Bone marrow is a so-called "semi-solid" tissue, soft and gelatinous, which is located in the bones and more precisely in the spongy parts of the bones. In mammals, the bone marrow is the main site of production of new blood cells, also called hematopoiesis. The human bone marrow can produce about 200 billion blood cells per day.

The bone marrow is composed of different types of cells: hematopoietic cells, fat cells, stromal cells and stem cells. The bone marrow is crossed by blood vessels and capillaries that allow the hematopoietic cells to be distributed in the body. Bone marrow stem cells are of 2 types: mesenchymal and hematopoietic.

There are 2 types of bone marrow: red bone marrow and yellow bone marrow. The yellow bone marrow contains more fat cells than the red bone marrow but both contain blood cells.