Development of C.R system

Development of C.R system

1. Learning objectives

   • Describe formation of the heart and major vessels, including heart folding/looping, artrial spetation and ventricular septation
   • Describe function of the ductus arteriosus and ductus venosus
   • Describe fetal and post-fetal circulation
   • Describe ventral septal defects, atrial septal defects, Tetralogy of Fallot, patent ductus arteriosus
   • Describe formation of lungs, pleura and diaphragm

2. Early stages, diffusion sufficient

   • CV system needs to be one of FIRST functioning system
   • BVs can develop via:
     • 1. VASCULOGENESIS
       • cells form blood islands -> vessels
     • 2. ANGIOGENESIS
       • new vessels sprout from existing ones

3. At cranial pole, blood islands fuse to form horseshoe shaped endothelial tube surrounded by myocardial muscle cells

4. NB: cardiogenic area is very cranial

   • as embryo develops its moves caudally
   • explains length of phrenic and recurrent laryngeal nerves

5. Lateral folding brings two lateral endocardial tubes together in the midline

   • Fuse by programmed cell death to from single PRIMITIVE HEART TUBE
     • apoptosis

6. Primitive heart tube

   • Formed by fusion of two lateral endocardial tubes during lateral folding
   • Tube has 5 expanded portions
     • TRUNCUS ARTERIOSUS
     • BULBUS CORDIS
     • VENTRICLE
     • ATRIUM
     • PERICARDIAL ACTIVITY
     • SEPTUM TRANSVERSUM
     • SINUS VENOSUS intiates contraction of heart tube and eventually becomes SA node
     • SINUS VENOSUS
     • Blood at this stage simply flows in and out, no circuit as no chambers

7. Folding of heart

   • http://www.columbia.edu/itc/hs/medical/humandev/2004/Chapt6-Heart1.pdf
   • 1. Truncus arteriosus forms aorta and pulmonary trunk
   • 2. Bulbus cordis forms aorta, pulmonary trunk and R.ventricle
   • 3. Primitive ventricle forms L.ventricle
   • 4. Primitive atrium forms atria (muscular part)
   • 5. Sinus venosus forms atria (smooth part) & coronary sinus.
   • Atrium folds to where truncus arteriosus is, sinous venosus stays in the same place

8. ROUGH part of ATRIA/auricles derived from PRIMITIVE ATRIUM

   • SMOOTH part of RIGHT ATRIUM derived from INTUSSUSCEPTION of RIGHT SINUS HORN (sinus venosus)
   • SMOOTH part of LEFT atrium formed by INTUSSESCEPTION of NEWLY-SPROUTING PULMONARY VEINS

9. Septal devlopment

   • On the superior, inferior, and lateral walls of the AV canal, thickenings known as endocardial cushions (EC) appear
     • mesenchymal cells migrate into the cardiac jelly that separates the endocardium and myocardium to produce mesenchyme that ultimately differentiates into connective tissue- forms cushions
       • Superior and inferior cushions fuse forming septum intermedium. The atrioventricular canal is now divided into a right AV canal (between the right primitive atrium and primitive right ventricle) and a left AV canal (between the left atrium and the left ventricle)
     • ENDOCARDIAL CUSHIONS form SEPTUM INTERMDIUM
   • SEPTUM PRIMUM grows dwon to meet SPETUM INTERMEDIUM
     • As OSTIUM PRIMUM closes OSTIUM SECUNDUM appears
       • SEPTUM SECUNDUM grows down to RIGHT of SEPTUM PRIMUM, leaving FORAMEN OVALE gap
         • Blood flows through staggered openings of FORAMEN OVALE and OSTIUM SECUNDUM - valves closes at birth
           • Abnormalities - atrial septal defects
             • often associated with other developmental defects
   • MUSCULAR ventricular septum grows upwards, stops short of SEPTUM INTERMEDIUM - completed by MEMBRANOUS SECTION

10. Separation of conus cordis

    • Spirally truncoconal swellings fuse to form truncoconal septum
      • Inside TRUNCUS ARTERIOSUS
      • Has to twist or else would be mixed up
        • Uneven seperation between two trunks
          • Right ventricles larger
    • divides outflow tract into ascending aorta and pulmonary trunk
    • Forms membranous interventricular septum
    • Abnormalities = Tetralogy of Fallot (ventral septal defect, pulmonary stenosis, enlarged aorta, hypertrophy of right ventricle)

11. Aortic arches & dorsal aortae

    • cardinal veins - drain head, trunk and limbs
      • Vitelline a & v - yolk, gut tube and its derivative
        • Umbilical a & v - veins will bring oxygenated blood from placenta
          • At start - paired aorta fuse and come together as one

12. FETAL CIRCULATION

    • Ductus venosus
      • shunts a portion of the left umbilical vein blood flow directly to the inferior vena cava. Allows oxygenated blood from the placenta to bypass the liver.
    • Foramen ovale
      • Another passage to aid divercation of blood to the left atrium so that it does not go to the lungs via the pulmonary artery in the fetus
    • Ductus arteriosus
      • Blood vessel connecting the pulmonary artery to the proximal descending aorta. It allows most of the blood from the right ventricle to bypass the fetus's fluid-filled non-functioning lungs.
    • After birth
      • Foramen ovale = fossa ovale
      • Ductus arteriosus = ligamentum arteriosum
      • Abnormalities = Patent ductus arteriosus/ patent foramen ovale

13. Development of the respiratory system

    • Trachiobronchial diverticulum buds off gut tube (foregut)
    • The endodermal lung primordium that will give rise to the epithelial lining of the respiratory tract.
    • Tracheoesphageal septum grows to seperate the trachea and oesphagus
      • Buds branch into primary then secondary bronchi
      • Further development produces segmental bronchi forming what will = BRONCHIOPULMONARY SEGMENTS
        • Respiratory bronchioles and alveolar ducts form. - VACULARITY INCREASES. -- cells still CUBOIDAL in shape
          • cuboidal cells flatten and become associated with BVs - respiration can sustain life
        • internal lining of lung from endoderm
        • Cartilage rings, trachealis muscle etc form from lateral plate mesoderm

14. PLEURA

    • Visceral pleura from visceral (splanchnic) lateral plate mesoderm
    • Parietal pleura from parietal (somatic) lateral plate mesoderm
    • Pleuro-pericardial folds invaginate from lateral walls = seperation of pleural and pericardial sacs
    • Seperation of pleural and peritoneal sacs
      • pleuroperitoneal folds invade from lateral wall
      • Fuse with oesophageal mesentry and septum transversum
      • Septum transversum forms central tendon of diaphragm
      • Lateral body wall forms outer part of diaphragm
      • Abnormality = diaphragmatic hernia

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