Figure 1

Four different tissue architectures that lead to 8 differentiated cells (green) and one stem cell (red). All tissue architectures require the same number of cell divisions, but individual cells divide different numbers of times. On the left the stem cell divides only once (n_s = 1) to produce a transit cell (black) that in turn divides binomially three times (n_t = 3), which produces a total of eight differentiated cells (k = n_s × $2^{n_{\text{t}}}$ = 1 × 2³ = 8). On the right the stem cell divides eight times and no transit cells are produced (n_s = 8, n_t = 0, k = 8 × 2⁰ = 8). The other two tissue architectures are intermediate cases (centre left: n_s = 2, n_t = 2, k = 2 × 2² = 8; centre right, n_s = 4, n_t = 1, k = 4 × 2¹ = 8). The complete tissue will consist of N stem cells and thus be able to produce T = N × k differentiated cells. If the tissue is a testis each of these differentiated cells will go through the two meiotic divisions and will thus produce T = N × 4 × k sperm.