<P> Bone marrow is a semi-solid tissue which may be found within the spongy or cancellous portions of bones . In birds and mammals , bone marrow is the primary site of new blood cell production or hematopoiesis . It is composed of hematopoietic cells , marrow adipose tissue , and supportive stromal cells . On average , bone marrow constitutes 4 % of the total body mass of humans ; in an adult having 65 kilograms of mass ( 143 lb ) , bone marrow typically accounts for approximately 2.6 kilograms ( 5.7 lb ) . Human marrow produces approximately 500 billion blood cells per day , which join the systemic circulation via permeable vasculature sinusoids within the medullary cavity . All types of hematopoietic cells , including both myeloid and lymphoid lineages , are created in bone marrow ; however , lymphoid cells must migrate to other lymphoid organs ( e.g. thymus ) in order to complete maturation . </P>
<P> Bone marrow is a semi-solid tissue which may be found within the spongy or cancellous portions of bones . In birds and mammals , bone marrow is the primary site of new blood cell production or hematopoiesis . It is composed of hematopoietic cells , marrow adipose tissue , and supportive stromal cells . On average , bone marrow constitutes 4 % of the total body mass of humans ; in an adult having 65 kilograms of mass ( 143 lb ) , bone marrow typically accounts for approximately 2.6 kilograms ( 5.7 lb ) . Human marrow produces approximately 500 billion blood cells per day , which join the systemic circulation via permeable vasculature sinusoids within the medullary cavity . All types of hematopoietic cells , including both myeloid and lymphoid lineages , are created in bone marrow ; however , lymphoid cells must migrate to other lymphoid organs ( e.g. thymus ) in order to complete maturation . </P>
where is the bone marrow found in a long bone
where is the bone marrow found in a long bone
within the spongy or cancellous portions of bones
within the spongy or cancellous portions of bones