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March 2001

Reactive Pericytes vs Myofibroblastic Tumor Cells

Arch Ophthalmol. 2001;119(3):459. doi:

I read with interest the article by Allaire et al1 that appeared in this journal on malignant fibrous histiocytoma of the conjunctiva, but would like to express reservations on certain aspects of the ultrastructural interpretation.

In Figure 6 (top), the authors describe "subplasmalemmal fusiform densities suggesting myofibroblastic differentiation." In my view, the multiple small subplasmalemmal densities clearly illustrated in the figure are not a feature of myofibroblasts. Although these cells have submembranous densities, they do not appear in large numbers and tend to be associated with the microtendon. This is the fibronectin-containing fibril believed to mediate contact between the surface of the myofibroblast and the extracellular matrix, and is regarded as an important marker of myofibroblastic differentiation.2,3 Further, the cell in Figure 6 clearly displays a continuous, well-formed lamina. Despite citations in the literature that myofibroblasts have a lamina,4 this idea is controversial; most authors accept that if the lamina is present, it is mostly focal.4,5 My own belief is that the principal structure on the surface of the myofibroblast is the fibronexus junction2,3 and that cells with a well-defined and continuous lamina are more likely to show an alternative line of differentiation. In the context of the authors' Figure 6, I think the cell in question is a pericyte—moreover, a pericyte of a nonneoplastic vessel. It has a close association with a lamina-bearing cell, which would be endothelium (although the micrograph does not include a lumen), and has the features of a poorly differentiated smooth-muscle cell (subplasmalemmal densities and lamina). This, in a cell biological sense, is what a pericyte is.

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