Coelomic cavities

Chapter 10
Coelomic cavities


At the end of gastrulation, the embryonic mesoderm consists of three regions, paraxial, intermediate and lateral mesoderm. As development proceeds, clefts develop within the right and left lateral mesoderm. Later these clefts coalesce, forming a cavity which splits the lateral mesoderm into an outer layer of somatic mesoderm and an inner layer of splanchnic mesoderm (Fig 10.1). The cavities between the two layers of mesoderm on the left and right sides are referred to as coelomic cavities. The left and right developing coelomic cavities located on either side of the midline extend cranially, meet and fuse in front of the developing neural and cardiogenic plates, forming a horseshoe‐shaped coelomic cavity (Fig 10.2). The lateral walls of the coelomic cavity are composed of somatic mesoderm which fuses with ectoderm, forming somatopleure. The medial walls are composed of splanchnic mesoderm which fuses with endoderm, forming splanchnopleure. The mesodermal cells lining the coelomic cavity differentiate into a simple squamous epithelium, referred to as mesothelium. Following cranial, caudal and lateral folding of the embryo, the convex region of the horseshoe‐shaped coelom occupies a position ventral to the foregut and the developing heart, and gives rise to the primordium of the pericardial cavity. The right and left limbs of the coelomic cavity are connected to the pericardial cavity by the pericardial–peritoneal canals (Fig 10.3). Lateral body folding results in division of the developing embryonic coelom into an intra‐embryonic and an extra‐embryonic region. Subsequently, the intra‐embryonic coelom gives rise to the pericardial, pleural and peritoneal cavities. The extra‐embryonic coelom is associated with the developing foetal membranes. The intra‐embryonic and extra‐embryonic coelomic cavities, which are initially continuous at the umbilicus, subsequently become separated from each other.

Two diagrams illustrating cross-sections of an embryo at an early stage of development with formation of the intra-embryonic coelom.

Figure 10.1 Cross‐sections through an embryo at an early stage of development showing formation of the intra‐embryonic coelom (A and B).

3 Diagrams in dorsal views of an embryo at an early stage of development illustrating formation of the coelomic cavity with labels coelomic clefts, neural plate, neural groove, neural fold, etc.

Figure 10.2 Dorsal views of a mammalian embryo at an early stage of development showing formation of the coelomic cavity (A to C).

Left lateral view of an embryo displaying the arrangement of the pericardial and peritoneal cavities and the pericardial – peritoneal canal with labels heart, foregut, dorsal aortae, peritoneal cavity, etc.

Figure 10.3 Left lateral view of an embryo showing the arrangement of the pericardial and peritoneal cavities and the pericardial–peritoneal canal.


Pleural and pericardial cavities


Pleural cavities


The primordia of the developing lungs and heart are surrounded by the left and right segments of the pleuro‐pericardial cavity (Fig 10.4). Gradually, folds of mesoderm, the pleuro‐pericardial folds, containing the left and right common cardinal veins and left and right phrenic nerves, grow medially into both segments of the pleuro‐pericardial cavity. When these folds meet, they divide the left and right segments of the pleuro‐pericardial cavity into pleural cavities dorsally, and pericardial cavities ventrally. The left and right pleural cavities remain separate and communicate with the left and right limbs of the intra‐embryonic coelom, located on either side of the developing foregut, through the left and right pleuro‐peritoneal canals. The developing lungs, which grow into the pleural cavities, subsequently expand into the pleuro‐peritoneal canals. The mesothelium in direct contact with the developing lungs is referred to as visceral pleura while the mesothelium in contact with the wall of the pleural cavity is called the parietal pleura. As the lungs continue to enlarge, the pleural cavities extend into the lateral body walls, dividing them into thin inner layers and thicker outer layers, the latter destined to become the definitive thoracic walls (Fig 10.4).

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Sep 27, 2017 | Posted by in GENERAL | Comments Off on Coelomic cavities
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