ABSTRACT
The primitive vasculature is forming during the first 3 gestational weeks. Angioblasts form from mesenchymal cells that separate from the primary trophoblast at the same time as other mesoderm is forming. These angioblasts appear in the body stalk and form isolated vascular spaces or blood islands which become lined with endothelium.1 Growth and union of these isolated spaces form plexi, which eventually unite to form a system of closed vessels. Mesenchyme laid down around the endothelium forms the intima, media, and adventitia of the vessels. Once this system of closed vessels is in place, any new vessels that arise do so as outgrowths of existing vessels. Definitive vessels arise by selection, enlargement, and differentiation. The selection of resulting channels is by virtue of inherited patterns and hemodynamic factors. Vasculogenesis factors, such as vascular endothelial growth factor (VEGF) and erythropoietin (EPO), have been extensively studied in various animal models with correlative studies ongoing in human embryogenesis and fetal placental angiogenesis.2-4 It is during this period that paired dorsal aortae arise. In addition, paired postcardinal veins form to return blood to the heart from the caudal end of the body via a common cardinal vein. These postcardinal veins develop as vessels of the mesonephroi and run dorsal to them. The previously paired dorsal aortae fuse at the level of the fourth thoracic vertebra to the fourth lumbar vertebra to form a single descending aorta in the 4th week. Lateral branches of the dorsal aorta appear during the 5th week. These irregularly segmented vessels supply structures arising from the nephrotome region, specifically the mesonephros, gonads, metanephros, and suprarenal glands. The number of lateral branches is reduced with involution of the mesonephros; these lateral branches ultimately become the inferior phrenic, adrenal, renal, and gonadal arteries.
