Separating the Pericardial and Pleural Cavities

00:01 In this talk, we're gonna discuss how the intraembryonic coelom which has already been partitioned into a pericardial cavity and a peritoneal cavity will give rise to the pericardial cavity in its mature state and the overlying pleural cavities both on the right and left.

00:18 Now, the lungs are developing off the foregut and initially, the heart is the only structure in the embryonic pericardial cavity.

00:27 But as the lungs develop, they're going to grow outward from the foregut and initially develop posterior to the heart.

00:34 So here, we can see the heart still fairly far interior in the pericardial cavity but the lungs have grown in and are in the same space.

00:43 Now, once the lungs are in that space, we refer to the area around them as the pleural cavity but at the moment, this pleural cavity is continuous with the pericardial cavity surrounding the heart.

00:54 But the lungs continued to grow and they don't stay posterior to the heart, they're gonna grow laterally and anteriorly around the heart.

01:03 Now, as this happens, there's gonna be a fold of tissue on the lateral body wall that gets stuck between the developing lungs and the heart.

01:12 These are called the pleuropericardial folds.

01:15 And the pleuropericardial folds will eventually wrap around the heart and keep it separate from the developing lungs.

01:22 Inside each pleuropericardial fold is the phrenic nerve.

01:27 That nerve that innervates the diaphragm so it's travelling through these folds to get inferiorly to the diaphragm along with the branch of one of the common cardinal veins.

01:36 So the pleural cavities are going to eventually sculpt their space out of the body wall by pushing this pleuropericardial fold ahead of them.

01:46 So we move a little further ahead.

01:48 We can see that the lungs are enlarging, they're moving around the heart, and as they're posterior to it, they're gonna push the folds ahead of them and eventually, wrap around the heart and as they do so, the pleuropericardial folds fuse to the surrounding of the heart and are gonna become the pericardial sac.

02:08 And that's one reason we find the phrenic nerve on the left and right in the wall of the pericardial sac.

02:14 It's because it got pushed there as the lungs expanded around it.

02:18 So as the lungs developed from the foregut, they enlarge, expand in the pleural cavity, and then, push the pleuropericardial folds ahead of them.

02:27 This will essentially sculpt the pleural space out of the body wall and push the pleuropericardial folds up against the heart.

02:36 And the pleuropericardial folds will create the outer fibrous lining of the pericardium and that's one reason you find the phrenic nerve on both the left and right travelling inferiorly through the pericardium to get to the heart.

02:50 As the lungs expand into the pleural cavity, they're covered by a thin membrane which is gonna be the visceral pleura of the lungs.

02:58 Congenital pericardial defects are believed to be defective formation or fusion of the pleuropericardial membranes. They are usually more common on the left side. A communication between the pericardial cavity and the pleural cavity is seen. In more severe cases, intermittent herniation of the atrium into the pleural cavity might be observed with each heartbeat. Thank you very much for your attention and I'll see you on our next talk.