Embryonic Development of the Diaphragm

Tube within a Tube

• During weeks 3 and 4, the top ectoderm layer develops into the brain and spinal chord through neurulation.
• Simultaneously, the endoderm layer folds to create the gut tube.
• The embryo becomes a tube-within-a-tube structure
• The neural tube is positioned dorsally and the gut tube ventrally.
• The mesoderm layer secures the neural and gut tubes.
• The lateral plate of the mesoderm divides into the visceral (splanchnic) and parietal (somatic) layers.
• The visceral layer encloses the gut tube.
• The parietal layer, together with the ectoderm, gives rise to the lateral body wall folds to close the ventral body wall.
• The space between the visceral and parietal layers is the primitive body cavity. -This space is initially continuous without subdivisions into pericardial, pleural, and abdomino-pelvic regions.

Formation of the Body Cavity

• At the third week's end, the intraembryonic mesoderm develops into the paraxial, intermediate, and lateral plate mesoderm. -Paraxial mesoderm plays a key role in the formation of the skull and vertebrae. -Intermediate mesoderm contributes to the urogenital system. -Lateral plate mesoderm is crucial in forming the body cavity.
• Clefts arise in the lateral plate mesoderm and combine, dividing it into parietal and visceral layers.
• (1) the parietal (somatic) layer is next to the surface ectoderm and connects with the extra-embryonic parietal mesoderm across the amnion.
• Together, the parietal layer and the ectoderm are called the somatopleure.
• (2) the visceral (splanchnic) layer is next to the endoderm, which forms the basis of the gut tube, and connects to the visceral layer of extraembryonic mesoderm that covers the yolk sac.
• Together, the visceral layer and the endoderm form the splanchnopleure.
• The space between the two layers becomes the primitive body cavity.
• During the fourth week, the embryo begins to grow ventrally and forms two lateral body wall folds.

Further Body Cavity Development

• The folds consist of the parietal layer of lateral plate mesoderm, ectoderm, and cells that move from nearby somites into the mesoderm layer across the lateral somitic frontier.
• As the folds grow, the endoderm layer also folds ventrally to create the gut tube.
• By the fourth week's end, the lateral body wall folds meet and merge in the midline to shut the ventral body wall under the head and tail regions (folds), causing the embryo to curve into a fetal position.
• The ventral body wall closes completely, except where the connecting stalk exists to be the future umbilical cord.
• The gut tube also closes, but a connection from the midgut to the yolk sac persists.
• The Vitelline (yolk sac) duct is ultimately incorporated into the umbilical cord, narrows extensively, and degenerates along with the yolk sac between the second and third gestation months.
• The parietal and visceral layers of lateral plate mesoderm remain connected where the gut tube meets the posterior body wall during body cavity and gut tube development.

Serous Membranes

• Parietal layer cells of the lateral plate mesoderm turn into mesothelial cells, which form the parietal layer of the serous membranes around the peritoneal, pleural, and pericardial cavities.
• Visceral layer cells of lateral plate mesoderm form the Visceral layer and cover the abdominal organs, lungs, and heart.
• The visceral and parietal layers connect as the dorsal mesentery, suspending the gut tube into the peritoneal cavity from the posterior body wall.
• The dorsal mesentery extends from the end of the foregut to the hindgut.
• The ventral mesentery exists between the caudal foregut and the upper duodenum and arises from mesoderm thinning of the septum transversum.
• The septum transversum is a mesoderm block creating connective tissue in the liver and a the base the diaphragm
• The mesenteries are double peritoneum layers that allow blood arteries, nerves, and lymphatic vessels to reach the organs.

Diaphragm and Thoracic Cavity

• The septum transversum is a solid mesodermal tissue plate sitting in the area involving the thoracic cavity and stalk of the yolk sac.
• The septum develops visceral (splanchnic) mesoderm around the heart.
• The septum settles between the thoracic and abdominal cavities when the top end of the embryo curves.
• The septum only partly separates the thoracic and abdominal cavities, forming pericardioperitoneal canals on each side of the foregut.
• Lung buds grow caudolaterally inside the pericardioperitoneal canals.
• Due to lung development, the pericardioperitoneal canals shrink, so lungs grow into the body section mesenchyme on the dorsal, lateral, and ventral sides.
• Ventral and lateral growth occur behind the pleuropericardial folds.
• These pleuropericardial folds are small ridges projecting into the primitive thoracic cavity.
• As the lungs grow, mesoderm produces the definitive thorax wall and pleuropericardial membranes.
• Descent of the heart and sinus venosus shifts the cardinal veins, drawing the pleuropericardial membranes.
• They merge with each other and the lung roots, splitting the thoracic cavity into the pericardial cavity and pleural cavities.
• In adults, the fibrous pericardium results from pleuropericardial membranes.

Diaphragm Formation

• The pleural and pericardial cavities remain connected to the abdominal cavity through the pericardioperitoneal canals.
• During development, the opening between the pleural and peritoneal space closes partially by pleuroperitoneal folds at the pericardioperitoneal canals.
• In the seventh week, the folds thin into membranes that grow to merge with the esophagus mesentery (the diaphragm crura around the aorta.)
• The pleuroperitoneal membranes grow to coat the septum transversum completely.
• Cervical segments 3-5 (C3-C5) are where muscles originate for the diaphragm.
• The phrenic nerves arise from the ventral primary rami of spinal nerves innervating the diaphragm.
• Diaphragm development begins in the cervical fourth week.
• In the sixth week, differential embryo growth causes the diaphragm to move ventrally and caudally, resulting in phrenic nerves having a long rout.
• The two pleuroperitoneal membranes is what forms the central diaphragm tendon
• Muscular components are derived from somites at cervical segments three to five.
• The crura of the diaphragm are derived from the mesentery of the esophagus.

Questions cover the embryological origin of the diaphragm, including the crura, phrenic nerve innervation, pleuroperitoneal membranes, and differential growth. Also covers the formation of the gut tube and somatopleure.