114 H. COTTIER, B. RODS, AND S. BARANDUN 



whole-body X-irradiated mice and rats during the first 24 hours after 

 exposure. 



MATERIAL AND METHODS 



Six-week-old Swiss albino mice and rats (inbred strains, Institute of 

 Eadiology , University of Bern) were given 600, 700 and 1 ,000 r air doses of 

 X-ray delivered by a 250 kVp X-ray machine with the following conditions: 

 250 kVp X-iays, filter Thoraeus I, HLV 1-52 mm Cu, 15 mA, t.s.d. 60 cm, 

 average dose-rate 24 r/min. The dose-rate was calibrated by Victoreen dosi- 

 meter. 



Lymphatic organs (thymus, spleen, lymph nodes, Peyers patches), bone- 

 marrow (mice) and liver tissue (rats) were taken 1, 5, 10, 15, 26, 34 and 60 

 minutes, 2, 4, 6, 12 and 24 hours after exposure. The material was fixed 

 immediately in 2% buffered osmic acid and embedded in butyl methyl 

 methacrylate according to Bernard's (personal communication) procedure. 

 The blocks were sectioned with glass or diamond knives on a Porter-Blum 

 microtome, placed on copper grids covered with a formvar film and examined 

 in a Siemens ElmiscojDe I electron microscope. 



RESULTS AND DISCUSSION OF RESULTS 



Nuclear chromatin 



The first detectable nuclear change in small lymphocytes consists in a 

 derangement of the normally rather diffusely arranged and finely granular or 

 fibrillar chromatin (diameter of granules in intact cells approximately 

 100 A) into larger and denser aggregates, leaving osmiophobic spaces between 

 the latter (Figs. 1(a), (b), 2(a)-(d)). This may be seen throughout the nucleus, 

 without a noticeable predeliction for chromatin located near the nuclear 

 membrane. With the doses and dose-rate used in these experiments some very 

 discrete alterations of the same quality may show immediately after the end 

 of exposure (e.g. 24 to 40 minutes after the start of irradiation); but on blind 

 examination of the electron micrographs it was often impossible to differen- 

 tiate between irradiated and non-irradiated lymphatic tissue taken before 15 

 minutes after radiation ( = 39 to 55 minutes after start of exposure; cf. the 

 independent studies of Bauer and Stodtmeister, 1961). The difficulty in 

 detecting very early ultrastructural changes may in part be due to slight 

 variations in the electron microscopic aspect of non-irradiated lymphocytes 

 as they occur even under standardized conditions of fixation. Later there is, in 

 many cells, a marked progression of this peculiar chromatin condensation 

 resulting in larger aggregates and/or clumping in some parts and oedema or 

 vacuolation in other pasts of the nucleus (Fig. 2(d)), with formation of 

 perinuclear halos (Figs. 1(b), 2(c)) and margination of the chromatin 



