The origin of projections from the medullary reticular formation to the spinal cord, the diencephalon and the cerebellum at different stages of development in the North American opossum: studies using single and double labeling techniques.

We have employed the retrograde transport of horseradish peroxidase alone or conjugated to wheat germ agglutinin, to label neurons within the medullary reticular formation which project to the spinal cord, the diencephalon and the cerebellum at different stages of development in the North American opossum. At selected ages, the fluorescent markers Fast Blue and Diamidino Yellow were also used in double-labeling experiments to determine if single neurons innervate both the spinal cord and diencephalon or the spinal cord and cerebellum, presumably via axonal collaterals. The opossum was employed because it is born in a very immature state, 12 days after conception, and is thus available for injections at early stages of development. At all ages studied, the location of retrograde labeling within the medullary reticular formation after spinal, diencephalic or cerebellar placements of horseradish peroxidase or its conjugate appeared similar to that obtained in the adult animal. Such results suggest that the origin of projections from the medullary reticular formation to the areas injected is specified early in development. At some ages, however, the labeling density appeared greater than in the adult animal. When either Fast Blue or Diamidino Yellow was injected into the spinal cord and the other marker was placed into the diencephalon at such ages, relatively few neurons of the medullary reticular formation were double-labeled. When one marker was injected into the spinal cord and the other was placed within the cerebellum, no double-labeled neurons were found. These results indicate that at the ages studied, relatively few neurons of the medullary reticular formation provide collateral innervation to either the spinal cord and diencephalon or the spinal cord and cerebellum. Similar conclusions have been reached previously for the adult opossum. We have interpreted our results to suggest that the organization of reticular projections, at least to the areas injected, may not be shaped by the selective elimination of axonal collaterals as in certain other areas of the brain.