Coelom & Mesentery

• Ventral cavity

  The larger of the two body cavities, subdivided into the thoracic, abdominal and pelvic cavities
• Dorsal cavity

  The smaller of the two body cavities, divided into the cranial cavity (upper part which houses the brain) and vertebral cavity (lower part containing the spinal cord)
• Intraembryonic coelom

  Soon after the formation of the 3 bands of mesoderm, small cavities – known as the intra-embryonic coelom – will appear in the lateral plate mesoderm.

  >Coelom means cavity, and this intraembryonic coelom will form the body cavities.
• Lateral folding of the embryo

  1. Endoderm-part of the yolk sac gets incorporated into the embryo

  2. Intra-embryonic coelom becomes continuous with the extra-embryonic coelom

  3. Amniotic cavity expands while yolk sac gets smaller

  4. Edges of the amniotic cavity fuse at the midline
• Partitioning of the body cavities

  1. Septum transversum partially separates the thoracic and abdominal cavities

  2. Two pericardio-peritoneal canals posterior to the septum transversum on either side of the oesophagus remain between the cavities

  3. Partitions form to separate the pericardial cavity from the pleural cavities and the pleural cavities from the peritoneal cavity will form concurrently on the two pericardio-peritoneal canals.
• Formation of the diaphragm

  The diaphragm is a dome-shaped, musculotendinous partition that separates the thoracic and abdominal (peritoneal) cavities.

  
• Innervation of the diaphragm

  • During the 4th week of development, the septum transversum is positioned opposite the 3rd, 4th & 5th cervical somites.
  • During the 5th week of development, myoblasts (which form muscle) will migrate from the cervical somites into the developing diaphragm and bring nerve fibres with them.
  • Therefore, the phrenic nerve (which supplies the motor innervation to the diaphragm) arises from the 3rd, 4th & 5th cervical spinal nerves
• Mesenteries

  Double layers of peritoneum that enclose an organ and connect it to the body wall

  • Such organs are called intraperitoneal, whereas organs that lie against the posterior body wall and are covered by peritoneum on their anterior surface, such as kidneys, are called retroperitoneal.
• Types of mesenteries

  • Dorsal mesogastrium (greater omentum) in the region of the stomach.
  • Dorsal mesoduodenum in the region of the duodenum
  • Dorsal mesocolon in the region of the colon
  • Mesentery proper (of jejunal and ileal loops)
  • Ventral mesentery (of terminal esophagus, stomach, upper duodenum)
• Mesenteries provide pathways for vessels, nerves, and lymphatics to and from abdominal viscera
• Abnormalities of the diaphragm

  • Agenesis of the diaphragm
  • Posterior hiatus (of Bochdalek)
  • Parasternal hiatus (of Morgagni)
• Agenesis of the diaphragm

  Congenital diaphragmatic developmental anomaly where all or part of diaphragm fails to form
• Posterior hiatus (of Bochdalek) AKA Bochdalek hernias

  Congenital diaphragmatic defect resulting from failure of posterolateral diaphragmatic foramina to fuse, allowing abdominal organs to protrude into thoracic cavity

  >Occur more commonly on the posterior left side

  >Symptomatic Bochdalek hernias in adults are infrequent and may lead to GIT dysfunction or severe pulmonary disease.
• Parasternal hiatus (of Morgagni)

  Anterior defect of the diaphragm characterized by herniation of abdominal contents through the foramina of Morgagni which are located immediately adjacent and posterior to the xiphoid process of the sternum
• Ventral cavity cont.

  While the abdominal cavity is continuous with the pelvic cavity, it is separated from the thoracic cavity by a diaphragm.

  • The thoracic cavity itself is divided into 3 parts: the pericardial (containing the heart) cavity and two pleural cavities (containing the lungs).
• Division of intra-embryonic coelom

  The intra-embryonic coelom will divide the lateral plate mesoderm into two types of mesoderm:

  • The mesoderm adjacent to the ectoderm is known as somatic mesoderm
  • The mesoderm adjacent to the endoderm is known as splanchnic mesoderm.
• Pleuropericardial folds

  • When the lungs form, they will grow inside the pericardio-peritoneal canals and with further growth, the lungs will expand into the mesenchyme of the body wall.
  • As the lungs expand, the mesenchyme of the body will give rise to two extensions known as the pleuropericardial folds. At first, these folds appear as small ridges which project into the undivided thoracic cavity.
• Pleuro-pericardial membrane

  As the pleuropericardial folds enlarge, they form pleuropericardial membranes, which will fuse in order to separate the pericardial cavity from the pleural cavities.

  • Pleuroperitoneal membranes contain the common cardinal veins.
  • In the adult the pleuropericardial membranes form the fibrous [layer of the] pericardium
• The diaphragm develops from the fusion of four embryonic components:

  1. Septum transversum
  2. Pleuroperitoneal membranes
  3. Dorsal mesentery of esophagus
  4. Muscular ingrowth from lateral body walls
• Descent of the diaphragm

  The dorsal part of the embryo rapidly grows, resulting in a positional change of the diaphragm.

  • The diaphragm appears as if it is descending in a caudal direction [‘the descent of the diaphragm’].
  • By the 6th week, the diaphragm is at the level of the thoracic somites.
  • When the diaphragm migrates caudally, the nerves supplying it will also lengthen.
  • By the beginning of the 8th week, the dorsal part of the diaphragm lies at L1, and the phrenic nerve will lengthen considerably to approximately 30 cm in the adult.
• Phrenic nerve

  The primitive phrenic nerves will enter the diaphragm by passing through the pleuropericardial membranes.

  >This explains why the phrenic nerves will subsequently lie on the fibrous layer of the pericardium
• Development of the diaphragm: Septum transversum

  o Forms at the end of the third week as a mass of mesodermal tissue cranial to the pericardial cavity.

  o After the head folds ventrally during the 4th week, it forms a thick incomplete partition between the pericardial and abdominal cavities.

  o It does not separate the thoracic and abdominal cavities completely.

  o There are large openings, the pericardioperitoneal canals, along the sides of the esophagus

  o It expands and fuses with the dorsal mesentery of the esophagus and the pleuroperitoneal membranes
• Development of the diaphragm: Pleuroperitoneal membranes

  o These membranes appear in the lateral walls of the pericardioperitoneal canals as pleuroperitoneals folds.

  o As the pleuroperitoneal folds enlarge they become pleuroperitoneal membranes.

  o These membranes fuse with the dorsal mesentery of the esophagus and the septum transversum.

  o This completes the partition between the thoracic and abdominal cavities and forms the primordial diaphragm.
• Development of the diaphragm: Dorsal Mesentery of the Esophagus

  o Septum transversum and pleuroperitoneal membranes fuse with the dorsal mesentery of the esophagus.

  o This mesentery forms the median portion of the diaphragm.

  o The crura of the diaphragm develops from myoblasts that grow into the dorsal mesentery of the esophagus.
• Development of the diaphragm: Muscular ingrowth from lateral body walls

  o Between weeks 9 and 12, the lungs and pleural cavities enlarge as they extend into the lateral body walls and split body-wall tissue into two layers:

  >External layer = part of the definitive abdominal wall; Internal layer: contributes to peripheral parts of the diaphragm, external to the parts derived from the pleuroperitoneal membranes.

  o Further extension forms the right and left costodiaphragmatic recesses; establishing characteristic dome-shaped shape of the diaphragm
• Peritoneal ligaments

  Double layers of peritoneum (mesenteries) that pass from one organ to another or from an organ to the body wall.
• Dorsal mesentery

  At first, the foregut, midgut, and hindgut are in contact with the mesenchyme of the posterior abdominal wall.

  >By the 5th week of development connecting mesenchyme has narrowed.

  >The caudal part of the foregut, the midgut, and a major part of the hindgut are suspended from the abdominal wall by the dorsal mesentery.

  >This mesentery extends from the lower end of the esophagus to the cloacal region of the hindgut.