Organisation and reorganisation of blood vessels in embryonic development.

The initial establishment of blood vessels is genetically predetermined but it is subsequently supplemented by local and haemodynamic factors. The earliest embryonic vessels arise independently in the mesenchyme of the yolk sac wall, chorion and embryonic disc. The angioblasts aggregate as cords which canalise by a process which includes intracellular vacuolation. Subsequent extension of the network occurs by a combination of (1) in situ formation of new vessels, (2) sprouting and fusion of established neighbouring vessels and (3) invading angioblasts from other regions. As capillaries mature, a basement membrane is established containing collagen IV, laminin, nidogen/entactin, heparan sulphate proteoglycan and a variable amount of fibronectin. Early capillary basement membrane contains more fibronectin which promotes proliferative and migratory activity of endothelium. As vessels mature, the endothelium switches to laminin secretion which causes differentiation including cell attachment, junction formation and establishment of tubes while cell proliferation is suppressed.