Cell rearrangement and directional migration in pronephric duct development.

The morphology of the directed migration of the pronephric duct rudiment of three vertebrates, the salamander, chick and sturgeon, has been examined by scanning electron microscopy. Of particular interest in this paper are the morphology of the duct tip, the role of cell rearrangement, and the relation of duct extension to somite segmentation. The duct rudiments of all three species have motile cell processes (lamellipodia and filopodia) largely confined to their posterior tips. The salamander and sturgeon embryos extend their duct rudiments by extensive cell rearrangements. A short, wide rudiment is elongated to form a long, thin one. The chick duct rudiment stays about the same width and apparently gains volume by cell proliferation. The salamander duct rudiment's posterior tip is always two somites behind the last formed somite. Both the sturgeon and chick embryo's duct rudiments lie well posterior of the last segmented somite adjacent to segmental plate mesoderm. There is still a close coupling, however, between the posterior progression of the duct rudiments and the advancing wave of somite segmentation.