The development of the long descending propriospinal projections in the opossum, Monodelphis domestica

Postnatal development of limb motor innervation in the opossum Monodelphis domestica: immunohistochemical localization of acetylcholine

The development of limb motor innervation was studied in the opossum Monodelphis domestica, a marsupial born with immature mobile forelimbs and immobile hindlimbs. Choline acetyltransferase (ChAT), the synthesis enzyme of acetylcholine, was evidenced on sections of the spinal enlargements, and the protein that transports acetylcholine (VAChT) on limb sections. In newborn, ChAT immunolabeling occurred in small, undifferentiated neurons of the ventral horn, presumably motoneurons, and intermediate and dorsal gray matter, and in the presumptive white matter, all less abundant at lumbosacral than brachial levels. Scant immunolabeling for VAChT marked small terminal-looking profiles, presumably growth cones or immature neuromuscular junctions, decreasing proximodistally in each limb and being less abundant in hindlimbs than forelimbs; it was absent distally in the foot where no muscle tissue was formed. ChAT labeling disappeared from the white matter within 1 week while cholinergic neurons increased in number and size. Motoneurons segregated in a medial and lateral group by 4-5 weeks. VAChT-labeled profiles increased in number and size and they flattened along a proximodistal gradient within each limb, but later in the hindlimbs than in the forelimbs. Labeling appeared in distal foot muscle at 1 week. The density, size, and shape of terminals became comparable in all segments of a given limb by 3-4 weeks. Their number and size increased, and by 8 weeks, they clustered in 3 or 4 along muscle fibers. Thus, limb motor innervation develops largely postnatally in the opossum, along rostrocaudal and proximodistal gradients. Its timecourse is compared to the development of motor behaviors.

Myelinogenesis in the Brachial and Lumbosacral Enlargements of the Spinal Cord of the Opossum Monodelphis domestica

Using immunohistochemistry in light microscopy, the myelin basic protein and proteolipid protein were localized on sections of the spinal cord enlargements of opossums, Monodelphis domestica, to determine the timecourse of myelinogenesis therein and compare it with other events of motor systems development. Additional tissue not processed for immunohistochemistry was prepared for transmission electron microscopy. No immunolabeling for either protein occurred on spinal sections from the newborn opossum, but in electron microscopy occasional fibers surrounded by loose, irregular membranous rings were seen on the outskirts of the ventral horn. Immunolabeling was detected first in the brachial enlargement during the second week, presumably on motoneuronal, vestibular and reticular axons. The areas of the dorsal columns, other spino-encephalic, reticulospinal and propriospinal projections became labeled in the third week, and the area of rubrospinal axons at 4 weeks. In the brachial gray matter, immunolabeling appeared along ventrodorsal and lateromedial gradients from the fourth to seventh weeks. Labeling developed similarly in the white and gray matter of the lumbosacral enlargement, but 3-5 days later than at brachial levels. Labeling intensity in the white and gray matter increased until at least 4 months, but remained light in laminae I-III. Thus, myelinogenesis in the spinal cord enlargements of the opossum is protracted and follows general rostrocaudal, ventrodorsal and lateromedial sequences. It occurs later than synaptogenesis at comparable levels of the cord, but earlier than myelinogenesis in the corresponding ventral and dorsal roots. Spinal myelinogenesis correlates with the development of sensorimotor reflexes, weight support and quadrupedal locomotion.

A correlative physiological and morphological analysis of monosynaptically connected propriospinal axon-motoneuron pairs in the lumbar spinal cord of frogs

Intracellular stimulation of single propriospinal axons evoked excitatory postsynaptic potentials (EPSPs) in lumbar motoneurons. Mean EPSP amplitudes differed by two orders of magnitude when measured in different connections. After analyzing the distribution of mean amplitudes of 47 single-fiber EPSPs, two populations of responses could be defined: (1) those with mean amplitudes between 0.1 and 1.2 mV (mean+/-S.D.: 0.48+/-0.30 mV, 34 pairs), which is in the range of values typical for single-fiber EPSPs evoked by stimulation of supraspinal fibers and primary muscle afferents, (2) those with mean amplitudes between 1.6 and 8 mV (4.2+/-2.0 mV, 13 pairs). Both populations of responses had similarly short latencies and rise times and responded similarly to paired-pulse stimulation, consistent with monosynaptic transmission. However, the high-efficacy connections had significantly smaller coefficients of variation of EPSPs, as well as increased quantal content and quantal size. Tetanic stimulation gradually depressed the amplitude of large EPSPs by 81-86%, but did not affect small EPSPs. Recovery of large EPSPs was exponential with a time constant of 3-5.6 min. During post-tetanic depression the amplitude ratio between the test and conditioned EPSPs evoked by paired-pulse stimulation was not changed but the coefficient of variation was increased, suggesting that the depression was due to depletion of synaptic vesicles available for release.Intracellular labeling of seven electrophysiologically studied propriospinal axon-motoneuron pairs revealed that the number of axon varicosities establishing close appositions with dendrites of the labeled motoneuron was higher for connections where large-amplitude EPSPs were recorded. These varicosities were more often located on proximal dendrites of motoneurons than those of low-efficacy connections. In addition, the number of boutons in highly effective connections was several times lower than the maximal number of available quanta estimated from physiological data, implying that the large EPSPs may be generated by multivesicular release from presynaptic boutons. We conclude that the efficacy and related mode of use-dependent modulation of propriospinal connections is determined by a number of factors, including the number and position of synaptic contacts and the number of active zones or vesicles available for release.

The development of vesicular acetylcholine transporter immunoreactivity in the hindlimbs of the opossum Monodelphis domestica

Vesicular acetylcholine transporter (VAChT) was revealed immunohistochemically in light microscopy on hindlimb sections of developing opossums, Monodelphis domestica. In the immobile hindlimbs of the newborn, which comprise cartilaginous bones and loose, unstriated myofibers, scant immunolabeled nerve segments and small spherical terminals, presumably growth cones or immature neuromuscular junctions, are found in the muscle tissue of the thigh, leg and proximal foot, decreasing in number and size proximodistally. When the hindlimbs start moving at 1 week, terminals are more numerous and larger, still decreasing proximodistally, and occur in the newly formed interosseous foot muscles. At 4 weeks, when the hindlimbs start supporting weight and quadrupedal locomotion appears, terminals are more numerous, flattened and in comparable size and density in thigh, leg and foot muscles. By 7 weeks, large and completely flat terminals are observed in groups of 3 to 4 at regular intervals along muscle fibers. VAChT expression develops largely postnatally in the opossum hindlimbs, along a proximodistal gradient that parallels somatic and reflex development.

The development of mammalian motor systems: the opossum Monodelphis domestica as a model

The opossum Monodelphis domestica is a marsupial born considerably immature 14-15 days after conception. It is possible to study postnatally, in this species, almost the entire development of its motor behaviors as well as of the nerve centers involved in their control. The lumbosacral spinal cord of the newborn comprises a thick ventricular zone containing mitotic figures, an intermediate zone of small and undifferentiated cells, and a thin marginal zone. The hindlimbs are little more than embryonic buds. The presumptive bones consist of cartilageneous or mesenchymal condensations and the presumptive muscles of immature myofibers mixed and surrounded with mesenchyme. Cholinergic fibers from lumbosacral motoneurons are already seen among the myofibers, but most of hindlimb motor innervation develops postnatally. The long descending and ascending projection systems connecting the lumbosacral enlargement to the cervical cord and the encephalon also form largely postnatally, but lateral vestibular and medullary reticular axons are present in the lumbosacral cord at birth. Synaptogenesis in the lumbosacral enlargement occurs largely postnatally, according to a general outside-in gradient, and the earliest evidence for it is on lateral motoneurons. Myelinogenesis therein is even later. These observations on neural development are correlated with observations on the development of simple reflex behaviors and locomotion.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Development of motoneurons and primary sensory afferents in the thoracic and lumbar spinal cord of the South American opossum Monodelphis domestica

The postnatal development of the primary sensory afferent projection to the thoracic (T4) and lumbar (L4) spinal cord of the marsupial species Monodelphis domestica was studied by using anterograde and retrograde neuronal tracers. Large numbers of primary afferents and motoneurons were labelled by application of the carbocyanine dye DiI into individual dorsal root ganglia (DRG) afferents in short-term organ cultures. Dorsal root axons had entered the cord at birth, but most primary afferent innervation of the grey matter and the establishment of cytoarchitectural lamination occurs postnatally. In addition to ipsilateral projections, some primary afferents that projected to the dorsal horn extended across the midline into the equivalent contralateral regions of the grey matter. Similarly, motoneuron dendrites occasionally extended across midline and into the contralateral grey matter. The first fibres innervating the spinal cord project to the ventral horn and formed increasingly complex terminal arbours in the motor columns between P1 and P7. After P5 many afferents were seen projecting to the dorsal horn, with the superficial dorsal horn being the last region of the spinal grey to be innervated. Histochemical labelling with the lectin Griffonia simplicifolia indicated that C fibre primary afferents had arborised in the superficial dorsal horn by P14. The sequence of primary afferent innervation is thus similar to that described in the rat, but this sequence occurs over a period of several weeks in Monodelphis, compared with several days in the rat.

Development of the facial and hypoglossal motor nuclei in the neonatal Brazilian opossum brain

The development of the facial and hypoglossal motor nuclei were examined in the neonatal Brazilian opossum (Monodelphis domestica), a marsupial in which postnatal central nervous system development has been well characterized. In this study, we utilized postnatal injection of the retrograde tracer cholera toxin subunit B (CtB) to characterize the formation of the facial and hypoglossal motor nuclei in the developing neonatal opossum brainstem. Injections of CtB were made into the cheek/lip region or tongue of opossum pups to retrogradely label the facial or hypoglossal motor nuclei, respectively. Following a 2 h survival time, facial motoneurons in newborn opossum pups (1 PN) exhibited CtB labeling, with their cell bodies localized near the developing cranial abducens nucleus. At 3 and 5 PN, following a 48 h survival time, CtB-labeled facial motoneurons were observed in and migrating to the region of the adult facial motor nucleus in the rostral medulla. Between 7 and 10 PN, almost all facial motoneurons had migrated to their destination within the facial motor nucleus. Hypoglossal motoneurons also exhibited CtB labeling from 1 PN; however, their cell bodies were localized within the hypoglossal motor nucleus at the earliest age examined. Double label studies, to examine guidance of facial motoneurons during migration, demonstrated that CtB-labeled facial motoneurons are in close proximity to vimentin-like immunostained radial glial fibers during migration. These results suggest: (1) migration of facial motoneurons to the facial motor nucleus is a postnatal event, (2) efferent projections from facial and hypoglossal motoneurons project into the peripheral region of their target muscles from the day of birth, and (3) facial motoneurons migrate to their destination in the brainstem thereafter, in close association with radial glial fibers.

The development of synaptophysin-like immunoreactivity in the lumbosacral enlargement of the spinal cord of the opossum Monodelphis domestica

The presence of synaptophysin in the lumbosacral enlargement of developing opossums, Monodelphis domestica, was studied immunohistochemically at the light microscopic level. In newborn, synaptophysin-labeling was observed in the presumptive white matter, presumably in growing axons, and was scant in the ventrolateral gray matter. Over the next 3 weeks the labeling filled the gray matter following a general ventrodorsal gradient. Labeling was found in the white matter until the fifth week. Synaptogenesis in the lumbosacral enlargement of the opossum thus occurs mostly postnatally, when many descending axons have already reached that level. It is particularly intense in the ventral horn when the hindlimbs begin to move, and in the dorsal horn when sensorimotor reflexes can be elicited.

Fetal spinal cord transplants support growth of supraspinal and segmental projections after cervical spinal cord hemisection in the neonatal rat

Cervical spinal cord injury at birth permanently disrupts forelimb function in goal-directed reaching. Transplants of fetal spinal cord tissue permit the development of skilled forelimb use and associated postural adjustments (, companion article). The aim of this study was to determine whether transplants of fetal spinal cord tissue support the remodeling of supraspinal and segmental pathways that may underlie recovery of postural reflexes and forelimb movements. Although brainstem-spinal and segmental projections to the cervical spinal cord are present at birth, skilled forelimb reaching has not yet developed. Three-day-old rats received a cervical spinal cord overhemisection with or without transplantation of fetal spinal cord tissue (embryonic day 14); unoperated pups served as normal controls. Neuroanatomical tracing techniques were used to examine the organization of CNS pathways that may influence target-directed reaching. In animals with hemisections only, corticospinal, brainstem-spinal, and dorsal root projections within the spinal cord were decreased in number and extent. In contrast, animals receiving hemisections plus transplants exhibited growth of these projections throughout the transplant and over long distances within the host spinal cord caudal to the transplant. Raphespinal axons were apposed to numerous propriospinal neurons in control and transplant animals; these associations were greatly reduced in the lesion-only animals. These observations suggest that after neonatal cervical spinal cord injury, embryonic transplants support axonal growth of CNS pathways and specifically supraspinal input to propriospinal neurons. We suggest that after neonatal spinal injury in the rat, the transplant-mediated reestablishment of supraspinal input to spinal circuitry is the mechanism underlying the development of target-directed reaching and associated postural adjustments.

Developmentally regulated markers in the postnatal cervical spinal cord of the opossum Monodelphis domestica

The aim of this study was to identify developmentally regulated immunocytochemical markers to assess development in the cervical spinal cord of Monodelphis domestica. We demonstrate that two commercially available antibodies exhibit altered patterns of distribution during early postnatal development. Although neurofilament staining was present at birth, only the phosphorylated form, recognised by monoclonal antibodies 2F11 or SMI31 could be detected. Non-phosphorylated neurofilament, recognised by monoclonal antibody SMI32, only became detectable around postnatal day 4 (P4) but was restricted to cells in the ventral horn until 5 weeks postnatum. By 7.5 weeks, SMI32 immunoreactivity (IR) was found throughout the grey matter in a pattern similar to that in the adult for both SMI32 and microtubule-associated protein 2 (MAP2). The intermediate filament proteins, glial fibrillary acidic protein (GFAP) and vimentin (VIM), were detectable at birth in radially oriented, fibrous cells, but GFAP-IR was restricted to the ventral half of the cord. This ventral to dorsal gradient of GFAP-IR diminished during the first week of postnatal life, disappearing by P8. Many astrocyte-like, GFAP-positive cells were clearly present by 38 days and, in the adult, were abundant in the white matter. A few VIM-IR cells remained in the adult cord, also within the white matter. We suggest that SMI32 and GFAP are useful, developmentally regulated markers for studies of opossum spinal cord development. We are currently using these markers to investigate the pronounced rostral to caudal gradient in the postnatal spinal cord and to assess development in the cultured spinal cord.

Development of spontaneous locomotor behaviors in the opossum, Monodelphis domestica

The development of spontaneous locomotor behaviors was studied in the opossum Monodelphis domestica. The newborn opossum performs alternate, rhythmic movements with its forelimbs to crawl on the mother's belly where it attaches to a nipple, and its hindlimbs are little more than embryonic buds. The forelimbs retain the above movements for about 3 weeks, while the hindlimbs begin to move late in the second week. When detached from the nipple at 2-3 weeks, the pup can support its weight on the forelimbs and pivot around its hindquarter. Around the fourth week, the young can detach from the mother, its hindlimbs can support weight and linear locomotion appears, but the four limbs are not well coordinated. However, it can swim with coordinated movements of all limbs. Coordination when walking appears around the sixth week. During development, the duration of the step cycle decreases significantly. The durations of the stance and swing phases of the step cycle decrease in absolute terms, but swing increases as a percentage of the step cycle. The results are discussed in relation to the development of nervous and skeletomuscular components as well as sensorimotor reflexes.

Descending propriospinal axons in the hindlimb enlargement of the red-eared turtle: cells of origin and funicular courses

Spinal neurons with descending axons are important components of spinal sensorimotor networks. We used an anatomical tracing technique to study the distribution of descending propriospinal axons and cell bodies in red-eared turtles. We injected horseradish peroxidase into a portion of one funiculus in the middle of the hindlimb enlargement and examined six spinal segments rostral to the injection site (dorsal 3 through dorsal 8) for labeled neuronal cell bodies. Injections into each region of the white matter labeled substantial numbers of descending propriospinal neurons. Each injection labeled cell bodies over most of the six spinal segments examined. Each injection also labeled cell bodies in the ipsilateral dorsal horn, intermediate zone, and ventral horn as well as the contralateral intermediate zone and ventral horn. Injections into each of four regions of the white matter, the dorsal funiculus, the medial part of the lateral funiculus, the lateral part of the lateral funiculus, and the ventral funiculus reliably gave rise to a distinct distribution of labeled cell bodies. These experiments establish that descending propriospinal axons in red-eared turtles are found in all regions of the spinal white matter. This finding contrasts with a popular contemporary view of the organization of descending propriospinal axons in mammals. These experiments also demonstrate that neurons in each region of the gray matter give rise to a different distribution of descending, funicular axons, although these distributions are widely overlapping. Different funicular axon distributions could be associated with different sets of synaptic contacts with the white-matter dendrites of spinal neurons.