44. Body Cavities: Coelomic Divisions

  1. Introduction: by week 4, the intraembryonic mesoderm on each side of the midline forms a paraxial portion, an intermediate portion, and a lateral plate. With the appearance and coalescence of many intercellular clefts, the lateral plates divide into 2 layers: a somatic mesoderm layer which continues with the extraembryonic mesoderm covering the wall of the amniotic cavity and a splanchnic mesoderm layer which is continuous with the mesoderm of the yolk sac wall. The spaces between these layers are the intraembryonic coelomic cavities*
    1. THE RIGHT AND LEFT INTRAEMBRYONIC COELOMIC CAVITIES, at first, are widely connected with the extraembryonic coelom, but with development and embryonic folding, they lose this connection. The two intraembryonic coelomic cavities are then separated by a double-layer membranous partition formed by fusion of the right and left splanchnic mesoderm layers, the so-called ventral mesentery, as the lateral folds move below the embryo and join each other
    2. THIS SEPTUM OR MESENTERY persists indefinitely in some portions of the body, particularly in the upper abdominal region, but in others, e.g., the thorax, it partly disappears to unite the right and left coelomic cavities
    3. IN WEEK 5, AS THE HEADFOLD FORMS, the heart and pericardial cavity move ventrally below the foregut, but open dorsally into the pericardioperitoneal canals which pass above the septum transversum on each side of the foregut, and the intraembryonic coelom consists of a thoracic and abdominal portion connected by the two canals, the pericardioperitoneal canals
    4. AFTER FOLDING OF THE EMBRYO, the caudal foregut, midgut, and hindgut are suspended in the peritoneal cavity by the dorsal mesentery. Thus, temporarily, the dorsal and ventral mesenteries divide the peritoneal cavity into 2 separate halves. However, the ventral one soon disappears except at its attachment to the caudal portion of the foregut, and the peritoneal cavity once again is a large continuous space
    5. IN THE ADULT, THE INTRAEMBRYONIC COELOM is divided into 3 well-defined compartments
      1. A pericardial cavity containing the heart
      2. The pleural cavities containing the lungs
      3. The peritoneal cavity with the viscera, caudal to the diaphragm
    6. THE SEPTUM between the thoracic and abdominal cavities is formed by the diaphragm, and between the pericardial and pleural cavities is found the pleuropericardial membranes
    7. *Provides short-term storage for excretory products and room for organ development and movement. May aid in transfer of fluid and nutrients to early embryo where intra-and extraembryonic coeloms communicate. The latter are occluded with folding of the embryonic disk.

body cavities: coelomic divisions: image #1