Caenorhabditis elegans

The complete structure of the anterior sensory nervous system of the small nematode C . elegans has been determined by reconstruction from serial section electronmicrographs. There are 58 neurons in the tip of the head. Fiftytwo of these are arranged in sensilla. These include six inner labial sensilla, six outer labial sensilla, four cephalic sensilla and two amphids. Each sensillum consists of ciliated sensory neurons ending in a channel enclosed by two nonneuronal cells, the sheath and socket cells. The amphidial channel opens to the outside as does that of the inner labial sensilla so that these probably contain chemoreceptive neurons. The endings of the other sensilla are embedded in the cuticle and may be mechanoreceptive. The cell bodies of all the neurons lie near the nerve ring and their axons project into the ring or into ventral ganglia. One of the ciliated sensory neurons in each of the six inner labial sensilla makes direct chemical synapses onto a muscle making these sensory-motor neurons. The anatomy of four isogenic animals was compared in detail and found to be largely invariant. The anatomy of juveniles is nearly identical to that of the adult, but males have four additional neuron processes. Nematodes have simple nervous systems composed of only a few hundred neurons. Seventy years ago, R. Goldschmidt made a detailed study of the nervous system of Ascaris by reconstruction from serial sections using optical microscopy. (Goldschmidt, '03, '08, '09). Among his elegant anatomical drawings he produced a set of enigmatic wiring diagrams which have never been interpreted. It is likely that some of the bizarre connections shown in these diagrams were simply the outcome of the low resolution of the method Goldschmidt used. A smaller nematode would offer the possibility of determining the complete structure of a nervous system by electron microscopy allowing reliable identification of connections. This would open the way to understanding how the nematode's behavior could be generated by the nervous structure. We have chosen the small soil nematode Caenorhabditis elegans for electron microscopical reconstruction of its nervous system. C. elegans is a favorable organism for studying the genetics of complex processes J. COMP. NEUR., 160: 313-338. and many behavioral mutants have been isolated (Brenner, '73; Ward, '73; Brenner, '74). Reconstruction of the wild-type nervous sytem will make i t possible to recognize anatomical lesions in such mutants. Such lesions should aid the task of understanding nervous system function by allowing correlation of behavioral and anatomical defects. In addition, the study of genetically induced lesions may help to understand how nervous system structure is specified genetically. This paper is the first of a series which will describe the structure of the nervous system of C. elegans. It deals with the anterior sensory nervous system. Although, in volume, this is only a small part of the nematode it accounts for 58 of the 300 or so neurons found in the animal. The neurons are relatively simple and the reconstruction, though laborious, has not posed any special problems. MATERIALS AND METHODS