Color enhanced light micrograph of bone marrow showing fat cells. Bone marrow is the flexible tissue in the interior of bones. In humans, red blood cells are produced by cores of bone marrow in the heads of long bones in a process known as hematopoiesis. Bone marrow is also a key component of the lymphatic system, producing the lymphocytes that support the body's immune system. Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat. Magnification unknown.