Development entails the ensemble of processes—molecular, cell, and tissue scale—that are necessary to transform a single cell into a complex multicellular organism. For development to occur, most, if not all, eukaryotic cells are destined to enter the cell cycle and divide at a given time and place. Ultimately, tissue and organ formation are primarily determined by when, where, and how much cells divide, move, and die.

To divide, a cell must first enter the cell cycle and transit through the G₁, S, G₂, and M phases in a unidirectional manner. However, cells will not stop whatever they are doing to undergo cell cycle progression; they continue performing their functions to ensure proper tissue morphogenesis. A better understanding of the link between cell behavior and cell cycle progression thus could provide novel insight into developmental biology. Unsurprisingly, the last few decades have witnessed a concerted effort put into the expansion of tools that allow us to follow cell cycle dynamics live. In the review “Imaging developmental cell cycles,” Kohrman et al. set out to discuss existing cell cycle reporters, with an emphasis on how these tools can contribute to new discoveries in the fields of cell and developmental biology. This review is part of the October 5, 2021 special issue of Biophysical Journal entitled “Biophysics of Development,” which focuses on how research in biophysics has contributed to developmental biology.

Among existing cell cycle reporters, perhaps the most commonly used are fluorescent ubiquitination-based cell cycle indicators (FUCCIs), which were first developed as dual-colored reporters that could distinguish G₁ and S-G₂-M phases. Current newer versions allow for live tracking of all four phases of the cell cycle. The adaptability of this reporter has led to numerous modified versions, such as FUCCI for use with fluorescence lifetime imaging microscopy as a lineage-restricted FUCCI reporter and as a dual-FUCCI-primary cilium sensor. To avoid exogenous protein expression, other reporters with direct protein tags of endogenous cell cycle proteins, like proliferating cell nuclear antigen, are also available. More recently, reporters of kinase activity like DNA helicase B-fluorescent protein, whose localization is dependent on cyclin-dependent kinase phosphorylation, have been developed. More recently, reporters of kinase activity, such as the human DNA Helicase B fused to a fluorescent protein (DHB-FP), whose localization is dependent on CDK phosphorylation, have been developed.

Although cell cycle reporters help us visualize cell cycle dynamics, they provide limited information if the acquired data sets are not analyzed quantitatively. Kohrman et al. describe how a generalized image-processing pipeline works, providing a thorough list of existing automated tracking and segmentation methods in 2D and 3D, the latter being particularly challenging but far more relevant to cell cycle analysis in living organisms.

With the improvement of biosensors and image-processing tools used to quantify cell cycle dynamics, our understanding of the link between cellular behavior and the specific cell cycle during development continues to grow. The authors highlight many notable findings: the relationship between mitogen sensing and the decision to proliferate or enter quiescence (G₀), the coordination of adult stem cell behavior and the cell cycle in response to tissue damage, the restriction of cell migration and tissue-scale movements to the gap phases of the cell cycle, and the complex relationship between cell cycle phase entry and length with cell fate decisions.

Naturally, for every question answered in the field, a cascade of unanswered questions ensues, but as Kohrman and colleagues emphasize, this also represents an exciting avenue for new discoveries in cell and developmental biology.