Final answer:
During embryonic development, embryonic stem cells switch from a fast cell cycle to a normal cell cycle. This transition is regulated by control checkpoints and signals, with the retinoblastoma protein playing a key role as a mitogen.
Step-by-step explanation:
The mechanism for switching embryonic stem cells from a fast cell cycle to a normal cell cycle involves several control checkpoints and signals. During embryonic development, cells differentiate and eventually stop cycling. This is signaled in the Gā phase, where the cells enter Go and stop dividing. However, in some circumstances, cells in Go can be recruited to resume cycling. A key factor in this process is the retinoblastoma (Rb) protein, which acts as a mitogen and turns on genes that lead to cell proliferation.