Heart Development

Heart Development

1. Primitive Heart Field

   • Established around day 18
   • Splanchnic Lateral Plate Mesoderm
   • Secondary heart field develops in surrounding mesoderm
     • Cells migrate - incorporate to outflow tract and venous pole
       • Populating venous flow/outer flow (Truncus Arteriosus/Conus Arteriosus)
   • Folding
     • Lateral folding brings two arms of C-shaped primitive field together (joins endocardial tubes and cardiogenic mesoderm)
     • Cranio-Caudal folding brings heart to the thoracic region
   • Heart Tube Looping
     • Bulboventricular Loop - right
     • Sinus Venosus - dorsally and to the left
     • Epicaridal Layers (3) are formed from myocardium (1) endocardium: internal endothelial lining of the heart; (2) myocardium: muscular wall; (3) epidcardium/visceral pericardium: covers outside of tube
     • Septation
       • Endocardial Cushions - mesenchymal cells comparmentalize structure by swelling in to ECM between myocardium and endocardium
         • Protrustions in the atrioventricular and conotruncal region assist in forming: atrial and ventricular septa, atrioventricular canals/valves and aortic/pulmonary channels
           • Atrioventricular cushions populate with overlying endocardial cells that move into matrix
             • Dorsal and Ventral cushions fuse and form left and right ventricles
           • Concotrunal cushions are derived from neural crest cells from neural folds - to outflow tract region
             • Bulbous Cordis - Right Ventricle
             • Conus Cordis - outflow of both ventricles
               • Grow in spiral - form pulmonary artery and trunk and fusion completes
             • Trucnus arteriosus - roots of aorta and pulmonary artery
             • Primitive Left Ventricle
           • Defects due to location of cushion - ASD, VSD, Great Vessels (Transposition, Common Truncus Arteriosus, Tetra of Fallot)
       • Atrial Septation
         • Crest grows from common atrium -- lumen forming septum primum (full covering over L/R atrial division)
           • Opening is foramen primum (soon closes)
           • Apoptosis perforates and forms foramen secundum (prevents blood flow between primitive atrium)
             • Cushions close foramen primum
               • Septum priumum partially degrades; leaving valve of ovale foramen (allows blood flow one direction R--L due to tough septum secundum and fuses at birth)
         • Right atrium expands - sinus horn incorporation; new fold forms septum secundum (never entirely partitions cavity)
           • More rigid; septum secundum overlaps foramen secundum and the opening left is -- foramen ovale
         • DMP grows toward AV canal at same time as septum primum growth
           • DMP forms pulmonary vein positioned by left atrium - 2 go to lung; 2 to posterior wall
     • Atrial portion - common atrium and connects to early ventricle
   • Divided into chambers by sulci

2. Timeline

   • Day 18 - Primitive Heart Established
     • Day 20 - Folding induces formation of single heart tube
       • Day 22 - Heart begins to beat
         • Day 23 - Heart begins to loop
           • Day 28 - Epicardial layers formed from myocardium - Loop is complete
             • Late Week 4 - 5 - Septation

3. Embryonic Origins

   • Primum atrium
     • Bulbous Cordis
       • Truncus Arterosus
         • Sinus Venosus
           • Conus Cordis
             • Vitelline Veins
               • Primitive Ventricle
                 • Anterior Cardinal Vein
                   • Superior Vena Cava
                 • Left Ventricle
               • Inferior Vena Cava
             • Conus Arteriosus
           • Coronary Sinus
         • Pulmonary Trunk, Ascending Aorta *Ligamentum Arteriosum
       • Right Ventricle
     • Left/Right Atrium