Autoradiographic Confirmation of the Mitotic Division of Every Mouse Jejunal Crypt Cell Labelled With ³h‐thymidine

The question was investigated of whether for crypt epithelia of the jejunum of the mouse all cells labelled after a single injection of 3 H‐TdR subsequently divide or whether cells exist in the crypt which synthesize metabolic DNA and, therefore, do not undergo division after labelling. A double labelling experiment was performed with a first injection of 3 H‐TdR followed 1 hr later by an injection of 14 C‐TdR. Then from double emulsion autoradiographs of isolated squashed crypts the number of 3 H‐only, 14 C‐only and double labelled cells and mitoses were counted. The double labelling produced a narrow, 1 hr wide sub‐population of 3 H‐only labelled cells. This subpopulation of S cells completed its division before labelled cells were lost from the crypts by migration onto the villi. The results showed that this subpopulation of 3 H‐only cells completely doubled within 3 hr and then remained constant through 6 hr. From this result it was concluded that every cell labelled after a single injection of 3 H‐TdR divides. From the same autoradiographs the flow rate through the end of mitosis was measured. From the flow rate and the mitotic index a mitotic duration of 0·5 hr was determined. The agreement of this measured mitotic time with the value calculated from the labelling index, mitotic index and S duration is also strong evidence that every labelled cell divides. Both experiments show that the intestinal crypt does not contain cells synthesizing metabolic DNA.