SUMMATION BY CHAIRMAN 



159 



DEVELOPING CEREBRAL WALL 

 INJ THE RAT 



6^& 



20 DAYS 



Fig. 1. Schematic representation of the neuroepithelium. (Adapted from Hicks 

 and D'Amato, 1960a.) 



principal radiosensitive ones, we believe. By radiosensitive we mean killed and 

 visibly necrotic in 2 or 8 hours. The threshold of this effect is about 20-30 r. 

 Above 200 r the selectivity is gradually lost, and more and more mature cells 

 are killed. Also, the time after radiation that they die may lengthen from hours 

 to a day or more. A good many young cortical neurons are killed by doses of 

 several hundred r, but most of them escape for a while. Even after 800 r some 

 members of the proliferative cell colony remain, and for the few days that an 

 embryo so exposed may live, these residual cells actually go on proliferating 

 brain or other neural structures as best they can. The little figures at the right 

 in Fig. 1 emphasize the changing ratios of mature to immature cells as the fore- 

 brain develops. At 13 days most of the primitive cells are involved in the pro- 

 liferative cycle, and a cell no sooner divides into two postmitotic cells than the 

 daughter cells enter the premitotic stage of the cycle again. This frenzied growth 

 sub.sides by term, although a series of bursts of mitotic activity occurs in the fore- 

 brain proliferative colonies between 13 days and birth, complicating the assess- 

 ment of just which cells are radiosensitive. 



A detailed account of the brain malformations, especially in the cortex, and 

 the mechanisms of their formation can be found in Hicks (1958), Hicks et al. 

 (1954, 1959), and Hicks and D'Amato (1960a). The patterns of cortex of a rat 

 irradiated with 200 r on day 13 is so completely difTerent from one irradiated 

 on day 20, or day 16, that from the neurologic standpoint, lumping such animals 

 together in behavioral experiments as "prenatally irradiated" would be absolutely 

 meaningless. Considerable data on the patterns of malformation of the retinas 



