Cardiovascular System I - Development of the Heart

Overview of Cardiovascular System Development

• Necessity of Early Vasculogenesis:

  • In early embryonic life, simple diffusion is sufficient for nutrient and gas exchange.

  • As the embryo grows rapidly in size and complexity, simple diffusion becomes inadequate to sustain metabolic needs.

  • Cardiovascular system ($\text{CVS}$) is the first organ system to develop, beginning in the $3\text{rd}$ week of intrauterine life ($\text{IUL}$).

• Molecular and Morphogenetic Control:

  • Cardiovascular development requires a complicated interplay of molecular communication to ensure precise spatial configuration, proper orientation, and timing.

  • Disruption of signaling pathways by genetic mutations or environmental factors (teratogens) leads to congenital heart defects ($\text{CHDs}$).

• Three Major Stages of Heart Development:

  1. Formation of the Heart Tube

  2. Cardiac Looping

  3. Septation and Chamber/Valve Formation

Primary and Secondary Heart Fields

• Primary Heart Field (PHF):

  • Cardiac development initiates during the $3\text{rd}$ week of $\text{IUL}$ with the migration of progenitor cardiac cells through the primitive streak into the splanchnic layer of lateral plate mesoderm.

  • Progenitor cells form a horseshoe-shaped area called the primary heart field ($\text{PHF}$) around day $16\text{--}18$.

  • The $\text{PHF}$ is situated cranial to the neural folds.

  • Cells within the $\text{PHF}$ are specified early to form the left and right sides of the heart, giving rise to:

  • Right and left atria

  • Left ventricle

  • Part of the right ventricle

• Secondary Heart Field (SHF):

  • Develops slightly later, around day $20\text{--}21$.

  • Resides in splanchnic mesoderm ventral to the posterior pharynx.

  • Provides cells that lengthen the outflow region of the heart, contributing to:

  • Remainder of the right ventricle

  • Outflow tract consisting of the conus cordis and truncus arteriosus

  • Regulation by Neural Crest Cells:

  • Neural crest cells ($\text{NCCs}$) migrate from cranial neural folds through pharyngeal arches to the heart outflow region.

  • $\text{NCCs}$ regulate $\text{SHF}$ proliferation and differentiation by controlling Fibroblast Growth Factor ($\text{FGF}$) concentrations.

  • Disruption of the $\text{SHF}$ leads to shortening of the outflow tract region, resulting in outflow tract malformations.

Formation of the Heart Tube

• Induction and Fusion:

  • Chemical signals stimulate the cardiogenic area to form two parallel primitive endocardial tubes.

  • By day $21$, the embryo undergoes lateral folding, bringing the two endocardial tubes into the midline where they fuse into a single primitive heart tube.

  • Fusion initiates caudally; the outflow tract and most of the ventricular region expand from the horseshoe crescent.

• Cephalocaudal Folding and Translocation:

  • Rapid growth of the central nervous system (brain) causes cephalocaudal folding of the embryo.

  • The cardiogenic area and pericardial cavity, initially located cranial to the oropharyngeal membrane, are rotated $180^\circ$ ventrally and caudally into the future chest region ($18\text{--}22\,\text{days}$).

• Layers and Organization of the Primitive Heart Tube:

  • Continuous with the intraembryonic and extraembryonic vascular networks.

  • Caudal end serves as the inflow tract for venous return; cranial end connects to aortic arch arteries directing blood to the dorsal aorta.

  • Consists of three distinct structural layers:

  • Endocardium: Internal endothelial lining.

  • Cardiac Jelly: Gelatinous extracellular matrix secreted by the myocardium, separating endocardium from myocardium.

  • Myocardium / Epicardium: Outer muscular mantle and mesothelial surface coat.

  • Divided into five primary anatomical segments along the craniocaudal axis:

  1. Sinus Venosus: Caudal venous entry (receives umbilical, vitelline, and common cardinal veins).

  2. Primitive Atrium: Gives rise to trabeculated walls of right and left atria.

  3. Primitive Ventricle: Develops into the left ventricle.

  4. Bulbus Cordis: Gives rise to smooth portion of right ventricle (conus arteriosus) and left ventricle (aortic vestibule).

  5. Truncus Arteriosus: Outflow region connecting to aortic arch arteries, giving rise to ascending aorta and pulmonary trunk.

Cardiac Looping

• Process and Directionality:

  • By day $22\text{--}23$, rapid cell proliferation causes the primitive heart tube to elongate within the confined pericardial cavity, driving cardiac looping.

  • Cranial Portion (Bulbus Cordis and Ventricle): Bends ventrally, caudally, and to the right.

  • Caudal Portion (Sinus Venosus and Atrium): Bends dorsally, cranially, and to the left.

  • Process requires approximately $5\,\text{days}$ and is complete by day $25$.

  • Looping places the primitive atrium dorsocranial to the primitive ventricle and establishes correct alignment of primitive heart segments.

Septation and Formation of Heart Chambers

• Overview of Septation:

  • Division of the common cardiac tube into four distinct chambers occurs between days $27$ and $37$ of development.

  • Involves growth and fusion of specialized extracellular matrix projections called endocardial cushions (dorsal and ventral / superior and inferior endocardial cushions).

Interatrial Septum Formation

• Septum Primum and Ostium Primum:

  • A sickle-shaped muscular ridge, the septum primum, grows from the dorsal roof of the common atrium down toward the endocardial cushions (septum intermedium).

  • The temporary gap between the advancing lower edge of the septum primum and the endocardial cushions is the ostium primum.

• Ostium Secundum Formation:

  • Before ostium primum is completely closed by fusion with endocardial cushions, programmed cell death (apoptosis) forms openings in the upper center of the septum primum.

  • These perforations coalesce to form the ostium secundum, maintaining an essential right-to-left blood shunt during fetal life.

• Septum Secundum and Foramen Ovale:

  • As ostium secundum enlarges, a second muscular fold, the septum secundum, grows from the roof of the atrium adjacent to the septum primum on its right side.

  • Septum secundum proliferates downward but leaves an oval aperture called the foramen ovale.

  • The lower remnant of septum primum forms the valve of the foramen ovale, acting as a one-way flap valve allowing blood flow from right to left atrium only.

  • At birth, increased left atrial pressure presses septum primum against septum secundum, closing the foramen ovale.

Interventricular Septum Formation

• Muscular Interventricular Septum:

  • A thick muscular ridge grows upward from the floor of the common ventricle toward the fused endocardial cushions (septum intermedium).

  • Leaves a temporary opening, the interventricular foramen, between the muscular septum and endocardial cushions.

• Membranous Interventricular Septum:

  • Formed by down-growth and proliferation of tissue from the fused endocardial cushions combined with right and left bulbar ridges.

  • Complete fusion of membranous and muscular septa closes the interventricular foramen by week $8$, fully dividing the ventricles into left and right chambers.

Formation of Heart Valves

Atrioventricular (AV) Valves

• Developmental Sequence:

  • Begins toward the end of the $4\text{th}\text{ week}$ with the appearance of endocardial cushions around the single atrioventricular canal.

  • Four endocardial cushions develop: superior (dorsal), inferior (ventral), and two lateral cushions (left/right).

  • By the end of the $5\text{th}\text{ week}$, superior and inferior cushions fuse, dividing the canal into right and left atrioventricular orifices.

  • Subendothelial mesenchyme around orifices proliferates and undergoes cavitation on its ventricular face, creating fibrous cusps connected to papillary muscles via chordae tendineae.

  • Left AV Valve: Consists of $2\,\text{leaflets}$ (bicuspid / mitral valve).

  • Right AV Valve: Consists of $3\,\text{leaflets}$ (tricuspid valve).

Semilunar Valves

• Developmental Mechanism:

  • Following division of the outflow tract by the aorticopulmonary ($\text{AP}$) septum, three subendothelial tissue swellings form at the bases of the aorta and pulmonary trunk.

  • Swellings derive from conotruncal endocardial cushions and neural crest mesenchyme.

  • Upper faces of the swellings are excavated to form pocket-like semilunar cusps (aortic and pulmonary valves).

Formation of the Great Vessels

• Aorticopulmonary Septum Formation:

  • Within the outflow tract (truncus arteriosus and conus cordis), opposing pairs of conotruncal swellings (bulbar/truncal ridges) develop.

  • These ridges grow spirally toward each other, rotating $180^\circ$.

  • Fusion of ridges forms the spiral aorticopulmonary ($\text{AP}$) septum.

  • Divides the truncus arteriosus into the ascending aorta and pulmonary trunk.

  • Spiral trajectory ensures the aorta aligns with the left ventricle and the pulmonary trunk aligns with the right ventricle.

Clinical Correlates

• Overview of Congenital Heart Diseases ($\text{CHDs}$):

  • Abnormalities arise from defects in looping, endocardial cushion fusion, or neural crest cell migration.

• Specific Anomalies and Features:

  • Dextrocardia:

  • Heart loops to the left instead of the right ($\text{L-looping}$ instead of $\text{D-looping}$).

  • Heart is located in the right side of the thorax. Can occur as isolated dextrocardia or part of situs inversus.

• Atrial Septal Defect (ASD):

  • Defect in interatrial septation caused by excessive resorption of septum primum, inadequate development of septum secundum, or failure of closure.

  • Ostium secundum defect is the most common form. Allows left-to-right blood shunting.

• Ventricular Septal Defect (VSD):

  • Most common congenital cardiac defect.

  • Stems from failure of membranous interventricular septum closure, leading to left-to-right shunting.

• Tetralogy of Fallot:

  • Classic cyanotic CHD resulting from unequal division of the conus cordis due to anterior/ventral displacement of the aorticopulmonary septum.

  • Characterized by four hallmarks:

    1. Stenotic pulmonary valve / Pulmonary stenosis

    2. Ventricular septal defect ($\text{VSD}$)

    3. Overriding aorta (straddles the $\text{VSD}$)

    4. Right ventricular hypertrophy (thickened muscle)

• Persistent Truncus Arteriosus:

  • Complete failure of aorticopulmonary septum development, resulting in a single shared outflow tract for both ventricles.

• Transposition of Great Vessels:

  • Occurs when $\text{AP}$ septum fails to follow its spiral course and runs straight down.

  • Aorta arises from the right ventricle and pulmonary trunk arises from the left ventricle.

• Tricuspid Atresia:

  • Absence or complete agenesis of right atrioventricular valve orifice.

• Ectopia Cordis:

  • Severe structural defect caused by failure of lateral ventral body wall folding, leaving the heart exposed outside the thoracic cavity.