The International Journal of Biochemistry & Cell Biology
Cells in focusThe mesothelial cell
Introduction
In 1827, Bichat observed that the serous cavities were lined by a single layer of flattened cells similar to those of the lymphatics. In 1880, Minot referred to this layer of cells as the “epithelial lining of mammalian mesodermic cavities” and subsequently proposed the term “Mesothelium”.
The mesothelium consists of a monolayer of specialised cells (mesothelial cells) which extends over the entire surface of the three serosal cavities (pleural, pericardial and peritoneal) and the organs contained within these cavities. Mesothelial cells rest on a thin basement membrane supported by connective tissue stroma, the stroma varying in quantity and quality depending on site and species. The cells are predominantly flattened, squamous-like, approximately 25 μm in diameter, with the cytoplasm raised over a central round or oval nucleus. The cells contain microtubules and microfilaments, glycogen, vesicles and vacuoles, few mitochondria, a poorly developed Golgi apparatus and little rough endoplastic reticulum (RER).
The luminal surface of the mesothelial cell has a well-developed microvillous border with occasional cilia. The boundaries between mesothelial cells are tortuous, with adjacent cells often overlapping. They have well-developed cell–cell junctional complexes including tight junctions, adherens junctions, gap junctions and desmosomes. Tight junctions in particular are crucial for the development of cell surface polarity and the establishment and maintenance of a semi-permeable diffusion barrier.
Mammalian mesothelium is considered essentially similar regardless of species or anatomical site. However, cuboidal mesothelial cells are found in the septal folds of the mediastinal pleura, the parenchymal organs (liver, spleen), the “milky spots” of the omentum and the peritoneal side of the diaphragm overlying the lymphatic lacunae. Morphologically similar cells are also seen after injury or stimulation of the serosal surfaces and contain abundant mitrochondria and RER, a well developed Golgi apparatus, microtubules and a comparatively greater number of microfilaments, suggesting a more metabolically activate state (Mutsaers, 2002; Mutsaers, Whitaker, & Papadimitriou, 2002) (Fig. 1).
Section snippets
Cell origin, plasticity and regeneration
Embryologically, the mesothelium develops from the mesodermal tissue between 8 and 18 days of gestation depending on the species. In humans, this occurs around day 14, with cells gradually differentiating from round or cuboidal cells to elongated flattened cells which line the coelomic cavities.
The mesothelium is a slowly renewing tissue with 0.16–0.5% of cells undergoing mitosis at any one time. However, if appropriately stimulated, its mitotic activity can be greatly increased. Within 48 h of
Functions
Historically, the main roles attributed to the mesothelium have been as a protective barrier against physical damage and invading organisms and a frictionless interface for the free movement of apposing organs and tissues. However, more recent studies have begun to elucidate many different roles for mesothelial cells. This review provides an overview of the functions of mesothelial cells. For a more comprehensive review see (Mutsaers, 2002) (Fig. 2).
Associated pathologies
Mesothelial cells are important clinically as impaired mesothelial regeneration is a likely cause of post-operative adhesion formation and they are most likely the progenitor cells of malignant mesothelioma (MM).
Acknowledgements
I would like to thank Dr. Cecilia Prêle for her help in preparing the artwork for this review. This work was supported by a Raine Medical Research Foundation Priming Grant no. 29/2001, Western Australia.
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