Ultrastructure of migrating spinal motoneurons in anuran larvae

Motoneurons in the course of migration to the lateral motor column (LMC) were identified by horseradish peroxidase (HRP) applied to lumbar ventral roots in anuran larvae (Rana catesbeiana). The ultrastructural characteristics of these migrating motoneurons were compared with motoneurons in the LMC that had completed their migration. Both migrating and LMC motoneurons tended to be bipolar with one process extending toward the ependyma and the other out towards the ventral root. Typically, centrally and peripherally directed processes in both migrating and LMC motoneurons contained many microtubules, mitochondria and rosette-like clusters of ribosomes, as did the soma. Four of the five types of synapses found on adult frog motoneurons were also found on migrating motoneurons in tadpoles. Radial glia, whose cell bodies are located in the ependyma, have processes extending to the spinal cord periphery. In less than 10% of the preparations, radial glia were labeled with the HRP reaction product. These labeled glia were further distinguished from migrating motoneurons at the ultrastructural level by the presence of abundant filaments.

Cell death of motoneurons in the chick embryo spinal cord. V. Evidence on the role of cell death and neuromuscular function in the formation of specific peripheral connections

Previous reports from this laboratory have shown that the chronic treatment of chick embryos with neuromuscular blocking agents (e.g., curare, alpha-bungarotoxin), during the period of naturally occurring cell death of spinal motoneurons (days 5 to 10), greatly reduces the amount of cell death in this system. The surviving motoneurons continue differentiation and innervate the peripheral musculature. Since cell death has been prevented in these preparations from the earliest stages of limb innervation on day 4 or 5, it was expected that any inappropriate synaptic connections present at that time, or formed later, would be retained as long as the cell and their axons were prevented from regressing. To test this possibility, small injections of horseradish peroxidase were made into specific leg and wing muscles on embryonic day 10 in order to label retrogradely motoneuron pools in the spinal cord. The location of labeled motoneurons was found to be the same in control and experimental embryos. The specific muscles examined included the gastrocnemius, peroneus, adductor, and sartorius in the leg and the biceps, triceps, extensor metacarpi radialis, and flexor carpi ulnaris in the wing. In virtually all cases, there was a greater number of labeled motoneurons in the experimental cases. Despite this difference, the location of motoneuron pools in the rostral-caudal and transverse planes were remarkably similar in control and experimental embryos. Thus, natural cell death in this system is not primarily designed to remove errors in synaptic connectivity. Since a normal pattern of neuromuscular connections was formed in the virtual absence of functional synaptic interactions between motoneurons and their targets, these data do not support the contention that function is involved in the developmental specificity of peripheral connectivity.

Uptake, intra-axonal transport and fate of horseradish peroxidase in embryonic spinal neurons of the chick

Horseradish peroxidase (HRP) was injected in ovo into the ventral muscle mass of the hind limb of 5- to 7-day-old chick embryos or into the gastrocnemius muscle of 8- to 18-day embryos and localized histochemically. HRP is extensively incorporated via endocytosis into axonal growth cones or presynaptic terminals in the proximity of the injection site. Much of the tracer is taken up in vesicles and small vacuoles. Most of these are smooth-surfaced and only a few are bristle-coated. A small amount of the tracer is also incorporated into the axon terminal through the openings between the axolemma and an intricate membrane channel. The majority of the tracer-laden vesicles and vacuoles rapidly fuse with one another to become large vacuoles, some of which are transformed into multivesicular bodies (MVBs). In axon shafts, many labeled vacuoles and MVBs are transferred to tubule-like organelles, which appear to be the primary carrier for transporting the tracer back to the cell bodies in the lumbar spinal cord. HRP arrives in the sensory ganglia about 0.5-1 hour earlier than in the motoneurons of the lateral motor column. The maximal rate of the retrograde axoplasmic transport is about 3.5 mm/hour. After arriving in the cell bodies, HRP is transferred from tubule-like organelles to discrete vacuoles of various sizes and appearance. Lysosomal dense bodies and HRP-labeled vacuoles can be distinguished ultrastructurally. A fusion of HRP-labeled vacuoles with lysosomal dense bodies or Golgi vesicles was occasionally observed and the density of HRP-labeled vacuoles diminished after 2 to 3 days. Most of the HRP-labeled organelles were found to contain acid phosphatase activity. Therefore, the complete disappearance of HRP by 4 days postinjection is most likely related to lysosomal degradation. Neuronal cell bodies diffusely labeled with HRP were only observed prior to day 6. After day 6, despite various attempts to injure the peripheral axons, only granularly labeled cell bodies were found. This difference may imply that "mature" neurons have a more efficient mechanism for the sequestration of "free" HRP in the cytoplasmic matrix into membrane-bounded organelles. A mature-like retrograde transport mechanism appears to exist at the earliest stages of axonal growth in vivo.

Ontogeny of behavioral sensitivity to glycine in the chick embryo

Strychnine (STR), glycine (GLY), and gamma-aminobutyric acid (GABA) were injected intravenously into 6- and 7-day chick embryos and subsequent effects upon spontaneous motility were observed. In 7-day embryos STR alone increased motility and when injected simultaneously with GLY antagonized the GLY-induced motility decrements; STR did not antagonize GABA-induced motility decrements. In 6-day embryos STR injected alone had no effect on motility, but did antagonize motility decrements induced by the simultaneous injection of GLY; again STR did not antagonize GABA-induced motility decrements.

Cell death of motoneurons in the chick embryo spinal cord. IV. Evidence that a functional neuromuscular interaction is involved in the regulation of naturally occurring cell death and the stabilization of synapses

Embryos immobilized with neuromuscular blocking agents for differing periods between 4.5 and 9 days of incubation had an increased number of motoneurons in the brachial and lumbar lateral motor columns. Treatment with alpha-cobratoxin (alpha-CTX) on days 4--9, for instance, was able to prevent virtually all natural cell death during this period; control embryos had an average of 22,500 lumbar motoneurons on day 5.5, and 13,500 on day 10, whereas treated embryos had approximately 21,000 cells on day 10. Curare, alpha-CTX, alpha-bungarotoxin (alpha-BTX) and botulinum toxin were all about equally effective in preventing cell death. Similar treatment begun after day 12, however, had no effect on cell number. If even a partial immobilization was continued after day 10 (in embryos totally immobilized earlier) most of the excess neurons were maintained, in some cases right up to hatching, at which time the embryos died due to respiratory failure. In contrast, when administration of the immobilizing agents was stopped, allowing the embryos' motility to return to control levels, the excess neurons underwent a delayed cell death and total cell number fell to below control levels by days 16--18. Limb muscles from embryos with excess motoneurons exhibited relatively normal differentiation and had acetylcholinesterase (AChE) stained endplates which were innervated. Following curare treatment the two wing muscles, anterior and posterior latissimus dorsi, were found to have an increased number of AChE-stained endplates, whereas the only leg muscle examined quantitatively--the ischioflexorius (IFL)--did not; the IFL, did, however, have a markedly reduced variance in endplate distance, as well as other apparent differences suggesting an altered pattern of innervation. Our findings imply that the number of motoneurons undergoing natural cell death is closely related to muscle activity. Thus, functional interactions at the developing neuromuscular junction seem to be critical in controlling cell death. If a retrograde trophic factor is involved its action is somehow related to muscle activity.

Behavioral and biochemical analysis of GABA-mediated inhibition in the early chick embryo

Exogenous gamma-aminobutyric acid (GABA) decreased spontaneous motility in 4-, 6-, 7-, 9-, and 13-day chick embryos; the younger embryos were more sensitive. Neither the positional isomers of GABA, alpha-aminobutyric acid (AABA) and beta-aminobutyric acid, nor the principle GABA catabolite, succinic acid, decreased motility in 4-day embryos. Several semi-rigid GABA analogues decreased motility in 4-day embryos with a potency that paralleled their effectiveness in displacing [3H]GABA in ligand-binding studies. The effects of AABA and GABA on hind-limb motility were quantitatively similar in thoracic spinal and sham-operated 7-day embryos. Bicuculline and picrotoxin elicited absolute motility increases at 6, 7 and 9 days of incubation. Picrotoxin and two bicyclophosphate GABA antagnoists elicited relative motility increases while bicuculline elicited an absolute motility increase at 4 days. The two bicyclophosphates increased motility with a potency that paralleled their electrophysiological effectiveness. L-Glutamic acid decarboxylase (GAD) activity was detected in the embryonic lumbar spinal cord at all ages examined (3--7 days) using a new radiometric cation-exchange method. Gamma-Aminobutyric acid transaminase (GABA-T) activity was detected in the lumbar spinal cord at the earliest age examined (day 5). Both GAD and GABA-T activity were detected at earlier ages than previously reported. GABA receptors, and the enzymes necessary for the synthesis and degradation of GABA, all appear to be present at (or before) the onset of spontaneous motility. GABA-mediated transmission appears to be present at 6 days and perhaps as early as 4 days.

The retrograde transport of horseradish peroxidase from the developing limb of the chick embryo

Chick embryos ranging in age from 4.0 to 18 days of incubation and 1-2-day-old hatchlings received injections of HRP solutions directly into the leg musculature. After survival periods of from 5 to 25 h motoneurons and cells in the spinal sensory ganglia were found to be stained with the HRP reaction product. It was found that the first appearance of a positive HRP reaction coincided with the time when nerve processes are first detected in the limb-bud by silver techniques (i.e. at 4.5 days of incubation). Only neurons with processes in the region of injection showed a positive reaction. The demonstration of a retrograde transport mechanism in neurons and axons which are still undergoing growth and differentiation provides a possible mechanism for the peripheral regulation of certain features of CNS neurogenesis. The application of this technique during development may also allow one to map neuroanatomical pathways during their formation.

Some aspects of synaptogenesis in the spinal cord of the chick embryo: a quantitative electron microscopic study

We have quantitatively examined the development of synapses in the ventral part of the lumbar spinal cord of the chick from embryonic day 4 until adulthood. The first synapses occur on day 4 and are of the axo-dendritic type; they are invariably located adjacent to the border between the intermediate and marginal zones. Initially there are more synapses in the presumptive white matter than in the motoneuron neuropil, but this trend is later reversed; however, we found numerous axo-dendritic synapses throughout much of the ventrolateral white matter even in the adult stage. The first axo-dendritic synapses always contain spherical synaptic vesicles and have symmetric membrane specilizations. By day 7 a few of these synapses were found to have mixed populations of spherical and flattened vesicles and asymmetric membrane specilizations. After hatching there are still considerably more axo-dendritic synapses with symmetric membrane specializations. Axo-somatic synapses were first found on embryonic day 6 and were typically located on motoneurons lying adjacent to the marginal zone. These axo-somatic synapses contain a few spherical synaptic vesicles and have symmetric membrane densities. Flattened synaptic vesicles were first found on day 10 and increased throughout development. Although a few axo-somatic synapses with asymmetric membrane specializations were found at practically all stages, the symmetric type was always in the majority. An attempt was made to relate these observations with physiological, behavioral and neuroembryological findings from birds and other forms. For example, the fact that axo-dendritic synapses always appear prior to axo-somatic contacts would seem to rule out the role of somatic synapsesin the initial induction of dendritic growth in the spinal cord.