The spinal cords of fetal (days 12 to 17 gestation) and neonatal (0 to 3 days postnatal) mice were transected and examined histologically to as long as 4 months postoperatively. Serial reconstructions were made of spinal cord sections which were prepared within the vertebral column. Based upon the application of several histological stains, we found: (i) Several months after transection the cut ends of the spinal cord were separated by a space of as many as one to two vertebral segments in transection animals and two to four vertebral segments in resection animals. (ii) After complete transection, the lesion site contained longitudinally oriented glial processes which occasionally connected the rostral and caudal segments of the spinal cord. (iii) Very few nerve fibers traversed the lesion site continuously from one stump to the other. (iv) Connective tissue scarring at the site of transection was absent and neuroglial scar formation minimal to both fetal and neonatal mice. (v) In the undamaged regions of the spinal cord, microcysts occurred more frequently in neonatal than in fetal animals. (vi) Dorsal root fibers regenerated across the gap between the ends of the cord and reentered the rostral stump of the cord. We conclude that despite the absence of connective tissue and glial scars, there was insignificant outgrowth of spinal nerve fibers. Thus, there is no greater regenerative capacity in the fetal and neonatal spinal cords than in the adult spinal cord.
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