Epithelial organs undergo steady-state turnover throughout adult life, with old cells being continually replaced by the progeny of stem cell divisions1. To avoid hyperplasia or atrophy, organ turnover demands strict equilibration of cell production and loss2–4. However, the mechanistic basis of this equilibrium is unknown. Using the adult Drosophila intestine5, we find that robustly precise turnover arises through a coupling mechanism in which enterocyte apoptosis breaks feedback inhibition of stem cell divisions. Healthy enterocytes inhibit stem cell division through E-cadherin, which prevents secretion of mitogenic EGFs by repressing transcription of the EGF maturation factor rhomboid. Individual apoptotic enterocytes promote divisions by loss of E-cadherin, which releases cadherin-associated β-catenin/Armadillo and p120-catenin to induce rhomboid. Induction of rhomboid in the dying enterocyte triggers EGFR activation in stem cells within a discrete radius. When we block apoptosis, E-cadherin-controlled feedback suppresses divisions, and the organ retains the same number of cells. When we disrupt feedback, apoptosis and divisions are uncoupled, and the organ develops either hyperplasia or atrophy. Altogether, our work demonstrates that robust cellular balance hinges on the obligate coupling of divisions to apoptosis, which limits the proliferative potential of a stem cell to the precise time and place that a replacement cell is needed. In this manner, localized cell-cell communication gives rise to tissue-level homeostatic equilibrium and constant organ size.

Organ renewal is governed by the dynamics of cell division, differentiation and loss. To study these dynamics in real time, we present a platform for extended live imaging of the adult Drosophila midgut, a premier genetic model for stem-cell-based organs. A window cut into a living animal allows the midgut to be imaged while intact and physiologically functioning. This approach prolongs imaging sessions to 12–16 hr and yields movies that document cell and tissue dynamics at vivid spatiotemporal resolution. By applying a pipeline for movie processing and analysis, we uncover new and intriguing cell behaviors: that mitotic stem cells dynamically re-orient, that daughter cells use slow kinetics of Notch activation to reach a fate-specifying threshold, and that enterocytes extrude via ratcheted constriction of a junctional ring. By enabling real-time study of midgut phenomena that were previously inaccessible, our platform opens a new realm for dynamic understanding of adult organ renewal.

Epithelial organs undergo steady-state turnover throughout adult life, with old cells being continually replaced by the progeny of stem cell divisions1. To avoid hyperplasia or atrophy, organ turnover demands strict equilibration of cell production and loss2-4. However, the mechanistic basis of this equilibrium is unknown. Using the adult Drosophila intestine5, we find that robustly precise turnover arises through a coupling mechanism in which enterocyte apoptosis breaks feedback inhibition of stem cell divisions. Healthy enterocytes inhibit stem cell division through E-cadherin, which prevents secretion of mitogenic EGFs by repressing transcription of the EGF maturation factor rhomboid. Individual apoptotic enterocytes promote divisions by loss of E-cadherin, which releases cadherin-associated β-catenin/Armadillo and p120-catenin to induce rhomboid. Induction of rhomboid in the dying enterocyte triggers EGFR activation in stem cells within a discrete radius. When we block apoptosis, E-cadherin-controlled feedback suppresses divisions, and the organ retains the same number of cells. When we disrupt feedback, apoptosis and divisions are uncoupled, and the organ develops either hyperplasia or atrophy. Altogether, our work demonstrates that robust cellular balance hinges on the obligate coupling of divisions to apoptosis, which limits the proliferative potential of a stem cell to the precise time and place that a replacement cell is needed. In this manner, localized cell-cell communication gives rise to tissue-level homeostatic equilibrium and constant organ size.

Cell-and tissue-level processes often occur across days or weeks, but few imaging methods can capture such long timescales. Here, we describe Bellymount, a simple, noninvasive method for longitudinal imaging of the Drosophila abdomen at subcellular resolution. Bellymounted animals remain live and intact, so the same individual can be imaged serially to yield vivid time series of multiday processes. This feature opens the door to longitudinal studies of Drosophila internal organs in their native context. Exploiting Bellymount's capabilities, we track intestinal stem cell lineages and gut microbial colonization in single animals, revealing spatiotemporal dynamics undetectable by previously available methods.

Though cell size varies between different cells and across species, the nuclear-to-cytoplasmic (N/C) ratio is largely maintained across species and within cell types. A cell maintains a relatively constant N/C ratio by coupling DNA content, nuclear size, and cell size. We explore how cells couple cell division and growth to DNA content. In some cases, cells use DNA as a molecular yardstick to control the availability of cell cycle regulators. In other cases, DNA sets a limit for biosynthetic capacity. Developmentally programmed variations in the N/C ratio for a given cell type suggest that a specific N/C ratio is required to respond to given physiological demands. Recent observations connecting decreased N/C ratios with cellular senescence indicate that maintaining the proper N/C ratio is essential for proper cellular functioning. Together, these findings suggest a causative, not simply correlative, role for the N/C ratio in regulating cell growth and cell cycle progression. Expected final online publication date for the Annual Review of Genetics, Volume 56 is November 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.