Embryonic Body Cavity Development
Body Cavities, Mesentery, and Diaphragm
The Embryonic Body Cavity
The intraembryonic coelom becomes the embryonic body cavity, which gives rise to three coelomic body cavities during Week 4:
- Pericardial cavity
- Two pericardioperitoneal canals
- Peritoneal cavity
These cavities are lined with mesothelium: a parietal wall and a visceral wall covered with mesothelium. The peritoneal cavity is connected to the extraembryonic coelom at the navel. The pericardial cavity opens to the pericardial-peritoneal canals.
Mesenteries
Peritoneum is a bilayer that begins as an extension of the visceral peritoneum covering an organ. The mesentery connects the organ with the body wall and contains the vessels and nerves leading to it.
Division of the Embryonic Body Cavity
The septum transversum forms the primordium of the central tendon of the diaphragm. Pericardioperitoneal canals form partitions separating the pericardial cavity from the pleural cavities and these from the peritoneal cavity.
As a result of lung bud growth to pericardioperitoneal canals, a few membranous flanges occur on the side wall of each channel:
- Pericardial folds: head flanges are located above the developing lungs.
- Pleuroperitoneal folds: flanges are located in an internal position to the lungs.
Pericardial Membranes
The pericardial folds increase in size and form partitions called pericardial membranes, containing the common cardinal veins. As the pleural cavity around the heart is exposed, it extends into the body wall, dividing the mesenchyme into:
- An outer layer that becomes the chest wall
- An inner layer which is the fibrous pericardium
The primitive mediastinum consists of a mass of mesenchyme that extends from the sternum to the spine and spreads to the lungs. In addition, development is formed by the junction of the pericardial membrane fusion with the mesenchyme.
Pleuroperitoneal Membranes
Pleuroperitoneal folds are membranous and form peluroperitoneal membranes. These membranes separate the pleural cavities from the peritoneal cavity. Myoblast migration into the pleuroperitoneal membranes collaborates with the closing of pleuroperitoneal communications.
Development of the Diaphragm
Diaphragm: dome-shaped musculotendinous division separating the thoracic and abdominopelvic cavities. It is composed of:
- Septum transversum
- Pleuroperitoneal membranes
- Dorsal mesentery of the esophagus (middle portion of the diaphragm)
- Muscle growth that separates chest and abdominal cavities
Pleuroperitoneal membranes fuse with the dorsal mesentery of the esophagus and the septum transversum, completing the division between the thoracic and abdominal cavities, and forming the primordial diaphragm.
The tissues of the body wall are divided into two layers:
- An outer layer that is part of the final abdominal wall.
- An inner layer that participates in the formation of the peripheral portions of the diaphragm.
Changes in Position and Innervation
- Week 4: The transverse septum is in the 3rd, 4th, and 5th cervical somites.
- Week 5: Myoblasts migrate to the diaphragm, along with nerves.
- Innervation: Phrenic nerves (C3, C4, C5)
- Migration of the diaphragm is due to the rapid growth of the embryo dorsally.
- Week 6: Diaphragm at the level of the thoracic somites.
- Phrenic nerves continue to decline.
- Merging of the parts of the diaphragm: the transverse septum mesenchyme forms myoblasts.
Eventration of the diaphragm is the superior displacement of the viscera to the diaphragm bag.
Gastroschisis and congenital epigastric hernia result from the lack of complete fusion of the body folds when forming the anterior abdominal wall during folding along the 4th week.