bone marrow
Meaning of bone marrow: Bone marrow exists in two forms, one actively producing blood cells and another composed mostly of fat, and the balance shifts with age.
Definition of bone marrow
Bone marrow is the soft tissue found within the internal cavities of bones, responsible for producing the body's blood cells. Red marrow, the active blood forming tissue, contains developing red blood cells, white blood cells, and platelets, along with the stem cells that give rise to them, while yellow marrow consists mostly of fat cells and has largely replaced red marrow within the shafts of long bones in adults.
At birth, red marrow fills nearly the entire skeleton, but with age it is gradually replaced by yellow marrow in many bones, so that in adults active red marrow is concentrated mainly within the pelvis, spine, breastbone, ribs, and skull, along with the ends of certain long bones.
Because bone marrow is the source of the body's blood cells, conditions affecting the marrow itself, such as certain cancers or a failure of normal blood cell production, can reduce the number of one or more blood cell types circulating in the blood.
Used in a sentence
A bone marrow sample was obtained from the hip bone to evaluate the low blood counts.
Good to know
Yellow marrow can convert back to active red marrow if the body's demand for blood cell production increases substantially, showing that it retains this capacity rather than losing it permanently with age.