Intracellular collagen and fibronexus in fibromatosis and other fibroblastic tumors

Myofibroblasts are mesenchymal cells with combined function and structure for contraction and collagen synthesis. They are found in reparative responses, nodular fasciitis, fibromatosis, and myofibroblastic sarcoma. Ultrastructurally, myofibroblasts are characterized by a specialized cell surface structure called the fibronexus (FNX). In addition, intracellular collagen fibers (ICF) have been described in nodular fasciitis and fibromatosis, but their origin and nature are still controversial. The aim of the present work was, first, to assess the frequency of FNX and ICF in proliferative myofibroblastic conditions compared to diverse mesenchymal tumors with spindle-shaped cells, and, second, to determine what kind of organelles contain ICF and if they are related to phagocytosis or cell synthesis. Forty-two cases of aggressive fibromatosis and 11 of nodular fasciitis (group A) were compared to 82 spindle-cell mesenchymal tumors of diverse nature (group B) by electron microscopy study. The presence and frequency of FNX and ICF was compared in both groups, and the organelles containing ICF were recorded. FNX and ICF were constantly found in group A (69.8 and 84.9%, respectively), and rarely in group B (0 and 5.12%, respectively). Most frequently ICF were contained in tunnels and phagolysosomes, but also were found in Golgi vesicles and cisternae of rough endoplasmic reticulum. In the majority of cases (75%), ICF were similar to collagen fibers of the extracellular space, but in some cases (22.5%), they were in dissimilar stages of fibrogenesis. Fibromatosis and nodular fasciitis are characterized by proliferation of myofibroblasts and constantly show FNX and ICF. These structures are rarely found in other mesenchymal tumors. The ICF are found in organelles of digestion and also in others related to synthesis and transport.