Irradiation at different times of the day. Morphology and kinetics of the small intestine

Effects of radiation damage on intestinal morphology

The current flow of papers on intestinal structure, radiation science, and intestinal radiation response is reflected in the contents of this review. Multiparameter findings and changes in compartments, cells, or subcellular structure all contribute to the overall profile of the response. The well-recognized changes in proliferation, vessels, and fibrogenesis are accompanied by alterations in other compartments, such as neuroendocrine or immune components of the intestinal wall. The responses at the molecular level, such as in levels of hormones, cytokines, or neurotransmitters, are of fundamental importance. The intestine responds to localized radiation, or to changes in other organs that influence its structure or function: some structural parameters respond differently to different radiation schedules. Apart from radiation conditions, factors affecting the outcome include the pathophysiology of the irradiated subject and accompanying treatment or intervention. More progress in understanding the overall responses is expected in the next few years.

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.

Cell proliferation and differentiation in the small intestine after irradiation with multiple fractions

Qualitative and quantitative morphologic changes in rat small intestine were studied after abdominal exposure to multiple fractions of gamma radiation. One group of animals received 3 X 2 Gy with one fraction every 4 hours. Another group received two courses of this type with a 16 hour interval between the courses (total dose 6 X 2 Gy). A marked decrease in the number of crypt epithelial cells, and in mitotic and labelling indices, was observed up to 24 to 36 hours after the end of both regimens. Repair and recovery occurred within 72 hours after the end of the last exposure, and the epithelium regained normal morphology. At 1 and 4 hours after the end of the treatment the frequency of S-phase cells along the crypt was greatly reduced and at the following intervals labelled cells occupied the region where differentiation occurs in control animals. During recovery labelled cell distribution showed a gradual return to normal. No substantial differences between the effects of total doses of 6 and 12 Gy were shown except for a greater reduction in crypt epithelial cells at the early time intervals after the larger dose.

Quantitative changes in the goblet cells of the rat small intestine after irradiation

In order to evaluate the process of cell differentiation in the crypt of the rat small intestine the goblet cells were quantitatively studied in controls and after irradiation of the abdomen. The effect of a single dose, administered at 4 different times of the day, and multiple daily fractionations (MDF) of 6 and 12 Gy with different doses per fraction and different time intervals, were compared. Both regimens caused an initial increase of the goblet cells (both in absolute and relative terms), followed by a decrease and finally return to nearly control levels. After MDF the increase was more marked and the return to a normal level occurred earlier than after the single dose.

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.

Behaviour of the proliferative compartment of the small intestine at different times of the day

The modifications of mitotic and labelling indices in animals killed at different hours of the day were analysed. The invertase activity was also assayed. This brush border enzyme, synthetized by the differentiating cells, showed a clear circadian rhythm with a maximum in the night and a minimum near the end of the light period. Mitotic and labelling indices showed the highest activity in the night and at noon. The positions of mitotic and labelled epithelial cells in the crypt were also determined. The frequency of labelled cells in the different positions of the crypt evidenced a shift of these cells towards the crypt-villus junction in the late afternoon with a consequent reduction of the differentiating compartment.

Modifications of S-phase cell distribution in the intestinal crypts after multiple daily fractionation

The effects obtained by multiple daily fractionation (3 Gy X 2 or 3 Gy X 2 X 2) on the distribution of S-phase cells along the crypt of the small intestine were investigated. The frequency of labelled cell distribution was reduced at early intervals; then the proliferating compartment gradually extended to the villus junctions. During recovery labelled cell frequency in the lower half of the crypts returned to control levels, while labelled cells were present in the differentiating area. With lower total dose modifications were milder and, as early as 72 h before exposure, distribution was already similar to controls. Invertase activity showed an initial increase and a higher reduction during acute damage when fractionation with higher doses was used. A lack of return to normal activity was present even 11 days after exposure when, however, the characteristic circadian pattern was observed.

S-phase cell distribution in the small intestine irradiated at different times of the day. II. Recovery phase

Modifications occurring during recovery in the small intestine of animals exposed to the same radiation dose given at different times of the day were evaluated. S-phase cell distribution along the crypts and invertase activity were evaluated to ascertain the functional capacity of epithelial cells. In animals killed between 5 and 6 days after exposure, S-phase cell distribution and functional conditions tended towards normality although recovery was not complete. Labelled cells occurred also at villus junctions, demonstrating limitation in size of the differentiating compartment. This was confirmed by reduced activity of the brush border enzymes. Animals irradiated at the end of the dark period recovered more quickly and efficiently. In this group, labelled cell distribution was almost the same as in the controls starting from 120 h, and invertase activity was also closer to the controls than in any other group.

S-phase cell distribution in the small intestine irradiated at different times of the day. I. Acute irradiation injury

The S-phase cell distribution has been analysed to evaluate the behaviour of proliferative cells in the intestinal epithelium after irradiation at different times of the day. A marked reduction of S cell frequency was observed at early intervals after abdominal irradiation; this reduction was particularly evident in the lower half of the crypts. At subsequent intervals a progressive extension of the proliferative compartment, with labelled cells also at the top of the crypt, was present. The irradiated groups generally showed a homogeneous behaviour even if a more marked reduction in S-phase cells was observed in group C. The invertase activity, a brush border enzyme synthetized during the differentiation process, presented a different behaviour at the early intervals in the irradiated groups. When the extension of the proliferative compartment occurred the invertase activity reached values close to zero. The modifications in brush border enzymes and in S-phase cell distribution, at early killing times, led to the hypothesis of an early differentiation.

Modifications in the brush border enzymes of the small intestine after irradiation at different times of the day

The behaviour of the brush border enzyme activity of the intestinal epithelium after the same sublethal radiation dose to the abdomen at different times of the day was investigated. Three previously observed post-irradiation phases (initial increase of activity, reduction and the return to control values) were confirmed, although with some differences. A later return to normal of lactase was also confirmed. The same dose produced different behaviour of the enzyme activities both during the initial and the recovery phase, depending on the time of the day when irradiation was performed, i.e. on the functional condition of the epithelial cells.