Ultrastructure of bone marrow tissue in so-called primary (idiopathic) myelofibrosis-osteomyelosclerosis (agnogenic myeloid metaplasia). II. The myeloid stroma (hematopoietic microenvironment).

An ultrastructural study was performed on bone marrow tissue in 8 patients revealing early and late stages of so-called primary (idiopathic) myelofibrosis - osteomyelosclerosis (agnogenic myeloid metaplasia) to evaluate the constituents of the hematopoietic microenvironment (myeloid stroma). A survey of the stroma cells disclosed an overall increase, particularly in so-called undifferentiated (primitive - pluripotent), but also in transitional (fibroblastic) reticular cells and myofibroblasts. The most primitive reticular cells were characterized by their stellate aspect with elongated slender cytoplasmic processes traversing the interstitial space, and by the scarcity of organelles. The transition into a fibroblast was preceded by the appearance of branching cisternal structures of the rough endoplasmic reticulum, extensively developed Golgi fields and an abundance in mitochondria. Frequently, so-called myofibroblasts were encountered displaying bundles of filaments along the subplasmalemmal region. Extracellularly fibrillar material with an irregular cross-banding as well as microfibrils could be observed. The many vascular structures (sinusoids and capillaries) exhibited a multilayered basement membrane-like material including many fibrils and adventitial cells (pericytes, smooth muscle and transitional reticular cells) with numerous cytoplasmic processes. Undifferentiated and transitional reticular cells as well as myofibroblasts seem to form an integral part of the hematopoietic microenvironment in OMF and are assumed to play an important role for the evolution of the disease-specific myelofibrosis in this disorder.