Intestinal crypt survival was studied in mice irradiated in air, 100% normobaric oxygen, 100% hyperbaric oxygen (30 psi), and 6% normobaric oxygen. Exposures from 900-2000 R at the rate of 47 R/min. Intestinal crypt survival was determined by the method of Hagemann, Sigdestad, and Lesher (4). The results demonstrated no difference in crypt survival for mice irradiated in air, normobaric oxygen, or hyperbaric oxygen. The crypt survival of mice was shown to have a D 0 from 377 to 380 R and a n from 7.9 to 9.7 for mice irradiated in air, 100% oxygen, or 100% oxygen at 30 psi, while mice irradiated in 6% oxygen had a <tex-math>$D_{0}=529$</tex-math> R with a n=8.1. The DMF (ratio of D 0 in 6% oxygen to D 0 in oxygen) was found to equal 1.4. The lack of difference in air and oxygen suggests the highly vascularized (oxygenated) gut is maximally sensitive. Animal lethality (<tex-math>${\rm LD}_{50(5)}$</tex-math>) is compared with crypt survival.

Intestinal crypt survival was studied using the method of Hagemann, Sigdestad and Lesher (8) in mice irradiated with 250 kVcp x-rays, cobalt-60 and fission neutrons. The D 0 's for the three treatment modalities were 377, 375 and 103 rads with extrapolation numbers of 6.2, 9.2 and 3.1, respectively. The RBE (ratio of D 0 's relative to x-rays) was 3.6 and 1.0 for neutrons and cobalt-60, respectively. The <tex-math>${\rm LD}_{50(5)}$</tex-math> values were found to be 1081 (945-1247), 1365 (1279-1456) and 252 (234-273) rads. The RBE [ratio of <tex-math>${\rm LD}_{50(5)}$</tex-math> relative to x-rays] values were found to be 4.3 and 0.8 for neutrons and cobalt-60 irradiation. Intestinal crypt survival varied from 25 to 34% of control at all of the <tex-math>${\rm LD}_{50(5)}$</tex-math> values. Total and per crypt cellularity was determined at 3 days postirradiation for each modality. The maximum proliferative response, measured at this time, occurred at or near the <tex-math>${\rm LD}_{50(5)}$</tex-math> dose for each type of radiation.

Mice were given whole-body exposures of 500 R, or abdomen-only exposures of 900 R. One or 4 days later, when crypt cellularity was either relatively low or high, respectively, a graded series of exposures was given, and crypt survival and total and per crypt levels of proliferative cellularity measured 3 days thereafter. Crypt survival was found to be markedly affected by crypt cellularity existing at the time of irradiation. Advantage may be taken of the compensatory proliferative intestinal response following a given radiation exposure to ameliorate somewhat, the efficacy of a subsequent exposure for crypt killing. A precise analysis of the relationship between proliferative and crypt progenitor compartment sizes was thwarted however, by an apparent increase in D 0 of cells surviving the conditioning exposure. The difference in D 0 's was marginally significant when determined 3 days after 500 R, and not significant 1 or 4 days after 900 R. At high radiation exposures, the proliferation rate of cells in surviving crypts was markedly reduced, in spite of a continued stimulus for accelerated proliferation. This suggests that a condition relating to growth, possibly analogous to small colony formation seen in vitro, exists in the heavily irradiated mammalian intestine.

Hydroxyurea was employed as a synchronizing agent for proliferative intestinal crypt cells in vivo. The drug was found to kill in excess of 90% of S-phase cells, and to block the G 1 to S transition for 2-3 hours. Two injections of drug were used to obtain enhanced synchrony. The age response for intestinal crypt survival has been obtained. The data indicate decreasing radiosensitivity of cryptogenic cells in the order $G_{1}\rightarrow G_{2}\rightarrow S$ . Total proliferative cellularity in the intestine, measured 3 days after irradiation, was found to be directly proportional to crypt survival. Cellularity per surviving crypt at this time was inversely related to crypt survival. Killing the relatively radioresistant S-phase cells by hydroxyurea greatly reduced crypt survival, and dramatically reduced the animal ${\rm LD}_{50(6)}$ from 1125 to 764 R.

The split-dose response of jejunal crypt survival has been measured for first exposures of 1000 or 600 R (1400 R total exposure). The curves obtained are analyzed in terms of repair of sublethal damage, progression through the cell cycle, and alterations in multiplicity commencing after the G 2 block. The maximum survival ratio for that portion of the curves which probably represents repair of sublethal damage is 3-4, which is considerably less than anticipated from previous studies on single-dose crypt survival. Total and per crypt levels of proliferative cellularity have been measured 3 days following the second exposure. These curves represent the net result of perturbing a complex steady-state in vivo system and are but partially conducive to temperate analysis at this time.

A radiation survival curve for intestinal crypts has been obtained. It is characterized by a D 0 of 330 R and n of 10. Crypt survival is quantitatively similar for assays performed at 3 and 4 days following irradiation, although crypt size varies considerably during this interval. An as yet unexplained discrepancy exists between the crypt survival curve obtained experimentally and that predicted from crypt cell survival parameters and compartment size. Proliferative crypt cells appear to constitute the intestinal stem cell compartment. Recovery in terms of cellularity occurs even following exposures as high as 1400 R. It seems critical for 5-day survival of the animal that proliferative cellularity reaches some requisite level by day 3 following exposure; later increases in cellularity are not reflected by increased 5-day survival. It appears from this that crypt survival is an important feature with respect to 5-day survival of the irradiated animal.

Mouse ascites tumor cells were irradiated in vivo and the delay in cell division determined by periodic examination of mitotic index. Fractional exposures of 200 R were given eight times for a total of 1600 R. The effect on survival was determined 9 days after an inoculation of 4000 cells which had been removed from the irradiated animal. Survival was determined by counting the cells and was expressed as percentage of controls. It was found that fraction intervals of 0.5 and of 1 hour gave minimal delay and maximal survival. As the fractional interval was increased to 3 and to 4.8 hours, the delay became directly proportional to total radiation exposure and there was little recovery from lethal action as compared to the effect of other methods of exposure. It appeared that fractionation at 4.8-hour intervals tended to partially synchronize cells into later stages of the generation cycle; increased mitotic delay; and reduced survival as compared with effects of shorter fraction intervals.

Topical ocular application of DMSO prior to head-only irradiation of mice prevented complete cataract formation. While some degree of radiation damage was detected in these lenses, it did not progress to complete lenticular opacity as occurred in the irradiated control animals. The minimum effective DMSO concentration was about 10%. One hundred percent DMSO was effective when applied only one minute prior to exposure. The drug protected against exposures up to 1400 R; 1800 R was lethal. It was not effective when administered following irradiation. The mechanism of DMSO protection may involve hypertonicity to some extent and may involve alteration in epithelial cell metabolism. DMSO, as administered in these experiments, did not result in systemic radioprotection. In the same animals wherein DMSO (in high concentration) protected against radiation cataract, it resulted in radiosensitization of the cornea. This action of DMSO may be due to a nonspecific stress on the irradiated cornea. No corneal lesions were observed in the DMSO treated unirradiated control animals. The reason for the dichotomy between DMSO effect on radiation damage to lens and cornea remains abscure.

The effect of X-irradiation on <tex-math>$\text{glycine-}1\text{-}{}^{14}{\rm C}$</tex-math> transport in Ehrlich ascites tumor cells was investigated immediately after in vitro radiation exposure in the presence and absence of oxygen, in the presence and absence of bovine serum albumin, and at different cell concentrations. The effect (reduction of transport capability) was more pronounced when the cells were irradiated as a 1:500 suspension than as a 1:10 suspension. There was an oxygen enhancement in both the 1:500 and the 1:10 experiments. Bovine serum albumin exerted a protective influence when present (1:500 cells) during irradiation. Neither the "passive" permeability to glycine (measured at 0°C) nor the fraction of viable cells in the incubation population (measured by aqueous eosin staining) was significantly different from control. Irradiation of the medium prior to addition of cells was without effect within the experimental period. The effect on transport probably involved the indirect action on the plasma membrane, since (1) it was immediate, (2) it was influenced by the volume ratio of cells to irradiation medium, and (3) a nonpenetrating substance afforded protection when it was present during irradiation.