Growth factors, which are regulatory proteins that stimulate cell division, trigger inter-cellular signaling systems in order to start the cell division process. Growth factors are recognized by certain cell surface receptors, which are shaped to fit the exact shape of a specific growth factor. Growth factors activate certain proteins within the cell in the course of this process, and these proteins initiate DNA replication, cell growth and cell division.
The initiation of cell division does not occur simply anytime; it has a very specific period during the cell cycle in which it can occur. The cell cycle includes these periods: mitosis, G1, G2 and the S phase. During the G1 phase, cells undergo growth, and, during this phase, the growth factors activate certain proteins. This activation will cause a cell to enter the S phase, which is the phase in which cells replicate DNA in order prepare for division.
In addition to growth factors, certain cellular receptors and chemical compounds in some cells may cause other surrounding cells to divide. Cytokines, molecules produced within certain cells, can be secreted by certain cells and promote division among other cells. Certain cells may not begin to divide due to a condition called contact inhibition, in which cells detect other cells next to them and do not divide. However, when cells die and there becomes a gap between these cells and other surrounding cells, then cell division may begin until the gap is filled.
Although scientists point to growth factors, contact inhibition and cytokines as the primary initiators of cell division, there are certain studies that indicate that a chemical called thrombin may also initiate cell division. Thrombin, a chemical found in a variety of processes within the body, may initiate cell division when placed in contact with a cell membrane. Certain preliminary studies on chickens indicate that thrombin contact of this sort may cause cell division in chicken cells.