Development of the Coelom
By definition, a coelom is a space lined with mesoderm. The mesothelial linings of the body cavities and outer coverings of visceral organs come from the somatic and splanchnic layers of lateral plate mesoderm, respectively.
At day 13 of development, the trophoblast cavity is lined by extraembryonic mesoderm, converting it into an extraembryonic coelom (aka chorionic cavity).
The somatic layer of extraembryonic mesoderm lines the trophoblast and the splanchnic layer of extraembryonic mesoderm covers the yolk sac and amnion.
A mucus plug seals the implantation site.
Day 13 - Implantation
After gastrulation, splitting of the lateral plate mesoderm creates the intraembryonic coelom.
Initially, the intraembryonic and extraembryonic coeloms are one continuous space.
Folding of the embryo in the sagittal and transverse planes separates the intraembryonic coelom form the extraembryonic coelom.
This folding narrows the connection between the gut and yolk sac as well as incorporating them into the umbilical cord along with the allantois.
This animation demonstrates how folding of the embryo in the transverse plane, incorporates the intraembryonic coelom into the body of the embryo, and separates it from the extraembryonic coelom.
Diaphragm
After folding of the embryo is complete, the intraembryonic coelom is divided into a thoracic cavity superiorly and an abdominal cavity inferiorly by the presence of a sheet of mesoderm, septum transversum.
The thoracic and abdominal cavities communicate with one another by a pair of pericardioperitoneal canals on either side of the gut tube and posterior to septum transversum.
Septum transversum makes a significant contribution to the development of the respiratory diaphragm.
Septum transversum begins development in the cranial end of the embryonic disc. As a result, the phrenic nerve that innervates it is composed of fibers fromvcervical spinal nerves, C3, C4, C5.
Cranial and caudal folding of the embryo displaces septum transversum caudally to occupy a position between the developing heart and liver.
Initially, pericardioperitoneal canals connect the thoracic and abdominal cavities posterior to septum transversum.
These are ultimately closed by the formation of pleuroperitoneal membranes.
Roll over the image
Ultimately, the diaphragm arrises from four primordia:
- septum transversum - forms the anterior part and much of the central tendon
- pleuroperitoneal membranes - complete the central tendon and close the pericardioperitoneal canals
- mesentery of the esophagus - form the crura
- myoblasts from mesoderm of the body wall - to form the posterior perimeter
Development of the Thoracic Cavity
The thoracic cavity is the portion of the intraembryonic coelom superior to the septum transversum.
During the 5th week of development, it becomes subdivided into a pair of pleural cavities, each containing a lung and a midline pericardial cavity containing the heart.
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Partitioning of the thoracic cavity is dependent upon the development, growth and fusion of a pair of membranes that arise from the lateral body wall.
Pleuropericardial folds, each containing a phrenic nerve and a common cardinal vein, are dissected from the lateral body wall by the cranial, caudal and lateral growth of the lung buds.
Eventually they are the pleuropericardial membranes.
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Splanchnic layer of lateral plate mesoderm over the lungs becomes mesothelium of the visceral pleura, while the somatic layer of lateral plate mesoderm, adherent to the body wall becomes the parietal pleura.
The pleuropericardial membranes approach one another in the midline until they fuse isolating the heart within the pericardial cavity and the lungs within the pleural cavities.
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As a result of fusion of the pleuropericardial folds, the phrenic nerves course between the mediastinal pleura, and the fibrous pericardium. The common cardinal veins fuse to form the superior vena cava.
Each lung is covered by a layer of visceral pleura and parietal pleura lines each pleural cavity.
Until completion of the diaphragm by growth of the pleuroperitoneal membranes, each pleural cavity is continuous with the peritoneal cavity through the pericardioperitoneal canals.
Development of the Peritoneal Cavity
The abdominopelvic cavity is the space inferior to the diaphragm. Like the thoracic cavity, the abdominal cavity is lined by a moist mesothelium derived from the somatic and splanchnic layers of lateral plate mesoderm. In the case of the abdomen, this membrane is called peritoneum. Parietal peritoneum lines the walls of the cavity and visceral peritoneum covers the outer surfaces of the organs contained within the abdomen.
Double folds of peritoneum, called mesenteries, attach the visceral organs to the anterior (ventral) and posterior (dorsal) body wall.
Some abdominal organs possess mesenteries. Such organs are said to be intraperitoneal because they have relative freedom of movement within the abdominal cavity.
Some of the organs on the posterior wall of the abdomen are covered by peritoneum only on their anterior surfaces. These are said to be retroperitoneal to describe their relatively fixed position on the body wall.
The greater momentum is a double fold of mesentery (quadruple fold of peritoneum) that forms as a result of gut rotation. Its role in development will be explored in greater detail in a later module.
Sagittal section through the abdominopelvic cavity of an adult showing the locations of the parietal and visceral peritoneal layers
There is a dorsal mesentery that suspends the gut tube from the posterior abdominal wall and a ventral mesentery that anchors it to the anterior body wall.
The ventral mesentery disappears from all but the foregut region as the ventral mesogastrium.
The dorsal mesentery is named for the regions of the gut tube with which it is associated.
- Dorsal mesogastrium - stomach
- Dorsal mesoduodenum - duodenum
- Mesentery 'proper' - jejunoileum
- Dorsal mesocolon - colon
As the developing organs of the abdomen come to their final locations, some parts of the dorsal mesentery will be absorbed.
Initially, the gut tube is suspended in the midline from the posterior body wall. Following rotation of the stomach, the liver is pushed to the right side of the abdomen and the stomach is pushed to the left. As a result, a small portion of the peritoneal cavity is located behind the stomach. It is called the lesser sac or omental bursa. The larger space within the peritoneal cavity, anterior to the stomach and liver, is called the greater sac.
Knowledge of these subdivisions is important in pathologies of organs that are located behind the stomach in the omental bursa. in particular, the pancreas, duodenum and aorta.
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Start Slide Show
Initially the abdominal cavity is a single continuous space referred to as the greater peritoneal sac or just greater sac.
As the gut tube develops and rotates, the liver is cast to the right side of the abdomen and the stomach to the left. The spleen and pancreas, developing in the dorsal mesentery are also pushed to the left side of the abdomen.
Before rotation of the stomach
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At the end of gut rotation, a small portion of the greater peritoneal cavity is isolated posterior to the stomach, thus forming the lesser sac. The natural communication between greater and lesser sacs is the epiploic foramen (of Winslow),.
Also a result of gut rotation, the pancreas and portions of the GI tract are pushed against the posterior abdominal wall with absorption of one side of the mesentery, fixing them into a retroperitoneal position.
After rotation of the stomach
Other parts of the mesentery form peritoneal ligaments between visceral organs as a means through which blood vessels can reach them. Two such ligaments, the splenorenal and gastrosplenic ligaments are shown.
Clinical Correlates
Congenital Diaphragmatic Hernia
Congenital diaphragmatic hernia (CDH) is a relatively common anomaly of the neonate, occurring is approximately 1/2000 births. It is usually due to failure of the pleuroperitoneal folds to close off the pericardioperitoneal canals.
In the case presented here, an ultrasound at 23 weeks gestation revealed the presence of abdominal viscera in the left thorax. These included stomach and a small part of the large intestine and left lobe of the liver. The MRI shows the fetus with hypoplastic right and left lungs and stomach in the left thorax. The bright signal in the lumen of the stomach is amniotic fluid swallowed by the fetus. The authors of the case provided information on estimated functional lung capacity and project a chance of mortality at <10%, suggesting a favorable prognosis.
A larger hernia wouldi be associated with a high rate of mortality, about 75%, from pulmonary hypoplasia and dysfunction of the system.
For more details of the case and additional imaging it is recommended to visit the link below.
