bone marrow
US /ˈboʊn ˌmer.oʊ/
UK /ˈboʊn ˌmer.oʊ/

1.
a soft fatty substance in the cavities of bones, in which blood cells are produced
:
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The doctor explained that bone marrow is essential for producing blood cells.
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A bone marrow transplant can save lives for patients with certain blood disorders.