Invited review: guidance cue patterns and cell migration in multicellular organisms.

In multicellular organisms, guidance cues are either diffusible molecules or cellular or extracellular surfaces that are found in reproducible locations and that orient migrating cells and cell processes. The pattern of the guidance cues usually determines the complex in vivo migration routes of motile cells and cell processes. Within organisms, guidance cues are found to be organized in two general patterns: a) broad gradients--such as diffuse chemotactic gradients; b) discrete routes (substrate pathways)--such as chemotactic gradients confined to long channels, and such as the axon surface which represents a long specific highway for migrating Schwann cells.