Extracellular control of cell size

Both cell growth (cell mass increase) and progression through the cell divi sion cycle are required for sustained cell proliferation(1). Proliferating cells in culture tend to double in mass before each division(2), but it is not known how growth and division rates are co-ordinated to ensure that cel l size is maintained(1,3-5). The prevailing view is that coordination is ac hieved because cell growth is rate-limiting for cell-cycle progression(6-10 ). Here, we challenge this view. We have investigated the relationship betw een cell growth and cell-cycle progression in purified rat Schwann cells, u sing two extracellular signal proteins that are known to influence these ce lls(11-13). We find that glial growth factor (GGF) can stimulate cell-cycle progression without promoting cell growth. We have used this restricted ac tion of GGF to show that, for cultured Schwann cells, cell growth rate alon e does not determine the rate of cell-cycle progression and that cell size at division is variable and depends on the concentrations of extracellular signal proteins that stimulate cell-cycle progression, cell growth, or both .