Avian vasculogenesis and the distribution of collagens I, IV, laminin, and fibronectin in the heart primordia

The heart‐forming regions of the early embryo are composed of splanchnic mesoderm, endoderm, and the associated ECM. The ECM of the heart‐forming regions in stage 7–9 chicken embryos was examined using immunofluorescence. Affinity purified antibodies to chicken collagens type I and IV, chicken fibronectin, and mouse laminin were used as probes. We report that (1) the basement membrane of the endoderm contains immunoreactive laminin and collagen IV; (2) the nascent basement membrane of the heart splanchnic mesoderm contains immunoreactive laminin, but not type IV collagen, and (3) the prominent ECM between the splanchnic mesoderm and the endoderm (the primitive‐heart ECM) contains collagen IV, collagen I, fibronectin, but not laminin. In addition, we describe microscopic observations on the spatial relationship of cardiogenic cells to the primitive‐heart ECM and the endodermal basement membrane.