Expression of alpha-smooth muscle actin in the stromal cells of bone marrow in fetuses in different stages of development, in multiple myeloma and monoclonal gammopathy of unknown significance.

Several disorders are associated with a monoclonal immunoglobulin detected by serum or urine electrophoresis, the most common being a monoclonal gammopathy of unknown significance, multiple myeloma, Waldenstrom's macroglobulinemia, and amyloidosis. Plasma cells, the immunoglobulin secretory cells of the immune system, are normal constituents of bone marrow (BM). Plasma cells are seen in small numbers in the stroma, surrounding blood vessels in the marrow. Their perivascular disposition is consistent with their secreting capacity. The hematopoietic microenvironment has a crucial role homing and regulating precursor cell growth both in physiologic and pathologic conditions. Cellular components such as branched adventitial reticular cells, macrophages, endothelial cells and fat cells constitute the supporting framework (stroma) for hematopoiesis, which takes place in the extravascular compartment. The presence of myiod cells (MCs) in human bone marrow has been observed during hematopoiesis in embryonic life, whereas during adult life, it is strictly related to various pathologic conditions. The aim of this study was to examine in the stroma of BM the presence, distribution and quantitation of cells expressing a-smooth muscle actin (MCs) from patients with monoclonal gammopathy of unknown significance, those with plasma cell myeloma and embryos (gestational age 15 to 25 weeks). For this reason, a series of 20 trephine bone marrow biopsies from adult patients and ten fetal specimens of the spine and femur were examined for the presence of stromal myoid cells using a monoclonal recognising alpha-smooth muscle actin, a contractile microfilament expressed solely by smooth muscle cells, myofibroblasts and related cells. Our results suggest that the appearance of MCs and subsequent fibrosis is not a feature of malignant BM disorders such as MM but it is also seen to a lesser degree in the BM stroma of individuals with monoclonal gammopathy of unknown significance (MGUS). Stromal cells with phenotypic smooth muscle features appear in bone marrow during pathological situations in a manner reminiscent of what occurs during normal development.