Hello, welcome to Bite Size Med. This video is on the development of the heart and part 1 is on the heart tube. The heart develops from the mesoderm. Remember the 3 germ layers?
There's the ectoderm, the endoderm and the mesoderm. The mesoderm has 3 parts. The paraxial mesoderm, the intermediate mesoderm and the lateral plate mesoderm. The lateral plate mesoderm develops cavities that fuse to form an intraembryonic cavity that splits the lateral plate mesoderm into two layers, the somatic layer and the splanchnic layer. Now the heart develops in the splanchnic layer of the lateral plate mesoderm.
It starts off as a cluster of cells. The endoderm underneath stimulates these cells to differentiate into blood islands. These blood islands form vessels and that process is called vasculogenesis. The vessel that they form here is called the heart tube.
And there are two, so paired heart tubes. If we go back to those three germ layers, these are the two heart tubes, one on each side. The heart sits in the pericardial cavity. Now that space forms from the intraembryonic cavity.
If you look at these sections as best as I could draw them, you'll notice that the sides are coming together. That's because the embryo is folding. I'm going to try and simplify what that is.
The hard part about embryology is the fact that so many things happen at once and things keep changing. That's why there are so many sections at so many different angles. Assume that this is the embryo and we're looking at it from above and we've removed the amnia. This is the head end of the embryo or the cranial end or the cephalic end.
and this is the tail end or the caudal end. This would be the dorsal surface of the embryo. Now the sections we saw earlier, they were across like this at different ages of the embryo.
Transverse sections are cross sections, so you can see the midline, the sides and the layers. A section going this way would be a longitudinal section or a sagittal section. The embryo folds during its development in two directions, laterally and cephalocaudally. Lateral folding would be the sides coming together, closing the ventral body wall. Cephalocaudal folding would be the bending of the head and the tail ends.
To see lateral folding, we're gonna have to use transfer sections. And when the embryo folds, the two heart tubes come together and unite. to form one heart tube in the pericardial cavity.
Now in a section like this you'll also see the dorsal aortae which continue from the heart tube but ignore those for now. So the lateral folding results in a single heart tube but the embryo folds cephalocaudally as well and for that we're going to use a sagittal section which would look something like this. This is the cephalic end and this is the caudal end. Now the fascinating thing about heart development is the fact that the heart starts developing at the cephalic end, in the head, making things just as confusing as two heart tubes. But the folding is how it gets sorted out.
Cephalo-caudal folding. So when the head end folds, the heart comes towards the thoracic region. Initially, the heart tube and the pericardial coelom are separate. and their relationship changes as it folds.
Finally, the heart tube gets pushed into the pericardial cavity. Now we'll pick up that heart tube and reorient it to look at it vertically. It has a venous end and an arterial end.
The heart tube was a vessel with endothelium. That endothelial lining forms the endocardium. It was surrounded by cardiac myoblasts. and they form the myocardial layer.
The myocardium produces a specialized connective tissue which separates the two layers. This is called the cardiac jelly. The outer epicardial layer, which is the visceral layer of the pericardium, is formed by migration of mesothelial cells, which then cover the myocardium.
And these are the layers of the heart tube. It still looks nothing like what the heart is supposed to look like. and it's incredible how it goes from this to that. But to understand how, let's first look at some of the structures of the heart that need to develop and we'll come back here.
The heart has four chambers, the right atrium, the left atrium, the right ventricle, and the left ventricle. First, let's look at the atria. The right atrium receives blood from the superior vena cava, the inferior vena cava, and the coronary sinus. The left atrium receives blood from the four pulmonary veins.
In both the atria, all these openings are on the posterior wall, making it smooth. So they have a smooth posterior wall. The anterior wall on the other hand is rough, with muscular ridges called musculi pectinati. And those are the two parts of the atria. Between the atria and the ventricles are the atrioventricular valves, the tricuspid valve on the right and the mitral valve on the left.
The free ends of the valve leaflets are attached to chordae tendineae, which in turn are attached to papillary muscles on the wall of the ventricles. The walls of the ventricles have muscular ridges and bridges called trabeculae carnii. That makes them rough or trabeculated.
This is the rough inflow portion of the ventricles. The right ventricle pumps blood into the pulmonary trunk and the left ventricle into the ascending aorta. The outflow tracts that lead into these vessels are smooth.
The outflow tract of the right ventricle is called the infundibulum. or the conus arteriosus, while that of the left ventricle is called the aortic vestibule. These are the smooth outflow tracts of the ventricles.
Between the ventricles and those big vessels are the semilunar valves, that's the pulmonary valve on the right and the aortic valve on the left. The ventricles pump blood into the ascending aorta and the pulmonary trunk. And now let's go back to the heart tube.
It has alternating dilations and constrictions which are going to form the different chambers of the heart. At the top here we have the arterial end which is going to lead into the aortic sac with the aortic roots. The first part of the heart tube is called the truncus arteriosus. Below that is the bulbous cordus and I'm going to divide that which I'll explain in a bit. Next we have the primitive ventricle and then the primitive atrium.
The venous end is the sinus venosus. Starting at the top, the truncus arteriosus, which sounds like an arterial trunk, forms the ascending aorta and the pulmonary trunk. Now these two will separate when a spiral aorticopulmonary septum forms between them. The bulbous cordis forms the smooth outflow tracts of the ventricles. That's the infundibulum on the right and the aortic vestibule on the left.
But the proximal third of the bulbous cordus forms the right ventricle, the rough, trabeculated part of the right ventricle. The bulbous cordus actually has three parts, a distal third, a middle third, and a proximal third. The distal third is the truncus arteriosus. The middle third is also called the conus cordus. And if you remember that the smooth outflow tract of the right ventricle is also called the conus arteriosus.
You'll remember that the outflow tracts of both the ventricles comes from the conus cortis. The proximal third doesn't have a special name, but it forms the trabeculated part of the right ventricle. The trabeculated part of the left ventricle is formed from the primitive ventricle.
Between the bulbous cortis and the primitive ventricle is a bulboventricular sulcus. This is the site of the interventricular foramen. because it's between the developing right ventricle and the developing left ventricle. This will be the site of formation of the interventricular septum, which will eventually separate the two chambers. The primitive atrium forms the two atria, the rough anterior walls of both the atria.
Between the developing atria and the developing ventricles is the atrioventricular canal, which won't close, but it will split. to separate the right and the left sides. What's left in the atria is the smooth posterior walls, which I'll explain when I talk about atrial development because that's when I'll talk about this sinus venosus.
But between the sinus venosus and the primitive atrium there is a sinoatrial orifice. Now let's see the path that blood takes through the heart tube. It starts at the venous end from the sinus venosus into the primitive atrium Let's put those two together as the atria and their inflow tracts. Next up on the path is the primitive ventricle, the bulbous cortis and the truncus arteriosus. which we'll put together as the ventricles and their outflow tracts.
Blood then gets pumped out of the arterial end. Now this heart tube has to go from looking like this to this and that happens by a process called looping, cardiac looping. And now you'll see why I grouped them.
The trunk is arteriosis, the bulb is cortis and the primitive ventricle, they bend ventrally. The ventricles enlarge and this part of the heart tube comes forwards and shifts lower down. Meanwhile the primitive atrium and the sinus venosus shift backwards, so dorsally, and upwards so that they come behind the ventricles. Now in front we have the choncus arteriosus, the conus cordis, the proximal third of the bulbous cordis, the primitive ventricle, and behind we have the primitive atrium.
If we flip it over to look at the back, the sinus venosus opens into the primitive atrium through that sinoatrial orifice. And now it's starting to look more like the heart on the outside. The truncus arteriosus forms the ascending aorta and the pulmonary trunk. The conus cordis forms the smooth outflow tracts of both the ventricles. The proximal third of the bulbous cordis forms the trabeculated part of the right ventricle.
The primitive ventricle forms the trabeculated part of the left ventricle and the primitive atrium forms the rough parts of both the right and the left atria. In part two we'll look at how these two atria develop. But all that was the heart tube.
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