Ultrastructural localization of laminin subunits during the onset of mesoderm formation in the mouse embryo.

Using ultrastructural immunogold histochemistry on LR-Gold-embedded 6- and 7-day-old mouse embryos we investigated the appearance of the A- and B1-chains of the laminin molecule during mesoderm formation. With the help of antibodies against the A-chain and the E4 fragment of the B1-chain of the laminin molecule we were able to detect the subunits in vivo. Staining for the E4 fragment of the short arm of the laminin molecule from day 6 was negative. In contrast, strong staining for the A-chain of laminin was observed. Our results show, that the A-chain of laminin appears before the B1-chain in the 6-day-old mouse embryo before a basement membrane is seen between the ectodermal and entodermal cell layers. Furthermore, the staining pattern indicates, that the laminin molecule changes its orientation in the basement membrane of the ectoderm during mesoderm formation. On day 7 staining for the A-chain of laminin and for the E4 fragment was seen in a random distribution throughout the entire basement membrane, whereas in areas were the onset of mesoderm formation was taking place, the E4 fragment was restricted to the edge of the disintegrating basement membrane.