Early in the fourth week of development of the embryo, the intraembryonic coelom appears as a horseshoe-shaped cavity ( Fig. 8.1 A ). The bend in the cavity at the cranial end of the embryo represents the future pericardial cavity , and its limbs (lateral extensions) indicate the future pleural and peritoneal cavities . The distal part of each limb of the intraembryonic coelom is continuous with the extraembryonic coelom at the lateral edges of the embryonic disc (see Fig. 8.1 B ). The intraembryonic coelom provides room for the organs to develop and move. For instance, it allows the normal herniation of the midgut into the umbilical cord ( Fig. 8.2 E ; see Chapter 11 , Fig. 11.14 ). During embryonic folding in the horizontal plane, the limbs of the coelom are brought together on the ventral aspect of the embryo (see Fig. 8.2 C ). The ventral mesentery degenerates in the region of the future peritoneal cavity (see Fig. 8.2 F ), resulting in a large embryonic peritoneal cavity extending from the heart to the pelvic region.
Embryonic Body Cavity
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The intraembryonic coelom becomes the embryonic body cavity, which is divided into three well-defined cavities during the fourth week ( Fig. 8.3 ; see Figs. 8.1 A and 8.2 ):
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A pericardial cavity
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Two pericardioperitoneal canals
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A peritoneal cavity
These cavities have a parietal wall, lined by mesothelium (future parietal layer of peritoneum), that is derived from the somatic lateral layer of mesoderm and a visceral wall, also covered by mesothelium (future visceral layer of peritoneum), that is derived from splanchnic mesoderm (see Fig. 8.3 E ). The peritoneal cavity is connected with the extraembryonic coelom at the umbilicus (navel; Fig. 8.4 A and D ). The cavity loses its connection with the extraembryonic coelom during the 11th week of gestation as the intestines return to the abdomen from the umbilical cord (see Chapter 11 , Fig. 11.13 C ). Genetic lineage tracing studies suggest that the coelomic epithelium is a unique and highly active layer of mesenchymal cells that contributes to the development of major organs and systems, including the heart, lungs, and gastrointestinal tract.
During formation of the head fold, the heart and pericardial cavity are relocated ventrally, anterior to the foregut (see Fig. 8. 2B ). As a result, the pericardial cavity opens into pericardioperitoneal canals , which pass dorsal to the foregut (see Fig. 8.4 B and D ). After embryonic folding, the caudal part of the foregut, midgut, and hindgut are suspended in the peritoneal cavity from the dorsal abdominal wall by the dorsal mesentery (see Figs. 8.2 F and 8.3 B, D, and E ).
Mesenteries
A mesentery is a double layer of peritoneum that begins as an extension of the visceral peritoneum covering an organ. The mesentery connects the organ to the body wall and conveys vessels and nerves to it. Transiently, the dorsal and ventral mesenteries divide the peritoneal cavity into right and left halves (see Fig. 8.3 C ). The ventral mesentery soon disappears (see Fig. 8.3 E ), except where it is attached to the caudal part of the foregut (primordium of stomach and proximal part of duodenum). The peritoneal cavity then becomes a continuous space (see Fig. 8.4 D ). The arteries supplying the primordial gut—celiac arterial trunk (foregut), superior mesenteric artery (midgut), and inferior mesenteric artery (hindgut)—pass between the layers of the dorsal mesentery (see Fig. 8.3 C ).
Division of Embryonic Body Cavity
Each pericardioperitoneal canal lies lateral to the proximal part of the foregut (future esophagus) and dorsal to the septum transversum —a plate of mesodermal tissue that occupies the space between the thoracic cavity and the omphaloenteric duct (see Fig. 8.4 A and B ).
The septum transversum is the primordium of the central tendon of the diaphragm. Partitions form in each pericardioperitoneal canal separating the pericardial cavity from the pleural cavities and the pleural cavities from the peritoneal cavity. Because of the growth of the bronchial buds (primordia of bronchi and lungs) into the pericardioperitoneal canals , a pair of membranous ridges is produced in the lateral wall of each canal ( Fig. 8.5 A and B ):
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The cranial ridges— pleuropericardial folds —are located superior to the developing lungs.
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The caudal ridges— pleuroperitoneal folds —are located inferior to the lungs.
Defective formation and/or fusion of the pleuropericardial membranes separating the pericardial and pleural cavities is uncommon. This rare anomaly results in a congenital defect of the pericardium, usually asymptomatic and more often on the left side. Consequently, the pericardial cavity communicates with the pleural cavity. In very unusual cases, a part of the left atrium of the heart herniates into the pleural cavity at each heartbeat.
Pleuropericardial Membranes
As the pleuropericardial folds enlarge, they form partitions that separate the pericardial cavity from the pleural cavities. These partitions—the pleuropericardial membranes —contain the common cardinal veins (see Figs. 8.4 C and 8.5 A ), which drain the venous system into the sinus venosus of the heart. Initially, the bronchial buds are small relative to the heart and pericardial cavity (see Fig. 8.5 A ). They soon grow laterally from the caudal end of the trachea into the pericardioperitoneal canals (future pleural canals). As the primordial pleural cavities expand ventrally around the heart, they extend into the body wall, splitting the mesenchyme into:
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An outer layer that becomes the thoracic wall
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An inner layer that becomes the fibrous pericardium, the outer layer of the pericardial sac enclosing the heart (see Fig. 8.5 C and D )
The pleuropericardial membranes project into the cranial ends of the pericardioperitoneal canals (see Fig. 8.5 B ). With subsequent growth of the common cardinal veins, positional displacement of the heart, and expansion of the pleural cavities, the membranes become mesentery-like folds extending from the lateral thoracic wall. By the seventh week, the membranes fuse with the mesenchyme ventral to the esophagus, separating the pericardial cavity from the pleural cavities (see Fig. 8.5 C ). This primordial mediastinum consists of a mass of mesenchyme that extends from the sternum to the vertebral column, separating the developing lungs (see Fig. 8.5 D ). The right pleuropericardial opening closes slightly earlier than the left one and produces a larger pleuropericardial membrane.
Pleuroperitoneal Membranes
As the pleuroperitoneal folds enlarge, they project into the pericardioperitoneal canals. Gradually the folds become membranous, forming the pleuroperitoneal membranes ( Fig. 8.6 and Fig. 8.7 ). Eventually, these membranes separate the pleural cavities from the peritoneal cavity. The pleuroperitoneal membranes are produced as the developing lungs and pleural cavities expand and invade the body wall. They are attached dorsolaterally to the abdominal wall, and initially their crescentic free edges project into the caudal ends of the pericardioperitoneal canals.
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