Characterization of stromal cells with myoid features in lymph nodes and spleen in normal and pathologic conditions.

Stromal cells with myoid features were identified in rat or human lymph nodes and spleen in normal and pathologic conditions, using antibodies to desmin, alpha-smooth muscle actin, and smooth muscle myosin. In normal lymph nodes, myoid cells (MCs) were present in the superficial and deep paracortex as well as in the medulla, and absent in lymphoid follicles. In the spleen, they were numerous in the red pulp, less abundant in periarteriolar lymphocyte sheaths of the white pulp, and absent in lymphoid follicles. On double immunostaining, alpha-smooth muscle actin and smooth muscle myosin were coexpressed with desmin only in the deep paracortex and parafollicular areas of the lymph nodes, as well as in the MCs of the periarteriolar lymphocyte sheaths and marginal zone of the spleen; the remaining MCs expressed only desmin. When examined by means of electron microscopy, MCs showed a dendritic shape and cytoplasmic bundles of microfilaments with dense bodies scattered between them. When compared with normal conditions, MCs showed changes of distribution and number in several pathologic situations. Additional findings were 1) staining of pericytes surrounding high endothelium venules of lymph nodes with alpha-smooth muscle actin antibodies in man and rat and with desmin antibodies in rats; 2) staining of endothelial cells in these venules with desmin antibodies in rats. It is concluded that a subset of reticular cells in lymph nodes and spleen, as well as pericytes and endothelial cells in high endothelium venules display cytoskeletal features suggesting a myoid differentiation and function.