Heart development and cardiac embryology
Initial events with the heart happen as early as the middle of the 3rd week.
§ The trilaminar disk in its flat form, there’s a region towards the cephalic end, the cardiogenic region that will develop into the region that will be the heart.
§ On Day 19, 2 very thin walled tubes develop one on either side of this trilaminar disk. These are called the endocardial tubes.
§ Through the procedure of lateral folding, as was discussed last time, the trilaminar disk folds around and goes from a flat shape to a 3D sphere.
§ As it does this, it brings these two tubes in contact with one another.
§ Through a process of apoptosis (programmed cell death), the cells along the two edges where the tubes meet die and form one continuous tube.
§ This process begins cranially (outflow) and extends downward.
§ Becomes one primitive tube called the heart-tube.
The components of the primitive heart tube
§ Initially the tube is suspended by the dorsal mesocardium.
§ At this time the vasculature is also developing.
§ This thin strip of tissue ruptures and leaves the heart suspended in the pericardial space by the vasculature.
§ At this point the primitive heart tube consists of 3 layers.
1. The endoderm on the inside
There comes around it and attached to it a thick layer of the splanchnopleuric mesoderm, which surrounds the heart tube.
2. This then differentiates into a thick acellular matrix called cardiac jelly
3. And differentiates into what will become the myocardium, at this point they are myocardial cells.
The primitive heart tube: forms into 5 regions by these sulci, the indentations that occur along the heart tube.
§ And these will each go on and become an important structure in the heart.
1. Sinus Venosus
Inflow region at the caudal region it is the sinus venosus it forms 2 horns, right and left.
2. The primitive atria are next and these will turn into the right and left atria in the adult.
3. The P.V. (primitive ventricle) Will turn into mostly the left ventricle.
4. The bulbous cordus will become the right ventricle.
5. The truncus arteriosus, also called the conus arteriosus, the primary outflow here will become the aorta and the pulmonary trunk.
§ So now we have this long thin heart tube divided into its 5 sections and
§ It undergoes the process of folding in order to bring what will be the 4 chambers of the heart into their correct alignment.
Folding of heart tubes occurs.
§ It was initially thought that similar to the formation of the gut, as it grew it had no choice, it was limited by the pericardial space and so it folded over.
§ Actually, that’s not true. If you excise these tissues from animals and keep them in an invitro solution, they will fold regardless of the space around them.
§ So it is started at Day 23, where the heart tube becomes longer, it loops, it folds, it comes into roughly the correct spatial relationship, and it’s complete within 5 days.
§ Typically loops to the right (dextral).
§ Fairly early on, begins to loop to the right convergence – the formation of the large left ventricle then wedging – repositioning the AV canals to their correct position.
Remodeling and developing the septum
§ From Day 28 on, the rest of the process of development is involved in remodeling and developing the septum, or the walls that grow to form the correct chambers of the heart.
Prior to this looping, you have 2 sides to this primitive heart tube and they are pumping blood
§ the right blood that comes in the right sinus venosus horn and goes out the right truncus arteriosus and the same for the left.
§ You have sort of a parallel circulation.
§ Which will not work to bring blood to all the tissues of the very rapidly developing embryo.
§ So this very primitive R-side-to-R-side and L-side-to-L-side circulation is replaced by the formation of these 4 septa,
The primitive heart becomes divided into the 4 chambers
§ By fairly early on, blood starts flowing as it does in the adult heart, into the atria and out through the ventricle.
§ Day 22 would be a good date to recall, the heart tube itself during this process of looping starts beating intrinsically and the cells begin to contract.
§ And by Day 24 the blood is actually circulating in the embryo.
§ 1st in the primitive form, and then in the more correct adult orientation.
The formation of the 4 septa
§ make possible the 4 chambers of the adult heart and the outflow through the great vessels.
The 1st septum: The aortic-pulmonary or the AP septum.
§ This is the septum that is going to divide the large outflow vessel (truncus arteriosus) into the aorta (which send blood out of the left ventricle) and the pulmonary trunk (the outflow of the right ventricle to the lungs).
§ So septum forms aorta and pulmonary trunk. They become repositioned towards the end of development.
The 2nd septum:
§ divide the primitive atrium into the 2 atria (right and left).
§ Occurs in 4 steps:
1. 1st growth of the septum primum (1st septum) grows from the wall of the primitive atrium and leaves a gap (foramen primum)
2. Eventually the foramen primum closes as the septum primum continues to grow.
3. Also you see the growth of the septum secundum from the right side of the primitive atrium.
4. As this grows, it covers up the foramen primum, but it leaves the foramen ovale, which is a hole that continues to have blood communicate between the right and left atria during the entire period of embryo development.
§ Through this, oxygenated blood can pass from right to left in the developing fetus before birth.
§ By the relationship of the foramen ovale to the 2nd septum that grows it acts as a barrier, so blood can only go 1 way.
§ It’s only going from right to left, but not going back.
§ The septum secundum acts as a primitive valve.
§ Normally after birth, this closes and it closes because the septum primum fuses with the septum secundum.
§ In 20-25% of cases studied, the foramen ovale remains patent shortly after birth. Not automatically a cause for surgery.
The 3rd septum: The atrioventricular septum
§ differentiate the ventricle from the atria themselves.
§ Named AV septum.
§ In the primitive heart tube it’s going to close off atrium from ventricle.
§ Primitive ventricle becomes primarily left ventricle,
§ bulbus cordus becomes smooth part of R and L ventricle
§ canal that connects the two is called the AV canal.
§ In order to form the AV septum, the mesechyma on either side begins to form endocardial cushions.
§ These are areas of tissue that grow and act like glue and these will glue together the central points where atria and ventricle come together and form AV septum.
§ At this point the AV canal grows and divides it’s own septum so that the right becomes the tricuspid orifice which becomes the tricuspid valve. The left becomes the mitral (bicuspid) orifice which becomes the mitral valve.
§ At this point the right and left atria have formed, having the foramen ovale through which oxygenated blood is passing, now have divided with valves in place to prevent the blood only from going from atria to ventricle and not back up, by the formation of the AV septum.
The interventricular septum
§ At this point, with the growth of the interventricular septum, a wall that grows within the ventricular space and divides it into the right and left ventricle.
§ It’s not that this develops as a structure own it’s own,
§ it begins as a sulcus in the original heart tube from the bulbus cordis and the primitive ventricle
§ and it becomes prominent and enlarged as the ventricles grow.
§ It grows towards the endocardial cushions, but leaves a gap called the primary IV foramen,
§ another hole where blood can continue to communicate at this point.
§ Eventually this foramen closes, about the 7th week
§ this is formed by the bulbar ridges (from bulbus cordis) and they fuse with the endocardial cushions and closes this foramen.
§ In addition with the coincident development of the AP septum, we now have ventricular space that consists of 2 chambers divided by the IV septum
§ now the heart assumes it’s correct orientation and you have the right ventricle sending its blood through the pulmonary trunk and the left ventricle through the aorta.
§ So now, the components of the ventricle are now as in the adult heart.
Inlet to each ventricle
§ You have the inlet to each ventricle, mitral-right, tricuspid-left, have trabeculated portion,
§ chamber heart outlet via the great arteries (aorta and pulmonary trunk).
§ at this point at the outlet, these valves (semilunar) develop so that blood can only go out and not come back.
§ They develop from the distal end of the very first outflow portion the truncus arteriosus, also called the conotruncus, so the semilunar valves develop similar to the way the mitral and tricuspid valves develop.
§ In that you have swelling from the endocardium on the side. As it swells they form a little indentation so they form the correct valve shape.
Tags: bulbar ridges, bulbus cordosus, conotruncus, conus arteriosus, endocardial cushion, endocardial cushion defect, endocardial tubes, foramen primum, four chambers of the heart, heart tube, mitral valve, myocardium, primitive atrium, pulmonary trunk, septum primum, sinus venosus, trilaminar disk, trilaminar disk folds, truncus arteriosus
