Fig. 4. Extracellular matrix components in wild-type and Eng mutant yolk sac. Immunostaining for fibronectin and collagen in sectioned whole mounts of yolk sacs. In wild-type (A) and Eng mutant (B) yolk sacs, fibronectin was observed between the layers of the yolk sac. No striking difference was observed in collagen expression in the wild-type (C) and the Eng mutant yolk sacs (D). Scale bar: 0.5 mm. Abbreviations: EC, endothelial cell layer; end, endoderm; mes, mesothelial cell layer.