DEVELOPMENT OF WALKING, SWIMMING AND NEURONAL CONNECTIONS AFTER COMPLETE SPINAL-CORD TRANSECTION IN THE NEONATAL OPOSSUM, MONODELPHIS-DOMESTICA

Development of coordinated movements was quantitatively assessed in ad ult opossums (Monodelphis domestica) with thoracic spinal cords transe cted by (1) crushing 7-8 d after birth [postnatal days 7-8 (P7-P8)]; a t 2-3 years of age, systematic behavioral tests (e.g., climbing, footp rint analysis, and swimming) showed only minor differences between con trol (n = 5) and operated (n = 10) animals; and (2) cutting on P4-P6; at 1 month these opossums exhibited coordinated walking movements but were unable to right themselves from a supine position, unlike control s (n = 6). When tested at 2 or 6 months, they could right themselves a nd showed remarkable coordination, albeit with more differences from c ontrols than after a crush. No animals with spinal cords that were cru shed at P14-18 survived because of cannibalism by the mother. Morpholo gical studies (n = 10) 3 months-3 years after crush at 1 week showed r estoration of structural continuity and normal appearance at the lesio n site. Animals with cut rather than crushed cords showed continuity b ut greater morphological deficits. That lesions were complete was demo nstrated by examining morphology and nerve impulse conduction immediat ely after crushing or cutting the spinal cord in controls. After lumba r spinal cord injection of 10 kDa dextran amine, retrogradely labeled cells were found rostral to the lesion in hindbrain and midbrain nucle i. Conduction was restored across the site of the lesion. Thus complet e spinal cord transection in neonatal Monodelphis was followed by deve lopment of coordinated movements and repair of the spinal cord, a proc ess that included development of functional connections by axons that crossed the lesion.