The effects of irradiation on the blood and blood forming tissues.

A review is given of the effects of external whole body and local irradiation in man and experimental animals. Internal irradiation by radioactive substances in man and experimental animals is considered. An abstract of pertinent findings is given in the summary and conclusions. THE ORIGIN AND DIFFERENTIATION OF BLOOD FORMING TISSUE All investigators are in agreement that blood formation occurs embryologically in an identical manner in all mammals. It consists of a condensation in the yolk sac of the embryonic connective tissue, the mesenchyme, into blood islands, the central cells of which "round" up into basophilic cells which give rise to the primitive erythroblasts. These first basophilic cells have been called large lymphocytes by Maximow, hemocytoblasts by Bloom, Maximow, and by many other names. The blood cells produced in this embryonic site are largely so-called primitive red corpuscles which acquire hemoglobin, remain nucleated, and carry oxygen. As the embryo grows, primitive cells migrate to other sites and they, as well as local mesenchymal cells in these sites give rise to the definitive generation of red corpuscles, granulocytes, megakaryocytes, lymphocytes and other blood cells. The production of the cellular elements of the blood in places other than the yolk sac proceeds in a definite pattern (see Bloom''), probably in the following order: liver, spleen, lymph nodes, and bone marrow. No division of function into production of myeloid and lymphatic cells occurs in any of these sites until late in embryonic life. In the human at the time of birth, with the exception of minute foci in the liver which soon disappear, erythropoiesis and myelopoiesis have been taken over entirely by the bone marrow, and the production of lymphocytes and monocytes by the lymphatic system—lymph nodes and spleen. Only under pathological conditions such as pernicious anemia, leukemias, etc., does erythropoiesis or myelopoiesis ever occur in the adult liver, spleen, or lymph nodes (extramedullary hemopoiesis). In some mammals, however, notably the mouse, and to some extent the rat and rabbit, the spleen is active in the formation of practically all blood elements even in the adult. An important fact to remember is that the bone marrow normally produces the erythrocytes, the granulocytes, and the megakaryocyte series, and the lymphatic system produces the lymphocytes. The site of production of monocytes is somewhat controversial, but experimental evidence compiled by Bloom'' and others seems to indicate that their production is also a function of the lymphatic system. Under normal conditions the body tends to maintain the cellular elements of the peripheral blood in fairly constant numbers. The mechanisms accounting for these phenomena are generally unknown except perhaps in the case of the erythrocyte where the degree of peripheral oxygenation regulates the rate of production, release from spleen, etc. Under normal conditions homoplastic hemopoiesis accounts for the supply of mature erythrocytes