Bone marrow is the vascular pulpy connective tissue found in the marrow cavity of the bones.
It is of two types
(i) Yellow marrow It is a yellow coloured tissue rich in fat cells called adipocytes. It is richly supplied with the blood vessels and nerve fibres. It is found in the shaft region of the bones.
(ii) Red marrow It is red coloured and blood cells are formed in it. It is found at the ends of long bones, ribs, sternum, vertebrae, etc. In a foetus, red marrow occurs in all bones. After birth, the red marrow becomes restricted to the above mentioned locations.