SEM_7_02_Allantoic Circulation in Embryonic Development

• Embryonic blood circulation involves three circulatory arcs: vitelline, allantoic/placental, and intra-embryonic.

• The vitelline circulation is the earliest to develop and carries blood from the yolk sac to the embryo and back.

• In birds and reptiles, vitelline circulation is important as they have a large amount of yolk in the sac.

• In mammals, the vitelline circulation is rudimentary and eventually regresses during early gestation.

• The allantoic circulation establishes a connection between the chorioallantoic membrane and the embryo, and in mammals, it forms the placental circulation.

• The intra-embryonic circulation is confined to the interior of the body and is the origin of the permanent circulation in the adult.

• The vitelline circulation undergoes remodeling as its distal portions transform into intestinal veins and liver vessels.

• The right vitelline vein becomes the portal vein, the caudal vena cava, and the main intestinal artery.

• The left vitelline vein mostly involutes, but some remnants contribute to the formation of permanent vessels.

• The allantoic circulation ramifies throughout the chorion and interchanges gases with the outside in birds, or becomes the placental circulation in mammals.

• In some species, the allantoic sac becomes rudimentary and the allantoic circulation is not needed for waste disposal.

• The development of umbilical vessels is always necessary for the expansion of allantoic vessels in the placenta.

• Blood from the placenta is brought towards it by the right and left umbilical arteries, which arise from the dorsal aorta and terminate in the chorioallantoic membrane.

• After passing through a rich plexus of chorionic capillaries, the blood is collected from the chorion and returned to the embryo by the allantoic veins.

• The umbilical veins undergo remodeling after birth, with the oxygenated blood entering the heart via the caudal vena cava, and the left umbilical vein being the only vessel from the placenta to enter the embryo.

• In some species, such as horses and pigs, the umbilical veins fuse before entering the umbilical cord and three vessels are present. In other species, such as ruminants and carnivores, the veins fuse on entering the abdominal wall and four vessels are present.

• During fetal development, the left umbilical vein carries oxygenated blood from the placenta to the liver, most of which is shunted through the ductus venosus to the caudal vena cava.

• The ductus venosus is a shunt inside the liver that allows oxygenated blood from the placenta to bypass the liver and reach the heart sooner.

• In some dogs, the ductus venosus fails to occlude after birth, resulting in a patent ductus venosus, which is a congenital portosystemic shunt that allows toxic digestive products to bypass the liver and affect the brain.

• The right atrium of the heart receives blood from both the cranial (lower oxygen saturation) and caudal (higher oxygen saturation) parts of the fetal body.

• The relatively-oxygenated blood from the caudal vena cava is predominantly directed towards the left atrium via the foramen ovale due to haemodynamic factors.

• The relatively-high-oxygenated blood is then pumped from the left atrium to the left ventricle and into the ascending aorta, ensuring that vital structures like the brain and coronary circulation receive the best-oxygenated blood.

• The less-oxygenated blood from the cranial vena cava is directed across the tricuspid valve into the right ventricle.

• This less-oxygenated blood is pumped towards the pulmonary trunk but does not reach the lungs due to their non-functionality in the fetus.

• Instead, it is diverted through a fetal blood vessel called the ductus arteriosus into the descending aorta.

• The placenta receives relatively-deoxygenated blood from the fetal systemic arterial circulation and returns blood with the highest oxygen levels to the fetal venous circulation.

• At birth, the placental circulation is switched off, leading to the activation of the lungs and the pulmonary circulation.

• The closure of the foramen ovale and the ductus venosus after birth is due to the changes in pressure in the right and left atria.

• The ductus arteriosus closes immediately after birth due to reflexive muscle contractions.

• The closure of these fetal circulatory shunts is crucial for the proper functioning of the post-natal cardiovascular system.

• The segmental (intersegmental) arteries are branches of the dorsal aorta extending between somites, contributing to visceral and body wall blood supply.

• The embryonic sinus venosus of the heart receives return blood from different parts: vitelline veins (yolk sac), umbilical veins (allantois), and cardinal venous system.

• The cardinal venous system is the origin of the definitive venous system, which includes the cranial vena cava and caudal vena cava.

• The primitive venous system in the embryo consists of four cardinal veins, which drain from different parts of the body and fuse to form common cardinal veins before entering the heart.

• The cranial cardinal veins are transformed into the adult internal jugular veins, with the larger external jugular and subclavian veins arising from them.

• The caudal cardinal veins are temporarily replaced by two new sets of paired veins, the supracardinal and subcardinal veins.

• The subcardinal veins form most of the caudal vena cava, draining the blood from the embryonic kidneys and gonads.

• The supracardinal veins develop to drain the dorsal region of the body, forming the azygos veins in the adult.

• Lymphatic vessels develop from lymph sacs, which arise from developing veins and are derived from mesoderm.

• The first lymph sacs to appear are the jugular lymph sacs, followed by the retroperitoneal lymph sac, cisterna chyli, and paired caudal lymph sacs.

• Lymph nodes develop from localised mesodermal invaginations that divide the lumen of the vessels into sinusoids, with the mesoderm then developing a reticular framework and lymphocytes accumulating within.

• The spleen and haemal nodes (in ruminants) develop in a similar way.

• The segmental (intersegmental) arteries are branches of the dorsal aorta extending between somites, contributing to visceral and body wall blood supply.

• The embryonic sinus venosus of the heart receives return blood from different parts: vitelline veins (yolk sac), umbilical veins (allantois), and cardinal venous system.

• The cardinal venous system is the origin of the definitive venous system, which includes the cranial vena cava and caudal vena cava.

• The primitive venous system in the embryo consists of four cardinal veins, which drain from different parts of the body and fuse to form common cardinal veins before entering the heart.

• The cranial cardinal veins are transformed into the adult internal jugular veins, with the larger external jugular and subclavian veins arising from them.

• The caudal cardinal veins are temporarily replaced by two new sets of paired veins, the supracardinal and subcardinal veins.

• The subcardinal veins form most of the caudal vena cava, draining the blood from the embryonic kidneys and gonads.

• The supracardinal veins develop to drain the dorsal region of the body, forming the azygos veins in the adult.

• Lymphatic vessels develop from lymph sacs, which arise from developing veins and are derived from mesoderm.

• The first lymph sacs to appear are the jugular lymph sacs, followed by the retroperitoneal lymph sac, cisterna chyli, and paired caudal lymph sacs.

• Lymph nodes develop from localised mesodermal invaginations that divide the lumen of the vessels into sinusoids, with the mesoderm then developing a reticular framework and lymphocytes accumulating within.

• The spleen and haemal nodes (in ruminants) develop in a similar way.

Test your knowledge on the role of the allantoic sac and circulation in embryonic development. Understand how the formation of the allantoic stalk is essential for the development of umbilical vessels and placental expansion.