Diaphragm structure + embryology

Morphology and major parts:

Right dome locates 5th rib (liver makes right side diaphragm higher lvl), Left dome at 5th ICS. 3 main parts exist which area sternal, costal, and lumbar parts. It looks front convex because lumbar part contains IVC and descending aorta so it has no choice to be bulky center.

Muscular parts:

  1. Median arcuate lig. has aortic hiatus (T10) and connected medial, lateral arcuate ligs.
  2. Medial arcuate lig. is thickening fascia covering Psoas Major muscle and its range is from tip of the lumbar vertebrae transverse process to body of L1.
  3. Lateral arcuate lig. covers Quadratus Lumborum muscle and its range is from L12 transverse process to 12th rib.

Aperture:

  1. Caval opening (T8~9) : locates at central tendon and contains IVC, terminal branch of right phrenic nerve and lymphatic vessels from liver to mediastinal lymph and middle phrenic. (Caval: relating to Vena Cava. and caval op. is loca. for the central lymph system and right phrenic nerve)
  2. Esophageal hiatus (T10) : locates at right crus. As esophagus turn left into the stomach, opposite side, right sided crus has to fix the esophagus direction. Since it is muscle, it function as muscle sphincter of esophagus. It contains (ant, post) vagal trunk because this esophageal hiatus is most centered foramen and vagus nerve comes down through the center, and esophagus is linked to the left side stomach, esophageal branch of Left gastric vein passes through this esophageal hiatus. It also has few lymphatic vessels (all foramen is passage of nearby nerve.
  3. Aortic hiatus (T12) : Since aorta doesn't pierce the diaphragm(very posterior foramen of diaphragm), blood flow is not disturbed by the diaphragm movement. Since thoracic duct crawls down from left clavicle to the middle of the vertebrae, it pass through this aortic hiatus. Sometimes, azygos and hemiazygos pass through this hiatus but usually it ends at the posteriorphrenic recess. Lymphatic vessels from the diaphragm surface of the liver and superior gastric vessel

Four embryonic components :

  1. Septum transversum: It is composed of mesodermal (since it's skin between the dermis) tissue, forming central tendon of the diaphragm. It grows dorsally from the ventrolateral body wall and separates heart and liver(pericardial cavity, peritoneal cavity). During its development, a large surface of the liver is embedded in the septum transversum. It is identifiable during the 3rd week in the form of large mass of mesodermal tissue cranial to the pericardial cavity and thick incomplete partition between pericardial and abdominal cavity during the 4th week (large opening at this point is pericardioperitoneal canal) Finally septum transversum fuses with dorsal mesentary of the esophagus and pleuroperitoneal membrane.
  2. Pleuroperitoneal membrane: It completes the partition between thoracic and abdominal cavity and forms the primordial diaphragm. The size of the pleuroperitoneal membrane take a larger portion than that in fetal diaphragm.
  3. Dorsal mesentery of esophagus: It constitutes the median portion of the diaphragm. The crura of the diaphragm is grown from the myoblast which grows into the dorsal mesentery of the esophagus (in short, esophageal mesentery). It explains how the right and left crura muscle exists in between the parts of the diaphragm.
  4. Muscular ingrowth from lateral body walls: This process is quite late step relative to other parts, which occurs 9th~12th week because the process occurs as the lung and pleural cavity is getting larger. During the process, lateral body wall tissue splits into two layers which are external layer becoming part of the definitive abdominal wall and internal layer which contributes to peripheral part of the diaphragm (external to the pleuroperitoneal membrane). This two layers explain why the sensory innervation of the costal rim of diaphragm (lower intercostal nerve) and other parts of diaphragm (phrenic nerve, this is also motor nerve of the diaphragm). Further extension of the developing pleural cavities into the lateral body walls forms the costodiaphragmatic recess.

Positional change and Innervation of the diaphragm :

  • During the 4th week, septum transversum lies opposite to the 3rd to 5th cervical somties.
  • During the 5th week, myoblasts from these somties migrate into the developing diaphragm and bringing their nerve fibers (phrenic nerve supplying motor and sensory neuron, which senses superior, inferior domes of the diaphragm, arises from the ventral primary rami of from the 3rd to 5th cervical spinal nerves)
  • By the 6th week, developing diaphragm locates at the level of thoracic somties (which explains why the phrenic nerves have descending course from the top whose length in adult is about 30cm long)
  • By the beginning of the 8th week, the dorsal part of the diaphragm lies at the level of L1
  • The embryonic phrenic nerve enters the diaphragm by passing through the pleuropericardial membrane (Important! : pleuropericardial membrane(more dorsal to the pleuroperitoneal membrane) is different from the pleuroperitoneal membrane; Moore 193pg) and it explains why the phrenic nerves subsequently lie on the fibrous pericardium which is derived from the pleuropericardial membrane.

Credits:

Created with images by Dreaming in the deep south - "diaphragm"

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