The fine structure of elements of the nervous systems of adults and protoscolices of Echinococcus granulosus is described. The lateral nerve trunks are composed of numerous unmyelinated fibers without a surrounding cellular sheath. The nerve fibers contain mitochondria and several sizes and types of vesicles. Synaptic junctions are commonly seen with clear vesicles accumulating on one side of the synapse. Dense vesicles resembling neurosecretory vesicles are seen at a presumed neuromuscular junction as well as in the nerve processes. A sensory ending in the tegument is described. This structure is composed of a ciliumlike terminal process, which extends beyond the tegumental surface from a bulbous expanded area. A rootlet structure is contained in the bulblike expansion. The rootlet is composed of filaments which emanate radially from the basal body and extend into the nerve process. The nervous system of cestodes has received little attention since the late 1800's and early 1900's (see Wardle and McLeod, 1952 for a review of early investigations with the light microscope). Previous work on the nervous system of this group of invertebrates has been confined largely to descriptions of gross anatomical features (Bullock and Horridge, 1965). There are several reasons for this limitation which have been summarized by these two authors. Histologically, the main features of the central nervous system of cestodes are its loose texture and usually the lack of a sheath or capsule to set it off from the other body tissues. In addition, nerve cells are said to be absent or scarce along the main longitudinal cords, except for peripheral sense cells (Bullock and Horridge, 1965). There is general agreement on several features of the tapeworm nervous system: (1) that it is made up of longitudinal nerve cords running the full length of the body with one set lying lateral to the osmoregulatory collecting canals on each side, (2) that the nervous tissue does not stain readily with differential staining techniques, and (3) that it is one of the most difficult anatomical features of tapeworms to study by light microscopy (Wardle and McLeod, 1952). The use of methylene blue and silver staining techniques has produced a considerable amount of information as to shape, size, and location of nerves and nerve endings but resoReceived for publication 9 November 1966. * Present address: Laboratory of Parasitology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania. lution of their fine detail is possible only with the electron microscope. This work is a continuation of observations on the fine structure of taeniid tapeworms (Morseth, 1965, 1966a, b). MATERIALS AND METHODS The brood capsules were removed from cysts of E. granulosus and processed for electron microscopy as previously described (Morseth, 1966a, b). Some of the protoscolices examined had been cultured in vitro for 42 days (Smyth, Howkins, and Barton, 1966) prior to being processed for electron microscopy. Adult worms (35 days old) were collected from the intestine of an experimentally infected dog. The method used has been described (Morseth, 1966a, b). The worms were fixed in 2% osmic acid in veronal buffer (pH 9.0) for 2 hr at room temperature, dehydrated in ethanol solutions, and embedded in araldite. All sections were cut with glass knives on an LKB microtome and collected on grids coated with a thin collodion-carbon film. Sections were stained with lead citrate (Reynolds, 1963) for 10 to 15 min and examined and photographed in a Philips EM-200 electron microscope.
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Jul 1, 1978
J M Reuben + 2
Open Access
