Embryonic Development of the Cardiovascular System: A Comprehensive Guide

Introduction and Overview of Embryonic Cardiovascular Development

• Speaker Profile: Dr. Alexandros Chardas DVM, MVetMed, dipACVP, ECVP-CFVP, Lecturer in Veterinary Anatomic Pathology at the Royal Veterinary College (RVC).
• Source Material Acknowledgement: Slides courtesy of Dr. Alan Kessell.
• Primary Lecture Objectives:
  • Fundamentals of embryonic cardiovascular development.
  • Fetal cardiac development, specifically focusing on partitioning and folding of the heart.
  • Mechanism of the closure of the Foramen Ovale (Distinction between functional/physiological vs. anatomical closure).
  • Particularities of fetal circulation and the physiological transitions occurring after birth.

The Primacy of the Cardiovascular System

• Functional Status: The cardiovascular system is the $1^{st}$ functional system to develop in the embryo.
• Physiological Rationale: The necessity for early development is driven by how the embryo receives nutrition.
  • Early Embryonic Phase: Nutrition is delivered via diffusion from fluids secreted by the uterine glands.
  • Growth Threshold: As the embryo rapidly increases in size and structural complexity, simple diffusion becomes insufficient to meet metabolic demands. This creates an "Urgent!" requirement for a functional circulatory system.

Early Tissue Differentiation and Background

• Cellular Lineage:
  • Mesoderm $\rightarrow$ Mesenchyme (pluripotent cells) $\rightarrow$ Angioblastic tissue (hemangioblasts) $\rightarrow$ Cardiovascular system (comprising blood and blood vessels).
• Angiogenesis: The specific term defining the process of blood vessel development.
• Anatomical Landmarks in Early Embryo:
  • Cardiogenic Field ($1$): A horseshoe-shaped structure located anteriorly and around the lateral portions of the neural plate. Viewed dorsally after the removal of amniotic folds.
  • Neural Plate ($2$): Adjacent to the cardiogenic field.
  • Germ Layers in Transverse Section: Neural ectoderm ($7$), Mesoderm ($8$), Endoderm ($9$).
  • Early Cavities: Intra-embryonic coelom ($10$), Visceral mesoderm ($11$), Pericardial cavity ($13$).

Formation and Folding of the Primitive Heart Tube

• Cardiogenic Field Characteristics: This area consists of blood-forming cavities within the visceral mesoderm. These cavities eventually coalesce to form the primitive heart, blood vessels, and blood cells.
• Antero-Posterior (Cranio-Caudal) Folding:
  • The cardiac tube ($2$) is moved from an anterior position to a caudo-ventral position.
  • Dorsal Aortae ($3$): Developing vessels that approach the cardiac tube.
  • Vitelline Veins ($4$): These approach the cardiac tube and eventually fuse ($5$) with its caudal portion.
  • Fusion Events: The two sides of the cardiac tube fuse ($7$), and the caudal portions of the dorsal aortae fuse ($6$).
• Resulting Primitive Structure:
  • Formation of the Dorsal Aortae ($8$) and Ventral Aortae ($9$).
  • The heart tube differentiates into the Bulbus Cordis ($10$), Ventricle ($11$), and Atrium ($12$).
  • Surrounding Structures: Brain vesicles ($16$), Primitive gut ($15$), Septum transversum ($14$), and Pericardial cavity ($13$).

Regional Differentiation of the Primitive Heart Tube

• The Five Regions and Their Adult Derivatives:
  1. Primitive Atrium: Divides into the anterior parts of the Left and Right Atria.
  2. Primitive Ventricle: Develops into the majority of the Left Ventricle ($LV$).
  3. Bulbus Cordis: Develops into the Right Ventricle ($RV$), the Conus Cordis, and the Truncus Arteriosus.
  4. Truncus Arteriosus: Divides into the outflow tracts of the heart, specifically the Aorta and the Pulmonary Trunk.
  5. Sinus Venosus: Develops into the posterior portion of the Right Atrium, the Sinoatrial ($SA$) node, and the coronary sinus.
• Clinical Relevance: Defects in the development of the aortic arches lead to defects in the great arteries of the body.

Comparative Anatomy: Heart Chambers across Species

• Fish: $2$-chambered heart (one atrium, one ventricle); blood flows to gills then body.
• Amphibians: $3$-chambered heart; results in mixed blood (oxygenated and deoxygenated).
• Reptiles (e.g., Turtles): $3$-chambered heart with beginnings of septation.
• Birds and Mammals: $4$-chambered heart with complete separation of oxygenated and deoxygenated blood.

Partitioning of the Atrio-Ventricular Canal

• Process Overview: The partitioning divides the large common canal into smaller, specialized chambers to facilitate fetal circulation and the eventual shift to neonatal circulation.
• Atrial Partitioning Steps:
  • Septum Primum ($2$): The primary atrial septum grows toward the endocardial cushions.
  • Ostium Primum ($4$): A foramen that allows the initial passage of blood from the Right Atrium ($RA$) to the Left Atrium ($LA$).
  • Ostium Secundum ($5$): As the ostium primum closes, this second opening forms in the upper part of the septum primum.
  • Septum Secundum ($3$): A secondary, thicker septum that grows to the right of the septum primum.
  • Foramen Ovale: The passage between the primary and secondary septa. The septum primum eventually serves as the physical valve for this foramen, preventing the backflow of blood into the $RA$.
• Ventricular Partitioning:
  • Interventricular Septum ($7$): Composed of a muscular portion and a membranous portion. This grows to meet the fused endocardial cushions ($6$).

Developmental Timeline

• $18$ Days: Presence of cardiogenic area and endocardial tubes.
• $21$ Days: Fusion into a single primitive heart tube (Truncus arteriosus, Bulbus cordis, Ventricle, Atrium, Sinus venosus).
• $28$ Days: Heart after loop formation; separation starts to become evident.
• $4$ Weeks: Atrioventricular canals and dorsal endocardial cushion visible.
• $8$ Weeks ($56$ Days): Four chambers fully formed with distinct valves (Tricuspid and Mitral).

Post-Natal Physiological Changes and Closure of the Foramen Ovale

• Trigger Event: The first breaths cause the lungs to expand.
• Pressure Dynamics: Lung expansion creates a sudden drop in pulmonary circulation blood pressure. This pressure differential pushes the primary septum (septum primum) firmly against the secondary septum (septum secundum).
• Closure Types:
  1. Physiological Closure: Occurs immediately after birth due to the pressure drop.
  2. Anatomical Closure: The septa eventually fuse together. The remnant of the closed foramen is the Fossa Ovalis.

Partitioning of the Truncus Arteriosus

• Spiral Aortico-Pulmonary Septum: This septum divides the truncus arteriosus into the ascending aorta and the pulmonary trunk.
• Function of the Spiral: The spiral configuration ensures that blood from the $RV$ is directed into the pulmonary trunk, while blood from the $LV$ is directed into the ascending aorta.
• Transposition of the Great Vessels: A fatal anomaly occurring if the septum develops straight rather than spiraling. This leads to deoxygenated blood from the $RV$ entering the aorta and oxygenated blood from the $LV$ entering the lungs.

Fetal Circulation and Shunts

• Environmental Context: In utero, the environment is aqueous; the placenta, not the lungs, oxygenates the blood.
• The Three Essential Shunts:
  1. Ductus Venosus: A vein connecting the umbilical vein to the inferior vena cava, allowing oxygenated placental blood to bypass the non-functional liver.
     • Adult Remnant: Ligamentum venosum.
  2. Foramen Ovale: An opening between the $RA$ and $LA$ to bypass the non-functional fetal lungs.
     • Adult Remnant: Fossa ovalis.
  3. Ductus Arteriosus: A vessel connecting the pulmonary artery to the aorta, ensuring most blood bypasses the fetal lungs.
     • Adult Remnant: Ligamentum arteriosum.
• Blood Pathway Summary: Oxygenated blood from the placenta travels via the umbilical vein $\rightarrow$ ductus venosus $\rightarrow$ inferior vena cava $\rightarrow$ Right Atrium $\rightarrow$ Foramen Ovale $\rightarrow$ Left Atrium $\rightarrow$ Left Ventricle $\rightarrow$ Aorta. Blood returning to the placenta travels via the umbilical arteries.
• Oxygen Saturation Levels: Fetal blood is characterized by high oxygen in the umbilical vein, medium oxygen in the heart/aorta after mixing, and low oxygen in the umbilical arteries returning to the placenta.