Regulation of epiblast cell movements by chondroitin sulfate during gastrulation in the chick

Avian gastrulation is dependent on the ingression of outer layer cells into the interior of the embryo by means of a transient structure referred to a s the primitive streak. As the growing streak progresses through the centra l area pellucida of the blastoderm, selective de-epithelialization of epibl ast cells results in the initial migratory cells of the primitive mesoderm and endoderm. Here, we have examined the possibility that extracellular mat rix molecules of the epiblast basal lamina influence the selection of strea k-specific epiblast cells. By using whole embryo culture, we have found tha t removal of chondroitin sulfate glycosaminoglycans at gastrulation stages leads to defective streak formation. In situ hybridization with streak-spec ific markers in these embryos reveals ectopic patterns of gene expression, suggesting that differentiation of primitive streak precursors in the prega strula epiblast is independent of normal streak morphogenesis. In addition, in vitro assays with chondroitin sulfate containing matrices suggest that specific cells of the epiblast are inhibited from joining the streak during gastrulation. Taken together, these results indicate that the presence of chondroitin sulfate in the epiblast basal lamina facilitates the allocation of cells to the primary germ layers by preventing ectopic axis formation. (C) 2000 Wiley-Liss, Inc.