Body Cavities (Embryonic)
From a tightly packed, flat trilaminar disc of cells the body cavities must form. This is initiated around 21 days in the lateral plate mesoderm, which splits into splanchnic and somatic divisions. Between these mesodermal divisions vacuoles form and merge creating a U‐shaped cavity in the embryo. This is the intra‐embryonic cavity (Figure 19.1) and initially has open communication with the extra‐embryonic cavity (or chorionic cavity).
When the embryo folds the connection with the chorionic cavity is lost resulting in a cavity from the pelvic region to the thoracic region of the embryo.
Of the two layers of lateral plate mesoderm that divided, a somatic layer lines the intra‐embryonic cavity and a splanchnic layer covers the viscera.
The septum transversum divides the cavity into two: the thoracic and abdominal (peritoneal) cavities. The division is not complete and there remains communication between these cavities through the pericardioperitoneal canals (Figure 19.2).
Membranes develop at either end of these canals. These membranes separate the thoracic cavity into the pericardial cavity and pleu- ral cavities and are called pleuropericardial folds (Figures 19.2 and 19.3). The folds carry the phrenic nerves and common cardinal veins and as the position of the heart changes inferiorly, the folds fuse. The pleuropericardial folds will formthefibrouspericardium (Figure 19.3).
The diaphragms of the septum transversum, pleuroperitoneal folds, some oesophageal mesentery and a little muscular ingrowth from the dorsal and lateral body walls (Figure 19.4).
The septum transversum originates around day 22 at a cervical level, but caudal to the developing heart. It receives innervation from spinal nerves C3–C5, the beginning of the phrenic nerve. With growth of the embryo the position alters to rest at the level of the thoracic vertebrae.
The septum transverum is a boundary between the abdominal cavity and the thoracic cavity. There are two connections between these cavities as mentioned above; the pericardioperitoneal canals. The pleuroperitoneal folds arise from the dorsal body wall and eventually close off the pericardioperitoneal canals and prevent communication between the abdominal and thoracic cavities.
The pleuroperitoneal folds fuse with the septum transversum, the oesophageal mesentery and the muscular ingrowth from the body walls to form the diaphragm. Muscle cells from the septum transversum and the body wall invade the folds forming the muscular part of the diaphragm (Figure 19.4). The septum transversum forms the central tendon and the mesentery of the oesophagus merges into the central tendon, thus allowing passage of the aorta, vena cava and oesophagus.
In a congenital diaphragmatic hernia, caused by a failure of the diaphragm to form completely, the abdominal contents herniate into the thoracic cavity negatively affecting lung development, leading to pulmonary hypoplasia and hypertension. Generally survival rates are about 50%, but if the liver is unaffected they are nearer 90%. Treatment involves mechanical ventilation and extracorporeal membrane oxygenation (ECMO) to perform gas exchange, and even a lung transplant has been successfully reported.
Gastroschisis is also a herniation of the bowel, but caused by an anterior abdominal wall defect, usually just to the right of the umbilicus. Viscera are not covered with peritoneum or amnion, and it is not associated with the same level of other abnormalities (unlike omphalocoele). Surgical is required and generally survival rates are good.