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In a simple CNS such as that of C. elegans the question of symmetry is of considerable interest. Superficially there is a hexagonal regularity to the cephalic region, as seen in the six-fold repeated gross lip structure and the identity of their internal labial papillae. More internally there is a four-fold regularity, as reflected by the identity of the submedial sensory structures and the positioning of the muscle cells within the four cephalic muscle bundles. Within the CNS itself, however, only a bilateral symmetry remains, but to which there are reproducible exceptions. In the anterior ventral ganglion (fig 18) for example, we have identified several asymmetrically located cells which as yet are incompletely characterized. Also, the multipolar DM cell sends a single process asymmetrically to the dorsal cord, itself asymmetrically constructed and asymmetrically innervating the cephalic musculature (fig. 25). Our observations thus support the conclusion of Schuurmans-Stekhoven ('37) that nematodes are constructed with fundamentally bilateral symmetry, but also show that at a fine structural level this is not perfect.
Of the possible function of the various anterior sensory receptors not a great deal can be said on the basis of morphology alone. Certainly the opening of the amphidial pouch to the external milieu and its content of an inhomogeneous matrix surrounding the dendrites strongly suggest a chemosensory function. The ciliary structure of the terminal amphidial neurons is also consistent with this conclusion (Han- sen and Heumann, '71).
The amphidial fine structure in C. elegans, not surprisingly, shows many gross similarities with that of the few other species which have been studied by electron microscopy, differences occurring mainly in details. The homolog of our pocket cell with its gland-like inclusions seems to be universal. McLaren ('70, '72) also finds the analog of our cap cell which completely surrounds the distal end of the pocket cell, extends into the amphidial pore and to the cuticle, and seals on itself with a very long longitudinal desmosome. Microtubules within the cilia have been found to show varying patterns of organization, from our 9 x 2 + m, to 8 x 2 + m in M. incognita (Baldwin and Hirschmann, '73), to a semi-regular pattern in D. immitis (Kozek, '71), to a complete lack of pattern in D. viteae (McLaren, '70, '72). The rudimentary rootlets which we invariably observe in longitudinal sections are variously described as occasionally present or missing. No distinct basal bodies have ever been described. Baldwin and Hirschmann describe some of the amphidial dendrites as leaving the pouch distally and migrating to the other lips [Because of this papillary organs are identified by the positional terminology of Goldschmidt ('03), Schuurmans-Steckhoven ('37), and Chitwood and Chitwood ('50) in order to avoid implications of homology suggested by the more commonly used terminology of de Coninck ('50)]. In C. elegans we see these as filamentous extensions of the dendrites contained completely within the pocket cell. Binary branching of some of the dendrites within the pouch is also mentioned, and dendrites are always described as ending at various levels and being surrounded within the pouch by an amorphous electron dense material. The presence of vesicles at the point of entry of the amphidial dendrites into the pocket cell has also been noted in D. viteae (McLaren, '70, '72), and the desmosomal junction with the pocket cell at this point has been shown in D. immitis (Kozek, '71). The terminal opening of the internal labial papillary organ presents a problem in trying to deduce a possible function solely on the basis of morphological evidence, On the one hand, the structure and configuration of the cilia and associated cells produce a sensilium similar in appearance to that of a chordotonal organ, responsive to motion parallel to its axis, described for scolopedia in crustacean stretch receptors and in insects (Mill and Lowe, '71; Whitear, '62; Howse, '68). Although the characteristic and presumably functionally important scolopale is absent, there is a cuticular darkening within the protrusion (fig. 5A) which, if rigid, could serve the same purported function. The similarity is further observed in the structural differences between the two dendrites, one resembling the ciliary type with along rootlet and a more or less organized array of microtubules, the other the paraciliary type in which the rootlet is short and illdefined and the ciliary microtubules are not as regularly arrayed. As in the Johnson's organ of Chrysopa (Schmidt, '69), it is this latter cilium which protrudes slightly from the cuticular opening, whereas the former ends somewhat more proximally. The opening does not immediately disqualify this organ as a pure mechano-receptor since such openings have been described in association with the company form sensilia in insects (e.g, Schmidt and Gnatzy, '71), in which case they are interpreted as an artifact of molting, In addition, the small diameter of the IL ciliary endings and their basically 9 x 2 + O microtubular arrangement is quite different from the larger diameter of the amphidial cilia and of the cilia of the phasmids (D. Hall, unpublished observations) and their 9 x 2 + m microtubular pattern. On the other hand, the fact that the dendrites do have direct access to the external milieu through a very narrow cuticular channel does raise the possibility of either a bimodal function, as in insect contact chemoreceptors, or a purely chemosensory function.
It is easier to exclude a possible chemosensory function for the MSM, LSM, and VL papillae and the deirids, since they possess no cuticular opening. In addition, their inclusions of very electron dense bodies at the dendritic terminations are much like similar specializations in insect mechanoreceptors. Of these inclusions, only those of the LSM organs have a close homolog in insects with the very common tubular body structure of campaniform sensilia originally described by Thurm and linked experimentally with detection of motion perpendicular to the dendritic axis (Thurm, '64).
The often striking similarities in the basic construction of the nervous systems of C. elegans and Ascaris have been pointed out in the description as they occurred. That these exist in such two disparate forms at opposite ends of the size and habitat scale is a validation of Chitwood's hypothesis that the nemic nervous system is conservative (Chitwood and Wehr, '34). In spite of the fact that Hyman stated "the greatest divergences from the usual plan are seen in Ascaris, which is also the form in which the nervous system has been most thoroughly studied" (Hyman, '51), these similarities clearly point out that different superficial structures may belie a unity on a much more fundamental level. Table 1 lists the papillary sensory cells which we have identified and, for comparison, those identified by Goldschmidt ('03, '08). It can be seen that the correspondence as to cell numbers is close but not identical. Most of our numbered cells have exceedingly fine fibers in the anterior part of C. elegans. Thus despite the size difference between it and Ascaris it is possible that some fibers could have been overlooked in a light microscopic study, even if the animals were constructed identically. Cell bodies were unlikely to be missed in the earlier study, however, so it seems reasonable to conclude, especially in light of the markedly different aggregations of cells into ganglia in Ascaris and in C. elegans, that there are in fact differences in the nervous systems of these two nematodes. A major disagreement with the earlier light microscopic work has been the finding that all sensory fibers of C. elegans maintain their discreteness throughout the neuropil and that they do not, as Goldschmidt ('08) stated for Ascaris sensory neurons, anastamose with one another after entering the nerve ring.
The cell body positions of all neurons, in particular the sensory neurons, have been found to be highly reproducible in different animals. Figure 18 shows this for two of the animals which we have studied extensively, and these are by no means exceptional. The only other investigation of comparable extent in a different invertebrate, the rotifer Asplanchna brightwelli, (Ware, '71) has likewise shown that every cell body is unique and has a homolog, both in position and in detailed ultrastructural anatomy, in all cases. This poses an interesting developmental problem which might well have been solved differently in different classes of animals. For those where cell economy is of primary importance, the nervous system must be constructed exactly if the species is to survive. For higher animals, where there is evidence of a high degree of redundancy in cell type; it may be that proper function can be achieved through an overall averaging process of many elements which are connected only approximately but into appropriate "fields" of influence. Consequently it is anticipated that the study of deviations from a normal structure in simple animals may reveal developmental principles which are obscured in higher, perhaps less well specified, systems.
The cephalic musculature has been shown to divide itself into four submedial quadrants, all of which are symmetrical in structure, The muscle innervation of the nerve ring has been found to be highly reproducible from animal to animal and, except for the possible detailed terminal arrangements which have not been investigated, similar for each quadrant. The peculiarity of the nematode muscle innervation has been shown to possess another unusual feature, namely that the muscle processes assume the shape of broad sheets within the central neuropil. What functional or developmental advantage this pattern possesses over the more commonly observed one of multiple branchings is unknown. The possibility of an interesting control mechanism arises in the different types of innervation of the four more anterior and the four more posterior muscle cells of each cephalic quadrant. The anterior four are innervated only by the ring, with very broad bellies present immediately before their sheets. The posterior four, on the other hand, have very slender processes, about 350 nm in thickness, interposed between their bellies and their sheets within the ring. They are also innervated by their respective medial cords, where much broader entry processes are present (fig, 25). In the event that the thin connectives to the ring have a relatively impared function in the well fed adult animal as we have examined it, one may well ask as to their significance. Only further examination of animals in various states of nutrition or in different stages of development will indicate whether the animal can perhaps utilize this bimodal innervation of the posterior cephalic musculature as a switch mechanism, that is, to alter the posterior cephalic muscle activity by alternating between pure cord and ring plus cord control.
There is no existing literature which deals extensively with the neuropil of the nematode CNS. This is unfortunate because, since the light microscopic work of Goldschmidt ('08) describing an apparent total anastamosing of fibers, there has been the tendency to think uncritically of the nematode nervous system as something "special," not conforming to the normal structural-functional pattern of other animals which have been investigated by modern electron microscopic techniques. At least in the case of the anterior sensory fibers, in contrast to the statement of Goldschmidt ('08), we have found this not to be the case. The nematode nervous system does possess one unique feature among the invertebrates in that it is also the location of the neuromuscular synapses. These contacts occur on branches of the muscle cells which are sent centrally to the motoneurons, rather than on branches of motoneurons which are sent peripherally to the muscle cells, It has been found that the synapses to the cephalic musculature occur largely at the borders of four delimited capsules, two lateral and two medial, within the nerve ring which we have called muscle plate regions. These regions contain only muscle cell processes, which are observed to have many gap junctions among themselves. A possible interpretation for the function of these junctions will have to await a determination of which muscle cells are coupled by them in all four quadrants as well as a description of any neighboring chemical synapses which occur on one or the other of the participating muscle processes. They cannot mediate a complete coupling, for the cephalic region of C. elegans is capable of very tight bending motions in all directions. At present all that can be said is that these junctions are numerous, cover large areas of the muscle processes within the plate, and in some cases have been found between homologous muscles subdorsally and subventrally. In the neuropil external to the muscle plates we have found a rather common variety of vesicles and synapses, though the presence of large numbers of gap junctions involving fibers containing irregularly shaped clear vesicles may perhaps prove unusual. Certainly a much clearer picture of the exact distribution and possible functions of these synaptic interactions will emerge from histochemical and connectivity analyses. In addition to the commonly observed structures, we have identified an unusual chemical synapse type, of the axosomatic variety, repeatedly observed on the same two bilaterally paired cells of the ventral ganglion (fig. 32A).
Our investigation has also added another animal to the list of those which possess the presumed intermediate in the evolutionary development of more complex nervous systems (Parker, '19), the sensory-motor cell (fig. 33). Doubtless other examples will be found as more systematic investigations are carried out in other invertebrates. Only two other instances of this have been shown. In Aplysia californica, Coggeshall ('71) described a cell containing large (120 nm) electron dense granules in the secretory epithelium of the accessory genital mass. The ciliated end of this cell, in which no rootlet was observed, extends into the lumen of the oviduct. The basal portion, which has granule-filled swellings, is found directly under large secretory cells. Although no membrane specializations resembling classical synapses were found, accumulations of vesicles were observed adjacent to the presumed post-synaptic effector cell. In Hydra littoralis, Westfall ('73) described a neuron which has at one end a cilium with two perpendicular centrioles and a faintly appearing striated rootlet. Both its cell body and neurite processes contain 100 nm dense core vesicles and many localized membrane specializations without swellings, taken to indicate areas of synaptic contact, opposite several different cell types. In contrast to these two examples, the ciliated sensory-motor cell of C. elegans has been found to have a typical presynaptic swelling filled with small clear synaptic vesicles and a membrane specialization adjacent to an identified muscle cell not unlike that found at other synaptic sites, both neuromuscular and neuroneuronal.
The description of the CNS of C. elegans presented here has concentrated on the gross organization of its main mass, the nerve ring, in relation to the primary sensory input and cephalic muscle motor output. Clearly much more work would be required to establish the anatomical basis for behavioral patterns of the animal in even the crudest sense. The diffuseness of the nervous system as a whole, composed as it is of numerous peripheral cords which themselves innervate muscles, provides a formidable challenge for serial section analysis. Nevertheless the results presented provide a fine structural basis with which one can begin to form associations of observed behavioral abnormalities with alterations in the central controlling nervous tissue, and lay the groundwork for electron microscopic examination of detailed connectivities in a central neuropil.
Thanks is given to the Environmental Sciences Department, California Institute of Technology, for extensive use of, and permission to modify, their Zeiss EM9 electron microscope, and to Chris Irving for illustrations.
This research was supported by a Neurosciences Study Grant from the Sloan Foundation and by U. S. Public Health Service grants NS-09654 and NS-10628.
Web adaptation, Thomas Boulin, for Wormatlas, 2002