1. Early development of heart & vessels A,Dorsal view of embryo (about 18 days). B,C, T.S and L.S. showing relationship of angioblastic cords in cardiogenic mesoderm, to the pericardial coelom, and septum transversum.  The primordial heart & blood vessels development- Angio-genesis- appear during 3 rd week. Heart appear as paired endothelial strands -angioblastic cords- in cardiogenic mesoderm. (B,C)  These cords canalize to form heart tubes, which fuse to form one tubular heart at the end of 3 rd week.  Heart starts to beat and to function (at 22-23 days) at the begnning of 4 th week and blood flow can be visualized by Doppler ultrasonography.

2.  Drawing of embryonic cardiovascular system(26 days), at 4 th week showing left vessels :  Umbilical vein.  Vitelline vein.  Cardinal veins : anterior, posterior,& common cardinal v.

3. Development of veins associated with the heart of 4 th week embryo  Vitelline veins return poorly oxygenated blood from yolk sac to the sinus venosus of heart.  Umbilical veins carry well- oxygenated blood from primordial placenta to sinus venosus.  Common cardinal veins carry poorly oxygenated blood from body of embryo to sinus venosus

4.  Dorsal views of the developing heart.  A, during 4 th W. showing primordial atrium & sinus venosus and veins draining into them.  B, 7 th W. showing venous circulation through liver.  C, 8 th W,adult derivatives of anterior cardinal veins.

5. Vitelline Veins  Lie in the yolk stalk, they carry poorly oxygenated blood from yolk sac to sinus venosus passing through septum transversum.  In the region of developing liver in septum transversum, hepatic sinusoids develop from vitelline veins, and Hepatic veins develop from remains of right vetilline vein.  Proximal left vitelline vein degenerates, but / proximal part of right vitelline vein form hepatic part of I.V.C.  Portal vein develops from an anastomotic network of distal parts of right & left vitelline veins around the duodenum.

6. Umbilical Veins  They carry well-oxygenate blood from placenta to sinus venosus.  As the liver develops, umbilical veins lose their connection with heart.  Right umbilical vein disappears completely.  Proximal (cranial) part of left umbilical vein between liver & sinus venosus degenerates.  Distal (caudal) part of left umbilical vein persist and becomes the umbilical vein, which carries blood from placenta to embryo. How?  A large venous shunt- ductus venosus- develops within liver and connects umbilical vein with I.V.C. so blood pass directly from placenta to heart.

7. Anterior Cardinal Veins  Cardinal veins constitute the main venous drainage of the embryo.  Anterior & posterior cardinal veins drain cranial & caudal parts of embryo,respectively.  A,4 th week, anterior & posterior cardinal veins join the common cardinal veins, which enter sinus venosus.  Anterior cardinal veins are connected by anastomosis, which shunts blood from left to right. B,7 th week.  This anastomotic shunt between anterior cardinal veins becomes left brachiocephalic vein when caudal part of left anterior cardinal vein degenerates. C, 8 th week.  S.V.C. develops from right anterior cardinal vein + right common cardinal vein. C, 8 th week.

8. Posterior Cardinal Veins Ventral views of primordial veins of the embryos’s trunk. A, 6 weeks. D, adult.  Posterior cardinal veins develop primarily as vessels of mesonephroi and disappear with transitory kidneys.  The only adult derivatives of posterior cardinal veins are : root of azygos vein + common iliac veins.

9. Subcardinal & Supracardinal Veins  Drawings illustrating ventral views of primordial veins of trunk : vitelline,umbilical & cardinal veins, and also subcardinal & supracardinal veins.  A, 6 weeks.  B, 7 weeks.  Subcardinal veins appear first, followed by supracardinal veins to replace gradually the posterior cardinal veins.  Subcardinal veins are connected together through subcardinal anastomosis and with posterior cardinal veins through anastomosis through mesonephros (early kidney), Also sub-supracardinal anastomosis develops.  These anastomosis shunt blood from left to right veins, as a result, right veins enlarge while left veins become small and may disappear.

10. Fate of Subcardinal veins  Left subcardinal vein cranial to the anastomosis disappears leaving small left suprarenal vein, while caudal to anastomosis it becomes left gonadal vein.  Right subcardinal vein cranial to the anastomosis forms the pre-renal part of I.V.C. + right suprarenal vein, while caudal to the anastomosis it develops into right gonadal vein.  Sub-supracardinal anastomosis forms right & left renal veins + renal part of I.V.C.  Ventral views of primordial veins of trunk’s embryo.  C, 8 th week.  D, adult.

11. Fate of Supracardinal veins  Ventral views of primordial veins of trunk’s embryo.  C, 8 th week.  D, adult.  They are the last pair of vessels to develop.  The middle part of the 2 veins in the region of kidney disappears.  Cranial part of left supracardinal vein + transverse anastomosis form Hemiazygos vein.  Cranial parts of right supracardinal vein + right posterior cardinal vein form Azygos vein.  Caudal to the level of kidney : right supracardinal vein forms postrenal part of I.V.C., while left supracardinal vein disappears.

12. Development of I.V.C.  Hepatic part : develops from hepatic vein (from proximal part of right vitelline vein) + hepatic sinusoids.  Prerenal part : develops from right subcardinal vein.  Renal part : develops from subcardinal- supracardinal anastomosis.  Postrenal part : develops from right supracardinal vein.