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 consisting chiefly of fat cells and predominating in the cavities of the long bones—called also yellow marrowb: a reddish bone marrow containing little fat, being the chief seat of red blood cell and blood granulocyte formation, and occurring in the normal adult only in cancellous tissue especially in certain flat bones—called also red marrow
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