Bone marrow, a semi-solid tissue located within the spongious portions of bones, is the primary site of new blood cell production or hematopoiesis. It is composed of hematopoietic cells, marrow adipose tissue, and supportive stromal cells. A newborn baby's bones exclusively contain hematopoietically active "red" marrow, and there is a progressive conversion towards inactive "yellow" marrow with age. In adult humans, active bone marrow is primarily located in the ribs, vertebrae, sternum, and bones of the pelvis.
ICRP Glossary entry - June 2019
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