Author response image 8. Options 1 and 2 are two different situations that can be underlying the measurements of cell number and cell division.
A and B are the two regions of tissue to be compared. The circles are cells; the green circles indicate mitotic cells. In option 1, the tissue A has much less number of cells and mitosis than tissue B, but the proliferation rate (mitotic index, the probability of a cell to divide) is not different between the two tissues. In this case, the reduction in mitosis in A is just a consequence of a reduced number of cells in this tissue. In option 2, the tissue A has a few less cells than tissue B but much less mitotic events. Thus, A and B present a big difference in proliferation rate. It is possible to think that some signal stimulate proliferation in B. The observed difference in the number of mitosis is not a simple consequence of a different number of cells in the tissue, but of a different proliferative activity