116 H. COTTIER, B. ROOS, AND S. BARANDUN 



to become clearly visible. This is consistent with observations made after 

 partial-ceU irradiation which do not seem to indicate that centrioles are 

 especially radiosensitive structures (Bloom, et al., 1955; Zirkle et at., 1955, 

 1960). It may be mentioned that we several times observed non-symmetrical 

 partition ("budding") of centrioles in irradiated cells (Fig. 3(b)). Doubhng of 

 centrioles in protozoa (Cleveland, 1957) and higher invertebrates (Mazia, 

 1960, 1961) has been described to take place by birth of a tiny "mfant 

 centriole" from a mother centriole. But until now only one such asymmetric 

 pair of centrioles connected by a strand has been reported in electron micro- 

 graphs (de Harven and Dustin, 1960). It remains to be seen whether the 

 relative frequency with which this peculiar picture was seen in our studies on 

 iiTadiated cells is not coincidental. Smce the spmdle apparatus most often is 

 not readily demonstrated in ultra-thin sections (reviewed by Bernhard, 1958), 

 we do not know whether radiation-mduced derangements of the spindle 

 fibres have anything to do with these nndmgs. Such a process is considered as 

 one of the possible causes of mitotic delay, a very sensitive and early 

 biochemical reaction of irradiated cells (reviewd by Bacq and Alexander, 

 1961). In view of Schrader's (1953) hypothesis that kinetochores may become 

 transformed into centrioles, this question deserves further attention. Wendt 

 (1961) reported that doses of 600-1,000 r delivered as short-term irradiation 

 to cultured chick fibroblasts produced within 3 to 5 hours a reversible vacuola- 

 tion and milky change of the centroplasm detectable by phase-contrast 

 microscopy. In our studies we were unable to see more than an occasional and 

 questionable halo around the centrioles. 



Mitochoyidria 

 Swollen, disrupted or partly dissolved mitochondria with focal loss of 

 visible inner cristae are readily found in irradiated cells showing advanced 

 nuclear damage (Fig. 3(a)); but we could not estabHsh beyond doubt that 

 discrete changes of the same quality occur very early after exposure and in 

 absence of detectable nuclear injury. The marked reduction in size and 

 increased over-all electron density of rat liver cell mitochondria 4 hours after 

 whole-body X-irradiation with 1,000 r (Fig. 4(a), (b)) does not necessarily, 

 and exclusively, reflect direct radiation damage since local exposure to the 

 same dose does not produce an equal degree of alteration. The difiiculties m 

 evaluatmg eventual damage to mitochondrial membranes (Figs. 1(a), 

 2(a)-(d)) are the same as those discussed above for the nuclear membrane. 



Endoplasmic reticulum, ergastoplasm and annulate lamellae 

 Except for an occasional widening or collapse of the vesicular, canahcular 

 and spatial system of the endoplasmic reticulum and ergastoplasmic cisternae 

 in several irradiated cells withm 1 hour after exposure, no definite early 



