Vasculogenesis in the chick embryo. An ultrastructural study

TLDR: The morphogenesis of the extra-embryonic circulatory system of the chick was studied by light and electron microscopy, from precirculatory stages until the beginning of the definitive circulation, which leads to completion of the adult vascular structure.

Abstract: The morphogenesis of the extra-embryonic circulatory system of the chick was studied by light and electron microscopy, from precirculatory stages until the beginning of the definitive circulation. Early mesodermal migrations of prospective vasoformative cells and formation of blood islands take place with minimal morphologically expressed cytodifferentiation. The earliest mesodermal clusters are not syncytial. Formation of vascular lumina is not due to lysis of central cells. Undifferentiated mesenchyme possesses the capacity to establish complex junctional devices at points of contact with like cells. The first formed blood islands show intimate relationships with endoderm. The initial capillary network is sinusoidal in type, the endothelium being unsupported by a basement membrane. The major vitelline vessels acquire a more complex anatomy but the capillaries retain this simple pattern throughout the period studied. Several phases in the histogenesis of vitelline vessels are recognized, which could be correlated with the known progression of arterial pressure patterns. Continuing differentiation and mesenchymal investment of vascular walls, after stages 22–23 of Hamburger and Hamilton ('51), in correspondence with the last sustained pressure elevation, leads to completion of the adult vascular structure.