960 RADIATION BIOLOGY 



ology, the nature of this relation is poorly understood. The manifesta- 

 tions of the radiation syndrome and possible modes of their development 

 will be the subject of the following discussion. 



BLOOD AND HEMATOPOIESIS 



The radiosensitivity of blood-forming tissues and the consequent 

 hazard of blood damage have attracted considerable attention since the 

 classic work of Heineke in 1903. The vast early literature on this sub- 

 ject has been reviewed by Dunlap (1942). Although blood and blood 

 formation are discussed in some detail by Jacobson in Chap. 16, present 

 concepts merit brief consideration here since the hematologic picture is 

 an integral part of the radiation syndrome. 



Peripheral blood changes depend, for the most part, upon the radio- 

 sensitivity of the parent or precursor cells, their ability to recover from 

 injury, and the life span or rate of utilization of the mature elements in 

 the blood. From morphological studies on acutely irradiated animals, 

 it may be seen that changes in the peripheral blood cells reflect to a con- 

 siderable extent alterations in the bone marrow, lymph nodes, and 

 spleen (Lawrence, Dowdy, and Valentine, 1948; Lawrence, Valentine, 

 and Dowdy, 1948; Bloom and Jacobson, 1948; Jacobson, Marks, and 

 Lorenz, 1949; Suter, 1947; Steamer et at., 1947a). This is not, however, 

 readily apparent after chronic irradiation (Jacobson, Marks, and Lorenz, 

 1949). While the site of action appears to be mainly in the blood-form- 

 ing organs, indirect or remote effects on hematopoietic tissue and direct 

 effects on the morphological constituents of peripheral blood are not 

 unknown. The alteration of lymphoid tissue in areas distant from the 

 site of irradiation is an example of the indirect type of action (Barnes and 

 Furth, 1943; Leblond and Segal, 1942). The utilization, destruction, 

 and production of blood cells following irradiation of the whole animal are 

 undoubtedly influenced indirectly by certain general derangements of body 

 economy. Thus, infection may lead to an increased demand, damaged 

 epithelial surfaces to an increased loss, nutritional disturbances to a 

 decreased formation of substances necessary for normal hematopoiesis, 

 and circulatory changes to a decreased effective blood cell mass. 



Blood exposed in vitro to moderate dosages of radiation, i.e., in the 

 lethal range for mammals, shows only slight changes. A direct cytocidal 

 effect on lymphocytes from the thymus and spleen has been observed in 

 the test tube with as little as 50 r and may contribute to the early decrease 

 in circulating lymphocytes (Schrek, 1946). Mature granulocytes, on 

 the other hand, are quite resistant and are not affected directly by radi- 

 ation except in excessive dosage. Degenerate cells are not found gener- 

 ally in smears of mammalian blood made during the first few days after 

 irradiation ; they may be noted, however, in smears prepared from chicken 



