Circulatory System: Heart Chambers
Heart septa appear during week 5 and divide the heart tube into four chambers between days 27 and 37. The septa form as inward growths of endocardium separating the atrial and ventricular chambers, splitting the atrium into left and right, and splitting the ventricle and bulbus cordis into left and right ventricles, respectively (Figure 28.1). The atrioventricular canal connects the primitive atrium and ventricle. At the end of week 4 the endocardium of the anterior and posterior walls of the atrioventricular canal thicken and bulge outwards into the canal’s lumen. These are the endocardial cushions and by the end of week 6 they meet in the middle, splitting the atrioventricular canal into two canals (Figure 28.1).
At the same time, new tissue forms in the roof of the primitive atrium. This thin, curved septum is the septum primum and extends down from the roof, growing towards the endocardial cushions. The primitive atrium begins to split into left and right atria. The gap remaining inferior to the septum primum is the ostium primum (Figure 28.2). Growth of the endocardial cushions and the septum primum cause them to meet.
A second ridge of tissue grows from the roof of the atrium, on the right side of the septum primum. This is called the septum secundum (Figure 28.2) and grows towards the endocardial cushions, but stops short. The gap remaining is the ostium secun- dum, and the two holes and flap of the septum primum against septum secundum form a one‐way valve allowing blood to shunt from the right atrium to the left but not in reverse. This is the foramen ovale (Figure 28.2) and is one of the routes that exist before birth allowing blood circulation to circumvent the developing lungs. A change in pressure between atria at birth holds the septum primum closed against the septum secundum, and the foramen becomes permanently sealed.
From the end of the fourth week a muscular interventricular septum arises from the floor of the ventricular chamber as the two primitive ventricles begin to expand (Figure 28.3). The septum rises towards the endocardial cushions, leaving an interventricular foramen. As the atrioventricular septum is completed late in the seventh week the endocardial cushion extends inferiorly (as the membranous interventricular septum) to complete the interventricular septum and close the interventricular foramen (Figure 28.3).
Now the heart is four connected chambers with two input tubes. The single outflow tract of the primitive heart must also split into two to pass blood from the ventricles to the pulmonary and systemic circulatory systems (Figure 28.4). The conotruncal out- flow tract, comprising the conus arteriosus and truncus arteriosus, develops a pair of longitudinal ridges on its internal surface. These grow towards one another and fuse to form the conotruncal septum, which meets with the muscular interventricular septum to link each ventricle with its outflow artery. The conotruncal septum spirals within the conus arteriosus and truncus arteriosus, giving the intertwining nature of the adult pulmonary trunk and aorta (Figure 28.5).
After the fusion of the endocardial cushions to form two atrioventricular canals, mesenchymal cells proliferate in the walls of the canals. The ventricular walls inferior to this erode, leaving leaflets of primitive valves and thin connections to the walls of the ventricles. These connections develop into the fibrous chordae tendinae with papillary muscles at their ventricular ends. The left atrioventricular valve develops two leaflets (the bicuspid valve) and the right atrioventricular valve usually develops three (the tricuspid valve).
The semilunar valves of the aorta and pulmonary trunk develop in a similar manner during the formation of the conotruncal septum.
Neural crest cells, appearing during neurulation, migrate from the developing neural tube to take part in the development of an astounding range of different structures, including the heart. In the heart they contribute to the conotruncal septum.
Heart defects are the most common congenital defects, generally occurring because of problems with structural development processes. Six in 1000 children are born with a heart defect.
A ventricular septal defect is the most common heart defect, and failure of the membranous interventricular septum to close completely allows blood to pass from the left to right ventricles. Most will close on their own but surgery may be required. This can be linked to other conotruncal defects. Atrial septal defects occur when the foramen ovale fails to close (patent foramen ovale), allowing blood to pass between atria after birth. Treatment is surgical.
Abnormal narrowing of the pulmonary or aortic valves can give pulmonary or aortic stenosis, forcing the heart to work harder. Stenosis of the aorta will limit the systemic circulation, with clear consequences. These arteries can be transposed if the conotruncal septum fails to form its spiral course, and the aorta will arise from the right ventricle and the pulmonary trunk from the left ventricle (transposition of the great vessels). Low oxygen blood is passed into the systemic circulation.
Tetralogy of Fallot describes four congenital defects resulting from abnormal development of the conotruncal septum: pulmonary stenosis, an overriding aorta connected to both ventricles, a ventricular septal defect and hypertrophy of the wall of the right ventricle. Poorly oxygenated blood is pumped in the systemic ith symptoms of cyanosis and breathlessness. urgical intervention is required.