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.

Cell population kinetics of 1,2-dimethylhydrazine-induced colonic neoplasms and their adjacent colonic mucosa in the mouse

The parameters of cell population kinetics of symmetrical 1,2-dimethylhydrazine-induced colonic neoplasms and their adjacent colonic mucosa in the mouse were analyzed using the fraction labeled-mitoses curve method and compared with those of three groups of epithelial cells in the crypt of the descending colon of normal mouse. The analysis of three groups of epithelial cells in the crypt of normal mouse indicates that differentiation of epithelial cells was associated not only with a smaller proliferative pool of cells but also with a shortening of the duration of G2 phase and a prolongation of mitotic time. Other parameters of cell cycle did not change significantly. The mean cell cycle time of neoplastic cells in chemically induced colonic neoplasms was similar to that of epithelial cells in normal colon, but the variance was much greater in neoplastic cells. In neoplastic cells, the proliferative pool was greater, the G1 phase prlonged, and the S phase and the mitotic time became shorter as compared to epithelial cells in normal colon. The duration of G2 phase of neoplastic cells fell between the values of presumptive stem cells and differentiating cells in normal colon, compatible with the hypothesis that neoplastic cells are transformed stem cells defective in cellular differentiation. In the colonic mucosa immediately adjacent to neoplasms, the fraction-labeled-mitoses curve showed a flat second wave, indicating that the group of cells initially labeled by the pulse became a mixture of cells, some continuing the proliferative cycle normally, some going out of cycle, some slowing down in their passage from S through G2 to M, and some being arrested in mitotic phase. Such heterogeneous behavior of cells may be closely related to expansion of neoplasms. With some assumptions, however, cell cycle parameters of those normally cycling cells were estimated: the cell cycle time and the duration of G1 phase and mitotic phase were prolonged as compared to neoplastic cells and epithelial cells of normal colon.

Spatial distribution of DNA-synthesizing cells in colonic crypts of the guinea pig

The ascending colon of a guinea pig injected with tritiated thymidine was cut serially, autoradiographed and stained with periodic-acid Schiff-hematoxylin. Maps of transversely sectioned crypts were prepared with the use of a microscope eye-piece projector. The number and angular positions of pulselabelled (DNA-synthesizing) cells around the circumference of transverse sections of the crypt were recorded. A method of 'statistics of the circumference' was applied in order to find the variances of angular distances between labelled cells and thereby to find the type of arrangement of DNA-synthesizinbg cells in the crypt. The spatial distribution of DNA-synthesizing cells both around the crypt circumference and along the crypt, was found to be non-random. While the pattern of non-randommess around the crypt circumference is such that the DNA-synthesizing cells tend to occupy positions in the crypt circumference at maximal distances from each other, DNA-synthesizing cells along the crypt tend to occupy positions at minimal distances from each other. DNA-synthesizing cells are arranged in the crypt in rows, each consisting of several cells and each parallel to the long axis of the crypt. Apparently the dividing cell of the crypt produces either two proliferating or two differentiating cells. No evidence of differential mitosis could be found.

Fasting and refeeding: cell kinetic response of jejunum, ileum and colon

Following a period of fasting, feeding a normal diet results in a burst of DNA synthesis in the crypts of the colonic epithelium. This is due largely to a prompt entry of cells, blocked in G1, into S. Peak levels of S cellularity exceed 4 times the fasting, and 2 times the normal fed control values. Refeeding a low residue diet (soluble casien, glucose and corn oil) results in a return to control levels of proliferative activity, but no hyperplasia. However, in jejunum and ileum, refeeding is followed by a return to near control levels of proliferation with only a slight overshoot in S phase cellularity. During the fasting period, the ileal crypt proliferative compartment (Pc-zone) and total crypt cellularity decline significantly. These changes are accompanied by an increase in the total cycle time, due to an equivalent lengthening of the G1 and S phase. Following refeeding, there is a reduction in the cycle time and a gradual return to the control values for the Pc-zone size and cellularity. In the colon, fasting has no effect on the Pc-zone size or total crypt cellularity. There is an approximate doubling of the cycle time due solely to an increase in G1. Following refeeding there is an increase in the Pc-zone size and crypt cellularity and a marked shortening of the cycle time. Evidence that a G1 cycle blockade is induced in the colon by fasting is given by a lenghening of the G1 period and by stathmokinetic studies employing vincristine.