The ras pathway: a model for the control of proliferation in animal cells

Scrupulous control over cellular proliferation is essential for the survival of a multicellular organism, yet little is known about the mechanism of this control. A variety of factors are known to influence cell division in both positive and negative ways. These include diffusible factors and the results of cell-cell contact. While the ultimate decision concerning cell division is most likely made at the molecular level within a specific cell, this decision must be based upon the signals received from outside the cell. In this study data and hypotheses are presented primarily regarding the means by which extracellular proliferative signals are transferred into the cell. This discussion, therefore, represents an important but limited aspect of the control of proliferation within the organism. While the discussion will center primarily upon a presentation of data, hypotheses relating to these data will also be included. In many cases, the hypotheses are only speculative at present.