The soft, spongy tissue found in the centre of most
large bones that produces the cellular components of blood:
white cells,
red cells and
platelets.
If you are having problems with the rough numbers of one of those cell lines, there are several things that might be going wrong: marrow replaced by tumor or hyperproliferation of one cell type; cells killed by radiation; autoimmune destruction of the cells; poor nutrition; lack of production-stimulating factors from the endocrine system; or poor blood flow (and thus nutrients/oxygen) to the bone.