46. Body Cavities: The Lesser Peritoneal Sac (omental Bursa) and Dorsal Mesogastrium

  1. Small intercellular clefts are seen first in the mesoderm lateral to the foregut in week 4. The clefts fuse and form the pneumatoenteric recess or cavity
    1. THE CAVITY is initially bilateral, but its left side is soon obliterated, and on the right side it comes to extend cranially between the esophagus and the right lung bud
    2. WITH PLEUROPERITONEAL MEMBRANE DEVELOPMENT, the cranial portion of the recess is isolated and forms a small supradiaphragmatic bursa, the infracardiac bursa
    3. SIMULTANEOUS WITH THE APPEARANCE OF THE PNEUMATOENTERIC RECESS is the formation of another recess on the right side of the mesogastrium which expands toward the left side of the body, resulting in the formation of the omental bursa (lesser peritoneal sac) (an extension of the right half of the peritoneal cavity)
    4. AFTER ROTATION OF THE STOMACH is complete, the dorsal mesogastrium continues to grow and forms a double-leaved apron which extends in front of the transverse colon and small loops of the intestine as the greater omentum
      1. Its leaves fuse to form a single sheet that hangs over the greater curvature of the stomach, with the upper portion of the posterior leaf fusing to the mesentery of the transverse colon
    5. THE FORMATION OF THE SPLEEN complicates the dorsal mesogastrium development
      1. Its primordium appears in week 5 as a mesenchymal condensation between the two leaves of the mesogastrium, but soon bulges into the left peritoneal cavity
      2. With formation of the omental bursa, a part of the dorsal mesogastrium, between the spleen and the dorsal midline, fuses with the posterior abdominal wall, while the remainder remains and connects the spleen to the kidney as the lienorenal ligament
        1. The connection of the spleen to the stomach forms the gastrolienal ligament
        2. Thus, the spleen always keeps an intraperitoneal position
    6. THE FORMATION OF THE OMENTAL BURSA also influences the position of the pancreas which initially grows into the dorsal mesoduodenum, but in time, its tail portion expands into the dorsal mesogastrium
      1. Since this portion of the mesogastrium (the left leaf) fuses with the peritoneum of the body wall during rotation, the pancreatic tail comes to lie retroperitoneally

body cavities:  the lesser peritoneal sac (omental bursa) and dorsal mesogastrium: image #1