The detailed temporal and spatial changes in the labelling index in crypts of the small intestine of the mouse have been analysed after 8.0 Gy gamma-irradiation. The labelling index was determined for each cell position in the crypts at 34 different times between 3 and 192 h after irradiation. The changes between consecutive time points have been analysed to determine the details of the crypt shrinkage and crypt repopulation phenomena. The following points can be made: (1) There is a dramatic reduction in the overall labelling of the crypt which begins within 3 h and is at its minimum by 15 h postirradiation. Most of this shrinkage can be attributed to continued near-normal emigration of cells from the crypt to the villus while mitosis is reduced or absent, and a possibly premature maturation within the transit population. (2) The labelling index never falls below 34 per cent of control, i.e. many labelled cells persist and continue to replicate their DNA at all times postirradiation. (3) Repopulation begins in the lower regions of the crypt. The first changes are an increase in labelling at cell positions 3-8 that begins at 3 h and reaches a peak at 12 h. There is a second increase in proliferation at the crypt base that begins at about 15 h and reaches a peak at 22-32 h postirradiation. There is a third peak which begins at about 46 h and reaches a peak at 60-70 h. (4) There is a reduction in proliferation at the crypt base that begins at about 72 h postirradiation. (5) The mid and upper crypt population shrinks initially to reach a minimum at about 15 h, after which there is a steady increase to reach a peak at about 72 h. The labelling spreads into the crypt-villus boundary area beginning at about 32 h. There is a reduction of proliferative activity in the mid-crypt region that begins at about 72 h. (6) There is a dramatic overshoot in overall labelling index at 72 h, which involves mainly the upper crypt. This does not revert to normal levels within the 192 h time scale of the present experiments. There is a mild overshoot in labelling at the crypt base at 48-78 h with a return to normal levels thereafter.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of epidermal growth factor (EGF), transforming growth factor ox (TGFa), insulin-like growth factor (IGF) I and II, acidic fibroblast growth factor (FGF), tumour necrosis factor a (TNFa), macrophage inhibitory protein loa (MIPlox) (LD78), and TGFI-1 on cell proliferation in the crypts of the small intestine of mice were investigated. Various

The stem cells in the crypts of the small intestinal mucosa divide about a thousand times during the lifespan of a laboratory mouse, and yet they show little evidence of any decline in proliferative potential and rarely develop carcinogenic mutations, suggesting that their genome is extremely well protected. Protection against DNA-replication-induced errors can be achieved by the selective sorting of old (template) and new DNA strands with all template strands retained in the stem cell line. The template strands in the stem cells can be labelled during development or during tissue regeneration using tritiated thymidine (3HTdR). Labelling newly synthesised strands with a different marker (bromodeoxyuridine, BrdUrd) allows segregation of the two markers to be studied. Template strand label is retained(3HTdR), whereas label in the newly synthesised strands (BrdUrd) is lost following the second division of the stem cell. Random errors may occur in the template strands owing to environmental elements. These are protected against by the altruistic cell suicide (apoptosis) of the cells incurring such errors. A final level of protection for the tissue compensates for excessive deletion of stem cells via the apoptosis pathway. This is achieved by a hierarchical age structure in the stem cell compartment, with some cells being able to efficiently repair DNA damage and hence being more radioresistant. The presence of these protective mechanisms ensures that the small intestine rarely develops cancer and that stem cells can sustain the extensive cell proliferation needed during life.

We have recently shown that a mild mechanical irritation (tape strip) of the epidermis on the back skin of adult mice induces a strong and long lasting increase in proliferative activity and cell production. This was revealed by following the fate of 3HTdR-pulse labelled cells within the basal and suprabasal layers. To obtain further insight into the dynamics of cell kinetic changes we also performed double labelling experiments with 3HTdR and BrdUrd at various times after tape stripping. The technique for analysing the data had to account for a non stationary cell flux. A novel biometrical technique was developed which provides parameter estimates on the S-phase duration, the cell cycle duration and a parameter characterizing the degree of nonstationarity. When applied to the mechanically irritated epidermis we observed that the cell flux through the S-phase in the basal layer was accelerated by a factor of 10 between 18 and 36 h post tape strip. This activation declined slightly in the subsequent days and remained 4-6 fold higher than in the normal steady state for over 7 days post tape strip. The duration of the S-phase was 3-5 h and showed little variation. We conclude that mild mechanical irritation only affecting the stratum corneum has major stimulatory effects on the cell kinetics of proliferative keratinocytes in the basal layer of the epidermis indicating the existence of a powerful regulatory mechanism.