Hello Everyone, Welcome to Medicoscopy.

Let's start Anatomy with the Cardiovascular Embryology.

We are going to broadly divide cardiovascular embryology into two parts.

1. Development of Heart
2. Development of Interarterial Septum and Interventricular Septum

Development of Heart

First let me tell you about the Pericardium,

Pericardium

It has grossly divided into two layers.

1. Fibrous Pericardium
2. Serous Pericardium 

And the serous pericardium is a double-layered structure composed of parietal and visceral walls. The space between these layers, called the pericardial cavity, contains serous fluid to reduce friction.

To understand the development of the pericardium, let me take you to general embryology.

In this diagram you can see the ectoderm, endoderm, and mesoderm from the transverse section of the embryo, and don't forget the Notochord. Now this mesoderm present over there is intraembryonic mesoderm; it's divided into three parts. 

1. Para-axial Mesoderm
2. Intermediate Mesoderm
3. Lateral plate mesoderm

 Now focus on lateral plate mesoderm (LPM); it is, in fact, divided into two parts or layers, we can say.

1. Somatopleuric LPM
2. Splanchnopleuric LPM

In this diagram, apart from these two, we can see a place where these two are merging together, which is undifferentiated, called the septum transversum. We have to take note that this septum transversum is not present throughout the embryo, only in the cranial part. So basically we are talking about the cranial most part of the embryo.

We can see a cavity there, called the intraembryonic coelom. It's very important because it forms the pericardial cavity. Not only the pericardial cavity, but all the cavities inside our body are formed from this intraembryonic coelom.

Now think, if this coelom is forming a cavity, then somatopleuric LPM will form the parietal layer of the serous pericardium, and splanchnopleuric LPM should form the visceral layer (epicardium).

This splanchnopleuric LPM is also going to form endocardial cushions (cardiac jelly); it helps in the development of the interatrial and interventricular septum; we will see it in my next blog.

Apart from that, this splanchnopleuric LPM forms the myoepicardial mantle, which is going to form cardiac muscle and the visceral layer (epicardium).

From this diagram, we are only left with the septum transversum; it forms the fibrous pericardium (the outermost layer of the heart).

So, we are done with Pericardium.

Heart

So, if you understand the previous topic where it's written Cardiac muscle is derived from splanchopleuric LPM, and the heart is nothing but cardiac muscle only. We will discuss it more clearly, but before that I want you to remember some facts.

• Development of heart begins on the 16th day of gestation.
• Heart starts beats by the 4th week onward, or the 22nd day to be specific.

Initially we have two heart tubes, which are derived from splanchopleuric LPM, called endothelial heart tubes. Now see the diagram.

This heart tubes will fuse cranially and there will be the formation of three dialatation,

1. Bulbus Cordis
2. Primitive Ventricle
3. Primitive Atrium

At first I want you to focus only on the ends of this diagram; we can see the cranial end and caudal end, called Truncus Arteriosus and Sinus Venosus, respectively. We  see the sinus venosus is divided into two parts. Right horns and left horns.

Let's not clutter all the information at once; let's deal with it one by one, first concentrating on the ends.

From these ends we have to decide which one will be inflow and outflow, meaning where the blood will enter and from where it comes out. If you observe the name, from the name itself we can decide this, like Sinus Venosus-Vein; from it the blood will enter, and Truncus Arteriosus-Arteries, from where blood will come out. From here we can learn which structure can be derived from which part.I am going to tell you one simple logic; for a minute, let's forget about embryology and follow this trick.

Let's divide the entire cardiovascular system into three parts: inflow, pump (heart), and outflow.

We all know that from the upper part of the body, blood is collected by the superior vena cava, and from the lower part, it's the inferior vena cava, and from the lungs, it will be the pulmonary veins, and my friends, how can we forget the coronary sinus, which collects blood from the heart?

All these vessels will give blood to the heart, and then it will be pumped out through the aorta and pulmonary trunks.

Now I request you all to concentrate on outflow, meaning the aorta and pulmonary trunk; on the previous diagram we have discussed the truncus arteriosus,which helps in the outflow of blood. Same as the aorta and pulmonary trunk, which helps in the outflow of blood in our body. You don't have to remember this part; just go with the logic. So the aorta and pulmonary trunk are derived from the truncus arteriosus.

From the Inflow part, let's cut Superior and Inferior Venacava, because we will discuss it in our further blog. For now, keep the pulmonary veins aside; we will discuss them in some time. So, what remains is its coronary sinus, which is definitely derived from the sinus venosus. But the sinus venosus is not one structure; it has right and left horns. So you should remember that the coronary sinus is mainly derived from the left horn of the sinus venosus.

What remains now is its heart in the middle. We all know that the heart has four chambers, two atria and two ventricles; we got to learn about these structures.

Before explaining the derivation, let's know the names of the structure. You can see there are three dilatations in the middle in diagram 2. The first dilatation here is called Bulbus Cordis. This Bulbus Cordis is in fact divided into three parts: the first, most cranial part is the Truncus Arteriosus; the second is the Conus; and the third is the proximal 1/3 part.

Then after that next dilatation are the primitive ventricle and primitive atrium.

Let's see what they are going to form. We are not going to learn in a textbook way; let's not mug up; put logic into it.

I hope you remember that in the atrium and ventricle there are rough and smooth parts.

Inside atria, there will be a rough anterior wall and a smooth posterior wall. Let us similarly open the ventricle; here the inflow part will be rough, whereas the outflow part will be smooth.

Logically thinking, the heart pumps blood, so to reduce friction, the outflow part should be smooth, and as the blood is coming with some velocity there, there should be some stoppage there; the inflow should be rough. I am talking about the smooth and rough parts because we need to learn where this rough and smooth part will be derived.

Let's begin.

If the truncus arteriosus is forming the aorta and pulmonary trunk, the outflow, then the conus should form the smooth outflowing part of the right ventricle and left ventricle. This smooth outflowing part is also referred to as the infundibulum. Proximal 1/3 is the one that is going to form the rough inflowing part of the right ventricle. And Primitive Ventricle will form the rough inflowing part of the left ventricle. In some books this rough part is also called the trabeculated part. So with this we have completed our ventricles.

The primitive atrium is the one that is going to form the rough anterior wall of both the right atrium and the left atrium. Remember Sinus venosus? There I told about the left horn, which is going to form the coronary sinus, but what about the right horn? This right horn is going to form the smooth posterior part of the right atrium (note: only the right atrium).

For the posterior part of the left atrium, we have to see another diagram, which is kind of interesting.

We are left with the posterior part of the left atrium and pulmonary veins. In this diagram, both topics will be covered.

Pulmonary veins are composed of 4 veins, two from each lung, and drain the blood into the left atrium; we all know this basic. But let's see it in another way.

Initially, its left atrium, which is going to form a bud-like structure, which later on forms these 4 veins. You can see it in the diagram. The interesting part of this topic is that these ramified branches of pulmonary veins, the middle part, will again be absorbed back into the left atrium, thus forming the smooth wall of the left atrium and the pulmonary veins.

Now, if you observe the truncus arteriosus, it's going to form two structures: one is the ascending aorta and the pulmonary trunk. A simple doubt arises here: how are they going to be divided?

  

So, the answer is, by formation of a septum in the middle, called the aortico-pulmonary septum or spiral septum. It is derived from the neural crest cell. But why is it called a spiral septum? Because during the development of the embryo, this septum is going to take a turn or it's going to spiral, because of that turn, the blood from the left ventricle goes to the ascending aorta and blood from the right ventricle goes to the pulmonary trunk.

It's very interesting because, from this, you can understand heart anomalies, for example,

If the spiral septum doesn't spiral, then what's going to happen? The aorta is going to begin from the right ventricle, and the pulmonary trunk will be from the left ventricle. What's this condition called—transposition of great vessels?

Here are some other examples.

By this, my dear friends, we have completed development of Heart.Hope you like this explanation. 

Thank you for your understanding.