Ultrastructural and cytochemical studies on the sensory organelles and nervous system ofDipetalonema viteae(Nematoda: Filarioidea)

Abstract

The sensory organelles and some components of the nervous system of the adult filarial wormDipetalonema viteaehave been studied by means of electron microscopy. The sensory organelles contain modified cilia innervated by nerve axons. The cilia lie within cuticle-lined channels. The amphids each contain nine such cilia, but the phasmids and the papillae contain only one cilium. The amphidial and phasmidial cilia are exposed to the external environment and it is suggested that I they function as chemoreceptors. The cilia within the cephalic and caudal papillae are modified so as to produce a greater area of sensitivity. The ciliary channels in the cephalic papillae do not open to the exterior and these papillae therefore probably function as mechanoreceptors. The caudal papillae of the male worm are structurally similar to the cephalic papillae, except that the ciliary channels are open to the external environment. It is suggested that these papillae function both as mechanoreceptors and chemoreceptors. The terminal pair of caudal papillae differ from all the other papillae. The ciliary channels of all the sensory organelles are surrounded by supporting cells, but multivesicular cells are only associated K with the amphids, phasmids and cephalic papillae. The nerve-ring encircles the muscular region of the oesophagus; it is composed entirely of nerve axons. The nerve axons contain two kinds of vesicle. The nervering is surrounded by an ensheathing cell. The major nerve bundles are carried by the hypodermal cords, but they are not embedded in the hypodermal cytoplasm. The nerve axons at the surface of the cord form synapses with the innervation processes of the somatic muscle cells. Electron cytochemical techniques have been used to investigate the distribution of cholinesterases in the structures described. Acetylcholinesterase has been identified in the amphids, phasmids, cephalic papillae and nerve-ring; it is suggested that acetylcholine functions as a chemical transmitter at these sites. Pseudocholinesterase has been identified in the amphids, phasmids and cephalic papillae, but not the nerve-ring; it is present in considerably smaller quantities than acetylcholinesterase.