Study of kinetics of epithelial cell populations in normal tissues of the rat's intestines and in carcinogenesis. I. A comparison of enterocyte population kinetics in different segments of the small intestine and colon.

The peculiarities of enterocyte proliferation in the duodenum, jejunum, ileum, caecum, ascending, transverse and descending colon in the rat were studied by different methods of analysis of cell population kinetics (percentage-labelled mitosis curves, cumulative labelling curves, distribution of labelling index curves, etc.). The majority of proliferating cells in the small intestine are homogenous, as far as mitotic cycle mean duration (11-12 hrs) is concerned. All proliferating cells in all the zones of the colonic crypts and the bottom of the small intestine crypts are divided into subpopulations, having different mean durations of the mitotic cycle. It is suggested that, in the crypt bottom in all intestines as well as the crypt's maximum proliferation zones in most of the colonic segments, a considerable fraction of cells has a very long mitotic cycle or enters resting phase R1. The average value of the mean durations of the mitotic cycle of all colonic enterocyte subpopulations is 18-22 hours. On the basis of the authors' findings and literature data, a model for the enterocyte life cycle is proposed, according to which the cell flux is branched during the mitotic cycle and the crypt develops from a stem enterocyte population located at its bottom.