Bone marrow is the most important blood forming (hematopoietic) organ in the man.
Through the processes of differentiation, proliferation and maturation, mature blood cells of different lineages are constantly formed from the pluripotential stem cells in the bone marrow
It is an organ about the size of the liver, functionally spread in the flat and long bones. There is active red and inactive yellow marrow.
Active function - red bone marrow fills all bones until the age of 5. From then on it gradually disappears and at the age of 20 it still persists in the breast bone (corpus sterni), ribs (costae), backbone (columna vertebralis), flat bones of skull (ossa cranii), flat pelvic bones (os coxae), and proximal parts of the thigh bone (femur) and humeral bone (humerus). During bone marrow aplasia, the preservation of active marrow has a specific predictive value.
Reticular cells and bone marrow fibers form microenvironment and supportive network for the hematopoietic and immunopoietic cells. They direct stem cells to the process of differentiation.
Inactive function - yellow bone marrow mainly fills the diaphyses of the long bones. It is made of connective tissue and large, round fat cells. In normal conditions it does not function hematopoietically, but during some pathological processes it can be transformed into active (red) bone marrow.