Chapter 48. Development of The Diaphragm - Review of Medical Embryology Book

The diaphragm is a musculotendinous, dome-shaped partition between the thoracic and abdominal cavities and develops from 4 major structures

THE SEPTUM TRANSVERSUM (most important component) forms the central tendon and is first seen as a thick mesodermal plate cranial to the pericardial cavity between the base of the thoracic cavity and the stalk of the yolk sac

The septum does not separate the thoracic and abdominal cavities entirely, but after the headfold forms (week 4), it becomes a thick incomplete partition between the cavities with an opening on each side of the gut, the pleural canals
The septum fuses dorsally with the primitive mediastinal mesenchyme below the esophagus and later with the pleuroperitoneal membranes

PLEUROPERITONEAL MEMBRANES fuse with the dorsal mesentery of the esophagus and with the dorsal part of the septum transversum to complete the partition between the thoracic and abdominopelvic cavities to form the primitive diaphragm. They represent only a small portion of the final adult structure
THE DORSAL ESOPHAGEAL MESENTERY (mesoesophagus) fuses with both A and This mesentery forms the median portion of the diaphragm. The crura of the diaphragm develop from muscle fibers which grow into the esophageal mesentery
THE BODY WALL: during weeks 9 to 12, the pleural cavities enlarge and invade the lateral body walls. Body wall tissue, at this time, splits off medially to form the peripheral parts of the diaphragm outside that formed by the membranes (B)

Extensions of the pleural cavities into the body walls form the costodiaphragmatic recesses

DURING WEEK 4, THE SEPTUM TRANSVERSUM lies opposite the upper cervical somites, and during week 5, nerves from the cervical spinal segments, C3, C4, and C5 grow into the septum and form the phrenic nerve. These nerves pass to the septum via the pleuropericardial membrane, thus, the nerves lie in the fibrous pericardium
RAPID GROWTH OF THE DORSAL EMBRYO BODY compared to its ventral part results in an apparent descent of both diaphragm and nerves, by week 6, to thoracic somite level
BY WEEK 8, the dorsal part of the diaphragm lies at the level of the first lumbar vertebrae, thus, its nerve has been carried down with it from the cervical region

CONGENITAL DIAPHRAGMATIC HERNIA: a common malformation in the newborn seen in 1/2200 births and usually as a posterolateral defect of the diaphragm

Usually results as a defective formation and/or fusion of the pleuroperitoneal membrane(s) which normally separate(s) the pleural and peritoneal cavities
Defect is usually unilateral with a large opening (foramen of Bochdalek) in the posterolateral part of diaphragm. It is seen more often on the left as a result of an earlier closure of the right pleuroperitonea1 opening
If the pleuroperitoneal membrane is not fused when the intestines return to the abdomen from the umbilical cord (week 10), the intestines may pass into the chest
Occasionally see stomach, spleen, cecum, appendix, and parts of colon in the chest cavity. If present at birth, may interfere with respiration
Heart and mediastinum are often displace Lungs are small and hypoplastic

CONGENITAL HIATAL HERNIA: rare; abdominal viscera herniate through a large esophageal hiatus or opening. Usually an acquired lesion seen in adult life
ESOPHAGEAL HERNIA: if esophagus is shorter than normal, part of stomach may appear in the thorax and be constricted as it passes through the enlarged esophageal hernia
RETROSTERNAL OR PARASTERNAL HERNIA (of Morgagni): a rare defect between sternum and sternocostal parts of diaphragm. A small peritoneal sac with intestinal loops often seen in chest
CONGENITAL EVENTRATION OF DIAPHRAGM: rare; half of diaphragm has defective muscles and balloons up into chest cavity. Upward displacement of abdominal contents