Observations on the early development of ascending spinal pathways. Studies using the North American opossum

The development of ascending spinal pathways has been studied in the North American opossum using degeneration methods and the retrograde transport of horseradish peroxidase. Axons from caudal thoracic and/or lumbosacral levels of the spinal cord reach the lateral reticular nucleus, the inferior olivary complex, the reticular formation of the medulla and pons as well as the cerebellum very early in development. Innervation of the nucleus gracilis occurs somewhat later. Spinal axons grow into most of the caudal brain stem areas they occupy in the adult animal, including the nucleus gracilis, before there is convincing evidence that they reach the thalamus. Although spinal axons enter the cerebellum early in development their adult distribution with its characteristic discontinuities appears relatively late.