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. Not all blood cells formed in the bone marrow are released into the blood stream after maturation.
Megakaryocytes remain in the bone marrow when mature.
They lie close beside the blood sinuses, as do many developing cells, and extend processes through holes in the endothelial lining of these vessels. Textbook of Medical Physiology. W.B. Saunders, Philadelphia Heller, Craig H., et al. The Science of Biology. Inc. Utah Wichramasinghe, S.N. 1975.