Early development of descending pathways from the brain stem to the spinal cord in Xenopus laevis.

The early development of descending pathways from the brain stem to the spinal cord has been studied in Xenopus laevis tadpoles. The relatively protracted development of this permanently aquatic amphibian as well as its transparency during development make this animal particularly attractive for experimental studies. Between the 5th and 10th myotome the spinal cord was crushed with a thin needle and dry horseradish peroxidase (HRP) crystals were applied. After a survival time of one day the tadpoles were fixed and the brain and spinal cord were stained as a whole according to a modification of the heavy metal intensification of the DAB-reaction, cleared in cedarwood oil and examined as wholemounts. At stage 28 (the neural tube has just closed) the first brain stem neurons projecting to the spinal cord were found in what appear to be the nucleus reticularis inferior and -medius. At this stage of development the first, uncoordinated swimming movements can be observed. At stage 30/31 (the tailbud is visible) both Mauthner cells project to the spinal cord as well as the interstitial nucleus of the fasciculus longitudinalis medialis situated in the mesencephalon. Towards stage 35/36 (the tail is now clearly visible), a more extensive reticulospinal innervation of the spinal cord appears, now including cells of the nucleus reticularis superior. At this stage also the first vestibulospinal and raphespinal projections were found. At stage 43/44 (the tadpoles have now a well-developed tail) the pattern of reticulospinal projections appears to be completed with the presence of labeled neurons in the nucleus reticularis isthmi. From stage 43/44 on, the number of HRP-positive cells is steadily increasing. At stage 47/48, when the hindlimb buds appear, the descending projections to the spinal cord are comparable with the adult situation except for the absence of a rubrospinal and a hypothalamospinal projection. The observations demonstrate that already very early in development reticulospinal fibers and, somewhat later, Mauthner cell axons and vestibulospinal fibers innervate the spinal cord. Furthermore, a caudorostral gradient appears to exist with regard to the development of descending projections to the spinal cord. However, the interstitial nucleus of the fasciculus longitudinalis medialis forms an exception to this rule.