Bone lining (endosteal) cells and hematopoiesis: A light microscopic study of normal and pathologic human bone marrow in plastic‐embedded sections

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Abstract
Human trabecular bone that encloses the bone marrow (BM) is covered by a single layer of thin, sometimes inconspicuous, flat, elongated (spindle-shaped) endothelium-like cells with a round or oval nucleus. These “bone lining” cells, or endosteal cells (EC), form a continuous membrane (endosteum) over the trabecular bone surfaces. In most cases, the composition and thickness of these cells do not vary unless the cells are in intimate contact with hematopoietic tissue. In that instance, they are seen as a single layer adjacent to hematopoietic tissue or as a zone of tightly packed or loosely arranged mononuclear (hematopoietic) cells, some apparently originating from the endosteum. In a reparative process, such as following BM harvest, during which bony trabeculae (BT) are mechanically fratured, these cells are seen giving rise to osteoprogenitor (osteoblasts and osteoclasts) cells. Occassionally, the EC appear similar to BM stromal cells (morphologically and by their association with collagen/reticulin fibers) and are best seen at or near the BT that are cut tangentially. Short processes extending from the EC towards the underlying osteocytes have also been observed, suggesting that a channel of communication exists between them and osteocytes. Our observations, coupled with the experimental findings of others (i.e., that hematopoietic stem cells are concentrated near the endosteum, that cells responsible for BM and stroma regeneration are derived from the endosteal layer, and that high concentrations of hematopoietic colony-stimulating factors are produced there), indicate that, in addition to functioning as a simple membranous covering layer for BT, the endosteum helps to support osteocytes and maintains mineral homestasis. In addition, these cells contribute to the hematopoietic microenvironment and thereby play a role in regulating hematopoiesis. We also believe that the EC are reminiscent of embryonal-stage, undifferentiated mesenchyme, which under appropriate regulatory influence, can modulate and transform into cells with hematopoietic potential, stromal cells and osteoprogenitor cells.