CYTOPLASMIC DNA IN IRRADIATED NEURAL TUBE 91 



tion of contieuons solid material? What use is made of all of this cytoplasmic 

 DNA? Surely a ready made supply of its own brand of DNA is a material 

 too valuable to rapidly di\idino cells to be wasted. What is the role of 

 DNase, or its absence, in connection with cytoplasmic DNA? Finally, there 

 remains the unsolved problem of the great radiosensitivity of the differentiat- 

 ing neinoblast. What change has suddenly come over this cell, probably still 

 with mitotic potentiality, to increase its sensitivity and, with neurofibril ap- 

 pearance, to lea\e it again to make it among the most resistant of cells? 



Summary 



The material consisted of 2- to 4-day chick embryos subjected to 200 to 500 

 r of x-irradiation or labeled with thymidine-H"* of high specific acti\ity. 



Moderate doses of radiation led to a structural change in the cytoplasm 

 in numerous cells of the early neiual tube. A striking featvne a few hoins 

 following exposure to ionizing radiation was the presence within the cyto- 

 plasm of one or more dense, basophilic bodies approximately 2-5 fx in diam- 

 eter. These typically consisted of one or several Feulgen-positive centers, 

 surrounded by an RNA-containing rim. The centers were digested by DNase. 

 They represent a relatively large amount of extranuclear DNA. Electron 

 microscopy demonstrated their great density and confirmed their cyto- 

 plasmic location. At the lower dosage of x-ray, the process was completely 

 reversible, without an inteivening period of degeneration. The bodies were 

 not confined to the neural tube, although they attained great prominence 

 there, but were widely distributed throughout the embryo. 



It is concluded that the bodies are of se\eral types. It is possible to dis- 

 tinguish between those resulting from degeneration of mitotic stages and 

 those of interphases on the basis of their morphology. Micronuclei are an- 

 other type, sometimes present in considerable number. 



Refp:rences 



.'Mberti. W.. and Politzer. G. 1924. Uber den Einfluss dcr Rontgenstrahlcn auf die 

 Zellteilung. Arch, mikroskop. Anat. u. Entivicklungsmech. 100. 83-109. 



Alfert, M. 1955. Changes in the staining capacity of nuclear components during cell 

 degeneration. Biol. Bull. 109, 1-12. 



Baeckeland. E., Chevrcmont-Comhaire. S., and Chevremont, M. 1957. Dosages cyto- 

 photometriques d'acides desoxyribonucleiques (nucleaires et cytoplasmiques) dans 

 des cellules traitees \i\antes par une desoxyribonuclease acide. Compt. rend. acad. 

 sci. 245, 2390-2393. 



Bajer, A. 1958. Cinc-micrographic studies on chromosome movements in B-irradiated 

 cells. Chromosoma 9, 319-331. 



