3 Lymphocytes develop from lymphoblasts in bone marrow and lymphoid organs. 4 Some circulating lymphocytes appropriately stimulated can also become lymphoblasts. The naked-eye appearance of fresh, unstained marrow may be red from many developing RBCs, or yellow from mainly fat cells. The iron recovered is stored, combined with protein as ferritin granules, before release to the labile pool and reuse. Blood cells develop extravascularly, are stored, then released through the sinusoidal wall into the circulation.
Fat cells are present, large and empty of fat in embedded sections.
3 Microscopic methods for marrow include sections, and smears of aspirated sternal marrow stained with a blood stain. Although the marrow smear picture appears static, it represents part of a dynamic system of cell populations. The population of each compartment at any time represents the balance between the numbers entering and leaving. Many of these leucocytes in the vessels hug the wall as a barely moving, marginated reserve. 2 Erythropoietin is a humoral factor, released from the kidney in response to hypoxia, that increases RBC production.