The (red) bone mar­row is a com­plex struc­tu­re of cells and tis­su­es found in the lar­ge bones of the body. All the cells that appe­ar in the blood are con­stant­ly pro­du­ced here. The bone mar­row can rege­ne­ra­te its­elf so that the por­ti­on taken for a trans­plant is repla­ced in only a few weeks. It is important to dis­tin­guish bet­ween the bone mar­row and the spi­nal cord, which is part of the cen­tral ner­vous sys­tem and is never transplanted.