Diurnal variations in radiosensitivity of mouse intestine

Does chronomodulated radiotherapy improve pathological response in locally advanced rectal cancer?

The predominant mode of radiation-induced cell death for solid tumours is mitotic catastrophe, which is in part dependent on sublethal damage repair being complete at around 6 h. Circadian variation appears to play a role in normal cellular division, and this could influence tumour response of radiation treatment depending on the time of treatment delivery. We tested the hypothesis that radiation treatment later in the day may improve tumour response and nodal downstaging in rectal cancer patients treated neoadjuvantly with radiation therapy. Recruitment was by retrospective review of 267 rectal cancer patients treated neoadjuvantly in the Department of Radiation Oncology at the Canberra Hospital between January 2010 and November 2015. One hundred and fifty-five patients met the inclusion criteria for which demographic, pathological and imaging data were collected, as well as the time of day patients received treatment with each fraction of radiotherapy. Data analysis was performed using the Statistical Package R with nonparametric methods of significance for all tests set at p < 0.05. Of the 45 female and 110 male patients, the median age was 64. Seventy-three percent had cT3 disease and there was a mean tumour distance from the anal verge of 7 cm. Time to surgical resection following radiotherapy ranged from 4 to 162 days with a median of 50 days, with a complete pathological response seen in 21% of patients. Patients exhibiting a favourable pathological response had smaller median pre- and postradiotherapy tumour size and had a greater change in tumour size following treatment (p < 0.01). Patients who received the majority of their radiotherapy fractions after 12:00 pm were more likely to show a complete or moderate pathological response (p = 0.035) and improved nodal downstaging. There were also more favourable responses amongst patients with longer time to surgical resection postradiotherapy (p < 0.004), although no relationship was seen between response and tumour distance from the anal verge. Females were less likely to exhibit several of the above responses. Neoadjuvant radiotherapy for locally advanced rectal cancer performed later in the day coupled with a longer time period to surgical resection may improve pathological tumour response rates and nodal downstaging. A prospective study in chronomodulated radiotherapy in this disease is warranted.

Acute gastrointestinal syndrome in high-dose irradiated mice

The most detailed reports of the response of the gastrointestinal system to high dose acute radiation have focused mainly on understanding the histopathology. However, to enable medical countermeasure assessment under the animal rule criteria, it is necessary to have a robust model in which the relationship between radiation dose and intestinal radiation syndrome incidence, timing, and severity are established and correlated with histopathology. Although many mortality studies have been published, they have used a variety of mouse strains, ages, radiation sources, and husbandry conditions, all of which influence the dose response. Further, it is clear that the level of bone marrow irradiation and supportive care can influence endpoints. In order to create robust baseline data, the authors have generated dose response data in adult male mice maintained under identical conditions and exposed to either total or partial-body irradiation. Partial-body irradiation includes both extensive (40%) and minimal (5%) bone marrow sparing models, the latter designed to correlate with an established primate model and allow assessment of effects of any medical countermeasure on all three major radiation syndromes (intestinal, bone marrow, and lung) in the surviving mice. Lethal dose (LD(30), LD(50), and LD(70)) data are described in the various models, along with the impact of enteric flora and response to supportive care. Correlation with diarrhea severity and histopathology are also described. These data can be used to aid the design of good laboratory practice (GLP)-compliant Animal Rule studies that are reflective of the conditions following accidental radiation exposure.

The stem cells of small intestinal crypts: where are they?

Recently, there has been resurgence of interest in the question of small intestinal stem cells, their precise location and numbers in the crypts. In this article, we attempt to re-assess the data, including historical information often omitted in recent studies on the subject. The conclusion we draw is that the evidence supports the concept that active murine small intestinal stem cells in steady state are few in number and are proliferative. There are two evolving, but divergent views on their location (which may be more related to scope of capability and reversibility than to location) several lineage labelling and stem cell self-renewing studies (based on Lgr5 expression) suggest a location intercalated between the Paneth cells (crypt base columnar cells (CBCCs)), or classical cell kinetic, label-retention and radiobiological evidence plus other recent studies, pointing to a location four cell positions luminally from the base of the crypt The latter is supported by recent lineage labelling of Bmi-1-expressing cells and by studies on expression of Wip-1 phosphatase. The situation in the human small intestine remains unclear, but recent mtDNA mutation studies suggest that the stem cells in humans are also located above the Paneth cell zone. There could be a distinct and as yet undiscovered relationship between these observed traits, with stem cell properties both in cells of the crypt base and those at cell position 4.

Intestinal crypt clonogens: a new interpretation of radiation survival curve shape and clonogenic cell number

Estimates of the clonogen content (number of microcolony-forming cells) of murine intestinal crypts using microcolony assays show an apparent dependence on the radiation dose used in the assay of clonogen content. Crypt radiation survival curves often show increased curvature beyond that expected on the basis of the conventional linear-quadratic model. A novel form of crypt survival curve shape is proposed based on two contributory mechanisms of crypt killing. Six previously published sets of microcolony data were re-analysed using a dual-kill model, where target cells are killed by two contributory mechanisms, each described by a linear-quadratic function of dose. The data were analysed as two series--high-dose rate and low-dose rate irradiation. The data were fitted to the models using direct maximization of a quasi-likelihood, explicitly allowing for overdispersion. The dual-kill model can reproduce both the apparent dose-dependence of the clonogen estimates and the high-dose curvature of the dose-response curves. For both series of data the model was a significantly better fit to the data than the standard linear-quadratic model, with no evidence of any systematic lack of fit. The parameters of the clonogenic cell component of the model are consistent with other studies that suggest a low clonogen number (somewhat less than five) per crypt. The model implies that there is a secondary mechanism decreasing clonogen survival, and hence increasing clonogen number estimates, at high doses. The mechanisms underlying the modification of the dose-response are unclear, and the implied mechanisms of, for example, slow growth, induced either directly in the surviving cells or indirectly through stromal injury or bystander effects are only speculative. Nevertheless, the model fits the data well, demonstrating that there is greater kill at high doses in these experimental series than would be expected from the conventional linear-quadratic model. This alternative model, or another model with similar behaviour, needs to be considered when analysing in detail and interpreting microcolony data as a function of dose. The implied low number of < or = 5 of these regenerative and relatively radioresistant clonogenic cells is distinct from a similar number of much more radiosensitive precursor stem cells which undergo early apoptosis after doses around 1 Gy.

Review article: effects of radiation on the cell proliferation kinetics of epithelial tissues--therapeutic implications

The cell kinetic responses of the epithelia of the skin, oral mucosa and intestine to photon irradiation are reviewed. One of the fundamental assumptions made in the development of mathematical models, used to predict the acute response of normal tissues to changes in fractionation protocols, is "equal effect per fraction". There is now accumulating cell kinetic data to indicate that this assumption is unlikely to be valid. The implications of these findings are discussed.

Cell kinetics in rat small intestine after exposure to 3 Gy of gamma-rays at different times of the day

Qualitative and quantitative morphological changes in rat jejunum were studied after a whole-body exposure to 3 Gy of gamma rays. Four groups of animals were irradiated at different times of the day, namely midnight, 06.00, 12.00 and 18.00 hours. The number of epithelial cells, labelling and mitotic indices were evaluated in crypt sections and the spatial distribution of S-phase cells was determined. At 12 h after irradiation a marked reduction was observed in all parameters, but the proliferative activity was restored quickly and at 36 h after irradiation the values were significantly higher than the controls. The frequency distribution of labelled cells at different positions in the crypt was reduced at 12 h but a clear expansion of S phase cells to positions near to the crypt villus junction was observed during the recovery phase. The animals irradiated at different times of the day showed a similar general post-irradiation response in the number of cells along the side of the crypt, labelling and mitotic indices and in the distribution of S phase cells along the crypts. It is worth noting that the animals exposed at midnight had a distribution of S phase cells similar to controls at 72 h post-irradiation, i.e. earlier than the other groups.

Radiation hypersensitivity of LEC strain rats controlled by a single autosomal recessive gene

LEC strain rats (LEC rats), which are known to develop hereditarily spontaneous fulminant hepatitis 4-5 months after birth, were highly sensitive to whole-body X-irradiation when compared to WKAH strain rats. The radiosensitivity of F1 hybrids of LEC and WKAH rats was similar to that of WKAH rats and significantly lower than that of LEC rats. Segregation data of backcross hybrids (F1 x LEC and LEC x F1) suggested that the hypersensitivity of LEC rats to whole-body irradiation is controlled by a single autosomal recessive gene. The radiosensitivity of fibroblasts from LEC rats was higher than that of fibroblasts from WKAH rats. The repair process of DNA double-strand breaks in LEC cells was slower than that in WKAH cells. LEC rats could provide a useful animal model to assist in understanding the mechanism of radiation-induced DNA damage and repair.

Influence of fractionation schedule on development of intestinal complications following localized irradiation. An experimental study in the rat

This study examined the relationship among time-dose-fractionation, development of intestinal complications, and histopathologic radiation injury in rat small bowel. In 177 rats, a functional loop of rat ileum was surgically transposed to the left scrotum. Three weeks later, fractionated irradiation was delivered to the transposed intestine as 2.8 Gy or 5.6 Gy daily fractions, or as 2.8 Gy twice daily with 12 hours' interval. Eleven experimental groups received total doses ranging from 28 Gy to 90 Gy. The animals were observed for intestinal complications, and groups of animals were killed 2 and 26 weeks after completion of irradiation for assessment of injury. Radiation injury was assessed by a semiquantitative histopathologic scoring system and by the frequency of lethal intestinal complications. Both increased fraction size and reduced overall treatment time increased the severity of subacute and chronic radiation injury, as well as frequency of intestinal complications. We conclude that rapidly proliferating cells (mucosal epithelium) play a pivotal role in the pathogenesis of radiation enteropathy and mechanisms other than radiation-induced mitotic cell death are pathogenetically involved.

Influence of circadian rhythms on radiosensitivity: single and fractionated dose studies in mouse skin

In rodent skin significant increases in labelling and mitotic indices have been reported during the period of maximum nocturnal activity. It has been suggested that sparing of radiation damage in fast-proliferating normal tissues could be achieved if treatments were given at the time of day when the maximum number of normal cells were most radioresistant. If changes in radiosensitivity do occur in tissues with circadian fluctuations in the cell kinetic parameters, then the magnitude of these changes should be dependent on the size of dose per fraction. Because of the implications for clinical radiotherapy, especially in regimes where multiple fractions per day are given (MFD), possible diurnal variations in radiosensitivity were investigated using single dose and fractionated X-ray regimens (5F/5 days, 8F/8 days), in rodent skin. Treatments were delivered at 1, 3, 5 a.m. (time of highest DNA synthetic activity in mouse epidermis), 6 and 7 a.m. (highest mitotic activity) and at 5 p.m. (minimum labelling and mitotic indices). To investigate a large range of doses per fraction, fractionated X-rays were given alone or followed by neutron top-up doses. Using a range of doses per fraction of 30 Gy down to 1 Gy, we did not detect any change in radiosensitivity with any of the schedules. Our results suggest that a decrease in normal tissue tolerance is unlikely to be observed in patients even if irradiated at a time of day at which a maximum increase in radiosensitivity might be predicted on the basis of a high mitotic index.

The circadian rhythm for the number and sensitivity of radiation-induced apoptosis in the crypts of mouse small intestine

Apoptosis is a mode of cell death involving nuclear pycnosis, cytoplasmic condensation and karyorrhexis. Changes in the number of apoptotic cells at various times (3-12 h) after a single dose of either 0.5 or 9.0 Gy given at 09.00, 21.00 or 03.00 h were studied in histological sections of small intestinal crypts of mice. The incidences of apoptosis were examined 3 or 6 h after irradiation at different times of day with different doses of gamma-rays ranging from 0.15 to 9.0 Gy. Survival curves were constructed from the dose-incidence curves for apoptosis, using the number of apoptotic cells after high doses (NM) as the maximum cell population size. The mean lethal doses (Do) for the dose range 0-0.5 Gy were calculated for each time of day. A circadian rhythm in both Do and NM values was detected, indicating that both the number and sensitivity of radiation-induced apoptosis were changing throughout the day. A possible explanation based on the cell-cycle states of the target cell population for apoptosis (presumably functional stem cells) was drawn. Most of the target cells were assumed to be in an extended G1 phase. Around 21.00 h a transition from G1 to S phase takes place in some of these cells (approximately seven or eight cells per whole crypt). The S phase then lasts till around 06.00 h. They may be at G2 and M around 06.00-09.00 h, and then they re-enter G1. The circadian rhythm for the number and sensitivity of the cells susceptible to apoptosis obtained in the present report agrees well with this pattern of cell-cycle phases of target cells.

Effect of radiation on glucose absorption in the mouse jejunum in vivo

In an attempt to develop a clinical assay for intestinal damage, the relationship between radiation dose and glucose absorption in the jejunum has been investigated. Lightly anesthetised C3D2F1 mice had their upper abdomen irradiated. The intestine was exteriorized and a 10-cm segment of the jejunum was isolated and included in a closed circuit perfusion loop. An isotonic solution containing 2 mg/ml D-glucose was perfused in 2 h and the rate of glucose absorption calculated. The absorption decreased as a function of dose and time after irradiation with the most prominent reduction after 4-5 days, returning to normal within a week after irradiation. A dose-response relationship was found after single doses between 5 and 17.5 Gy. Split-dose recovery of approximately 7.5 Gy was found within 2-4 h. This absorption insufficiency was only found to be present acutely, and no late abnormalities were seen for times greater than 12 months after irradiation. The results correlated with other functional endpoints such as absorptive surface. This assay seems suitable to demonstrate (acute) functional intestinal radiation damage and has obvious clinical applicability.

Repair kinetics in mouse lung after multiple X-ray fractions per day

The potential advantage for sparing normal tissue damage by hyperfractionation of low-LET radiation may be limited by the repair kinetics of tissues in the irradiated field. Tissues with slow repair kinetics will limit the number of fractions that may be given on the same day. Results are presented for mouse lung treated with a range of doses per fraction using either two or three fractions per day in multiple X-ray fractionation schedules. The results are analysed to determine whether the repair kinetics follow a single exponential function of time. The calculated repair rate (T1/2) was about 1.2 h for two fractions per day of 2 Gy (10F/5d) but slightly less (T1/2 = 0.8 h) for two fractions of 9 Gy (2F/1d). For smaller doses per fraction of 1.1 Gy, given three times per day (39F/13d), the T1/2 was not significantly less (T1/2 = 0.3-0.7 h). For three fractions per day of 1.1 Gy per fraction an unsatisfactory fit is achieved using a single exponential function of time, and a better fit is obtained using two components of repair. The repair kinetics are slow for lung, in comparison to acute reacting tissues (except skin), and may require that 6-8 h (i.e. four or five half-times) should be allowed between fractions on the same day so that more than 95 per cent of the repairable dose is repaired. At present the variation in repair kinetics with doses per fraction between 1.1 and 9 Gy are not significantly different, so no reduction of interfraction interval should be proposed.

Circadian rhythms in the incidence of apoptotic cells and number of clonogenic cells in intestinal crypts after radiation using normal and reversed light conditions

Variations in the number of radiation-induced morphologically dead or dying cells (apoptotic cells) in the crypts in the small intestine of the mouse have been studied throughout a 24-h period under a normal light regimen (light on, 07.00-19.00 h; light off, 19.00-07.00 h). A clear circadian rhythm was displayed in the apoptotic incidence 3 or 6 h after irradiation for each gamma-ray dose studied (range 0.14-9.0 Gy). The most prominent circadian rhythm was obtained after 0.5 Gy. The peak time of day for inducing apoptosis was 06.00-09.00 h, and the trough occurred at 18.00-21.00 h. Some mice were also transferred to a room with the light cycle reversed, and were irradiated on different days after the transfer. The apoptosis induced by 0.5 Gy or 9.0 Gy, or the number of surviving crypts (microcolonies) after 11.0 Gy or 13.0 Gy was examined. The transition point for reversal (i.e. the switch time from the normal-light pattern to the reversed-light pattern) of the circadian rhythm in apoptosis (after 0.5 Gy) occurred 7 days after the transfer and the rhythm was reversed by 14 days. The rhythm for crypt survival (i.e. for clonogenic cell radiosensitivity) was disturbed on 1 day and the transition point for reversal occurred 3 days after the transfer. The rhythm became reversed by 7 days. These observations are discussed in relation to the identity of clonogenic cells, (functional) stem cells, proliferating transit cells and the cells sensitive to small doses of radiation (i.e. hypersensitive cells) in the crypt.

Estimates of the number of clonogenic cells in crypts of murine small intestine

There is a proliferative cell hierarchy in the mouse intestinal crypt with ancestral stem cells which can regenerate all components of the lineage after injury (clonogenic cells). The number of these clonogenic or regenerative cells per crypt can be estimated from radiobiological experiments where doses of radiation are used to kill cells and ablate crypts. Various approaches can be adopted which provide different estimates of this number of cells. One of the conventional approaches used in the past provided estimates of about 70-80 clonogenic cells per crypt (i.e. about 50% of the proliferative or 30% of all crypt cells). A technically simpler approach has recently been suggested. This has been used here to provide many independent estimates of the number of crypt clonogenic cells. These suggest about 32 clonogenic cells exist per crypt i.e. about half the previous estimate and about twice the number of putative "functional" stem cells (those which operate as stem cells in the normal steady-state crypt). The reasons for the differences are discussed. The new estimates are compatible with the hypothesis that the crypt contains a ring of about 16 functional stem cells which are expected to be clonogenic, besides which there is a second ring of 16 clonogenic cells which represent early transit cells (the immediate daughters of the stem cells) which can act as clonogenic cells if required after radiation injury.

Radiobiology and stathmokinetics of intestinal crypts associated with patches of Peyer

The stathmokinetics and radiobiology of intestinal crypts directly adjoining the lymphoid patches of Peyer, have been compared with those of non-patch-associated crypts. Patch crypts contain an additional one to two rings of cells, the Mitotic Index for the whole crypt is higher than in non-patch crypts, and the apparent cell cycle time is insignificantly lower. Using single and split doses of gamma-rays, dose-survival curves were obtained for whole intestinal crypts, from which single-cell survival curves were derived for the clonogenic cells of the crypt. For a single-hit, multitarget, model, the extrapolation numbers of the cell survival curves for patch and non-patch crypts were the same (approximately 35) but the final D0 for cells of the patch crypts was significantly higher (2.1 versus 1.7 Gy). A linear-quadratic fit gave a similar ratio of alpha/beta (approximately 10) for the two curves. For a given level of crypt depletion, the number of clonogenic cells per crypt derived by the use of equal split doses of radiation, was the same for patch and non-patch crypts. This number is a function of the dose regime employed: the higher the level of crypt depletion, the higher the derived number of cells (range 10 to 45, for non-patch crypts).

Circadian phenomena and irradiation. Modifications of enzyme activity in the small intestine after sublethal exposure

Irradiation effects after a 3 Gy dose administered at four different times of a 24 h light/darkness cycle were studied: the modifications in the brush border enzyme activity of epithelial cells of the small intestine were determined. In controls the activity of these enzymes showed circadian oscillations with the maximum during the dark period and the minimum during the light period. The trend after irradiation in the various groups was very similar but some differences were present specially at the initial intervals when the effect appeared to be different depending on the enzyme level at the time of exposure. Lactase activity in animals irradiated at 0.00 and 18.00 o'clock returned to control levels later than in the other groups.

The effects of cis-diamminedichloroplatinum (II) and radiation on the proliferation kinetics of mouse duodenal crypt cells and on a partially synchronized crypt cell population

The effects of c-DDP and radiation, given alone or in combination, on the proliferation kinetics of mouse duodenal crypt cells were investigated by repeated labelling with 3H-thymidine. Crypt cells surviving a single dose of X-rays or c-DDP alone, appeared to proliferate faster than untreated controls. When both agents were given in combination, the proliferation rate of surviving crypt cells was markedly reduced as a result of a significant increase in the DNA synthesis time. The influence of the proliferation rate of crypt cells on the effects of c-DDP and X rays was analyzed using the crypt microcolony assay. Mice were given single doses or 2 fractions of X rays with 2 different time intervals (4 and 48 hours) either alone or in combination with c-DDP given 30 minutes before the second fraction. The lethal effect of the drug was approximately the same in cells that were in accelerated proliferation after the first irradiation as in unstimulated cells. A tendency for less cell killing by the drug was observed 3.5 hours after the first irradiation. To rule out cell age dependent effects of c-DDP, crypt cell survival was assessed after irradiation was given alone or in combination with the drug on a partially synchronized crypt cell population, obtained by repeated injections of hydroxyurea. A predominant lethal effect of c-DDP was found in cells in the late G1-phase. These findings are important to consider both from a mechanistic point of view as for clinical application of the agents.

Radiobiological evaluation of a newly synthesized cysteamine derivative

A new cysteamine-based compound, I109 (N-glycylglycyl-S acetylcysteamine trifluoroacetate) was tested on both normal tissues and tumors to evaluate its radioprotective potential. I109, which is three times less toxic than WR-2721, was injected at a dose approximately equal to half the LD50(30 days). The Protection Factor (PF = gamma ray dose ratio) after whole body irradiation was 1.3-1.4 for intestinal death and 1.4-1.5 for hemopoietic death when intervals of 40 or 20 min elapsed between injection and the end of irradiation. A crypt cell assay was done for both I109 and WR-2721; PFs were 1.1 and 1.4, respectively. I109 was then tested on five solid tumors. For each cell line (4 human, 1 murine) one dose of radiation was delivered. Surviving fraction ratios with and without drug ranged between 2.4 and 14.1 when an interval of 20 min elapsed between injection and the end of irradiation. The degree of radioprotection proved time dependent for EMT6 and HRT18; radioprotection afforded by WR-2721 on these tumors is either similar or greater than radioprotection afforded by I109 depending on the time interval between injection and irradiation.

The effect of neutron dose rate on jejunal crypt survival

The effect of dose rate of fast neutrons over a range from 0.02 Gy/min to 22.5 Gy/min on the survival of mouse jejunal crypts was investigated. A small but significant decrease in sensitivity with decrease in dose rate was observed. A 10-fold decrease in dose rate gave a decrease in effectiveness equivalent to 0.39 Gy. The sensitivity to changes in neutron dose rate is much smaller than the sensitivity to changes in dose rate of photons.

Digestive tract cell proliferation and food consumption patterns of Ha/ICR mice

The relationship between the daily pattern of food consumption and the proliferation rate of the oesophagus, stomach, forestomach, small intestine and colon of Ha/ICR mice was examined. Proliferative activity was determined by [3H]TdR incorporation on a wet weight tissue basis, along with selective counting of labelled nuclei. Under conditions of ad libitum feeding with a 12 hr light cycle (lights on at 0600) mice eat most of their food during the dark period. A distinct circadian rhythm was observed in the oesophagus, stomach, forestomach and colon with the peak of [3H]TdR incorporation between 0400 and 0600 and the nadir between 1600 and 1800. Although a circadian fluctuation was observed in the small intestine, its amplitude was much less than in other areas. This rhythmic change in proliferation rate could be phase shifted by allowing the mice to feed only between 0800 and 1600 for 14 days. Under these conditions the peak in proliferative activity occurred between 1800 and 2000. Fasting reduced the daily level of proliferative activity in all of the digestive tract sites studied, and for all areas except the oesophagus greatly reduced or eliminated the circadian fluctuation. The forestomach and colon were the most influenced by fasting with 24 hr [3H]TdR incorporation reduced to 30-40% of the control value. Refeeding following a 48 hr fast produced a rapid increase in proliferative activity peaking at levels well above the control value at 16 hr after the onset of refeeding. The major exception to this was the small intestine which slowly returned to the control value during the first 24 hr. Partial refeeding produced a diminished refeeding response. Once the normal pattern of food consumption was re-established following refeeding the normal proliferative fluctuations were again observed.

Diurnal variation in the response of gamma-ray-induced apoptosis in the mouse intestinal epithelium

The appearance of apoptosis has been used as an index of carcinogen-induced damage incurred by the intestinal epithelium. Here we report that the cytotoxic response elicited by gamma radiation in the duodenal and colonic epithelium of the mouse is markedly affected by the time of day at which the animals are irradiated or sampled. The diurnal pattern in radiosensitivity of the crypt cells, as measured by the appearance of apoptosis is reflective of circadian rhythms in mouse intestinal cells. Circadian effects should therefore be considered when interpreting apoptosis data obtained at one time point only or when comparing data obtained at different times of the day.

Diurnal variation in proliferative compartments and their relation to cryptogenic cells in the mouse colon

The depth of the crypts in mouse descending colon varied diurnally, between twenty-six cells at 24.00 hours and thirty-eight cells at 12.00 hours. Cell loss from the colon was greatest immediately before the maximum faeces production, at the beginning of the dark period. The labelling index of the colon also changed, from 9% at 20.00 hours to 16% at 12.00 hours. The greatest variation in labelling index occurred at the top of the zone of proliferative cells, between the ninth and eighteenth cell position up the crypt. In this region a synchronized cohort of about forty cells apparently entered S phase once a day. Although the length of the proliferative zone doubled at 12.00 hours, that of the non-proliferative zone remained fairly constant all day. The number of cryptogenic cells per crypt was estimated by comparing single and split-dose X-ray survival curves. This gave a mean value of two cryptogenic cells per crypt. Crypts rarely regenerated from the base after irradiation. The cryptogenic cells probably lay between cell positions Nos 9 and 18 up the crypt and probably did not function as stem cells in the normal crypt.

Radionecrosis of normal tissue: studies on mouse tails

Studies on the radionecrosis of mouse tails demonstrate the following modifications to the dose necessary for necrosis in 50 per cent of tails (the ND50): (a) There is very little reduction in ND50 values for irradiated lengths of tail from 2 cm to almost the whole tail, but there is a sharp increase in dose for lengths less than 1.5 cm. (b) The ND50 is high for unanesthetized mice irradiated in air, due to tissue hypoxia. (c) The hypoxia can be reduced by varying amounts by applying heat to the tail, or by flowing oxygen over the tail surface, or by anesthetizing the animal. (d) The ingress of oxygen through the surface can be reduced by placing a clamp round the proximal tail. These features are discussed with reference to the state and possible position of the target cells, and to the use of this assay technique in comparative studies.

Differences in survival of mouse colon crypts after whole- or partial-body irradiation

The survival of mouse colon crypts after X-irradiation has been studied by the microcolony technique. The Do for crypt survival was 266 rad after whole-body irradiation in air, but when the colon alone was irradiated, the Do was 340 rad. In mice which were breathing 95 per cent oxygen, the Do values were 238 rad for whole-body and 302 rad for colon irradiation. The survival curves were all extrapolated to the same number, and the DQ was also increased with colon irradiation. There was, therefore an enhancement of colon crypt survival of about 27 per cent by local as opposed to whole-body irradiation. These results might be explained by a circulating repair promoter or by the production of a toxin after whole-body irradiation.

The proliferative status of intestinal epithelial clonogenic cells: sensitivity to S phase specific cytotoxic agents

High concentrations of tritiated thymidine and cytosine arabinoside (Ara-C) have been used to selectively kill cells in the crypts of Lieberkuhn that are synthesizing DNA. The effect of these agents on the number of regenerating microcolonies seen 3 1/2 days after a range of radiation doses indicates that a majority of the clonogenic cells are proliferating rapidly and that the slowly proliferating cells at the base of the crypt do not represent the whole clonogenic population.