Structural and functional post-embryonic development of a non-rectifying electrical synapse in the crayfish.

The post-embryonic development of the non-rectifying septate synapse between homologous lateral giant (LG) fibre segments has been investigated using electron microscopy and electrophysiology. In adults, the LG-LG synapse is characterized by closely apposed membranes (approximately 4 nm separation) traversed by regularly spaced particles, and large (60-80 nm) spherical vesicles on both sides of the junction. In newly hatched crayfish the junction between lateral giant fibre segments comprises regions of close membrane apposition as seen in the adult along with non-specialized areas of wide (10-15 nm) membrane separation. Vesicles associated with these junctions are small (25-40 nm) and pleomorphic. The number of vesicles is low by comparison with adult junctions; in most sections of hatchling junctions there are normally fewer than five vesicles, although as many as 30 have occasionally been seen. During development the non-specialized areas of wide membrane separation become rare and the vesicle population changes to a mixture of small pleomorphic forms and larger (60-80 nm) spherical ones. However even at two months the number of large spherical vesicles is markedly less than that at the adult synapse, while small pleomorphic vesicles are still abundant. Despite the difference between the adult and hatchling vesicle populations, intracellular recordings have shown that the synapse is fully functional as a non-rectifying electrical junction on hatching and that the intracellular marker Lucifer Yellow can pass between adjacent lateral giant fibre neurons.