• Mesoderm divisions Þ chorda, paraxial, intermediate and lateral mesoderm (heart, blood vessels, somatopleure, splanch)
• Heart forms bilaterally paired vascular tubes from the splanchnic mesoderm
• Tubes fuse Þ primitive heart; Then twist to orient inflow and outflow
• Lateral mesoderm becomes divided by a space (coelom) that gives rise to the body cavities
• The body cavities are lined with somatopleure (body wall and parts of limbs) and splanchnopleure
• Paraxial Þ somites; Intermediate Þ urogenital system; Lateral Þ split Þ body wall, wall of digestive tract, limbs

• The earliest functioning organs ~week 4
• cardiogenic region (horseshoe shaped) – from the splanchnic mesoderm Þ heart and blood vessels
• Cardiovascular malformations are most common type of life threatening congenital defect
• accounts for 20% of congenital defects in infants; ventricular septal defects – most common
• Dextrocardia: bilaterally symmetric heart Þ asymmetric right/left side; heart folds clockwise (right) not counterclockwise (left) Þ situs inversus
• The heart forms from cords of cardiogenic mesoderm
• primitive heart tube Þ fusion of endothelial heart tubes
• the paired dorsal aortae develop and connect with endocardial tubes before folding begins Þ form 1^st aortic arch
• 3 pairs of vessels supply inflow to heart:
• Common cardinal veins (venous blood from embryo body)
• Vitelline veins (blood from yolk sac)
• Umbilical veins (oxygenated blood from placenta)
• During week 5-8, heart tube undergoes folding and remodeling (see figure below)
• develops bulges and sulci Þ primordial heart chambers
• Inferior (inflow end) Þ sinus venosus – common cardinal veins drain here; primitive atrium; primitive ventricles – future left ventricle; bulbis cordis – future right ventricle, outflow regions of both ventricles
• all systemic venous blood drains into right atrium
• splanchnic mesoderm Þ primitive myocardium

• Food absorption Þ yolk sac or placenta; Lungs are not used
• 5^th – 6^th week (Fig 3)
• septum primum and septum secundum separate right and left atria Þ allows right to left shunting in heart atria; foramen ovale (shunts partially oxygenated blood to the body, with less blood forced to the lungs); septa fuse ~2 – 3 months of life

• Blood vessels begin in the extra-embryonic mesoderm covering the yolk sac, connecting stalk and chorion
• Embryonic blood vessels develop ~2 days later
• Blood vessel formation (mesodermal) – do not grow like trees, grow in segments which become connected
• angioblasts form blood islands Þ cavities
• angioblasts flatten, form endothelial cells that line the cavities
• cavities fuse to form networks of channels
• Contraction of heart ~day 22; Embryonic blood circulation ~day 24; Primitive blood flow ~4 weeks

• (1) Aorta – carry partially deoxygenated blood to chorion thru umbilical arteries
• (2) Common cardinal veins – drain embryo proper
• (3) Vitelline vein – takes blood from yolk sac Þ heart
• (4) Vitelline artery – takes blood from aorta Þ yolk sac
• (5) Umbilical vein – carry oxygenated blood to embryo
• (6) Umbilical artery – carry deoxygenated blood to placenta

Blood Cell Formation – Begins in wall of yolk sac and wall of chorion

• Extra-embryonic splanchnic mesoderm of yolk sac Þ blood islands Þ hemocytoblasts (hemopoeitic cells)
• Blood formation Þ yolk sac (3-6 wks); Liver (6-9 wks); bone marrow, spleen, lymph nodes (10+ wks)