Development of the Heart

In my last post, I spoke mainly about the lungs. This time I'm going to talk about another major organ in the thorax: the heart! (Note: there are a LOT of bits in this that I'm not too sure about, especially because we had two lectures that seem to have some subtle differences. Any help correcting things will be greatly appreciated :) )

Basics about the Heart

Before reading the rest of this post, it might help you to refresh your memory about the main bits and pieces that make up the heart. You might find the following posts helpful:

• The Circulatory System
• Some more notes on the cardiovascular system
• Birth (contains a few points about the fetal circulation)

Development of the Heart

In the words of Julie Andrews: "Let's start from the beginning, it's a very good place to start." To my understanding, the cardiovascular system starts at around 13-15 days' gestation as mesodermal cells form "blood islands" on the surface of the yolk sac. These blood islands are made of haemoblasts, which eventually become blood cells, and endothelial cells, which eventually become the blood vessels.

A few days later (day 18), blood vessels appear in the intraembryonic mesoderm as well, especially at the cardiogenic area, which is actually located at the cranial end of the embryo at this point (the heart and diaphragm start above the head, and only move down as the embryo folds). These vessels include two large parallel vessels that will eventually become the heart.

As I just mentioned, folding occurs in week 4, which changes around where the heart and diaphragm are. The heart originally starts above the head, and the diaphragm above that, but after folding the heart and diaphragm are in more "normal" positions. This folding is also why our aorta has an arch, and why the nerves supplying the heart and diaphragm descend from the neck.

Now, back to the heart specifically. The heart starts as two vessels that fuse and start to develop specialisations. These specialisations are as follows:

• Sinus venosus- Smooth-walled. This is where the major veins join up.
• Primitive atrium- Rough-walled. Eventually becomes the atrium, along with the sinus venosus (this is why part of the right atrium is rough-walled and part is smooth-walled).
• Primitive ventricle- Rough-walled. Eventually becomes the ventricle, along with the bulbus cordis.
• Bulbus cordis- Smooth-walled. The part of the ventricle near the pulmonary valve.
• Truncus arteriosus- The part that eventually becomes the arteries.

The slides also mention something called the "conus" which appears to be a part of the bulbus cordis. However, as I mentioned earlier, I'm not 100% sure on all of the details regarding heart development (which is why I've asked Hazel to cover this in the review lecture :P).

Anyway, some other bits and pieces that you need to know about the primitive heart, which starts beating at day 22. There is a boundary that separates the primitive atrium from the sinus venosus. This line is called the crista terminalis (on the inside) or the sulcus terminalis (on the outside). The ventricle doesn't have any nice divisions like this (that I know of) but the thing that is special about the ventricle is that it grows really fast, which causes the heart to fold (kinda like how the growth of certain embryonic germ layers cause embryonic folding). This also causes the heart to grow into the pericardial cavity. By day 28, a primitive 2-chambered heart has formed, with the primitive atrium posterior and the truncus arteriosus anterior. A common atrioventricular canal passes through.

Of course, we eventually have to develop a proper four-chambered heart. This happens through the growth of several partitions.

The partition in the atria is known as the interatrial septum, which is easy enough to remember. The actual formation of the septum, however, is a bit trickier as it actually involves the formation of two septa. The first one, septum primum, has two holes: the ostium primum, which usually disappears, and the ostium secundum, which stays open. Then septum secundum, which is thicker than septum primum, grows from a different direction, and doesn't close off completely, so it has a hole of its own too. Hence, the "foramen ovale" which allows fetal blood to flow directly between two atria is actually made up of two holes: the ostium secundum of the septum primum as well as the septum secundum's hole. These holes usually do not line up so that when the lungs start working and the increasing pressure squishes the two septa together, the holes functionally close (i.e. they're still there, but no blood can pass through).

The ventricular septum is slightly less complicated (or at least we don't have to know about it in as much detail). It is made up of three parts: a membranous part made up of atrioventricular "cushions" (from endocardium), a spiral conotruncal septum and a muscular interventricular septum. The spiral septum, to my understanding, is caused by the spiralling blood flow in the primitive folded heart. Also, from my limited understanding, the spiral septum also partitions the truncus arteriosus, and this spiralling is also responsible for the pulmonary trunk and aorta appearing to wrap around each other.

Now, a few notes on the other bits and pieces. The atria and ventricles are separated from each other by endocardial "cushions" (a.k.a. atrioventricular cushions) that divide the atrioventricular canal. These cushions also give rise to valve cusps. The sinus venosus is also worth talking about briefly, as it is originally symmetrical but then the left part becomes smaller, giving the overall effect of the opening shifting to the right. This is why all of our big veins, like the superior and inferior vena cava, are located on the right.

After all that is done, a 4-chambered heart is finally achieved at around 10 weeks gestation!

Notes on Fetal Circulation

Even after all this is done, the embryo still has a different circulation compared to what they will have after birth. For starters, they have a couple of extra vessels. One of these is the ductus venosus, which bypasses the liver. You see, the mother does most of the job of detoxifying and so forth, so that the embryo doesn't have to do much. Instead, that blood is much better diverted fresh to the heart. The other vessel is called the ductus arteriosus, which bypasses the lungs. In fetal life, the lungs do not function so they are collapsed and offer a high resistance. Hence there is a pathway for blood to flow directly from the pulmonary trunk into the aorta. Eventually this degenerates to form the ligamentum arteriosum, which is just a ligament holding the pulmonary trunk and aorta together.

When the baby is born and starts to breathe, other changes start happening. The lungs expand, allowing blood to go through the pulmonary circuit. The ductus arteriosus closes after a few months as it isn't being used. As more blood is passing through the lungs, more blood is returning to the left atrium, and this increased pressure forces the foramen ovale closed. It functionally closes (i.e. doesn't let blood through), but it may not structurally close for over a year, if at all. Cutting of the umbilical cord also causes degeneration of the umbilical vessels as well as the ductus venosus. The ductus venosus is the fastest to disappear- it's gone after only a few weeks.

Yay! Finally finished this post! (I'd typed up the first few paragraphs and had it sitting on "draft" for ages...)