Scanning electron microscopy studies of bone marrow.

This paper reviews scanning electron microscopy (SEM) observations of bone marrow, based on our findings on the marrow of rat femur and tibia. The marrow sinuses are lined by an endothelium, of which the cell juncture is undulated and overlapping. The endothelium is backed by a thin sheet of adventitial cells which is discontinuous because of large perforations. Mature blood cells pass through the discontinuous parts of the adventitial cells and then penetrate the endothelium at its parajunctional zone. Mature megakaryocytes reveal the demarcation membrane system in their cracked surfaces, and its orifices in their surface. The platelet liberation is visualized by a bunch of ribbons projected from a megakaryocyte into the sinus lumen; these ribbons become beaded and then separate into platelets. After puncture perfusion into the parenchyme of marrow, erythroblastic islands can be clearly demonstrated by SEM. Erythroblasts with rhopheocytotic pits are embraced by a nursing cell which, with its filopodia and lamellipodia, corresponds to the macrophage and clearly differs from the reticular cell. The nursing cell differs in size from island to island and occasionally shows indications of cell death. The erythroblasts in an island may be in a uniform maturation stage or in a few different stages. Adipose cells in the marrow are observed and their relation to reticular cells is discussed.