Abstract. Cell replacement in the crypt of the murine small intestine has been studied and modelled mathematically under steady‐state conditions. A great deal of information is available for this system, e.g. cell cycle times, S phase durations, the rate of daily cell production, the Paneth cell distribution etc. the purpose of the present work was to consider simultaneously as much of these data as possible and to formulate a model based upon the behaviour of individual cells which adequately accounted for them. A simple mathematical representation of the crypt has been developed. This consists of sixteen stem cells per crypt (Tc= 16 hr, Ts= 9 hr), and four subsequent transit cell divisions (Tc= 11 to 12 hr, Ts= 8 hr) before maturation. Experimental data considered to test the modelling were LI and data on the number of vertical runs of similarly labelled cells. All data were obtained from the ileum after 25 μCi [3H]TdR given at 09.00 hours. A number of alternative assumptions have been considered and either accepted or rejected. Two alternative model concepts of cell displacement explain the data equally well. One is dependent upon strong local cell generation age determinance while the other could accommodate any weak local cell displacement process in conjunction with an environmental cut‐off determinant at the middle of the crypt. Both models provide new interpretations of the data, e.g. certain rates of lateral cell exchange between neighbouring columns (250 to 350 per crypt per day out of a total of 420 cell divisions per day) can be concluded from run data, while LI data provide information about the mechanisms involved in maintaining a position‐related age order in the crypt.

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Abstract. The position‐dependent mitotic index before, and 1, 2 and 3 h after vincristine was scored. the accumulation of cells in mitosis leads to an increase in the mitotic index from 0.06 to 0.34 at crypt positions 8‐12. Surprisingly, the leading edge of the position‐related mitotic index distribution moves to higher crypt positions although cell division was stopped. In addition, the vertical clustering of mitotic figures in sections was recorded. the data were examined using a previously described computer crypt model. We conclude: the average mitotic phase duration is about 0.7 h (40 min) and varies little with cell position; the geometrical correction factor for overscoring mitoses in crypt sections is about 0.6‐0.7 and adjacent cell columns can merge. Lateral cell displacement after mitosis, as predicted in a previous model analysis, would be a mechanism to counteract other forces that tend to reduce the crypt circumference. In the normal steady state merging and expansion processes would just balance each other. This would not follow if one mechanism was blocked. Thus we propose a new concept in which the crypt geometry would be dynamically determined by cell proliferative activity in connection with lateral positioning of new cells on one hand and contracting forces on the other hand.

A statistical analysis of the distribution of [ 3HlTdR-labelled cells in longitudinal and transverse sections of crypts from the ileum of the mouse, indicated that there was a strong tendency for labelled or unlabelled cells to be associated in short vertical runs and lateral clumps, suggesting the presence of clusters of labelled cells on the sides of the crypts. A model is discussed for the cellular spatial organization of the crypt that proposes a vertical alignment of the cells within branches of the proliferative cell lineage. The model would predict vertical alignment of partially synchronized cells as well as some lateral clumping.In the present studies mitoses were not observed at higher levels in the crypt than labelled (S phase) cells. This observation would be predicted by the non-random spatial organization suggested by the model. The model would also make certain predictions concerning cell migration. These are discussed in relation to cell migration studies which include evidence that migration continues in the absence of mitotic activity.

The simultaneous immunohistochemical detection of bromodeoxyuridine (BrdU) and [3H]-thymidine ([3H]TdR), by conventional autoradiography, was performed on the mouse small intestine (ileum). Proliferation was studied under normal conditions as well as after 3 Gy of y-rays. The BrdU method in conjunction with [3H]TdR autoradiography appears to be reliable and useful for the study of cell kinetics especially in disturbed states, on condition that [3H]TdR is delivered to the animals before BrdU. It has been found that cells in the crypt are delayed by irradiation in their progression through the cell cycle predominantly in late S phase. The cells at the bottom of the crypt are more affected than the more differentiated but proliferating cells in the upper part of the crypt. Bromodeoxyuridine (BrdU), the halogenated 5-substituted derivative of deoxyuridine, is a thymidine analogue specifically incorporated into the DNA (via the salvage pathway) of proliferative cells during the S phase. Bromodeoxyuridine incorporation (BrdU/Hoechst method) has been a traditional tool in cell kinetics and cytogenetic studies. The recently introduced monoclonal antibody which can distinguish between halogenated pyrimidines and thymidine (Gratzner, 1982) inspired numerous new cell kinetic studies. The immunocytochemical detection of BrdU as a non-radioactive technique can be used in vim in humans (Wilson et al., 1985), as a rapid assay using flow cytometry (Dolbeare et al.