Figure 1. The nervous system of the C. elegans hermaphrodite. (a) Schematic representation of the neuronal cell bodies in the head (green circles), ventral cord (red circles), tail (blue circles) and lateral positions (black circles) in the adult hermaphrodite. The worm is shown lying on its right side and therefore only neurons on the left side or middle of the worm are represented. (b) Confocal image of a transgenic worm that expressed GFP in the nervous system under the regulation of a pan-neuronal promoter. Neuronal cell bodies in the head and tail, and in ventral and lateral positions are evident, as are the neural processes of the ventral, lateral and dorsal nerve cords. Commissures running circumferentially can also be seen.

Figure 2. Cell differentiation in the lineages of the P neuroblasts. (a) Schematic of a hermaphrodite showing the approximate position of the P neuroblasts along the ventral cord (filled ovals). Below are the patterns of cell division and differentiation of P1–P12. The vertical lines connect to horizontal lines that represent division of the cell shown above. X indicates programmed cell death. (b) Expression pattern of lin-39 (red underline), mab-5 (blue underline) and egl-5 (yellow underline) in the P cell lineages. Those cells that express both lin-39 and mab-5 are shown in purple and those that express both mab-5 and egl-5 are shown in green. To demonstrate that patterning along the A/P axis is lost in lin-39, mab-5 and egl-5 mutants, the identity of neurons affected by these mutations is shown. Mutations in lin-39 affect P3–P8, mutations in mab-5 affect P11 and P12, and mutations in egl-5 affect P12.

Figure 3. Differentiation of the touch cells. (a) Schematic diagram of a worm showing the approximate locations of the neuronal cell bodies (circles) of the touch cells and their processes (lines). Neurons in red mediate backwards avoidance responses, while those in green mediate forward avoidance responses. (b) Genetic pathway for touch cell differentiation. vab-15 is thought to act upstream of lin-32, which activates expression of unc-86. unc-86 drives mec-3 expression, which in turn regulates mec-4 and mec-7. mec-3 also maintains its own expression by autoregulation. lin-22 restricts touch cell differentiation in other neurons by negatively regulating lin-32. (c) Cell division in the Q neuroblasts. Red lines indicate cells that express UNC-86 in wild-type worms. Grey lines represent cells that no longer express unc-86 in mutant worms.

Figure 4. Amphid sensory neurons and the differentiation of AWA, AWB and AWC. (a) Schematic of an adult hermaphrodite and the approximate position of neuronal cell bodies of the amphid neurons (filled circles) and their processes (lines). (b) Labelled cell bodies of the amphid neurons. Anterior is to the left and posterior to the right. Only neurons on the left side of the worm are represented. Neurons in red are involved in the repulsion from specific chemical compounds, while those in green are required for attraction. The ASJ neurons, as well as ASI, ADF and ASG, are involved in detecting sensory cues that regulate the entry and exit from an alternative larval stage called the dauer state. (c) Differentiation of AWA (yellow), AWB (purple) and AWC (blue). Black text represents genetic pathways that lead to the appropriate differentiation of the AWA, AWB and AWC neurons. Pathways in red text indicate those in either lin-11, lim-4 or odr-7 mutants that lead to inappropriate differentiation of the AWC default state.