Bone marrow is found in the medullary cavities - the centres of bones. The bone marrow is where circulating blood cells are produced – a process known as haematopoiesis. Early on in a human’s life, this takes place in many bones, but during development haematopoiesis increasingly centres on flat bones so that by puberty, blood production takes place predominantly in the sternum, vertebrae, iliac bones and ribs. Bone marrow undergoing haematopoiesis is coloured red due to the presence of red blood cells, whereas bone marrow that is not undergoing haematopoiesis is yellow. The red marrow consists of long trabeculae (beam-like structures) within a sponge-like reticular framework. Spaces around this framework are filled with fat cells, stromal fibroblasts and blood cell precursors