Myelination of the ventral and dorsal roots of the C8 and L4 segments of the spinal cord at different stages of development in the gray 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.

Computer-assisted morphometric study on the postnatal growth of axon size and myelin thickness in the ventral and dorsal roots of the rabbit

The morphometric analysis of the myelinated fibers is of great value in developmental, experimental, and pathological studies. The present study was performed on the ventral and dorsal spinal roots (L7) of new-born, 10-, 15-, 20-, 30-, 60-, 120- and 240-day-old rabbits. Semi-thin cross-sections were investigated by OLYMPUS Video image analyser. The average axonal diameter was measured as an average length of Feret's diameters and the specific width of the myelin sheaths as a total width of the myelin sheath. In the ventral roots, the increase of the axon size is more intensive than the increase of the myelin thickness. In adult rabbits, the mean axonal diameter for the ventral root is approximately 15% greater than the mean axonal diameter for the dorsal root, whereas the mean myelin thickness values are practically equal for both roots. For the period newborn-adult (240-day-old) rabbits, a full correlation was found in respect to the increase of the dorsal root mean axonal diameter and mean myelin thickness (380% and 380%). In the ventral roots, the mean axonal diameter increased 420% whereas the mean myelin thickness increased 350%. It is intriguing whether the greater axonal size of the ventral roots might compensate their relatively thin myelin sheaths in comparison to the dorsal roots. Our findings of essential increase (16%) of the mean myelin thickness in the last (120-240 days) period in both, ventral and dorsal roots, points to a protracted period of myelinization in the rabbit.

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 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.

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.