Figure 1

Cell type is determined by a cell’s position in the divide gradient. The virtual crypt is shown on the left. Non-proliferating cells are blue, proliferating cells are red. The various cartoons of crypts on the right illustrate the various possible locations of progeny cells after cell division at time 2, and the cell type of the progeny. The cell type of the progeny cells is determined by their position in the divide gradient (green triangle). Below the red line in the green triangle is the region of quiescent cells. In the top row of cartoons, a differentiated cell (blue) at the very top of the crypt may remain undivided at time 2; or a differentiated cell lower in the crypt may divide and one progeny may become a proliferative cell (red). In the center row of cartoons, the progeny of a proliferative cell (red) that arrives low in the gradient may become a quiescent stem cell (blue); or if it arrives high in the gradient may become a differentiated cell (red). In the bottom row of cartoons, a quiescent stem cell (blue) may arrive in the region of proliferating cells and become an active stem cell (red). Note the possibility of bidirectional conversion of cell types.