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The pleural space, derived from the intraembryonic coelom, is limited by a serous membrane including the mesothelium formed by cells possessing not only the characteristic features of epithelial cells but also the potential of secretory cells (cytokines and growth factor). Blood supply to visceral pleurae differs depending on the species while the lymphatic circulation is directly connected to the pleural space via pores in the parietal pleura. Pleural physiology and movement of pleural fluid are directly related to the particular structures of the pleura.

During the third week of embryonic life, segmentation of the paraxial intraembryonic mesoderm commences and the embryo enters the somite period of development. At the same time clefts appear within the lateral plate mesoderm, and gradually they coalesce to form a ‘U’ shaped intraembryonic coelom. The arms of the cavity lie within the lateral plate mesoderm and meet in the midline, within the cardiogenic area of mesoderm, at the rostral limit of the embryonic disc. The cells that line the primitive coelom and its derivatives (the pericardial, pleural and peritoneal cavities) constitute ‘The Mesothelium’. Initially they have a cuboidal form and the layer may appear pseudostratified (Fig. 1.1) Three events characterise their subsequent development: 1. The formation of a basal lamina that separates the mesothelium from the underlying mesenchyme 2. The appearance of intercellular junction 3. The change in cell form from cuboidal to squamous

Mononuclear cells in the extraembryonic and intraembryonic spaces of mice were examined qualitatively and quantitatively by semithin light microscopy. At 9 and 10 days of gestation, the extraembryonic serous cavities contained a small number of mononuclear free cells. These cells had an elongated or kidney-shaped nucleus and the cell surface showed many villous projections. The cytoplasm occasionally contained small lucent vesicles but no phagocytic vacuoles. The average cell diameter was 8.4 +/- 0.9 microns and N-C ratio, 0.51 +/- 0.21. Cell larger than 10 microns in diameter constituted only 0.4% of the total. In vitelline vessels at 9 days, mononuclear cells bearing a close morphological resemblance to extraembryonic free cells were observed. At 12 days of gestation, extraembryonic and peritoneal cavities contained mature macrophages and a few small mononuclears which had the same morphological features as those in the extraembryonic coelom and vitelline vessels.