The mesothelial cell

Abstract

Mesothelial cells form a monolayer of specialised pavement-like cells that line the body’s serous cavities and internal organs. The primary function of this layer, termed the mesothelium, is to provide a slippery, non-adhesive and protective surface. However, mesothelial cells play other pivotal roles involving transport of fluid and cells across the serosal cavities, antigen presentation, inflammation and tissue repair, coagulation and fibrinolysis and tumour cell adhesion. Injury to the mesothelium triggers events leading to the migration of mesothelial cells from the edge of the lesion towards the wound centre and desquamation of cells into the serosal fluid which attach and incorporate into the regenerating mesothelium. If healing is impaired, fibrous serosal adhesions form between organs and the body wall which impede vital intrathoracic and abdominal movement. Neoplastic transformation of mesothelial cells gives rise to malignant mesothelioma, an aggressive tumour predominantly of the pleura. Although closely associated with exposure to asbestos, recent studies have implicated other factors including simian virus 40 (SV40) in its pathogenesis.
Cell facts

• Mesothelial cells are derived from the mesoderm but express both mesenchymal and epithelial cell intermediate filaments.

• Mesothelial cells form a monolayer (mesothelium) lining the serosal cavities (pleural, pericardial and peritoneal) and the organs contained within these cavities.

• The mesothelium is a slowly renewing tissue with 0.16–0.5% of cells undergoing mitosis at any one time.

• The primary function of the mesothelium is as a protective non-adhesive surface but it is also involved in transport of solutes and cells across serosal cavities, antigen presentation, inflammation and tissue repair, coagulation and fibrinolysis and tumour cell adhesion.

• Mesothelial regeneration involves migration of cells from the wound edge and attachment and incorporation of free-floating mesothelial cells from the serosal fluid onto the wound surface.

• Impaired healing and cell transformation lead to the formation of serosal adhesions and malignant mesothelioma, respectively.

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).