CELLULAR COMPONENTS: ELECTRON MICROSCOPIC STUDY 115 



(Fig. 2(d)). When focal or complete nuclear pyknosis occurs, the corresponding 

 part of the perinuclear space widens and/or a clear zone or vacuole often 

 forms in the adjoining cytoplasm, suggestive of transfer of fluid into the 

 latter (Fig. 3(a)). 



Nucleolus 



Swollen, but morphologically intact cells may show within 1 hour after 

 the end of exposure unusually large nucleolar areas (Fig, 2(a)). But with 

 osmic acid fixation alone distinction between an increase in size of the 

 nucleolus and a condensation of nucleolus-associated chromatin may be 

 very difficult. 



Nuclear membrane 



A lack of sharpness, or discontinuities in the osmiophilic inner or outer 

 nuclear membrane has been observed (Figs. 1(a), 2(a), (b), (d)) but must be 

 interpreted with great caution; images looking like membrane defects are 

 often due to tangential sectioning and folding of the nuclear surface (cf. 

 review by Bernhard, 1958). To overcome part of this difficulty a great number 

 of serial ultra-thin sections would be necessary; unfortunately the technical 

 possibiHties for such a procedure have been unsatisfactory until lately, and 

 we have not yet sufficient data in this respect. It may be mentioned, however, 

 that an early loss of distinct membrane structures was more often seen in 

 irradiated than in non-irradiated cells; if all these findings were due to tangen- 

 tial sectioning, the frequency would be expected to be equal. With these 

 restrictions in mind we may conclude from our present findings that: 



1. No definite and clearly detectable radiation damage to the nuclear 

 membrane was observed on single sections prior to the appearance of 

 chromatin derangement. 



2. Wherever, apart from "pores", discontinuities in the nuclear membrane 

 are found within minutes after exposure, these single observations as such 

 cannot be acknowledged as proof of membrane injury before serial sectioning 

 has shown them to be real defects. 



3. It remains to be seen whether circumscribed cytoplasmic and/or 

 nuclear oedema in spatial connection with an apparent partial lack or 

 "fading" of the nuclear membrane (Figs. 1(a), 2(d)) can be regarded as 

 circumstantial evidence for membrane damage. 



4. No morphologic alteration of the so-called pores of the nuclear mem- 

 branes is observed prior to radiation-induced chromatin derangement. 



Centrosome and spindle apparatus 



Morphologically intact centrioles are found throughout the first hour 

 after exposure (Fig. 1(a)), e.g. the period of time it takes the nuclear damage 



