Comparative Electron Microscopy of Synapses in the Vertebrate Spinal Cord

Synapses with a cleft with ‘thickened’ membranes and presynaptic vesicles and mitochondria occur commonly throughout the grey matter of the spinal cord of goldfish, frog and various mammals studied. Such synapses are generally thought to have a chemical mode of transmission. The absence or rare occurrence of presynaptic neurofilaments in fish and frog accounts for the failure to detect boutons by silver methods, and there is no need to postulate morphologically unspecialized synaptic contacts in the lower vertebrates as some light microscopists did. Both fish and frog show axo-somatic or axo-dendritic tight junctions, which could be sites of electrical synaptic transmission. No neuronal tight junctions have yet been seen in the mammalian spinal cord. Axo-axo-dendritic synapses have been seen in the frog and mammalian cord, but not so far in the fish. Such serial synapses may be responsible for presynaptic inhibition. Neuroglia of fish, frog and mammals have tight junctions at their apposed surfaces. These differ structurally from neuronal tight junctions. Neuroglia in fish cord have, in addition, desmosomes at their apposed surfaces.