Expression of different regional patterns of fibronectin immunoreactivity during mesoblast formation in the chick blastoderm

The appearance and distribution of the extracellular material glycoprotein, fibronectin, was investigated in gastrulating chick embryos using affinity-purified anti-human plasma fibronectin antibodies. Preservation of tissue structure and immunoreactivity was carried out by ethanol/acetic acid fixation or by formaldehyde/glutaraldehyde fixation. Using the former fixation method, fibronectin immunoreactivity was detected (1) at the ventral surface of the upper layer or epiblast, mainly anterior and lateral to Hensen's node, in regions where middle-layer or mesoblast cells are not yet present, and (2) sparsely in extracellular spaces of the deep layer. Using the latter fixation method, fibronectin immunoreactivity was, moreover, found at the entire ventral surface of the upper layer, i.e., also at the epithelial-mesenchymal interface, where a basement membrane was previously described. At the light microscope level, we could not detect significant immunoreactivity in the middle layer. Treatment of sections of ethanol-fixed blastoderms with testicular hyaluronidase before immunostaining for fibronectin partially demasked the antigenic sites of this glycoprotein at the epithelial-mesenchymal interface. The present report indicates that the different regional patterns of fibronectin immunoreactivity in the basement membrane of the upper layer are spatially and temporally correlated with migration and positioning of mesoblast cells. These regional patterns are probably due to differences in the composition of fibronectin-associated material such as chondroitin sulfate A and/or C proteoglycans, and/or hyaluronate, before and after mesoblast expansion, rather than to differences in the distribution of fibronectin itself. In this respect, it is noteworthy that the chemical composition of the basement membrane of an epithelium changes as mesenchyme cells migrate over it. The results also favor the idea that fibronectin is a structural component of the whole basement membrane which is used as a substrate for migration of mesenchymal cells.