Extracellular Signals and The Restriction Point
Except for early embryonic development, most cells in multi-cellular organisms persist in a quiescent state known as G0, where proliferation does not occur, and cells are typically terminally differentiated; other specialized cells continue to divide into adulthood. For both of these groups of cells, a decision has been made to either exit the cell cycle and become quiescent (G0), or to reenter (G1).
A cell’s decision to enter, or reenter, the cell cycle is made before S-phase in G1 at what is known as the restriction point, and is determined by the combination of promotional and inhibitory extracellular signals that are received and processed1. Before the R-point, a cell requires these extracellular stimulants to begin progressing through the first three sub-phases of G1 (competence, entry G1a, progression G1b). After the R-point has been passed in “G1b”, however, extracellular signals are no longer required, and the cell is irreversibly committed to preparing for DNA duplication. Further progression is regulated by intracellular mechanisms. Removal of stimulants before the cell reaches the R-point may result in the cell’s reversion to quiescence. Under these conditions, cells are actually set back in the cell cycle, and will require additional time (about 8 hours more than the withdrawal time in culture) after passing the restriction point to enter S phase.
Read more about this topic: Restriction Point
Famous quotes containing the words signals, restriction and/or point:
“The term preschooler signals another change in our expectations of children. While toddler refers to physical development, preschooler refers to a social and intellectual activity: going to school. That shift in emphasis is tremendously important, for it is at this age that we think of children as social creatures who can begin to solve problems.”
—Lawrence Kutner (20th century)
“If we can find a principle to guide us in the handling of the child between nine and eighteen months, we can see that we need to allow enough opportunity for handling and investigation of objects to further intellectual development and just enough restriction required for family harmony and for the safety of the child.”
—Selma H. Fraiberg (20th century)
“I stand here tonight to say that we have never known defeat; we have never been vanquished. We have not always reached the goal toward which we have striven, but in the hour of our greatest disappointment we could always point to our battlefield and say: “There we fought our good fight, there we defended the principles for which our ancestors and yours laid down their lives; there is our battlefield for justice, equality and freedom. Where is yours?””
—Anna Howard Shaw (1847–1919)