The C. elegans nerve ring (NR), along with the ganglia of the head, is considered to be worm's brain. It is a tight axon bundle made of processes from over half of the animal's neurons and is the most synapse-rich part of the nervous system. These synapses made en passant between neurons within the NR govern most of the animal's behaviors. In postembronic animals neuron processes are seen to enter the NR from distinct ventral, lateral, subdorsal and dorsal routes (EmbryoNRDevFIG 1 and EmbryoNRDevTABLE 1). Neuron processes follow either direct paths to enter the NR or travel through indirect trajectories from their cell bodies, i.e. via commissures. Most neurons in the lateral ganglia reach the NR from the ventral side via the amphid commissures while a subset of them take a more direct lateral route to enter the NR (see Hermaphrodite Chapter-Nervous System General Description and Ganglia and Longitudinal Tracts). In the early stages, head muscle cells migrate from more central positions around the NR to the periphery of the embryo and separate the lateral ganglia from the ventral as they do so (see Hermaphrodite Chapter-Muscle System Introduction). Commissural axons must therefore travel under the muscle to reach the ventral ganglion. Entry to the NR from the the ventral route is the earliest, while lateral, subdorsal and dorsal routes are established later (see below). The initiation and positioning of the presumptive NR, as well as the establishment of these tracts, lay down the pattern for later interactions between neurons within the final organization of the worm's largest neuropil.

EmbryoNRDevFIG 1: Routes of Entry to the Nerve Ring (shown in postembryonic head). Most neuron processes enter the NR from the ventral side either directly or indirectly. However, a subset of the head neurons have entry to the NR from three other routes; dorsal, subdorsal and lateral. Note the neurons that are anterior to the nerve ring and have posterior processes that enter the NR are not indicated in this picture.

EmbryoNRDevTABLE 1: Neurons that enter the nerve ring from the dorsal, subdorsal and lateral routes. Note, neurons that are anterior to the NR are not listed.

In wild type C. elegans, after elongation, the NR is positioned around the center of the isthmus of the pharynx (EmbryoNRDevFIG 2). CWN-2, the C. elegans Wnt5 homolog, and SIA and SIB neurons play an essential role in maintaining the placement of the NR correctly in this position, likely during the elongation stage (Kennerdell et al., 2009). CWN-2 is required after 1.5-fold stage of embryogenesis for correct positioning, and functions through the Wnt receptor CAM-1. CWN-2 is a ligand for CAM-1 (Ror receptor) and CAM-1 expression in SIA and SIB neurons governs this function, although the CWN-2 expression site is not as important. MIG-1 may be acting as a co-receptor with CAM. When this pathway is perturbed the NR is displaced towards the anterior along with neuronal cell bodies, muscle arms and the excretory pore (EmbryoNRDevFIG 2) (Kennerdell et al., 2009). In cwn-2-mutant animals SIA and SIB process guidance is also defective such that these sublateral axons often fail to exit the nerve ring or exit in the wrong place (Kennerdell et al., 2009). The anterior misplacement of the NR is also observed when SAX-3 axon guidance receptor is mutated. Also, in these mutants the NR is no longer an intact single bundle but instead defasciculates into several bundles (Zallen et al.,1998 and Hao et al., 2001).

EmbryoNRDevFIG 2: Positioning of the Nerve Ring. In wild type animals the NR is placed around the isthmus of the pharynx. However, in CWN-2/CAM-1 pathway mutants, both the NR and the neurons are misplaced towards the anterior and the ring is placed around the anterior bulb of the pharynx (indicated by white dashed line). Differential interference contrast (DIC) image of the head of a wild type young adult, left lateral view. Magnification, 600x.

In the embryo, the NR assembly initiates between bean and comma stages and continues up until the end of embryogenesis, The NR is visible around the embryonic pharynx by the 1.5-fold stage (EmbryoNRDevFIG 3) (Rapti et al., 2017) (for stages of embryonic development see Hermaphrodite Chapter-Introduction to C. elegans Anatomy and IntroFIG 7). Its assembly is hierarchical; processes from five bilateral sublateral commissure neurons (SIAV, SIAD, SIBV, SIBD, SMDD) and the CEPshV glia merge to form a pioneer process bundle to enter and initiate the presumptive NR. The growth of follower processes occurs in waves subsequently. At the onset of NR formation, sublateral neuron processes fasciculate and navigate towards the CEPshV, they then coalesce with the dorsally directed process of CEPshV, which likely defines a turning point for the sublateral process bundle. Early neuronal axons and glia processes forming this pioneer nerve-fascicle of 10 or fewer processes then cooperatively direct follower axon entry to the newly born NR. Supporting their pivotal role in patterning the developing NR, SIA and SIB processes follow a complex path around the NR in the adult, occupy positions near the middle of the NR neuropil and have few synaptic connections. In CEPsh-ablated animals the pioneer sublateral process bundle still enters the presumptive NR, however, it is defasciculated and extends anteriorly in an aberrant fashion compared to the fascicle in nonCEPsh-ablated animals. Ablation of CEPshD or CEPshV glia correlates with AIY, AWB and AWC axon entry defects towards the NR. SIA and SIB ablation also leads to defects in AIY and amphid neuron process entry to the NR, although CEPsh glia process growth and wrapping of the NR are not affected. KPC-1 (a Kex2/subtilisin-like proprotein convertase) and CHIN-1 (a GTPase activating protein) function in CEPsh glia to guide the sublateral pioneers during the earliest NR assembly, and function in both the sublaterals and CEPsh glia for the navigation of the follower axons (Rapti et al., 2017). An axon guidance molecule, UNC-6, is produced by CEPshV cells and when mutated leads to defects in ventral NR processes and synapses (Wadsworth et al., 1996; Colon-Ramos et al., 2007, Yoshimura et al., 2008). For pioneer-process guidance, UNC-6/Netrin expression in CEPsh glia is essential and for follower-process guidance, MAB-20/Semaphorin and FMI-1/CELSR expression in both the glia and the sublaterals are essential (Rapti et al., 2017). The majority of follower amphid axons first start to extend from their cell bodies around 350 min (between bean and comma stages) after first cell division and have already completed their path to the nerve ring around 430 min after first cell division (around about 1.75-fold stage, 8 amphid commissure axons are seen entering the NR). At the 1.75-stage, the neuron processes that take the lateral route to the NR and the two processes of ALA which enter the NR via the dorsal route are only beginning to grow out. The latter two route may be blocked until muscle cell migration to the periphery opens up access by these processes to the NR (Norris C. and Hall D.H. unpublished observations 1997) (EmbryoNRDevTABLE 2) (EmbryoNRDevFIG 4). As the amphid commissure is forming from the lateral ganglion neurons extending towards the ventral, the amphid axons may utilize a large neuron (possibly SMBD or SMDD) situated between muscle and hypodermis as substrate for growth (unpublished observations, C. Norris and D.H. Hall, 1997).

4 References

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Rapti, G., Li, C., Shan, A. and Shaham, S. 2017. Glia initiate brain assembly through noncanonical Chimaerin–Furin axon guidance in C. elegans. Nat. Neurosci. 20: 1350-1360. doi:10.1038/nn.4630. Article

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Zallen, J.A., Yi, B.A. and Bargmann, C.I. 1998. The conserved immunoglobulin superfamily member SAX-3/Robo directs multiple aspects of axon guidance in C. elegans. Cell. 92: 217-227. doi: 10.1016/S0092-8674(00)80916-2. Article