Epithelial cell kinetics in the descending colon of the rat.

Epithelial cell kinetics were investigated in the descending colon of the rat. The number of cells per crypt was found to be approximately 625, with 33 cells per cell column and 19 cell columns per crypt circumference. The growth fraction of the colonic crypt was 0.42, and proliferating cells were situated largely in the lower half of the crypt. The cell cycle time was 50.5 h, with values for the G1, S and G2 phases of 40.0, 7.6 and 2.9 h respectively. Cell migration studies showed that it took 60-72 h for a cell to migrate from the upper border of the proliferative cell compartment in the crypt to the luminal surface of the colon. Data were also obtained from continuous labelling with tritiated thymidine and from studying the circadian rhythm of proliferative activity, which suggest that the cells in the bottom of the crypt may constitute a separate, more slowly cycling (stem)cell compartment.

The effect of transposition to jejunum on epithelial cell kinetics in an ileal segment.

Epithelial cell kinetics were studied in an ileal segment after transposition to proximal jejunum. The number of cells per villus column in the transposed ileum increased after 4--7 days to reach values normal for jejunum after 14--30 days. This increase was accompanied by a simultaneous increase in the number of cells per crypt column up to 130% of values in jejunum and ileum in situ. The percentage of labelled crypt cells, after labelling with 3H-thymidine, and the relative size of the proliferative cell compartment in the crypt in the transposed ileum did not differ from values in the ileum in situ at any time interval after surgery. The total proliferative activity per crypt, which was determined by scintillation counting of isolated crypts after 3H-thymidine labelling, increased two-fold from 7 days after surgery. Cell migration studies showed that the increase in the number of villus cells was probably not caused by a change in the life span of the epithelial cells. It seems that the increase in the number of villus cells in ileal epithelium after transposition to proximal jejunum is brought about by an enlargement of the crypt, while the relative size of the proliferative cell compartment in the crypt remains unchanged.

Influence of experimental bypass on cellular kinetics and maturation of small intestinal epithelium in the rat.

The effect of experimental bypass on the intestinal epithelium of the rat was investigated from 2 to 360 days after creating a Thiry-Vella fistula. Within 7 to 14 days the number of cells per villus column and, to a lesser degree, the number of cells per crypt column decreased and subsequently these values remained constant up to 1 year. Radioautography did not show any changes in the percentage of labelled cells per crypt column, after incorporation of [3H]thymidine. In addition, the relative size of the proliferative cell compartment in the crypt, the number of crypts per unit length of small intestine and the crypt-villus ratio remained unchanged. Scintillation counting of isolated crypts from the Thiry-Vella fistulas showed that the number of cells per crypt was markedly reduced. Scanning electron microscopy revealed that the three-dimensional size of the villi was also reduced. The activities of nonspecific esterases and neutral alpha-glucosidase remained unchanged in crypt cells and were above control values in villus cells. The development of these biochemical characteristics of the villus cells seems to be independent of the luminal contents. It was concluded that deprivation of food passage and gastric, pancreatic, and liver secretions from a small intestinal segment primarily causes a reduction in the number of cells per crypt, which leads to a lower cell production. In turn, this is responsible for the reduction of the functional villus cell compartment.

The effect of ischemic villus cell damage on crypt cell proliferation in the small intestine: evidence for a feedback control mechanism.

In recent years the hypothesis that the number of villus cells regulates crypt cell proliferation in the epithelium of the small intestine has been brought forward by a number of investigators. To test this hypothesis, the villus cell population was reduced by clamping the superior mesenteric artery and vein in rats for 1 hr and the effects on the intestinal epithelium were studied during the first 24 hr. It was shown that temporary interruption of the blood flow to the small intestine led to a marked decrease in the number of functional villus cells within 2 hr; preferentially, cells from the upper part of the villus were lost and the number of crypt cells was not affected. This reduction in the number of villus cells led to an increase in the percentage of labeled crypt cells after pulse labeling with [3H]thymidine, and an expansion of the proliferative cell compartment in the crypt. After a peak of proliferative activity at 16 hr, the investigated crypt cell kinetic parameters approached control values after 24 hr, as did the number of villus cells. The enzyme activities of nonspecific esterase and neutral alpha-glucosidase showed marked decreases in isolated crypt and villus cell compartments as crypt cell proliferation increased. These data support the view that the feedback control of crypt cell proliferation by the functional villus cells, and confirm earlier data on the influence of changing cell kinetics on crypt cell maturation. Additional data which were obtained after creating temporary ischemia in part of the small intestine support the hypothesis of a feedback control of crypt cell proliferation by the functional villus cells, and confirm earlier data on the influence of changing cell kinetics on crypt cell maturation. Additional data which were obtained after creating temporary ischemia in part of the small intestine support the view that the feedback control of proliferation by the villus cells is a local control mechanism.

The effect of continuous irradiation on cell proliferation and maturation in small intestinal epithelium.

Autoradiographic studies and scintillation counting of crypt material after pulse labelling with 3H-thymidine showed that during continuous irradiation with 290 rads/day a reduced proliferative activity is present in the crypts of rat small intestine after 1 day of irradiation and of normal activity during the remaining period (5 days) irradiation. After cessation of irradiation an increase in proliferative activity can be observed after 1 day of recovery. From the time (36-48 hr after starting of the irradiation) that the number of villus cells is reduced an expansion of the proliferation zone in the crypt was observed. Both effects last until 1 day of recovery after cessation of irradiation. The process of crypt cell maturation and of villus cell function has also been studied during and after continuous irradiation by micro-chemical enzyme analyses in isolated crypts and villi. It was found that the expansion of the proliferation zone in the crypt is accompanied by a decrease in activity of only those enzymes (i.e. non-specific esterases) which normally become active during crypt cell maturation. The activity of enzymes normally present mainly in the functional villus cells remained relatively unaffected by changes in crypt cell kinetics. A hypothesis of different regulation mechanisms of the proliferative activity in the intestinal crypt and a possible explanation of the different behaviour of various enzyme activities as a result of changes in crypt cell proliferation is discussed.