A cellular investment of bone marrow.

We describe a method for clearly separating the cell layers at the bone-marrow interface, which reveals that the myeloid tissue is invested by an epithelial-like layer of specialized squamous cells we call the marrow sac. The scanning electron microscope showed that the sac was fenestrated and that some of its cells pass as perivascular elements with the marrow capillaries that penetrate the bony cortex. The transmission electron microscope (TEM) showed that the cells comprising the marrow sac are less than 0.1 micrometer thick, overlap at their margins without specialized cell junctions, and are more electron dense than the reticular or fibroblastic cells of the marrow stroma. The fenestrations in the sac were intercellular and were usually occupied by cells having an ultrastructure compatible with an osteoprogenitor cell (OPC) lineage. The observation of a close proximity between the cells of the marrow sac and the osteogenic cells that line the endosteal surfaces of bone suggest that the sac cells, along with the OPCs of the superficial marrow stroma, should be included in any morphological or functional definition of an endosteum.