9.5Regulation of Cell Growth
Roughly speaking, unicellular organisms keep multiplying as long as their living environment permits. On the other hand, the cell growth of eukaryotic multicellular organisms is regulated to avoid uncontrolled growth despite the well-maintained internal environment the cells are in (e.g., nutrients and oxygen are supplied, waste materials and carbon dioxide are eliminated, and temperature and pH are appropriately maintained (internal homeostasis)). Tissues consisting of somatic cells contain many cells that do not grow by default (i.e., they are in the G0 phase) but do so as necessary.
Positive and Negative Regulation
The cell growth of eukaryotic multicellular organisms is strictly regulated, which is achieved by balancing anti-cell-growth signals (negative growth signals) and pro-cell-growth signals (positive growth signals). As an example, cells are firmly attached to each other in the epithelial tissue of mammals including humans (see Chapter 11), thereby negatively regulating cell growth. When a cell is detached from the adjacent cell (e.g., by injury), the negative regulation is lost and cell growth is enabled. This, however, is still not sufficient for the initiation of cell growth, which begins only when positive growth signals are transduceed to cells by growth factors (many of which are proteins) precommitted to particular cell types. If cells adhere to each other, the presence of growth factors does not induce cell growth. In this way, cell growth is regulated both positively and negatively.