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 G₁ 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.