Density-independent factors, such as weather and climate, exert their influences on population size regardless of the population's density. In contrast, the effects of density-dependent factors intensify as the population increases in size. For example, some diseases spread faster in populations where individuals live.
Here's basically what I could come up with. Hopefully this gives are a little better understanding on your subject and might even lead you to the right answer!
Cells could sense and respond to electromagnet signals such as light and mechanical signals such as touch. Cells often communicate with each other using chemical signals.