Cardiovascular System

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Cardiovascular System 
Ass. prof. Malak A. Al-yawer
The vascular system appears in the middle of the third week
B. Transverse section of a late presomite embryo (approximately 18 days) to show the position of the blood islands in the splanchnic mesoderm layer 
C. Cephalocaudal section of a late presomite embryo (approximately 18 days) showing the position of the pericardial cavity and cardiogenic field
With time, the islands unite and form a horseshoe-shaped endothelial-lined tube surrounded by myoblasts. This region is known as the cardiogenic field the intraembryonic cavity over it later develops into the pericardial cavity
Formation and Position of the Heart Tube
As a result of cephalic folding of the embryo,
the Buccopharyngeal membrane is pulled forward, while the heart and pericardial cavity move first to the cervical region and finally to the thorax
As a result of lateral folding ,
the caudal regions of the paired cardiac primordia merge except at their caudal most ends. Simultaneously, the crescent part of the horseshoe-shaped area expands to form the future outflow tract and ventricular regions.
Ventral views of the developing heart and pericardial region (22-35 days) showing fusion of the two heart tubes to form a single tube. The fusion begins at the cranial ends of the heart tubes and extends caudally until a single tubular heart is formed
Dorsal mesocardium
Schematic drawing of the heart (approximately 28 days) showing degeneration of the central part of the dorsal mesocardium and formation of the transverse sinus of the pericardium
Endocardium
The endothelial tube becomes the internal endothelial lining of the heart-endocardium
The myocardium
thickens and secretes a thick layer of extracellular matrix, rich in hyaluronic acid, that separates it from the endocardium.
Epicardium or Visceral pericardium
Is derived from (1) Mesothelial cells on the surface of the septum transversum migrate over the heart to form most of the epicardium . (2) The remainder of the epicardium is derived from mesothelial cells originating in the outflow tract region.
It is responsible for formation of the coronary arteries, including their endothelial lining and smooth muscle.
Formation of the Cardiac Loop
Formation of the Cardiac Loop
The heart tube continues to elongate and bend on day 23 . The cephalic portion of the tube bends ventrally, caudally, and to the right and the atrial (caudal) portion shifts dorsocranially and to the left . This bending is complete by day 28.
The process of looping
It seems that the process of looping is a genetic property of the myocardium (may be due to cell shape changes ) and not related to differential growth.
Concurrently, the tubular heart elongates and develops alternate dilations and constrictions
Bulbus cordis primitive ventricle Primitive atrium sinus venosus
bulbus cordis
Clinical Correlates
Dextrocardia
Abnormalities of Cardiac Looping Dextrocardia
The heart lies on the right side of the thorax instead of the left, is caused when the heart loops to the left instead of the right. Dextrocardia may coincide with Situs inversus, a complete reversal of asymmetry in all organs.
The primordial heart tube during the fourth weekA, Normal bending to the right, B, Abnormal bending to the left
Development of the Sinus Venosus
Development of the Sinus Venosus
the left sinus horn rapidly loses its importance
with obliteration of the left vitelline vein and the left common cardinal vein the right umbilical vein is obliterated also
Thank you
Next lecture Formation of cardiac septa
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