A study of the expansion of the chick area vasculosa

The radial expansion of the area vasculosa of the chick embryo was studied with regard to the location of the cells which generate the mesodermal movement. The expansion of stage 12 area vasculosae was shown to be autonomous from either continued blastoderm expansion or the continued presence of the embryo. Small glass rods were used as barriers to area vasculosa expansion. When placed peripheral to the terminal sinus, glass rods blocked the vascular expansion; however, when placed just central to the terminal sinus, glass rods had no effect on vascular expansion. In addition, removal of large amounts of tissue within the area vasculosa had no significant effect on vascular expansion. We conclude that the majority of the mesoderm within the boundaries of the terminal sinus plays no essential role in the expansion of the area vasculosa, and that the cells which generate the force for expansion are located at or very near the terminal sinus. A histological study of the area vasculosa and adjacent blastoderm was performed with light and electron microscopy. This survey showed that, as the terminal sinus and a group of mesenchymal cells just peripheral to it (“edge cells”) move out into the blastoderm, morphological changes occur in the epiblast overlying the terminal sinus and edge cells. Three major changes in the epiblast cell layer were observed: (1) The cells change from a squamous, monolayered arrangement to a cuboidal or columnar, bilayered arrangement; (2) the epiblast basal lamina is thrown into convoluted folds; (3) an electron-dense extracellular matrix becomes associated with both the epiblast basal lamina and mesenchymal edge cells. Histochemical staining (Alcian blue at pH 2.5 and 1.0 and two-step PAS) shows positive reactions at the epithelial-mesenchymal interface located just over the terminal sinus and edge cells. These results suggest that glycosaminoglycans are present in relatively large amounts at the advancing mesoderm edge.