the sympathetic ganglia (SG) and ciliary ganglion (CG). The CG was chosen for analysis because there was a temporal separation between cell proliferation and death in the CG but not in the SG. Ganglion cell proliferation and migration were unaffected by ganglionic blockade. The increased ganglion cell loss that followed ganglionic blockade was accompanied by an increased number of degenerating cells. These results indicate that the decreased number of healthy ganglion cells that followed ganglionic blockade was the result of enhanced naturally occurring cell death.

Naturally-occurring neuron death in the ciliary ganglion of the chick embryo following removal of preganglionic input: evidence for the role of afferents in ganglion cell survival

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)

Distribution and development of proenkephalin-like immunoreactivity in the lumbar spinal cord of the chicken

Met5-enkephalin- (Met-ENK), Leu5-enkephalin-, Met5-enkephalin-Arg6-Phe7-, metorphamide- and BAM 22P-like peptides could be detected in the lumbar spinal cord of the chicken by immunocytochemistry and/or high performance liquid chromatography. However, a peptide identical to Met5-enkephalin-Arg6-Gly7-Leu8 could not be detected in the lumbar spinal cord of the chicken using an antiserum that was capable of detecting the octapeptide in mammalian tissues. Nerve fiber- and terminal-like processes containing proenkephalin-derived peptides were concentrated in the superficial laminae of the dorsal horn and along the midline rostral to the central canal. A lesser concentration of processes containing proenkephalin-derived peptides occurred in the medial and lateral motor columns of the ventral horn. The level of total radioimmunoassayable Met-ENK in the lumbar spinal cord of the chicken embryo increased more than 1000-fold between day 4.5 and day 18. A schedule of curare administration that had previously been shown to prevent naturally occurring somatic motoneuron death in the chicken lumbar spinal cord resulted in a two-fold increase in total radioimmunoassayable Met-ENK in the lumbar spinal cord.

Behavioral development in the absence of neural activity: effects of chronic immobilization on amphibian embryos

Embryos of Xenopus laevis and Ambystoma mexicanum were continually immobilized from premotile stages of development to stages at which normally reared embryos were swimming well. Immobilization was achieved through exposure to solutions of chloretone, lidocaine, or alpha-bungarotoxin. At a number of stages after recovery from the drugs, spontaneous and stimulated behaviors were extensively quantified. Immobilization of Ambystoma embryos resulted in temporary defects in musculoskeletal development. In contrast, treated Xenopus embryos could not be distinguished from controls by simple visual observation within minutes to hours after removal from the drug solutions. Quantifications of behavior revealed, however, a transient period of 24-48 hr during which treated embryos exhibited consistently reduced measures of stimulated swimming, while showing an increase in frequency of spontaneous movements. Detailed behavioral testing could detect no permanent effects of chronic immobilization in either species after this initial period of recovery. The results are discussed in reference to the classic works of Harrison (1904), Carmichael (1926, 1927), and Matthews and Detwiler (1926).

The absence of significant postnatal motoneuron death in the brachial and lumbar spinal cord of the rat

Motoneuron numbers in the brachial and lumbar lateral motor column (LMC) of the rat were examined quantitatively on embryonic day (E) 15 and postnatal days (P) 1-5. There was a significant (40-45%) loss of motoneurons in both regions between E15 and P1. No further loss was observed between P1 and P5. The decline of motoneurons between E15 and P1 is very likely due to cell death since there were considerable numbers of dying motoneurons present on E15. By contrast, few dying motoneurons were observed on P1-P5. Possible reasons for discrepancies between the present results and previous investigations are discussed. It is concluded that there is not a significant loss of either brachial or lumbar motoneurons in the postnatal rat and thus that postnatal rearragements of muscle innervation are most likely the result of axon elimination, not cell death.

The development of motoneurons in the embryonic spinal cord of the mouse mutant, muscular dysgenesis (mdg/mdg): survival, morphology, and biochemical differentiation

Motoneuron development was studied in the spinal cord of the mouse mutant, muscular dysgenesis, between embryonic days (E) 13 and 18. Dysgenic embryos are characterized by the absence of neuromuscular activity (motility) and exhibit a number of other striking changes in neuromuscular development. Many of these changes have also been observed in chick embryos chronically treated with neuromuscular blocking agents that suppress motility. Motoneuron survival, as well as several other aspects of neuronal development, was examined in the thoracic and lumbar spinal cords of mutant and control embryos. There was a significant decrease in motoneuron numbers in control embryos indicating the presence of naturally occurring cell death in the mouse spinal cord. At all ages examined, the dysgenic embryos had significantly more healthy and significantly fewer degenerating motoneurons than controls. There were no differences in the number of dorsal root ganglion neurons or in any of the other morphometric parameters examined between mutant and control embryos. Creatine kinase activity, a marker for myofiber maturation, was significantly reduced in the limb musculature of mutant embryos. Choline acetyltransferase activity was significantly increased in the spinal cord of mutant embryos. No significant differences were observed in spinal cord levels of acetylcholinesterase activity between control and mutant embryos. The absence of muscle contractions in the dysgenic mouse is associated with a number of changes in neuromuscular development, including a substantial reduction of naturally occurring motoneuron death.

Further behavioral analysis of GABA-mediated inhibition in the early chick embryo

Five mumol of gamma-amino-N-butyric acid (GABA) completely inhibited the spontaneous motility of 5-day-old chicken embryos. A molar equivalent dose of baclofen, a specific GABAB receptor agonist, produced a much smaller (but still significant) inhibition of motility. The (-)-isomer of baclofen was significantly more potent than the (+)-isomer. GABAB receptors controlling overt behavior appear in the chick embryo at least as early as day 5. Neither 5 mumol of histamine nor 5 mumol of bradykinin, both potent cerebral vasodilators, inhibited spontaneous motility in 5-day-old chicken embryos. One mumol of serotonin, a potent cerebral vasoconstrictor, did not antagonize the significant (but submaximal) inhibition of motility caused by 1 mumol of GABA, GABA-mediated inhibition of overt behavior in the early chick embryo cannot be the result of GABA-mediated cerebral vasodilation.

Behavioral analysis of opiate-mediated inhibition in the early chick embryo

Morphine (opiate agonist) produced a dose-dependent decrease in the spontaneous motility of 5- and 9-day chick embryos. Naloxone (opiate antagonist) appeared to reverse competitively the inhibition of motility caused by morphine. The effects of morphine on spontaneous motility in 5-day embryos were also reversed stereospecifically by the opiate antagonist pairs WIN 44441-3/WIN 44441-2 and levallorphan/dextrallorphan. Levorphanol (opiate agonist) also produced a dose-dependent decrease in the motility of 5-day embryos while its inactive (+)-isomer, dextrophan, was not effective. Etorphine (opiate agonist) was more than 1000-fold more effective than morphine in inhibiting the motility of 5-day embryos. The effectiveness of several opiate agonists and antagonists on the spontaneous motility of 5-day embryos was similar to their effectiveness in radioligand-binding studies on isolated membrane receptors from either adult mammalian brain or ileum. Levorphanol was more effective than dextrophan and etorphine was substantially more effective than morphine in decreasing the spontaneous motility of 4-day embryos. WIN 44441-3 was more effective than WIN 44441-2 in reversing the inhibition of motility in 4-day embryos caused by morphine. Morphine inhibited spontaneous hind-limb motility in both thoracic spinal and sham-operated 7-day embryos; the inhibition of motility caused by morphine was reversed by WIN 44441-3 in both thoracic spinal and sham-operated 7-day embryos. [Leu5]enkephalin-like immunoreactivity in the lumbar spinal cord was concentrated in the superficial laminae of the dorsal horn and along the midline rostral to the central canal. A lesser concentration of immunoreactive processes occurred in the medial and lateral motor columns where labelled varicosities appeared to contact motoneurons. Opiate receptors appear to be present at least as early as day 5 (and perhaps as early as day 4) in the chick embryo. Opiate receptors appear to be present in the lumbar spinal cord of the chick embryo at least as early as day 7. The structural requirements for ligand binding to opiate receptors in the 5-day chick embryo are similar to the requirements for ligand binding to opiate receptors in the adult.

Novel sources of descending input to the spinal cord of the hatching chick

Nuclear groups contributing supraspinal input to the spinal cord of the hatching chick (Gallus domesticus) were determined by using the enzyme tracer horseradish peroxidase processed with tetramethylbenzidine histochemistry. Five sources of projections to the spinal cord were found which have not been previously described in any species. All are probably related to autonomic function. They include ipsilateral hypothalamic projections from the lateral mamillary n., suprachiasmatic n., and n. of the lateral tubercle. There is a bilateral projection from the large interstitial cells of the mesencephalic posterior commissure, and in the myelencephalon, a mainly contralateral projection from interstitial cells of the vagus-glossopharyngeal nerve. Two other projections observed here have not been described in other avian species, one from the accessory vestibular n., the other, from the n. ambiguus. In the cerebellum, projections arise from the main and ventrolateral divisions of the fastigial n., and from "border cells" between the fastigial and interpositus n. The large-celled submedial vestibular n. projects bilaterally. Several projections previously described only in the pigeon, were confirmed here: the hypothalamic nucleus over the supramammilary decussation, the n. intercollicularis, the tangential n., and the n. alatus, a cell group between the hypoglossal and vagal nuclei. Four sources of input projected only as far as mid-cervical cord. These are n. intercollicularis, fastigial n., accessory vestibular n., and tangential n. All remaining projections reached to lower lumbosacral cord. Sources of descending input are remarkably similar in mammals and avians. Where homologous nuclei exist, virtually identical projections to the cord are present.

The onset and development of descending pathways to the spinal cord in the chick embryo

The ontogenetic development of afferent (supraspinal and propriospinal) as well as efferent (ascending) fiber connections of the spinal cord was examined following the injection of horseradish peroxidase (HRP) or wheat germ agglutinin HRP (WGA-HRP) into the cervical and lumbar spinal cords (or brains) of embryos ranging in age from 4 to 14 days of incubation. A few cells were first reliably retrogradely labelled in the pontine reticular formation on embryonic day (E) 4 and E5 following the injection of WGA-HRP into the cervical and lumbar spinal cord, respectively. Propriospinal projections to the lumbar spinal cord, originating from brachial spinal cord, were found by E5, and from the cervical spinal cord by E5.5. Ascending fibers arising from neurons in the lumbar spinal cord could be followed to rostral mesencephalic levels in E5 embryos. Thus, the earliest supraspinal, propriospinal, and ascending fiber connections appear to be formed almost simultaneously. Retrogradely labelled cells were found in the raphe, reticular, vestibular, interstitial, and hypothalamic nuclei in E5.5 embryos following lumbar injections of WGA-HRP. Except for neurons in cerebellar nuclei, all the cell groups of origin that project to the cervical spinal cord of posthatching chicks were also retrogradely labelled by E8. There was a delay in the time of appearance of the projections from various regions of the brain stem to the lumbar versus the cervical spinal cord, ranging from 0.5 to 7 days, but typically of about 3 days duration. A large number of cells located in the ventral hypothalamic region, just dorsal to the optic chiasma, were found to be labelled following cervical HRP injection between E6 and E10. These cells may represent transient projections that are present only during embryonic stages since no labelled cells were found in this region in the newly-hatched chick.

Cell death of motoneurons in the chick embryo spinal cord. IX. The loss of motoneurons following removal of afferent inputs

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.

Cell death of motoneurons in the chick embryo spinal cord. VIII. Motoneurons prevented from dying in the embryo persist after hatching

A chronic neuromuscular blockade during those embryonic stages when naturally occurring spinal motoneuron death occurs, results in the prevention of this cell loss. The excess motoneurons are maintained as long as the neuromuscular blockade is continued; once embryonic neuromuscular activity resumes, however, the excess motoneurons undergo a delayed period of cell death. By contrast, the resumption of neuromuscular activity in these same preparations after hatching did not result in a delayed cell death. The excess motoneurons, prevented from dying in the embryo, persisted for as long as 4 days postnatally despite the presence of considerable limb motility. The maintenance of motoneurons may be regulated differently before and after hatching.

Behavioral analysis of benzodiazepine-mediated inhibition in the early chick embryo

Diazepam, a depressant benzodiazepine, produced a dose-dependent decrease in the spontaneous motility of 5-day embryos while Ro 5-3663, a convulsant benzodiazepine, had no apparent effect. Diazepam and 4 other benzodiazepines inhibited motility in 5-day embryos with a potency that paralleled their effectiveness in ligand-binding studies. Ro 11-5073/Ro 11-5231, depressant benzodiazepine enantiomers, stereospecifically inhibited motility in 4- and 5-day embryos. Ro 15-1788, a benzodiazepine antagonist, reversed the decrease in the motility of 4- and 5-day embryos caused by a depressant benzodiazepine, clonazepam. Ro 5-6945, a potent agonist for the non-neuronal (but not the neuronal) benzodiazepine receptor, had no apparent effect on motility in 5-day embryos. Benzodiazepine receptors appear at least as early as day 5 (and perhaps as early as day 4) in the chick embryo.

Cell death of motoneurons in the chick embryo spinal cord. VI. Reduction of naturally occurring cell death in the thoracolumbar column of Terni by nerve growth factor

A discrete preganglionic cell column (column of Terni, CT) in the caudal thoracolumbar (segments 21-23) spinal cord is first discernible in a avian embryos on day 8 when it contains approximately 9,300 visceral motoneurons. By day 10 there are about 6,900 motoneurons in the thoracolumbar (sympathetic) CT and this number remains constant until at least day 15. Daily injections of nerve growth factor (NGF) (1-20 micrograms) on the chorioallantoic membrane from day 3 to day 9 produced a dose-dependent increase in both the volume of the caudal thoracolumbar sympathetic ganglia and the number of motoneurons in the corresponding CT on day 10. The number and size of the neurons in the paravertebral sympathetic ganglia was also increased by NGF. Nerve growth factor decreased the number of pyknotic (dying) neurons in both the thoracolumbar sympathetic ganglia and the corresponding CT on days 8 and 10. The increased number of neurons in both the thoracolumbar sympathetic ganglia and the corresponding CT following chronic administration of NGF is the result of decreased cell death. The highest dose of NGF reduced naturally occurring cell death in the thoracolumbar CT by more than 60%. Daily injections of cytochrome C (20 micrograms) from day 3 to day 9 had no effect on either the volume of the sympathetic ganglia or the number of motoneurons in the Ct on day 10. Daily injections of NGF (10-20 micrograms) from day 10 until day 14 increased the volume of (and the number of neurons in) the caudal thoracic sympathetic ganglia on day 15 but no effect on the number of motoneurons in the corresponding CT. Nerve growth factor also had no effect on the number of motoneurons in either the somatic lateral motor column, the sacral (parasympathetic) CT, or the "abortive" visceral efferent column at the cervical level. Daily administration of NGF produced a similar increase in the volume of the dorsal root ganglia at the cervical, thoracic, and sacral levels. The reduction of cell death in the thoracolumbar CT by NGF is the result of neither a direct effect on spinal motoneurons nor an indirect effect of increased sensory innervation. Naturally occurring cell death of sympathetic preganglionic neurons is regulated by the size of their innervation field.

Reduction of naturally-occurring cell death in the thoraco-lumbar preganglionic cell column of the chick embryo by nerve growth factor and hemicholinium-3

A preganglionic cell column (column of Terni, CT) in the thoraco-lumbar spinal cord of avian embryos is first discernible on day 7.5-8. Between days 8 and 10 the number of visceral motoneurons in the thoraco-lumbar CT is reduced by 25-30%. Chronic administration of either nerve growth factor (NGF) or hemicholinium-3 spanning the period of naturally-occurring cell death (day 8 through day 10) increased the number of healthy neurons and decreased the number of degenerating neurons in the thoraco-lumbar CT on day 10. The number of healthy neurons in the corresponding sympathetic ganglia were also increased by NGF. Chronic administration of NGF after day 10 increased the number of healthy neurons in the thoraco-lumbar sympathetic ganglia on day 15 but had no effect on the number of motoneurons in the corresponding CT.