Figure 6.
A possible finite state model for spermatogenesis. Germline lineages are traced through the model, entering at the START state, from which they can transition to either of the GSC states Ad or Ap. The cell death state (X) can be reached from either GSC state, whereas we assume here that spermiogenesis (S) is possible only from Ap. Transitions from the GSC states correspond to cell divisions, so the model describes a branching process representing an entire genealogy of spermatozoa, each descended from a single primordial germ cell (PGC) and having an associated age and total number of cell divisions (depth) from the root of the genealogy. In the established model (model 1 in figure 4), the transition Ap → Ad has zero probability under normal conditions.