Cardiovascular Development
last authored:
last reviewed:
Introduction
Due to increasing physiological demands for energy, the cardiovascular system is the first to function in the embryo.
During the third week, the heart and great vessels form from mesenchymal cells in the cardiogenic area at the head of the embryo. Paired heart tubes develop during the third week and fuse to form a primordial heart tube, which joins with vessels in the embryo, connecting stalk, chorion, and yolk sac. 'Cardiac' neural crest cells contribute to much of the cardiac system.
The cardiovascular system begins bilaterally and symmetrically, but many components fuse and become asymetric.
Blood is circulating and the heart begins to beat at the beginning of the fourth week, thus making the cardiovascular system the first to reach a functional state.
As the embryo folds longitudinally during the fourth week, the heart and pericardial coelom move to the ventral surface of the embryo.
The embryonic heart beat can be detected using a Doppler ultrasound during the 5th week, or 7 weeks after the LMP.
Vitteline Stage
vessels from extraembryonic tissues: islands that hollow out to produce vessel walls and free floating cells too: vasculogenesis is this new thing. angiogenesis is sprouting.
fetal: hematopoiesis in liver and spleen
later, bone marrow comes in
lateral plate mesoderm gives rise to the intraembryonic coelom, giving rise to pericardial and cardiac cavities.
the splancnic mesoderm gives rise to cardiogenic area
Precirculatory Nutrition
Prior to implantation, ATP is generated via uptake and metabolism of nutrients present in tubal and uterine fluids. Following implantation, and with decidualization, the active glands of the endometrium produce large amounts of glycogen and lipids along with hormones such as prolactin and insulin-like growth factor (IGF-1).
During the early embryonic stage, the embryo grows too large to receive nutrients, gases, and wastes via simple diffusion and begins to develop the umbilical vesicle (yolk sac) and vasculature.
During the third week, vessels and blood cells appear in the umbilical vesicle. During the 2nd month, this circulation is replaced by placental circulation.
Development of Cardiovascular Structures
- heart
- veins
- arteries
Heart Development
The heart tube is composed of a thin endothelial tube and a layer of cardiac jelly surrounded by thick myocardium.
The heart is first present in one tube in series, which then needs to be converted into a system in parallel this is accomplished by twisting while the two systems divide to connect to pulmonary and systemic circulations.
The bulging out leads to entry into the pericardial cavity
Moving from head to toe, the heart tube is composed of:
- truncus arteriosus, bulbus cordis, ventricle, primordial atrium, sinus venosus
During the 4th week, the atria separate, with the sinus venosus being absorbed into the right atrium.
Endocardial cushions are present between the atria and the ventricle.
After outflow is established, it is bidirectional. unidirectinal flow begins in the fifth week with the development of valves.
The septum previum separates the atria and soon becomes continuous with the endocardial cushions. Blood passes from the right to left atria through the ostium primum and then ostium secundum. The septum secundum begins to form, though it will never completely replace the septum previum. This results in the foramen ovale. The interventricular septum grows up from the apex and is soon completed by the membranous part of the septum.
The exit of the ventricles through the truncus arteriosus c becomes divided by the conotruncal ridges, which fuse to form the aorticopulmonary septum and provide blood to the aorta and pulmonary artery.
Fetal Cardiac Shunts
Oxygen rich blood flowing from the IVC is directed through the foramen ovale and to the rest of the body, while oxygen-poor blood from the SVC is aimed at the ventricle.
Lungs arent expanded and there is high resistance to flow, so bypass occurs from the RV through the ductus arteriosus and into the aorta.
Vein Development
Three paired veins drain into the tubular heart of a four-week embryo:
- vitelline veins return poorly oxygenated blood from the yolk sac
- ubilical veins carry well-oxygenated blood from the chorion
- common cardinal veins return poorly oxygenated blood from the embryo
The vitelline veins contribute to the hepatic and portal veins.
The umbilical vein becomes connected with the inferiot vena cava by the ductus venosus, forming a bypass of the liver and allowing most blood to travel directly from the placenta to the heart.
Cardinal veins form anastemoses and develop into the venae cava and other veins of the body.
Artery Development
Aorta
Paired dorsal aortae initially run the length of the embryo, but they soon fuse to form a single dorsal aorta.
Aortic Arch Development
As pharyngeal arches form during the fourth and fifth weeks, they are supplied by aortic arches that between the aortic sac, leaving the heart tube, and the dorsal aorta.
The fourth arch forms the definitive aortic arch.
Intersegmental Arteries
Thirty or so arteries leave the dorsal aorta to form intersegmental arteries, which carry blood to the somites. These become vertebral, intercostal, common iliac, and other arteries.
Changes in Circulation at Birth
Loss of placental circulation and expansion of the lungs results in immediate functional and then structural changes to the fetal shunts.
Umbilical arteries are ligated, become constricted, and then become the internal iliac arteries and medial umbilical ligaments.
The umbilical vein is ligated, constricts, and becomes the ligamentum teres of the liver.
The ductus venosus closes within hours and becomes the ligamentum venosum.
The ductus arteriosus closes 1-4 days after birth and becomes the ligamentum arteriosum.
The foramen ovale is closed by increase dpressure of the left atrium. Probe patency can last for months and on occuasion for a lifetime.
Resources and References
 |