Caenorhabditis elegans, or C. elegans, is a small roundworm (nematode) usually found in the soil in temperate climates all around the world. It feeds on microorganisms like bacteria and can be most easily isolated from rotten fruit.
In the 1960s Sydney Brenner at the MRC laboratory in Cambridge, England, decided to use this particular species as a new model organism to study embryonic development. Specifically he was looking for a multicellular organism that was small, had a simple anatomy with only few cells, was easily cultivated and would reproduce quickly - hallmark features of many nematodes and C. elegans in particular. Since then C. elegans has become one of the most intensively studied animals and the number of researchers working with C. elegans today exceeds the number of cells this tiny animal has (959 in an adult hermaphrodite).
C. elegans embryos develop rapidly and hatch after 14 hours. The first larval stage is completed after another 12 hours and the animals proceed through four molt cycles before becoming adults. Under crowded conditions or in the absence of food larvae can choose an alternative developmental pathway leading to the dauer larva, which does not feed but can survive adverse conditions for several months. When life gets better normal development is resumed, the animals exit the dauer larval stage and develop into the normal fourth larval stage before becoming adult. Adult animals are hermaphrodites and produce both sperm and eggs. Over the course of 3-4 days some 300 eggs are laid. The overall life span of C. elegans is 2-3 weeks. The short generation cycle facilitates genetic experiments and is a major advantage for researchers working with this organism.
C. elegans has a simple anatomy with a small number of tissues and internal organs (see Figures 3 and 4). The head contains the brain and the prominent feeding organ - the pharynx. The main body is filled with the intestine and - in the case of an adult hermaphrodite - the gonad consisting of the uterus and spermatheka. Embryos start to develop inside the mother and are laid through the vulva around gastrulation stage.
The body is cylindrical, surrounded by a sheet of epithelial cells (hypodermis) and protected by a secreted cuticle consisting mainly of collagens. Body wall muscle cells are arranged in four rows, two each on the ventral and dorsal sides (green in Figure 4). Major nerve cords run along the entire length of the body on the dorsal and ventral midline. The pseudocoelomic cavity surrounding the intestine and gonad is filled with fluid. C. elegans completely lacks skeletal elements and has no circulatory system. Adult animals are only 1mm in length about 0.2mm in diameter, small enough to allow oxygen from the air to diffuse through the body. Nutrients from the gut are simply released into the pseudocoelmic cavity and taken up by other cells. The animals are under internal hydrostatic pressure, which acts as 'hydrostatic skeleton'. Muscle cells are tightly connected to the external cuticle through the hypodermal cells. Contraction of muscle cells on one side leads to bending of the rigid body. Coordinated contractions allow movement in elegant sinusoidal waves (hence the name C. elegans).
C. elegans nervous system
Figure 5: C. elegans nervous system: all neurons labelled with a fluorescent marker (GFP)
The nervous system is by far the most complex organ in C. elegans. Almost a third of all the cells in the body (302 out of 959 in the adult hermaphrodite to be precise) are neurons. 20 of these neurons are located inside the pharynx, which has its own nervous system. The remaining 282 neurons are located in various ganglia in the head and tail and also along the ventral cord, the main longitudinal axon tract. The majority of the neurons develops during embryogenesis, but 80 neurons - mainly motoneurons - develop postembryonically. The structure of the nervous system has been described in unprecedented detail by electron microscopic reconstruction (White et al., 1986). The high resolution obtained with electron microscopic images allowed White and colleagues to identify all the synapses (about 5000 chemical synapses, 2000 neuromuscular junctions and some 500 gap junctions), map all the connections and work out the entire neuronal circuit.
The brain
Figure 6: C. elegans head region, ventral view, anterior to the left
various classes of neurons labeled with different GFP variants.
The majority of the neurons is located in the head, where they are organised in a number of ganglia surrounding the pharynx, forming the brain of the animal (Figure 6, pharynx not visible). 68 neurons are sensory neurons detecting various soluable and volatile chemicals, tactile stimuli and temperature. These sensory neurons, especially chemosensory neurons (all the white and most of the blue neurons in Figure 2), make up a large fraction of the neurons in the head ganglia. They send their dendrites to the tip of the nose (to the left, but outside the actual picture in Figure 6), which is richly innervated with several sensory structures. Sensory axons join a large axon bundle, the nerve ring, where they make synaptic connections with interneurons. Some of these interneurons (red neurons in Figure 6) in turn send long axons into the ventral cord, which runs the entire length of the animal. The command interneurons of the motorcircuit connect to motoneurons located in the ventral cord, which in turn connect to muscle cells allowing the animal to respond to sensory input by changing its movement pattern.
We use the ventral nerve cord (VNC) in C. elegans is the major longitudinal axon tract comparable in function to the spinal cord in vertebrates (Figure 7). The VNC contains inter- and motor neuron axons that form integral parts of the circuit that controls movement of the animal. The VNC runs along the length of the body and consists of two axon tracts flanking the ventral midline. The right VNC axon tract contains 50 axons, whereas the left tract has only four. Outgrowth of axons into the VNC is sequential and coordinated. The first neuron to extend an axon into the right VNC axon tract is the AVG neuron. The left tract is pioneered by the PVPR neuron. VNC pioneers are important for the correct navigation of follower axons. If the pioneer PVPR is missing, the left VNC tract does not form and all follower axons join the right axon tract. In mutants with PVPR navigation defects, the follower axons of PVQL and HSNL strictly follow the misguided pioneer, suggesting a complete dependence of the followers on the pioneer. Eliminating AVG, the pioneer of the right VNC axon tract, results in a disorganised VNC with axons frequently crossing the midline to extend in the wrong axon tract. We use the VNC as a simple system to study the molecular basis of pioneer navigation and pioneer-mediated navigation of follower axons.
