Live-cell mass profiling: an emerging approach in quantitative biophysics

Abstract

Cell mass, volume and growth rate are tightly controlled biophysical parameters in cellular development and homeostasis, and pathological cell growth defines cancer in metazoans. The first measurements of cell mass were made in the 1950s, but only recently have advances in computer science and microfabrication spurred the rapid development of precision mass-quantifying approaches. Here we discuss available techniques for quantifying the mass of single live cells with an emphasis on relative features, capabilities and drawbacks for different applications.

Figure 1

Some methods for single-cell or single-cluster mass measurements. (a) In a microfluidic cantilever, single nonadherent cells (yellow circle) pass through a microchannel contained within a vibrating cantilever. The cell buoyant mass is detected as a shift in the vibration frequency of the cantilever^,. (b) Example of phase-shifting interferometry based on a Michelson interferometer. Incoherent light incident on the sample is split into two paths and passes through the reference and sample chambers. The phase shift of light as it passes through a cell is determined from the interference of light from these two paths when they are reflected back and recombine at a charge-coupled device (CCD) camera. (c) Example of digital holographic microscopy using an off-axis Mach-Zehnder interferometer. Coherent light from a reference beam interferes with light from a sample beam in an off-axis configuration as the reference beam is tilted relative to the sample beam. This forms a hologram at a CCD camera that can be interrogated computationally to generate images of the phase shift of light as it passed through a biological sample.

Figure 2

Specific refractive index increment of biomolecules. (a) Changes in refractive index, n, of various biomolecules in solution relative to that of the solvent (water) as a function of biomolecule mass concentration. The slope of this line, dn/dc, is the specific refractive increment used in determinations of cell dry mass from quantitative phase images. (b) Representative specific refractive index increments of selected biomolecule groups, with the averages of these selected values indicated as horizontal bars. Data are shown for whole-cell average values^,, proteins (multiprotein average^–, bovine serum albumin (BSA) and Evans bacteriological peptone), nucleic acids (DNA^– and RNA^,) and other components (CaCl₂, NaCl, KCl, sucrose and dipalmitoyl phosphatidylcholine (DPPC), a common phospolipid).

Figure 3

Live-cell mass quantification with interference microscopy. (a,b) Mass distributions within a single, adherent mouse L-cell fibroblast (a) and a multicellular human pluripotent stem cell colony (b) measured with phase-shifting interferometry. The color scale corresponds to mass per unit area, showing the intracellular or intracolony distribution of mass (red, high mass; blue, low mass or background level). (c) Mass distribution images captured by time-lapse interference microscopy can be integrated over the area of a cell or colony to yield mass over time data. A mass-versus-time plot is shown for a single, nonadherent mouse CH12 erythroleukemia cell measured with phase-shifting interferometry.