Diaphragm and Body Cavities
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Direct answer
The embryonic body cavities begin as coelomic spaces in the lateral plate mesoderm that fuse into a horseshoe-shaped intraembryonic coelom in week four: a pericardial cavity cranially, paired pericardioperitoneal canals dorsal to the septum transversum, and a peritoneal cavity caudally. The diaphragm then closes the canals from four sources — the septum transversum, pleuroperitoneal membranes, dorsal mesentery of the oesophagus and body wall. Failure of the left pleuroperitoneal membrane produces the classic posterolateral (Bochdalek) congenital diaphragmatic hernia.
What you must remember
- Coelom formation: lateral plate mesoderm splits into somatic and splanchnic layers; the cavity between them is the intraembryonic coelom, lined by mesothelium, which becomes the serous membranes.
- Partitioning: the septum transversum forms a partial barrier between pericardial and peritoneal cavities, leaving the pericardioperitoneal canals on either side; pleuroperitoneal membranes close these canals during week seven, the left closing last.
- Diaphragm sources: central tendon from the septum transversum; posterolateral muscle from the pleuroperitoneal membranes; crura and the mesentery around aorta and oesophagus from the dorsal mesentery; peripheral rim from the body wall.
- Descent: the diaphragm forms opposite the cervical somites — hence its phrenic nerve supply from C3, C4 and C5 — and descends as the thorax grows, carrying its nerve with it.
- Congenital hernias: Bochdalek — large posterolateral defect, usually left-sided, presenting with respiratory distress and bowel sounds in the chest, pulmonary hypoplasia being lethal; Morgagni — small retrosternal defect, often silent; hiatus hernia through the oesophageal hiatus.
- Adult openings: inferior vena cava at T8 in the central tendon, oesophagus with vagal trunks at T10, and aorta with thoracic duct and azygos vein at T12.
- Peritoneal cavity: the gut stays suspended on its dorsal mesentery and gains ventral mesentery only at stomach, duodenum and liver, where the lesser and greater sacs are defined.
Common confusion
Students give the diaphragm a single source; it has four, and each explains a part or a defect — septum transversum for the central tendon, pleuroperitoneal membranes for the Bochdalek hernia, dorsal mesentery for the crura, body wall for the rim. The side matters: the left canal closes last and herniates most. The phrenic nerve's cervical origin is the standard viva hook for the descent of the diaphragm.
Exam-focused takeaway
Theory questions ask for the intraembryonic coelom, derivatives of the septum transversum, the four sources of the diaphragm and the embryological basis of Bochdalek and Morgagni herniae. In the viva, explain the phrenic nerve level and name the structures at T8, T10 and T12. Practicals include a diaphragm specimen and imaging of congenital hernia.
Frequently asked questions
What are the pericardioperitoneal canals?
Paired passages between the pericardial and peritoneal cavities on either side of the foregut, closed during week seven by the pleuroperitoneal membranes.
From which sources does the diaphragm develop?
The septum transversum (central tendon), pleuroperitoneal membranes (posterolateral muscle), dorsal mesentery of the oesophagus (crura) and the body wall (peripheral rim).
Why is the phrenic nerve rooted at C3 to C5?
Because the diaphragm forms opposite the cervical somites before descending to the thorax; its myoblasts carry the cervical nerve supply downwards.
What is a Bochdalek hernia?
A posterolateral congenital diaphragmatic hernia from failure of the pleuroperitoneal membrane, usually left-sided, allowing bowel into the thorax and causing respiratory distress with pulmonary hypoplasia.
What passes through the oesophageal hiatus at T10?
The oesophagus, anterior and posterior vagal trunks and oesophageal branches of the left gastric vessels — a common site of hiatus hernia.