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.
Clinical relevance
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.
