Early Development of the Heart

00:01 Hello and welcome to the first of our discussions on development of the heart.

00:05 This amazing process is going to give rise to an incredibly complex pumping machine hanging out in our thoracic cavity.

00:13 But how we get there is very convoluted and you might ask yourself, why should we care? Reason is, heart malformations and heart defects are incredibly common and prevalent.

00:24 Some form of cardiac malformation tends to occur in about one percent of all births and is a leading cause of both infant morbidity and mortality, especially if it's not diagnosed prior to delivery.

00:38 In the United States alone, there were roughly 6000 deaths associated with cardiac anomalies and malformations.

00:45 So let's go into the process by which the heart moves from being a single tube to becoming the incredibly complex mechanism that it is.

00:54 So you may recall from the lecture we had on the organogenesis period that when the visceral layer of lateral plate museum folds anteriorly to create the gut tube, it has two tubes within it called endocardial tubes.

01:08 And these are going to be the early primordia of the heart as they're brought together, as we can see on the right side.

01:16 They're going to come into existence just ventral to the developing gut tube, and they're going to pull a small cavity with them.

01:23 That cavity is part of the intraembryonic coelom, and specifically in this region, we refer to it as the pericardial cavity, although it's slightly different from the mature pericardial cavity for reasons we'll go into in a subsequent talk.

01:36 The endocardial tubes are brought together, fuse and develop some what's referred to as cardiac jelly around them and around.

01:45 The cardiac jelly is developing myocardium or muscle of the heart. This myocardium is going to be able to be relatively early, but it's not until those tubes fuse that it's going to be able to do anything with the chamber that's inside and propel blood through it.

02:03 Now, the heart is suspended in the pericardial cavity by a dorsal mesocardium, which is essentially a mesentery of the heart connected to the dorsal side of the embryo. As the heart continues to develop, It moves from its initial location to its final location.

02:20 And one amazing thing about the heart is that it actually develops anterior to our face. It grows right about in this area early on, and as the embryo enlarges and the forebrain gets larger and larger, it folds the heart down into the thorax into its eventual mature position.

02:38 As it does so, it pulls the pericardial cavity with it, along with a strip of mesoderm called the septum transversum.

02:48 That mesoderm is going to travel with it and take up residence inferior to the heart, and the septum transversum is the earliest primordia we have of the diaphragm. So we'll discuss how the septum transversum becomes the diaphragm and separates the pericardial cavity from the peritoneal cavity in a subsequent talk.

03:09 Now, the heart has more or less found its normal position in the eventual thorax, and it starts beating about twenty two days into development.

03:17 This is important because at this point, the embryo can't get much larger without a heart. Simple diffusion of gases and nutrients from cells is no longer sufficient to handle the size of the embryo.

03:30 We can't just create cavity after cavity for these things to diffuse.

03:34 We need to have a circulatory system and the heart starts pumping at twenty two days because we absolutely have to have it in order to get any larger.

03:43 So the heart pumps bringing blood in from inferior peristaltic pumps it out early and into a paired set of dorsal aorta on the posterior body wall.

03:54 So blood that's de oxygenated comes in and really gets pumped superiorly and is going to travel on either side of the gut tube through what are called aortic arches, and those aortic arches carry the blood dorsally to a pair of aortae.

04:10 Now, at this point, the heart, instead of being a simple cylindrical tube, is going to develop some distinctive areas that bulge outward.

04:19 The first and most inferior really is the part that receives blood from the body.

04:24 You're going to be our left and right side of the sinus venosus and the sinus venosus pumps blood to the primitive or primordial atrium.

04:32 From there, the atrium is going to pump blood to the embryologic ventricle and then to an area called the bulbus cordis.

04:41 Now, thereafter, blood is going to travel through aortic arches to reach the dorsal aorta. And at this point day twenty one, we only have a single aortic arch.

04:50 But as we develop more aortic arches, we have a common chamber called the aortic sac that's going to receive the blood from the bulbus cordis and distribute it through aortic arches to the dorsal aorta.

05:03 Now, as the heart enlarges and gets those distinctive five segments, the dorsal mesocardium is going to start to break down, and that's going to allow the heart to essentially sag ventrally in the developing chest, and it's going to sag into the pericardial sac that's already present around it. As that happens, the ventricle and bulbus cordis wind up more anterior.

05:29 The sinus venosus and atria wind up more posterior.

05:32 And if you think about how the heart appears in the mature human, the atria are posterior to the ventricles, which are more anterior, so that little fold between them is called the bulbo-ventricular loop.

05:46 And you've got the primordial ventricle just on the backside of that bulge and the bulbus cordis on the anterior side of that bulge.

05:53 And as development proceeds, the ventricles continue to move a bit more anteriorly. Here's an early electron micrograph of a developing heart posterior. We have the atria, which has been straddled by the outflow, tracks its more posterior, and as we move more anteriorly, we see the ventricle and then the bulbus cordis.

06:14 So we're looking at this from an anterior view and can see that the bulbus cordis and ventricle are actually located in front of the atria, which are receiving the blood. Now, when we discuss formation of the heart, we're going to make a single linear flow of blood into four separate chambers and two completely separate circuits.

06:36 What we have to do is follow the development of each one of these areas as they move from the embryo logic structure to the adult structure.

06:44 The sinus venosus is going to form the right atrium along with the superior vena cava, the inferior vena cava and the blood drainage of the heart itself. The coronary sinus.

06:57 The primitive atrium or primordial atrium is going to form the oracles, the little dog eared appendages that are just hanging out on the side of each atrium, along with quite a bit of the left atriums wall.

07:10 The primordial ventricle becomes the left ventricle and the bulbous Cordis is going to transition to become the muscular portion of the right ventricle, along with the outflow tracks of both ventricles.

07:22 So the initial smooth portion of the aorta and the pulmonary trunk as they leave the ventricles and travel up to become the very proximal pulmonary trunk and proximal aorta.

07:33 Thereafter, the aortic sac is going to divide to become a very proximal, but just a little further along portion of the aorta and pulmonary artery.

07:44 Thereafter, the aortic arches will contribute most of the large vessels, leaving the heart. So at this point, I'd like to take a moment and follow the flow of blood through the embryologic heart on the left side of your screen, we've got a sagittal cut.

08:00 We've taken the lid off the left side of the heart and on the right side of your screen.

08:05 We've taken a coronal cut through the anterior wall of the heart.

08:09 So as blood enters, it's going to come in the sinus venosus From there, it's going to enter the atrium, the primordial atrium and then pass into the next chamber, which is the ventricle so atrium to ventricle. From there, it passes from the ventricle to the bulbus cordis, and the bulbus cordis has two subdivisions.

08:33 The conus cordis, a smoothened area and the truncus arteriosus.

08:37 The initial outflow track thereafter, it's going to move to the aortic sac and be distributed through however many aortic arches are present at that stage of development. So here we're going to take a break and then move on to the further subdivisions of the heart and how this single flow of blood in a relatively linear sense becomes two separate circuits in a four chambered heart.

09:00 Thank you very much for your attention.