Three known genes guide circumferential migrations of pioneer axons and mesodermal cells on the nematode body wall. unc-5 affects dorsal migrations, unc-40 primarily affects ventral migrations, and unc-6 affects migrations in both directions. Circumferential movements still occur, but are misdirected whereas longitudinal movements are normal in these mutants. Pioneer growth cones migrating directly on the epidermis are affected; growth cones migrating along established axon fascicles are normal. Thus these genes affect cell guidance and not cell motility per se. We propose that two opposite, adhesive gradients guide circumferential migrations on the epidermis. unc-5, unc-6, and unc-40 may encode these adhesion molecules or their cellular receptors. Neurons have access to the basal lamina and the basolateral surfaces of the epidermis, but mesodermal cells contact only the basal lamina. These genes probably identify molecular cues on the basal lamina that guide mesodermal migrations. The same basal lamina cues, or perhaps related molecules on the epidermal cell surfaces, guide pioneer neurons.
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