Evidence for the guidance of pronephric duct migration by a craniocaudally traveling adhesion gradient

These experiments were undertaken to identify the general nature of the mechanism that guides the migration of the Ambystoma pronephric duct along the ventral edge of the somite file from its anterior origin to the cloaca. Using scanning electron microscopy in conjunction with microsurgery, we have sought to distinguish among such possibilities as chemotaxis, contact guidance, and gradients of adhesiveness to the substratum. A pronephric duct primordium transplanted to the flank of a host ventral to the primary duct migrates dorsocaudally across the flank to fuse with the primary duct. Removal of potential sources of distant attraction does not alter this behavior, nor do migrating secondary ducts follow any visible structures. A variety of transplantation experiments reveal that the guidance information is not only oriented but also directionally polarized and travels caudad as a wave. These results militate against chemotaxis and contact guidance as guiding influences and indicate that the cells of the pronephric duct tip are directed in their migration by local information which passes caudad over the duct's mesodermal substratum as a wave in register with the advancing wave of somite segmentation. We propose that this duct-guiding information may be a traveling gradient of flank mesoderm cell adhesiveness.