Heart Development Explained: From Heart Tube to Cardiac Looping

Hello welcome to Byte Size Med. This video is   on the development of the heart  and part one is on the heart tube. The heart develops from the mesoderm.  Remember the three germ layers? There's  

the ectoderm, the endoderm and the mesoderm. The mesoderm has three parts. The paraxial   mesoderm, the intermediate mesoderm  and the lateral plate mesoderm.   The lateral plate mesoderm develops cavities  that fuse to form an intra-embryonic 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 intra-embryonic 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 now we're looking  at it from above and we've removed the amnion.

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 or 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, lateral and 

cephalocaudal. 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  transverse 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. Cephalocaudal 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 muscle 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 tendinae,  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 carneae.

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 bulbus cordis 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 bulbus 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 bulbus cordis forms the right ventricle. The rough trabeculated part of the right ventricle.

The bulbus cordis 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 cordis 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 cordis.   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 bulbus cordis 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 side. 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 bulbus cordis 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 truncus arteriosus, the   bulbus cordis and the Primitive ventricle. They  bend ventrally. The ventricles enlarge and this   part of the heart tube comes forward and shifts  lower down. Meanwhile the Primitive Atrium and  

the sinus venosis shift backward, so dorsally and  upwards so that they come behind the ventricles. Now in front we have the truncus arteriosus, the  conus cordis and the proximal third of the bulbus 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   bulbus 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. I hope this  video was helpful, if it was you can give  

it a like and subscribe to my channel. Thanks  for watching and I'll see you in the next one! :)