The diaphragm is formed form a number of composite parts in the embryo. The most important is the septum transversum. Understanding the history of the formation of the diaphragm explains why the heart, lungs, liver, gut tube, neck and fascia all resonate strongly with the diaphragm. Clinical points that arise from the embryology are summarised at the end.
The septum transversum is a thick mass of cranial mesenchyme that gives rise to parts of the thoracic diaphragm and the anterior mesentery of the foregut in the adult. After its descent, discussed below, the septum transversum merges with mesoderm surrounding the oesophagus, the growing pleura and peritoneum (‘pleuroperitoneal folds’) and the growing muscles of the abdominal wall.
The septum transversum originally arises as the most superior part of the mesenchyme on day 22. During head folding, due to the massive growth of the neural tube, it is folded underneath the developing heart at the level of the cervical vertebrae. At this stage the septum transversum picks up innervation from the adjacent ventral rami of spinal nerves C3, C4 and C5, thus forming the precursor of the phrenic nerve.
During subsequent weeks the posterior of the embryo grows much faster than its anterior counterpart resulting in an “apparent descent” of the anteriorly located septum transversum. At week 8 it can be found at the level of the thoracic vertebrae. During the descent of the septum, the phrenic nerve is carried along and assumes its descending pathway. In the adult the whole diaphragm is innervated by the paired phrenic nerves; ‘C3,4,5 keep the diaphragm alive’.
During embryonic development of the thoracic diaphragm, myoblast cells from the septum invade the other components of the diaphragm. They thus give rise to the motor and sensory innervation of the muscular diaphragm by the phrenic nerve.
The superior part of the septum transversum gives rise to the central tendon of the diaphragm and is the origin of the myoblasts that invade the pleuroperitoneal folds resulting in the formation of the muscular diaphragm.
The inferior part of the septum transversum is invaded by the growing liver, budding from the gut tube. These hepatic diverticulum divide within the septum transversum to form the liver and thus gives rise to the ventral mesentery of the foregut, which in turn is the precursor of the lesser omentum, the visceral peritoneum of the liver and the falciform ligament.
Summary of some clinical points that arise from the embryology:
- The growing diaphragm drags the phrenic nerves down from the cervical region to the thoracic region. Irritation to the diaphragm frequently presents as neck and shoulder pain due to facilitation in the cervical cord induced by the phrenic nerves. For example, rupture of the spleen or gallbladder issues are well known to cause referred pain to the shoulders. The reverse is also true, neck issues can cause issues with the diaphragm.
- The diaphragm is intimately linked with the heart. They are very early structures that form from the same area of mesenchyme at the cranial end of the embryonic disc. The pericardium is firmly attached to the diaphragm in the adult and the heart moves up and down with diaphragmatic breathing.
- The diaphragm is intimately linked with the liver. The liver grows underneath the septum transversum and parts of the septum transversum differentiate into the membranes around the liver.
- The septum transversum is a reference centre and meeting point for the embryological development of the cavities and their fascial coverings. The diaphragm is considered to be the natural fulcrum for fascia in the craniosacral paradigm. The septum transversum position as the most superior mesechyme, before the folding of the nerual tube, speaks to its early nature as a reference point for the generation of tissues.
- The septum transversum merges with tissues that form the pleura and the peritoneum. It follows that in the adult this shared history means that changes in the diaphragm will affect the function of the lungs and the gut tube.
References accessed Mar 2010