Mode of growth of the jejunal crypt cells of the rat: an autoradiographic study using double labelling with 3H- and 14C-thymidine in lower and upper parts of crypts.

The frequency distribution of cells through the mitotic cycle in lower and upper portions of jejunal crypts of the rat was examined by the 3H-14C-thymidine double labelling technique. Isolated crypts were cut perpendicular to the longitudinal axis so that the percentage of cells in the lower portion varied from 16 to 74%. The lower and upper portion of the same crypt were squashed separately on one microscope slide and the number of 3H- and 14C-only labelled cells were scored to determine the flow rate into and out of S for the two portions. The mitotic cycle and its phases of the crypt epithelial cells were also determined. For lower portions of crypts which contained less than 40% of the total cell number in that crypt the flow rate into S was about 1-7 times that of the flow rate out of S indicating that nearly every mitosis in this region produced two proliferative daughter cells. As the proportion of cells in the lower part of the crypt increased the quotient of the flow rate into S divided by the flow rate out of S decreased, and approached the steady state value of 1-0 in lower portions containing 60-74% of the cells. For upper portions of crypts which contained less than 40% of the total crypt cells the flow rate into S was about 0-2 times that of the flow rate out of S, indicating that in this region mitoses predominantly produced non-proliferative daughter cells. The results obtained were in good agreement with the model of crypt cell proliferation proposed by Cairnie, Lamerton & Steel (1965b).