Bone marrow

soft tissue at the center of the bones which produces blood cells

Bone marrow (or medulla ossea) is the soft tissue found in the hollow interior of bones. In adults, marrow in large bones produces new blood cells.
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Bone marrow n. The soft, fatty, vascular tissue filling the cavities of bones, having a stroma of reticular fibers and cells. [more]Bone marrow - Community and Resources

Noun
1. the fatty network of connective tissue that fills the cavities of bones
(synonym) marrow
(hypernym) connective tissue
(hyponym) red marrow, red bone marrow
(part-holonym) bone, os
2. very tender and very nutritious tissue from marrowbones
(synonym) marrow
(hypernym) dainty, delicacy, goody, kickshaw, treat
(part-holonym) marrowbone

The bone marrow is the soft substance that fills bone cavities. It is composed of mature and immature blood cells and fat. The blood cells include white blood cells, red blood cells and platelets. Diseases or drugs that affect the bone marrow can affect the "total counts" of these cells.