3D Body Cavities Embryology Part 1: Embryonic Folding - Pericardioperitoneal canals

In this video Dr. Aizaz from MedicoVisual talks about Embryogeneiss of Body cavities using 3D graphics. Body cavities include pericardial cavity, pleural cavity and peritoneal cavity

00:00 What is intraembryonic celom and how it develops during second and third week of early embryogenesis?
08:03 Craniocaudal folding of the embryo
10:03 Transverse folding of the embryo
13:26 How u-shaped intraembryonic celom is transformed to body cavities viz., pleural cavity, pericardial cavity and peritoneal cavity
16:20 Why is peritoneal cavity connected with extraembryonic celom at the umbilicus
18:46 What happens to septum transversum during embryonic folding
21:45 Complete overview of body cavities
22:54 Pericardial cavity, Peritoneal cavity and pericardioperitoneal canals
26:25 Origin of Septum Transversum

During the third week of development, the three germ layers are formed, viz., Ectoderm, mesoderm, and endoderm. The mesoderm is not a homogenous layer, but it gets divided into different components. The medial most is paraxial mesoderm, the lateral most is lateral plate mesoderm and in between them is the intermediate mesoderm.
Small spaces arise within the lateral plate mesoderm. These spaces then coalesce to form a larger, continuous cavity called intraembryonic celom. Thus, splitting the lateral plate mesoderm into two layers. The layer that underlies the ectoderm is intraembryonic somatopleuric mesoderm and the layer that covers the endoderm is intraembryonic splanchnopleuric mesoderm.
As you must be knowing, from the lecture of 2nd week of development (and if you don’t know please watch that lecture first), that the embryo with its amnion and yolk sac cavity is surrounded from the outside, by a mesoderm. Since this mesoderm is outside of the embryo proper, it is called EXTRA-Embryonic mesoderm. This mesoderm too, develops spaces that coalesce to form the celomic cavity termed as extraembryonic celom. No such space develops at the connecting stalk. Please note that the extraembryonic mesoderm that underlies that chorion is extraembryonic somatic mesoderm. The extraembryoic mesoderm’s layer that covers the amnion is also the somatic layer. However, one that covers the yolk sac is splanchnic layer.
Now the intraembryonic splanchnopleuric mesoderm is continuous with the extraembryonic splanchnopleuric mesoderm and similarly the intraembryonic somatopleuric mesoderm is continuous with the extraembryonic somatopleuric mesoderm.
The intraembryonic celom is an inverted U-shaped cavity that is peripherally continuous and connected to the extraembryonic celom, except at the cranial midline where the intraembryonic mesoderm is not split. Here, there is a block of tissue called septum transversum. There are two hypotheses for its origin. One is simple that here that intraembryonic mesoderm is not split between splanchnic and somatic and the other is that intraembryonic splanchnic mesoderm proliferates to form the septum transversum.
So intraembryonic and extraembryonic celom have wide communication peripherally except at the site of septum transversum. It is worth mentioning here that according to some well-known embryologists, it is only the distal part of the limbs of inverted U-shaped intraembryonic celom that is in direct connection with extraembryonic celom. The midline and the proximal part of the limbs are not connected to the extraembryonic celom. The midline part of the intraembryonic celom is the primordium of pericardial cavity while it’s limbs are the primordium of pleural and peritoneal cavities. And if you are wondering how is that so? Don’t worry it will make sense in a few minutes.
With the craniocaudal and lateral folding of the embryo, this intraembryonic celom is drastically remodeled.
With the cranial folding the, the central midline of inverted U-shaped intraembryonic celom is bend cranially in such a way that it lies in front of (or, you can say, ventral to) the foregut.
With the transverse of lateral folding, lateral margins of the intraembryonic celom comes to meet at the ventral midline. Here the fuse, except at the region of midgut. At the region of midgut, they cannot fuse due to presence of vitelline duct.