Early development of descending supraspinal pathways: a tracing study in fixed and isolated rat embryos

Early brainstem-spinal cord projections were studied in the rat using the carbocyanine dye DiI in fixed embryos and biotinylated dextran amine (BDA) in an isolated embryonic brain-spinal cord preparation. A system of staging embryos was used that allows direct comparison with data in other mammals. With both techniques it was shown that in embryos of at least 12 days of age (E12), i.e., at the time of closure of the posterior neuropore, already a variety of brainstem centers innervate the spinal cord. In the interstitial nucleus of the fasciculus longitudinalis medialis and various parts of the reticular formation - mesencephalic, pontine as well as medullary - DiI or BDA labelled neurons were observed. Mainly large immature, bipolar neurons were labeled. In later stages (E13, E14) the number of labeled neurons increased and more mature, multipolar cells were found. Labeled neurons were also observed in the vestibular nuclear complex and in the medullary raphe. Just below the cerebellum a conspicuous small group of neurons was found labeled in a position reminiscent of the locus coeruleus. Comparison with available data on the time of neuron origin of brainstem neurons suggests that interstitiospinal and reticulospinal neurons start projecting spinalwards shortly after they are generated. The earliest brainstem projections to the spinal cord all pass via the fasciculus longitudinalis medialis.