Typically the tumor presents as well-circum-scribed masses even though microscopic infiltration into the adjacent tissue is frequently present. Many cases of low-grade fibromyxoid sarcoma are located in the skeletal muscles indicating a possible derivation from the fibroblasts in the muscle associated fascia. As discussed in the skin and kidney sections, fibroblast heterogeneity is likely to a universal phenomenon as each tissue or organ has its unique mechanical and functional needs [1, 2].

In fact, the tumor is composed of bland spindle cells with low mitotic figures and no necrosis. The spindle cells are arranged in an alternating fibrous and myxoid pattern. The fibrous areas resemble the distribution of normal fibroblasts in the normal endomysium, whereas the myxoid areas correspond to the spaces occupied by muscle fibers in a muscle bundle. Ultrastructurally, the tumor cells show perineurial and myofibroblastic differentiation [3, 4]. Many cases are positive for EMA and cytokeratins [5, 6].

Myxofibrosarcomas typically present in the subcutaneous tissue in the extremities of the elderly and manifest as vague nodules composed of spindle cells distributed in a myxoid stroma. In this myxoid background, elongated curvilinear capillaries are prominent. The tumor cells are frequently CD34 positive and show tendency to perivascular condensation. It is thus indicated that the tumor cells are probably related to the vascular adventitial cells [7, 8]. At ultrastructural level, myofibroblastic differentiation is evident [6].

Whereas benign fibrohistiocytoma is related to the XIII- and CD10-positive dermal fibroblasts, dermatofibrosarcoma protuberans is derived from the CD34-positive dermal fibroblasts which manifest perineurial differentiation. The characteristic cartwheel (storiform) arrangement of tumor cells probably reflects the inherent cellular property to form a 3D mesh-like immunological barrier for the dermis.

Also included in the intermediate malignancy category of fibrohistiocytic tumors are angiomatoid fibrohistiocytoma and plexiform histiocytoma [9, 10]. The former actually represents a tumor of the lymph node stromal cell with characteristic cystic areas and chronic inflammatory infiltration. The intimate relationship between the fibroblasts and histiocytic cells in plexiform histiocytoma is different from that seen in most of other fibrohistiocytomas. Instead of having an admixture of both cellular components, the tumor is featured by nodules of histiocytic cells surrounded by fibroblastic fascicles.

Deep fibromatosis has characteristic somatic beta-catenin or adenomatous polyposis coli (HPC) mutations which allow intranuclear accumulation of beta-catenin with resultant cell proliferation [11]. This important feature can be diagnostically useful.

Deep fibromatosis appears to be a tumor of the fibroblasts in the deep fascia. The typical long sweeping fascicules are composed of bland fibroblasts and are less cellular with less defined cell borders than the V-shaped fascicules characteristic of fibrosarcoma. These differences are probably a reflection of the different mechanical stresses in which fascia and tendons are programmed to withstand. Deep fascia tissue binds the muscle tissue with the adjacent tissue, and in doing so, it diffuses mechanical stress over a large area. Long sweeping fascicules are more suitable than herringbone-like structures for this purpose. The main function of a tendon is to concentrate the force generated by skeletal muscle tissue and transmit it to the bone. A V-shaped pattern seems to suit the purpose.

Primarily a pediatric sarcoma, rhabdomyosarcomas are far more common than rhabdomyomas. One unique feature of the sarcoma is that it often arises at sites where skeletal muscle is not a normal component or only scant.

In an analogy to lipogenesis, rhabdomyogenesis is a complex multistage process during which dramatic metamorphogenesis occurs. Stuck at various differentiation stages as a result of botched execution of the metamorphogenic drama. Low-grade rhabdomyosarcoma includes the spindle cell and botryoid variants which bear resemblance to late-stage fetal myoblasts [12, 13].

In low-grade rhabdomyosarcoma (to some extent, embryonal variant also), identification of rhabdomyoblasts constitutes the most important aspect of making the diagnosis. There is a wide range of morphological variations for rhabdomyoblasts which might reflect the dramatic metamorphism in rhabdomyogenesis [14, 15]. Typically, they are spindle to plump cells with eosinophilic cytoplasm, cigar-shaped nuclei, and prominent nucleoli. Cross striations can be present, but is not required. Other shapes include ribbonlike, tadpole, racquet, spider, and broken straw.

Benign mimickers of rhabdomyoblasts are legion. They include macrophages, squamous cells, crystal-containing cells, and rhabdoid cells. Sometimes, immunostainings might be needed to ascertain the cell nature. To avoid confusing apoptotic cells for rhabdoblasts, the cell nuclei should be viable. To avoid calling entrapped muscle cells as rhabdoblasts, attention to the striation and nuclear feature offers help. In rhabdomyoblasts, the striation usually traverses only part of the cell which contains moderate to severe atypical nuclei with prominent nucleoli.