Synaptoid profiles in regenerating crustacean peripheral nerves.

Synapse-like structures occurring in regenerating crayfish peripheral nerves are characterized by aggregates of small (250 to 600 A) electron lucent vesicles adjacent to a thickened "presynaptic" membrane. Such synaptoid profiles are seen opposite other axons, glial processes or extracellular fibrous material. Junctions with cellular elements do not show "postsynaptic" specializations. These complexes are compared to axo-glial synapses in the developing spinal cord and to synaptoid configurations observed by others in vertebrate and invertebrate neurosecretory systems.