The onset of synapse formation in spinal cord cultures as studied by electron microscopy

Abstract

The development of synapses in cultured nervous tissue was studied by explanting cross sections of fetal rat spinal cord prior to first synapse formation. Periodic electron microscopic examination confirmed that the initial explants were free of synapses and indicated that synapses first appeared regularly after 70 h of culture of 14 day fetal cord. Initial synaptic profiles cover a small area, contain few vesicles, lack mitochondria, and are exclusively axodendritic. These early synapses coexist with a number of forms of plasma membrane junctions; some but not all of the junctions may represent precursors of synapses. The time at which the first synapses are seen in the electron microscope correlates well with the appearance of bioelectric activity indicative of the presence of functioning synaptic networks 7. The detection of this close correlation and the finding that synaptic networks form in tissue completely isolated from normal afferent and efferent connections are considered the most significant contributions of this communication.