Embryological Development of the Cardiovascular System (CVS)

OVERVIEW: The Three Venous Inputs in Week 4

FIGURE 13.38 - Main intraembryonic and extraembryonic arteries (red) and veins (blue) in a 4-mm embryo (end of fourth week). Langman's Medical Embryology

PART 1: EARLY HEART TUBE FORMATION

Week 3 - Cardiogenic Crescent

• Angioblastic cells in the splanchnic mesoderm form a horseshoe-shaped cardiogenic plate (crescent) in front of the prechordal plate.
• Two endothelial strands (lateral heart-forming regions) fuse at the midline to form a single primitive heart tube by days 21-22.
• The heart begins beating around day 21-22 - the first organ to function.

Week 4 - Cardiac Looping (D-loop)

1. Truncus arteriosus (outflow)
2. Bulbus cordis (future right ventricle outflow)
3. Primitive ventricle (future left ventricle)
4. Primitive atrium
5. Sinus venosus (inflow - receives all three venous pairs)

Septation (Weeks 4-8)

• Atrial septation: Septum primum descends, leaving ostium primum at bottom. As primum fuses with endocardial cushions, cell death creates ostium secundum in the upper part. Then septum secundum grows to the right of primum, leaving the foramen ovale - this closes at birth by pressure change.
• AV canal: Four endocardial cushions fuse to form the superior and inferior cushions, dividing the canal into right (tricuspid) and left (mitral) AV orifices.
• Ventricular septation: The thick muscular interventricular septum grows up from the floor. The remaining membranous portion is closed by contributions from the inferior AV cushion plus the right and left conus swellings.
• Conotruncal septation: The spiral aorticopulmonary septum (contributed by neural crest cells) divides the truncus arteriosus into the ascending aorta and pulmonary trunk. Because it spirals ~180°, the pulmonary trunk winds around the aorta.

PART 2: DEVELOPMENT OF THE ARCH OF THE AORTA

The Six Aortic Arches - Overview

• Ventral aortic roots (aortic sac) - feeding from the heart
• Paired dorsal aortae - receiving blood and fusing below to form the single descending aorta

FIGURE 13.39 - A: Aortic arches at end of week 4 (4-mm stage). B: Aortic arch system at beginning of week 6 (10-mm stage) showing the aorticopulmonary septum and pulmonary arteries. Langman's Medical Embryology

Fate of Each Aortic Arch

• The external carotid is a sprout from the 3rd arch
• The remainder of internal carotid beyond the 3rd arch comes from the cranial dorsal aorta
• The left subclavian arises from the 7th intersegmental artery (not an aortic arch)
• The brachiocephalic trunk is formed from the right 4th arch + right dorsal aorta + right 7th intersegmental

FIGURE 13.40 - A: Aortic arches and dorsal aortae transformation. B: After transformation (broken lines = obliterated). C: Adult great arteries. Note position of recurrent laryngeal nerves. Langman's Medical Embryology

The Recurrent Laryngeal Nerves - A Classic Clue

Common Anomalies

PART 3: DEVELOPMENT OF THE PORTAL VEIN

Vitelline (Omphalomesenteric) Veins - The Origin

1. Form a venous plexus around the duodenum
2. Penetrate the septum transversum, where liver cords grow into and surround them
3. Break up into the hepatic sinusoids (first hepatic sinusoids)

FIGURE - Development of vitelline and umbilical veins: (a) 4th week, (b) 5th week, (c) 2nd month, (d) 3rd month. THIEME Atlas of Anatomy

Step-by-Step Portal Vein Formation

1. Week 4: Both vitelline veins are symmetric and enter the sinus venosus.
2. Week 5: The vitelline veins form three anastomotic channels around the duodenum - one ventral and two dorsal. The liver sinusoids disrupt the distal portions of both veins.
3. Week 6-8: Through a complex series of obliterations and anastomoses, the distal portion of the RIGHT vitelline vein becomes the portal vein. It receives:
   • The superior mesenteric vein (formerly right vitelline)
   • The splenic vein
   • The inferior mesenteric vein
4. The proximal right vitelline vein (between liver and sinus venosus) becomes the hepatic veins and posthepatic IVC segment.
5. The left vitelline vein largely regresses.

The Umbilical Veins and Ductus Venosus

• Initially, two umbilical veins bring oxygenated placental blood alongside the liver.
• The right umbilical vein regresses completely by month 2.
• The left umbilical vein persists and connects with the hepatic sinusoids.
• A shunt - the ductus venosus - develops inside the liver, bypassing the sinusoids and connecting the left umbilical vein directly to the proximal right vitelline trunk (future hepatic/IVC segment), allowing most blood to bypass liver metabolism and reach the sinus venosus.
• At birth, the left umbilical vein becomes the ligamentum teres hepatis (round ligament of liver), and the ductus venosus becomes the ligamentum venosum.

PART 4: DEVELOPMENT OF THE INFERIOR VENA CAVA (IVC)

The Four Segments of the IVC

The Cardinal Vein Story

• Anterior cardinal veins - drain the head
• Posterior cardinal veins - drain the body
• Common cardinal veins (ducts of Cuvier) - join ant. + post. cardinals and enter sinus venosus

1. Subcardinal veins - drain the kidneys (medial to posterior cardinals)
2. Supracardinal veins - drain body wall via intercostal veins (lateral to posterior cardinals), future azygos system
3. Sacrocardinal veins - drain the lower limbs

How Left-to-Right Channeling Builds the IVC

• Subcardinal anastomosis → becomes left renal vein → left subcardinal disappears; right subcardinal becomes the IVC renal segment
• Sacrocardinal anastomosis → becomes left common iliac vein; right sacrocardinal becomes sacrocardinal IVC segment
• Anterior cardinal anastomosis → becomes left brachiocephalic vein → left anterior cardinal largely disappears; right side builds SVC

FIGURE 13.49 - Development of the inferior vena cava, azygos vein, and superior vena cava. A: Seventh week. B: At birth showing the three components of the IVC. Langman's Medical Embryology

FIGURE - Cardinal venous system development. (a) At 5-7 weeks, (b) at term (ventral), (c) lateral view at 5-7 weeks. THIEME Atlas of Anatomy

IVC Anomalies

PART 5: DEVELOPMENT OF THE SUPERIOR VENA CAVA (SVC)

• The anastomosis between the two anterior cardinal veins (left and right) becomes the left brachiocephalic vein
• Once this anastomosis forms, blood from the left head/arm drains across to the right
• The left common cardinal vein regresses into the coronary sinus (which drains the heart's venous blood into the right atrium)
• The right common cardinal vein + right anterior cardinal become the SVC
• The anterior cardinal veins form the internal jugular veins

SUMMARY TABLE: Key Embryonic Vessels and Their Adult Derivatives

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