Medical Definition of bone marrow
: a soft highly vascular modified connective tissue that occupies the cavities and cancellous part of most bones and occurs in two forms:a: a whitish or yellowish bone marrow that consists chiefly of fat cells and is found especially in the cavities of the long bones—called also b: a reddish bone marrow that contains little fat, is the chief site of red blood cell and blood granulocyte formation, and occurs in the normal adult in cancellous tissue especially of certain flat bones—called also
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