NeuroFIG 3: Branching patterns of C. elegans neurons.

A&B. Most C. elegans neurons have simple monopolar or bipolar structures.
A. Epifluorescent micrograph of an adult animal expressing the str-2::GFP reporter in the AWCL neuron, left lateral view. The dendrite of the sensory neuron (arrow) extends to the tip of the nose, whereas the axon (arrowhead) passes through the amphid commissure to enter the nerve ring, where it makes synaptic contacts. This is one of the few neurons in the worm that has a distinct dendrite and an axon. Magnification, 600x. (Strain source: C. Bargmann.)
B. Epifluorescent image, ventral view. An adult animal expressing the zig-2::GFP reporter in PVT, which is a single, monopolar neuron located in the preanal ganglion. Its process (arrow) travels anteriorly to the nerve ring within the right fascicle of the VNC. Magnification, 600x. (Strain source: O. Aurelio and O. Hobert.)
C&D. A few neurons have more complex branching patterns.
C. Ventral view. HSN axon outgrowth begins during L2 and L3, and its main axon reaches the nerve ring by the late-L4 stage (see Reproductive System - Egg-laying apparatus). Vulval precursor cells guide the emerging HSN processes and induce them to form several short branches (arrowheads [left side is slightly out of focus]) at the vulva. Epifluorescent micrograph of an adult animal expressing the arrestin::GFP reporter in HSN neurons. Magnification, 600x. (Strain source: G. Garriga and C. Bargmann.)
D. Lateral views. PVD neurons display the most complex arborization pattern in the worm. In larval stages (top), three major processes can be discerned—one anteriorly directed, one posteriorly directed, and one ventrally directed (arrowheads). These processes have short branches along their length. As the animal ages (bottom), this branching pattern is extensively elaborated and covers most of the body with secondary branches. Epifluorescent image of an animal stained with DEG-3 antibodies. (Image source: M. Treinin.)

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