Fibroblastic and myofibroblastic tumors (FMTs) are tumors which develop from the mesenchymal stem cells which differentiate into fibroblasts (the most common cell type in connective tissue) and/or the myocytes/myoblasts that differentiate into muscle cells.
The World Health Organization (2020) defined tumors as being FMTs based on their morphology and, more importantly, newly discovered abnormalities in the expression levels of key gene products made by these tumors' neoplastic cells.
[1] Histopathologically, FMTs consist of neoplastic connective tissue cells which have differented into cells that have microscopic appearances resembling fibroblasts and/or myofibroblasts.
Myofibroblastic cells are plumper with more abundant cytoplasm and more prominent nucleoli; they express smooth muscle marker proteins such as smooth muscle actins, desmin, and caldesmon.
[2] The World Health Organization further classified FMTs into four tumor forms based on their varying levels of aggressiveness: benign, intermediate (locally aggressive), intermediate (rarely metastasizing), and malignant.