Overview
This lecture covers the structure, development, nerve supply, and common congenital abnormalities of the diaphragm, with emphasis on key components and embryological origins.
Structure and Orientation of the Diaphragm
- The diaphragm is the primary muscle of respiration, separating the thoracic and abdominal cavities.
- Major parts include a central tendon, muscular portion, right and left crura, and openings for the inferior vena cava, esophagus, and aorta.
- The diaphragm is a musculotendinous organ.
Developmental Origins of the Diaphragm
- Four main embryological sources contribute to diaphragm development:
- Septum transversum (forms central tendon)
- Dorsal mesentery of esophagus (gives rise to crura)
- Pleuroperitoneal membranes (major fetal contribution, closes pleuroperitoneal canals)
- Mesoderm of lateral body wall (main adult posterior contribution)
- Pleuroperitoneal canals connect pleural and peritoneal cavities before closure.
- Pleuroperitoneal membranes close these canals to separate cavities during fetal life.
- Myoblast invasion into dorsal mesentery of esophagus forms the crura.
Nerve Supply and Migration
- The initial position of septum transversum is in the cervical region (C3, C4, C5), hence diaphragm’s motor supply is via the phrenic nerve.
- As development proceeds, the diaphragm descends to a position between T7 and T12.
- Sensory supply comes from lower intercostal nerves due to the contribution of lateral body wall mesoderm.
Congenital Abnormalities
- Congenital diaphragmatic hernia (Bochdalek hernia): Left-sided defect; abdominal contents enter thorax, causing lung hypoplasia; left side is more common since it fuses later.
- Retrosternal (Morgagni) hernia: Defect near the sternum; abdominal contents herniate into thorax anteriorly.
- Congenital hiatal hernia: Enlarged esophageal opening leads to herniation of the stomach; often presents as acquired later in life.
- Eventration of diaphragm: Thinned, membranous diaphragm (not absent); allows abdominal contents to push into thorax, more often on left.
Key Terms & Definitions
- Diaphragm — main respiratory muscle separating thoracic and abdominal cavities.
- Septum transversum — embryological structure forming central tendon.
- Pleuroperitoneal membrane — fetal membrane closing the pleuroperitoneal canal.
- Crura of diaphragm — muscular extensions formed from myoblast invasion into dorsal mesentery of esophagus.
- Phrenic nerve — supplies motor innervation to diaphragm (C3, C4, C5).
- Bochdalek hernia — left-sided congenital diaphragmatic hernia.
- Morgagni hernia — hernia near sternum through foramen of Morgagni.
- Eventration — thinning of diaphragm with preserved membrane.
Action Items / Next Steps
- Review diagrams showing the embryological development of the diaphragm and its components.
- Memorize the four developmental sources and their adult contributions.
- Study key congenital abnormalities and their clinical presentations.