Neurocytology of the cerebral ganglion of Fasciola hepatica (Platyhelminthes).

An ultrastructural study of the organization and fine structure of the nervous system of the parasitic flatworm Fasciola hepatica was undertaken. The brain consists of paired cerebral ganglia, located just posterior to the oral sucker, that are connected by a transverse commissure which crosses over the dorsal surface of the pharynx. The cell bodies of the cerebral ganglia are not organized into a clearly defined rind around the neuropile but are loosely scattered around and within the neuropile area. The neuropile consists of two morphologically distinct types of unmyelinated nerve processes. The small nerve processes, with smooth cell membranes, are less than 2 micron in diameter, whereas the "giant" processes are greater than 12 micron in diameter and have extensively invaginated cell membranes. Giant processes make up the bulk of the nerve fibers in the transverse commissure and longitudinal nerve cords. Four morphological types of vesicles are present in the small processes; small clear vesicles (which were found associated with synapses), spheroidal and ellipsoidal dense vesicles, and dense-core vesicles. Two types of synapses are found in the neuropile: simple synapses characterized by pre- and postsynaptic membrane densities, clusters of small clear vesicles, and a clear synaptic cleft; and wedge-shaped synapses or divergent diads each having one presynaptic process synapsing onto two postsynaptic processes. Synaptic contacts were observed only between small processes; no synapses were observed on the cell bodies or on giant processes.