45. Body Cavities: Pleuropericardial and Pleuroperitoneal Membranes

  1. Pleuroperitoneal membranes are a pair of membranes which gradually separate the pleural and peritoneal cavities, produced as the pleural cavities expand by invading the body wall
    1. THE MEMBRANES ARE ATTACHED dorsolaterally to the body wall, and their free edges project into the caudal end of the pericardioperitoneal canals
      1. During week 6, they grow medially and ventrally, and by the end of the week, their free edges fuse with the dorsal mesentery of the esophagus and with the septum transversum to separate the pleural and pericardial cavities
      2. Closure of the openings is further enhanced by the growth of the liver and muscle tissue extension into the membranes. The right-side opening closes before that of the left
  2. Pleuropericardial membranes initially appear as small folds or ridges of mesenchyme projecting into the primitive undivided thoracic cavity. The folds contain the common cardinal veins which drain the primitive venous system into the sinus venosus of the primitive heart
    1. AS A RESULT OF SUBSEQUENT GROWTH of the common cardinal veins, descent of the heart, and expansion of the pleural cavities, the membranes are drawn out in a mesentery like fold that extends from the lateral wall
    2. BY WEEK 7, the membranes fuse with the mesoderm ventral to the esophagus or primitive mediastinum (the dorsal mesocardium) and divide the thoracic cavity into a single pericardial cavity and 2 pleural cavities
      1. The mediastinum is filled with a mass of mesenchyme and separates the developing lungs as it extends from the sternum to the vertebral column
    3. THE RIGHT PLEUROPERICARDIAL OPENING closes before the left one since the right common cardinal vein is larger and produces a larger membrane
    4. SUBSEQUENTLY, THE LUNG BUDS grow into the medial walls of the pericardiopleural canals (primitive pleural cavities); and the pleural cavities expand around the heart into the body wall and split the mesenchyme into an outer layer that becomes the chest wall and an inner layer that forms the fibrous pericardium
  3. Congenital malformations
    1. CONGENITAL PERICARDIAL DEFECTS: defective formation and/or fusion of the pleuropericardial membrane(s) which normally separate(s) the pericardial from the pleural cavities may result in congenital defects of the pericardium, usually on the left side. It is a rare abnormality.
      1. The pericardial cavity then communicates with the pleural cavity, and in rare instances, a part of the atrium may herniate into the pleural cavity with each heartbeat

body cavities: pleuropericardial and pleuroperitoneal membranes: image #1