An investigation of the foetal rat spinal cord. I. Ultrastructural observations on the onset of synaptogenesis.

Ultrastructural studies on the foetal rat spinal cord show that during synaptogeneis there is difficulty in recognizing true synaptic precursors. Symmetric densities are found at unusual sites forming, for example, somato-dendritic and somato-somatic junctions. They are also found between neurons and possible glial processes. Symmetric densities occur between nerve cells but may be confused with desmosomes. Profiles exhibiting membrane density, cleft material and 50 nm vesicles, which are the most reliable indicators of presumptive synapses, are found between neurones, but also at junctions between neurons and what may be glial processes. The picture is further confused by the presence of degeneration axodendritic synapses at early foetal stages. Caution must be exercised in defining an apparent synapse or precursor in foetal cord as that of a presumptive functional synapse because of the observed degenerating profiles and because of our knowledge, somato-somatic, somato-dendritic, and neurono-glial synapses have not been observed in adult cord. It is not known whether these structures are an unwanted consequence of development or play a role in guiding development.