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 c arbocyanine dye DiI in fixed embryos and biotinylated dextran amine (BDA) i n 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 ag e (E12), i.e., at the time of closure of the posterior neuropore, already a variety of brainstem centers innervate the spinal cord. In the interstitia l nucleus of the fasciculus longitudinalis medialis and various parts of th e reticular formation - mesencephalic, pontine as well as medullary - DiI o r BDA labelled neurons were observed. Mainly large immature, bipolar neuron s were labeled. In later stages (E13, E14) the number of labeled neurons in creased 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 l abeled in a position reminiscent of the locus coeruleus. Comparison with av ailable data on the time of neuron origin of brainstem neurons suggests tha t interstitiospinal and reticulospinal neurons start projecting spinalwards shortly after they are generated. The earliest brainstem projections to th e spinal cord all pass via the fasciculus longitudinalis medialis.