Development of axosomatic synapses of theXenopus spinal cord with special reference to subsurface cisterns and C-type synapses

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Abstract
The relationships of highly flattened subsurface cisterns (SCCs) were investigated electron microscopically in the spinal cord at various devel‐ opmental stages of tadpoles and adult toads, Xenopus laeuis. In medial ventral motor cells (MVCs) of the adult, more than 90% of 156 SSCs examined were situated postsynaptically. Similarly, more than 909% of 540 SSCs in lateral motor column cells (LMCs) were postsynaptic. By contrast, in early developmental stages, the SSCs were initially formed by regional flattening of cisterns of rough‐surfaced endoplasmic reticulum just beneath the cell surfaces opposite to glial processes. Then, the glial processes were displaced by nerve endings with an elongated bouton, and thus the C‐type synapses were formed. The ratio of post‐ synaptic SSCs to the total SSCs reached the adult level at around Stage 60. This finding suggests that the SSCs in the MVCs and LMCs draw a certain type of nerve ending to form C‐type synapses. Such a mechanism is totally lacking in the dorsal and lateral small nerve cells, since the SSCs in these cells were always situated under the surface opposite to glial processes throughout the developmental stages and in the adult. In mature C‐type synapses, an aggregate of synaptic vesicles and a structural specialization of presynaptic membrane occurred only at the region where the postsynaptic membrane was associated with the SSC. The postsynaptic membrane itself of the C‐type synapse showed no marked structural specialization at any stage of development or in the adult. The postsynaptic SSC in the mature C‐type synapse seems to be involved in some way in the reception of synaptic transmission.