A longstanding question in biology is whether there is an intrinsic mechanism for coordinating growth and cell cycle in metazoan cells. We have examined cell size distributions in populations of lymphoblasts and applied a novel mathematical analysis to calculate accurately how growth rates vary with both cell size and the cell cycle. Our results show that growth rate is size-dependent throughout the cell cycle. After initial growth suppression there is a rapid increase in growth rate in G1, followed by a constant exponential growth phase. The probability of cell division varies independently with cell size and cell age. We conclude that proliferating mammalian cells have an intrinsic mechanism that maintains cell size.

Mitochondrial membrane potential provides a valuable indicator of cells' health and functional status. Cytometry- and microscopy-based analyses, in combination with fluorescent probes, are widely used to study mitochondrial behavior related to cellular pathways, most notably – apoptosis. The cyanine dye JC-1 (5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimi- dazolylcarbocyanine iodide) facilitates discrimination of energized and deenergized mitochondria because the normally green fluorescent dye forms red fluorescent aggregates when concentrated in energized mitochondria in response to their higher membrane potential. JC-1 fluorescence is usually excited by the 488 nm laser wavelength common in flow cytometers. In this study, we show that in practice this approach is not optimal for monitoring mitochondrial behavior. Investigation of fluorescence of JC-1 in solution and in cells using spectrofluorimetry, microscopy and flow cytometry reveals that excitation at 405 nm wavelength, now available on standard instruments, produces signals from aggregate fluorescence with considerably less spillover from dye monomer fluorescence than can be obtained using 488 nm excitation. The improved data are more accurate and eliminate the necessity for fluorescence compensation, making the use of the alternative excitation wavelengths beneficial for mitochondria-related biological and biomedial research.

We used a suspended microchannel resonator (SMR) combined with picoliter-scale microfluidic control to measure buoyant mass and determine the ‘instantaneous’ growth rates of individual cells. The SMR measures mass with femtogram precision, allowing rapid determination of the growth rate in a fraction of a complete cell cycle. We found that for individual cells of Bacillus subtilis, Escherichia coli, Saccharomyces cerevisiae and mouse lymphoblasts, heavier cells grew faster than lighter cells.