| Abstract No.: | A-D1148 |
| Country: | Canada |
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| Title: | THE LOCATION OF THE POOLS OF MOTONEURONS INNERVATING THE LIMBS IN ADULT AND DEVELOPING OPOSSUMS MONODELPHIS DOMESTICA. |
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| Authors/Affiliations: | 1 Syndia Champagne; 1 Amélie Petrou; 1 Stéphanie Lamoureux; 1 Thérèse Cabana*;
1 Université de Montréal, QC, Canada
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| Content: | Objectives: By using the marsupial Monodelphis domestica, an opossum born more immature than eutherians, motor systems development can be studied largely postnatally and without constraint of the uterus or marsupium, this species being pouchless. The forelimbs of a newborn are just sufficiently developed to allow the animal to climb on the mother from the birth canal to a nipple where it attaches. The hindlimbs are little more than buds, and they only start moving in the second week. In order to interpret results on axonal growth and synaptogenesis in the spinal cord, we need to know the identity of the neurons therein, starting with motoneurons. In newborn and young postnatal opossums motoneurons can be recognized by their position in the ventral horn, their slightly larger size and their expression of choline acetyltransferase (synthetic enzyme of acetylcholine), but they are undifferentiated and form a continuous mass bordering the marginal zone (presumtive white matter). Even in adult opossums, the functional identity of motoneurons has not been investigated. How does their organization compare with what is known of other mammals?
Materials and Methods: We have used the retrograde axonal tracing of WGA-HRP and PRV-152 injected in different segments of the limbs to label motoneurons in adult and developing opossums, and for each tracer the post-injection survival period was adjusted to the size of the animals (length of motor axons).
Results: In adult opossums, Lamina IX motoneurons innervating arm and forearm muscles form longitudinal columns of fusiforn shape extending in cervical segments 4 to 8, and those innervating thigh and leg muscles in lumbar segments 3 and 4. Within each spinal enlargement, motoneuronal columns related to the proximal segment of the limb (arm or thigh) start and may end more rostrally than those related to the distal segment (forearm or leg), and they tend to be somewhat more medial and ventral in the ventral horn. For each limb segment, flexor motoneurons tend to be slightly more rostral than extensor motoneurons. In both the cervical and lumbar enlargements, each of the four motoneuronal columns is about one segment shorter in opossums aged P1 or P10. The length of each column gradually increases, and by P20-P30 it surpasses the length observed in the adults. The length of each motoneural pool decreases gradually afterwards and reach adult value around P90. The columns innervating the proximal versus distal segment of a limb or the flexor versus extensor compartment of a segment show even more mediolateral and ventrodorsal overlap in developing than in adult opossums.
Conclusion: By using this marsupial model, it has been possible to describe postnatally the temporal evolution of the pools of motoneurons innervating the flexor and extensor compartments of the proximal and distal segments of the forelimb and hindlimb. The motor columns innervating the comparable hindlimb muscle groups are shorter in opossums (2 segments) than in rats (3 segments), rabbits (4 segments) or cats (5 segments). This feature together with a less clear somatotopic organization may represent a more primitive state. |
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