mesothelial cell hyperplasia
Tuesday 13 September 2011
Mesothelial cells respond to the presence of air, fluid, blood, infection or other irritants in the pleural cavity by undergoing proliferation.
This is recognized in pleural biopsy specimens by the presence of multiple layers and loose surface papillary aggregates of mesothelial cells.
In contrast to mesotheliomas, the papillae in benign proliferations tend to lack a vascular connective tissue core. Mitoses are often present, and some degree of atypia is not unusual.
When this is more marked, it may be very difficult to make a confident distinction from mesothelioma.
In larger specimens, the presence of invasion, with groups of cells infiltrating through the pleura, provides the best clue to malignancy, but this raises the question of what constitutes invasion.
In the normal pleura, there is no clearly identifiable basement membrane between the mesothelial cells and the underlying loose fibrocollagenous tissue with its irregular elastic layers.
At an early stage in benign conditions, hyperplastic mesothelial cells may form layers within the connective tissue that are parallel to the surface.
In biopsy specimens, cross cutting and folding of the pleura results in the false impression of deeper cell clusters, which may be seen to be continuous with the surface in deeper sections.
In longstanding effusions, where there is organization of inflammatory exudate, the mesothelial cell inclusions trapped within the layers tend to form small lumens, usually in a recognizable plane.
Where this pattern is disturbed and the cells become more numerous and more disorganized, particularly with infiltration perpendicular to the surface, there should be at least a suspicion of malignancy.
If invasion of fat can be clearly identified, this is regarded as a more conclusive indicator, although tangential cutting or cross-cutting through the tissue may make this difficult to interpret.
Neoplastic cells invading fat can usually be readily identified as they induce some formation of stroma, but more subtle infiltration between the fat cells may easily be missed.