Studying cell volume beyond cell volume.

The first part of the paper describes two simple microscopic techniques that we use in our laboratory. One measures cell volumes in adherent cultures and the other measures cell dry mass; both measurements are done on the same instrument (a standard bright-field transmission microscope with only one or two narrow-band color filters added) and on the same cells. The reason for combining cell volume with dry mass is that the ratio of the two-dry mass concentration (MC)-is an important and insufficiently utilized biological parameter. We then describe a few applications of MC. The available experimental data strongly suggest its critical role in biological processes, including cell volume regulation. For example, most eukaryotic cells have surprisingly similar values of MC. Moreover, MC (and not cell volume) is tightly controlled in growing cell cultures at highly variable external osmolarities. We review the results showing that elevation of MC is a direct cause of shrinkage-induced apoptosis. Also, by focusing on MC, one can study heterogenous processes, such as necrotic swelling, or discriminate between apoptotic dehydration and the loss of cell fragments.