Endothelial cells and hematopoiesis: A light microscopic study of fetal, normal, and pathologic human bone marrow in plastic‐embedded sections

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Abstract
The origin and morphological identity of hematopoietic progenitor cells, as well as their precursor, the pleuripotential hematopoietic stem cell (HSC), has not been established. Our studies of 2 μm sectioned undecalcified plastic-embedded bone marrow (BM) from healthy human fetuses; normal adults; patients with acute myeloblastic leukemia (AML), acute lymphoblastic leukemia (ALL), and chronic granulocytic leukemia (CGL) in various stages (chronic, accelerated, acute blastic phase, and after autografting); and patients recovering from therapy-induced marrow hypoplasia suggest that proliferative hematopoietic zones exist near the endosteum (endosteal marrow) and the vascular endothelium (capillary and sinus-lining endothelium) and a maturational zone distal to these regions. In some of these areas, morphologically recognizable hematopoietic cells were seen and interpreted as emerging and maturing in a sequential progression, suggesting an origin from the endosteal or endothelial progenitors. In other loci, early hematopoietic cells were seen in close contact with the endosteal or vascular endothelial (VE) cells. This latter relationship suggested that these areas of cellular contact were important and represented sites of cell to cell interaction that may be associated with the liberation of growth factors by endosteal and endothelial cells and their action on hematopoietic progenitor cells. Following treatmentinduced hypoplasia, the endosteal and VE cells were seen to modulate, transform, and migrate into the surrounding empty and edematous marrow space as fibroblasts. Later, as hemopoietic regeneration began, clusters of regenerating hematopoietic cells were seen adjacent to bone trabecule (BT) and near the vascular endothelium. We postulate that endosteal and VE cells are the equivalent of embryonal-stage, undifferentiated mesenchyme and, under the appropriate regulatory influence, are capable of modulation and transformation (differentiation) into stromal (fibroblast-like) cells and precursors of hematopoietic cells in normal (physiologic) and stressed (pathologic) conditions. Recently, human endothelial cells have been shown to express a large number of cell surface antigens in common with hematopoietic (myeloid and lymphoid) cells. It is also possible that, in some situations, the VE cells act to establish a microenvironment and liberate growth factor (s), enabling pleuripotential and progenitor cell differentiation into mature hematopoietic cells adjacent to the vascular endothelium. Indeed, vascular endothelium has been shown to elaborate growth factors that participate in normal hematopoiesis.