Endothelial cells line the sinusoids as well as the vessels leading to and from the bone marrow. Together with the basement membrane and the fibroblasts, they form the so-called bone marrow blood barrier.
Endothelial cells can rarely be clearly recognized with the optical microscope. They are large elongated cells of 20-30 µm in diameter with very large pale blue-grey cytoplasm. The nuclei are round to oval with 1 to 3 nucleoli. Occasionally, they are visible as whole vessels in the bone marrow.