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"Professional" and "nonprofessional" contractile cells in the lung.

Every student of biology learns that there are several clearly defined categories of mesenchymal cells, such as fibroblasts, chondrocytes, Schwann cells, and, of course, smooth muscle cells. The latter cells are characterized by the presence of a contractile apparatus that corresponds to ultrastructurally identified myofibrils and the immunochemically defined presence of proteins such as desmin and smooth muscle actins. But, to paraphrase Goethe, it is only when we know little about a subject that we can afford to be dogmatic about the facts. Recently, cells with "partial" smooth muscle phenotypes have been described in a wide range oflocations, including atherosclerotic plaques (1), the stroma of breast carcinomas (2), the interstitium oflung (3), and proliferative disorders such as Dupuytren's disease (4). The appellation "myofibroblast" has been given to some of these cells, as a reflection of the "partial" phenotype many of them display. Myofibroblasts, originally described as morphologically distinctive cells seen in granulation tissue, are now known to have the capacity to express many muscle-associated proteins such as smooth muscle actins and desmin (5). In addition, more recent studies have documented that muscle gene expression can be ephemeral in vivo cells expressing a "partial" muscle phenotype (which we can alternatively . define as "nonprofessional" contractile cells) appear and then disappear in granulation tissue during the course of wound healing (6). Several studies have also documented an analogous plasticity and reversibility of muscle phenotype in cultured smooth muscle cells as a function of initial plating density, passage number, and so forth (7). Developmental systems have been studied in the hope that cell lineage could be established and light could therefore be shed on the nature of the spectrum of "nonprofessional" contractile cells seen in the adult. Unfortunately, mesenchymal cell development and differentiation is less well understood than that occurring in epithelial cells and tissues. Epithelial cells, once determined developmentally, form self-contained replicating pools, complete with stem cells (usually undifferentiated) which give rise to a limited repertoire of fully differentiated daughter cells. The situation is less clear with respect to mesenchymal cells. Striated muscle cells seem to follow the stem cell:daughter cell paradigm as satellite cells or myoblasts fuse and apparently irreversibly form myotubes