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The small soil nematode Caenorhabditis elegans has received considerable attention as a model organism in which a genetic approach can be brought to bear on questions of eukaryotic development (Hedgecock et al., 1987, 1990; Desai et al., 1988; Wood, 1988; Chalfie and Au, 1989). In part, this attention is based on the relative genetic manipulability afforded by C. elegans' self-fertilizing, hermaphroditic mode of reproduction (Brenner, 1974; Herman and Horvitz, 1980), and, in part, it is based on the fact that C. elegans has a small and reproducible number of somatic cells whose lineage histories have been completely described from the fertilized zygote to the adult (Sulston, 1976; Sulston and Horvitz, 1977; Deppe et al., 1978; Sulston et al., 1983). The nervous system of C. elegans is of great developmental interest. Its functional organization must depend not only on general aspects of development, such as tissue-specific differentiation or selective adherence of cells of like tissue type, but also on differentiation of a considerable variety of specific neuronal types and especially on the establishment of highly specific patterns of outgrowth and synaptic interconnection among these types (White et al., 1983; Chalfie et al., 1985; Hedgecock et al., 1987, 1990; Chalfie and White, 1988). C. elegans mutants can remain viable despite catastrophic lesions to the nervous system (Hedgecock et al., 1987, 1990; Hall et al., 1989), offering access to many essential developmental genes. Molecular studies in both vertebrates and invertebrates have shown significant homologies in early neural development (Hedgecock and Hall, 1990; Way, 1990); these homologies suggest that C. elegans mutants may uncover basic developmental mechanisms of broad importance. As a necessary prerequisite to developmental study of the nervous system, previous reports have described parts of the adult C. elegans nervous system in considerable detail, including the anterior sensory cells (Ward et al., 1975; Ware et al., 1975), the pharynx (Albertson and Thomson, 1976), the ventral nerve cord (White et al., 1976), and most elaborately, the nerve ring (White et al., 1986). Analysis has been facilitated by the relative simplicity of the nervous system (302 neurons in total; White et al., 1986) and by its very limited physical size, which has made possible the complete reconstruction of portions of the nervous system by serial-section electron microscopy with series of, at most, a few thousand sections in length.
Here, we describe in detail the patterns of synaptic interaction observed in the tail of the adult hermaphrodite, cataloging all the synapses observed and establishing the unique identities of pre- and postsynaptic cells at most synapses. When combined with the data from White et al. (1986), our results allow inferences to be made about the degree of developmental reproducibility in synapse formation and about the probable relationships between bilaterally homologous cells. They also suggest interesting possibilities for the ways in which the observed synapses might serve to process sensory information.
Web adaptation, Thomas Boulin, for Wormatlas, 2002