Quantitative bioimage analysis of cell characteristics

CELLS are the building blocks of tissues, organs, and organisms. They represent organizational units that appear in certain morphologies and exhibit dynamic behavior to accomplish specific functions. Since the first microscopic visualization of cellular organisms by Antonie van Leeuwenhoek in the 17th century (1), new imaging techniques have been and are still being developed to open our eyes for life in the microworld. Today, we are not only able to observe cells with unprecedented accuracy but also can study their function, dynamics, and morphology in great detail. Scientific investigations—often initiated by individual observations originating out of pure curiosity—eventually have to go beyond the level of descriptive views by rigorously quantifying the observed phenomena. Quantitative bioimage analysis is a theoretical approach that is based on algorithmic solutions for the automated, objective, and accurate evaluation of biological processes in microscopic image data (2). This approach has already proven to yield significant advancements of our quantitative understanding by elucidating the characteristics of cells as being the essential organizational units of biological life. This Cytometry Part A Special Issue comprises eight articles—six research articles and two review articles— focusing on quantitative bioimage analysis. The biological systems considered in these studies are quite diverse, for example, including human blood samples, brain of zebrafish and mouse as well as the murine gut. However, as schematically shown in Figure 1, the tertium comparationis of articles in this Special Issue is given by the focus on the quantitative bioimage analysis of one or more cell characteristics in terms of cellular dynamics, function, and morphology. Regarding morphological characteristics of cells, the review article by Model (this issue, page 281) puts cell volume in the focus by summarizing the main methods for its quantitative determination. These methods include, for example, measurement of light scattering, estimation based on various cell dimensions, scanning of the cell surface, imaging of cell stacks, and several others more. The suitability of the various methods is discussed in the light of different applications. Similarly, intracellular properties—such as the concentration of specific proteins inside the cell—are an essential morphological cell characteristics as well. This is emphasized in the research article by Mudrak et al. (this issue, page 297). Using calibrated bright-field-based imaging, they describe the measurement of intracellular protein concentrations through the refractive index, which could be realized using two or more mutually defocused bright-field images that were then quantitatively analyzed in the theoretical framework of the transport-of-intensity equation complemented by a calibration procedure. In this way, accurate and reliable measurement of intracellular proteins was realized, as illustrated by measuring the long-term cell volume adaptation of HeLa cells in a hyperosmotic buffer. Two studies in this Special Issue applied an image-based systems biology approach, which generally comprises the