Septum transversum

It is a thick mass of mesoderm which partially separates thoracic cavity and abdominal cavity.

• Forms in neck by fusion of 3, 4, 5 cervical myotomes
• Motor nerve is phrenic nerve
• Embryonic disc folds and heart descends, therefore septum transversum is pushed caudally and pulls the phrenic nerve with it

Derivatives:

1. Superior layer – formation of fibrous pericardium
2. middle layer – diaphragm muscle, central tendon, diaphragmatic pleura and peritoneum
3. Inferior layer – fibrous capsule and connective tissue of liver, ventral mesentery of the gut

Diaphragm

Origin: Mesoderm

Diaphragm develops from:

1. Septum transversum – Central tendon, sternal and costal parts of diaphragm
2. 2 pleuro-peritoneal membranes – 2 mesodermal folds that project inwards from body wall. Close pleuro-peritoneal canals. Forms dorsilateral part of diaphragm
3. Mesoderm from chest wall – marginal part of diaphragm
4. Mesentery of esophagus – Posterior medial part and crura of diaphragm
5. Mesoderm around aorta – lumbar part of diaphragm

Congenital anomalies:

1. Parasternal hernia of Morgagni – failure to develop a small part of diaphragm between sternal and costal part
2. Esophageal hernia – Protrusion of stomach in thorax
3. Congenital diaphragmatic hernia of Bochdalek – failure of pleuro-peritoneal membranes to close the pleuro-peritoneal canals. Abdominal vicera enter pleural cavity, compress heart and lungs