Walsh & Hoyt: General Considerations

Agnogenic myeloid metaplasia is the most commonly used term for this neoplastic hematopoietic disorder, considered one of the chronic myeloproliferative disorders, and is characterized by extramedullary hematopoiesis and myelofibrosis. Myelofibrosisfibrosis of the bone marrowis not unique to this disease and may be seen in a variety of neoplastic and nonneoplastic disorders, both hematologic and nonhematologic. In agnogenic myeloid metaplasia, hematopoietic cell proliferation is clonal, and myelofibrosis is a secondary process, resulting from growth factor stimulation of adjacent polyclonal fibroblasts. Although the cause is unclear, current evidence points to involvement of a major putative cytokine, transforming growth factor-beta (TGF-beta).