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The distribution of synapses along the spinal cord of an amphibian embryo: An electron microscope study of junction development

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The rostro-caudal gradient of differentiation found in vertebrate embryos has been utilized to examine the sequence of synaptic junction development in the spinal cord of Xenopus laevis at a late embryonic stage. Uniform samples were taken at various points along the cord of a stage 27 embryo and examined in the electron microscope. The general ultrastructure of the cord demonstrated the rostro-caudal gradient of development. The sequence of synaptic junction development was like that in the cervical region (Hayes and Roberts, 1973). “Membrane-vesicle clusters” and “immature” synaptic junctions were found most caudally followed by synaptic junctions, first with cleft and subsynaptic membrane density, then with only cleft density and finally, most rostrally, with cleft, subsynaptic membrane, and subsynaptic cytoplasmic density. Mature synaptic junctions were found in increasing numbers from the mid to anterior trunk cord and could mediate alternating trunk flexions made by the embryos at this stage of development. “Membrane-vesicle clusters” were found near processes containing irregular vesicles and also near membrane outlines. These may be signs of dendritic growth. “Membrane-vesicle clusters” were also found in varicosities, facing the space around the spinal cord and in nerve fibres peripherally between the skin and myotomes. This suggests an association of early stages in synaptogenesis with axon growth. This and other possible inferences about axon and dendrite growth in relation to synaptogenesis are discussed.

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Correspondence to Dr. Alan Roberts.

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Supported by a grant from the Medical Research Council.

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Hayes, B.P., Roberts, A. The distribution of synapses along the spinal cord of an amphibian embryo: An electron microscope study of junction development. Cell Tissue Res. 153, 227–244 (1974). https://doi.org/10.1007/BF00226611

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Key words

  • Synapses
  • Embryo (Xenopus laevis)
  • Spinal cord
  • Electron microscopy