Corticospinal development in the North-American opossum: evidence for a sequence in the growth of cortical axons in the spinal cord and for transient projections.

The course and distribution of corticospinal axons have been traced in a series of pouch young and adult opossums using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Cortical axons enter the spinal cord approx. 28 days after birth (40 days after conception) and, at that time, are limited to those portions of the dorsal and lateral funiculi which contain them in the adult animal. Shortly thereafter, cortical axons are also found in regions of presumptive white matter where they are not seen in older pouch young or adult opossums. Those in the dorsal and lateral funiculi reach their caudal extent, the fourth thoracic segment, approx. 38 days after birth and do not significantly overgrow that level during development. Cortical axons grow into the gray matter exclusively from the dorsal and lateral funiculi and first innervate adjacent portions of laminae IV and V. They subsequently extend into laminae III and VI, then VII, VIII and X and finally, at the cervical enlargement, the medial edge of laminae I-II and lamina IX. There is a subsequent period of development during which the density of cortical innervation in all spinal laminae, particularly in the ventral horn, appears to exceed that in the adult opossum.