The walls of marrow veins are extremely thin but the veins of the marrow are disproportionately large. The vein wall consists of a single layer of endothelium, like the sinusoids. Because of the thinness of the marrow veins, they are easily compressible. When a blood cell is ready to exit the marrow, they are discharged into thin walled blood vessels called blood sinuses, which is found throughout the bone marrow. Generally, the cellular passage across the endothelium is transendothelial rather than interendothelial.
Cellular passage is done through newly formed pores.
Each passing cell opens a new pore, which is immediately closed after the passage is completed. A cell entering the sinus must press against the abluminal membrane of the endothelium bringing it into contact with the luminal membrane. The two membranes then fuse in one direction perpendicular to the first. This opens a pore through which the cell enters the lumen. Immediately after the passage is complete, a fusion-fission in a reverse direction leads to the closure of the pore.