Origin of Blood Cells

During the first month of intrauterine life the blood cells are formed outside the embryo in the blood islands of the yolk sac. These cells are large nucleated hemoglobiniferous cells resembling the rubriblast of the pernicious anemia type. As development continues, these primitive red cells are replaced by smaller cells of the erythrocytic series, which by the second month are being formed from the mesenchyme of the liver. At this time granular leukocytes and megakaryocytes can also be found in the liver. The granulocytes become quite numerous by the fifth month of fetal life. During the fourth month red blood cells are actively formed in the spleen, but erythropoiesis diminishes there during the fifth month. The spleen appears to be concerned with lymphopoiesis from the fourth month and monocytes appear during the fifth month. At about the fifth month granulocyte and thrombocyte formation occurs. In the adult, erythrocytes, granulocytes and thrombocytes originate in the bone marrow, lymphocytes in lymph nodes and spleen, and monocytes probably from the reticuloendothelial tissue, especially of lymph nodes and spleen. Plasma cells are believed to develop by metamorphosis from lymphocytes. The marrow of practically all the bones is red during the first few years of life, but between the ages of 5 and 7 years the long bones begin to form fatty marrow. By 18 years of age red marrow is confined to the vertebrae, ribs, sternum, skull, pelvic bones and to some extent, the proximal epiphyses of the humerus and femur. To meet unusual demands the fatty marrow can rapidly become replaced by hematopoietic marrow. The processes of blood manufacture and blood destruction are going on continuously in the normal individual and the blood is kept in a state of more or less constant cellular composition. Red blood cells survive from 100 to 128 days, granulocytes and thrombocytes can survive to 4 days. The lymphocytes' life span is still subject to study; some apparently live only hours, others days. The stimulus for red blood cell r elease into the circulation is probably anoxemia and for leukopoiesis probably some chemotactic factors liberated at the site of tissue injury or destruction of leukocytes. Certain disease processes result in the production of a large number of a particular type of cell, and this disease stimulus causes the blood forming tissues to pour forth immature cells into the blood stream. The presence of immature cells in the circulating blood contributes to the diagnosis of many diseases. For this reason, a knowledge of the morphology and staining characteristics of the mature and immature blood cells is necessary.