Anastasios Pavlopoulos

To build tissues, and ultimately functional bodies, cells in early embryos must arrange into specific patterns. In most animals, epithelial tissues exhibit a predominantly hexagonal space-packing geometry. However, in many species of the largest group of crustaceans, the Malacostraca, the embryonic epithelium takes on the striking form of a grid made up of predominantly square cells, sequential rows of which establish the adult segmented body plan. After square cells emerge, their organization appears to be maintained by specific cell division patterns. However, the mechanisms that initially generate square cells from hexagonal precursors are unknown. Here we address this problem by combining long-term multiview lightsheet microscopy, immunohistochemistry, laser ablation, and pharmacological perturbation. We show that in the emerging model crustacean Parhyale hawaiensis this highly unusual grid geometry is first initiated from two perpendicular axes that are established sequentially according to different cellular mechanisms. The first axis arises dorso-ventrally at a tensile lineage compartment boundary, while the second emerges at the anterior-posterior axis along the ventral midline through lineage-independent cell intercalation driven by tensile actomyosin cables. We show that these midline cables are necessary for organizing square-cell packing as well as for proper expression of the segmentation gene engrailed . Our findings show that both cell lineage-specific behaviors, as well as lineage-independent supracellular structures, are required to establish square grid epithelial organization and a segmented body plan.

How does growth encode form in developing organisms? Many different spatiotemporal growth profiles may sculpt tissues into the same target 3D shapes, but only specific growth patterns are observed in animal and plant development. In particular, growth profiles may differ in their degree of spatial variation and growth anisotropy; however, the criteria that distinguish observed patterns of growth from other possible alternatives are not understood. Here we exploit the mathematical formalism of quasiconformal transformations to formulate the problem of “growth pattern selection” quantitatively in the context of 3D shape formation by growing 2D epithelial sheets. We propose that nature settles on growth patterns that are the “simplest” in a certain way. Specifically, we demonstrate that growth pattern selection can be formulated as an optimization problem and solved for the trajectories that minimize spatiotemporal variation in areal growth rates and deformation anisotropy. The result is a complete prediction for the growth of the surface, including not only a set of intermediate shapes, but also a prediction for cell displacement along those surfaces in the process of growth. Optimization of growth trajectories for both idealized surfaces and those observed in nature show that relative growth rates can be uniformized at the cost of introducing anisotropy. Minimizing the variation of programmed growth rates can therefore be viewed as a generic mechanism for growth pattern selection and may help us to understand the prevalence of anisotropy in developmental programs. Published by the American Physical Society 2025

Lineage tracing based on modern live imaging approaches enables to visualize, reconstruct, and analyze the developmental history, fate, and dynamic behaviors of cells in vivo in a direct, comprehensive, and quantitative manner. Light-sheet fluorescence microscopy (LSFM) has greatly boosted lineage tracing efforts, because fluorescently labeled specimens can be imaged in their entirety, over long periods of time, with high spatiotemporal resolution and minimal photodamage. In addition, an increasing arsenal of commercial and open-source software solutions for cell and nuclei segmentation and tracking can be employed to convert data from pixel-based to object-based representations, and to reconstruct the lineages of cells in their native context as they organize in tissues, organs, and whole organisms. This chapter describes the preparation of LSFM image datasets and the use of three freely available platforms, namely, the Fiji/ImageJ plugins Massive Multiview Tracker (MaMuT), Mastodon and TrackMate, for small-scale and large-scale lineage tracing purposes using manual, semi-automated, and fully automated pipelines for nuclei or cell tracking. Lineage tracing with these tools is described on LSFM image datasets of fluorescently labeled embryos from the crustacean model Parhyale hawaiensis that lends itself to multi-scale investigations of development and regeneration.

Morphogenesis, the process through which genes generate form, establishes tissue-scale order as a template for constructing the complex shapes of the body plan. The extensive growth required to build these ordered substrates is fuelled by cell proliferation, which, naively, should destroy order. Understanding how active morphogenetic mechanisms couple cellular and mechanical processes to generate order—rather than annihilate it—remains an outstanding question in animal development. We show that cell divisions are the primary drivers of tissue flow, leading to a fourfold orientationally ordered phase. Waves of anisotropic cell proliferation propagate across the embryo with precise patterning. Defects introduced into the nascent lattice by cell divisions are moved out of the tissue bulk towards the boundary by subsequent divisions. Specific cell proliferation rates and orientations enable cell divisions to organize rather than fluidize the tissue. We observe this using live imaging and tissue cartography to analyse the dynamics of fourfold tissue ordering in the trunk segmental ectoderm of the crustaceanParhyale hawaiensisbeginning 72 h after egg lay. The result is a robust, active mechanism for generating global orientational order in a non-equilibrium system that sets the stage for the subsequent development of shape and form.

Parhyale hawaiensis is a marine crustacean which has emerged as a powerful model organism to study molecular and cellular mechanisms linked to embryonic development and regenerative processes. Recently, several fluorescence-based optical microscopy techniques have been employed for the study of Parhyale to obtain anatomical descriptions, analyze gene expression patterns and reconstruct cell lineages. Aiming at the expansion of the imaging repertoire for this emerging model organism, we introduce a low-cost hybrid diagnostic system which integrates confocal fluorescence and frequency domain photoacoustic (FDPA) microscopy modalities, concurrently capturing both the radiative and radiationless relaxations of molecules following their excitation by an intensity-modulated laser source. We initially characterize the hybrid microscope in terms of PA signal generation efficiency and lateral spatial resolution, and subsequently evaluate its capabilities for the in vivo imaging of unlabelled and fluorescently labelled Parhyale embryos found at different stages of development. The recorded hybrid images have revealed a remarkable contrast complementarity between the integrated imaging modes, providing valuable information regarding cells’ structure, nuclei location, cellular membranes and yolk distribution. Our findings may pave the way for the broader adoption of inexpensive hybrid optical and PA microscopy methods in developmental biology, significantly upgrading the capabilities of the currently used technologies.

Many animal embryos pull and close an epithelial sheet around the ellipsoidal egg surface during a gastrulation process known as epiboly. The ovoidal geometry dictates that the epithelial sheet first expands and subsequently compacts. Moreover, the spreading epithelium is mechanically stressed and this stress needs to be released. Here we show that during extraembryonic tissue (serosa) epiboly in the insect Tribolium castaneum, the non-proliferative serosa becomes regionalized into a solid-like dorsal region with larger non-rearranging cells, and a more fluid-like ventral region surrounding the leading edge with smaller cells undergoing intercalations. Our results suggest that a heterogeneous actomyosin cable contributes to the fluidization of the leading edge by driving sequential eviction and intercalation of individual cells away from the serosa margin. Since this developmental solution utilized during epiboly resembles the mechanism of wound healing, we propose actomyosin cable-driven local tissue fluidization as a conserved morphogenetic module for closure of epithelial gaps.

Many animal embryos pull and close an epithelial sheet around the spherical or ellipsoidal egg surface during a gastrulation process known as epiboly. The ovoidal geometry dictates that the epithelial sheet first expands and subsequently compacts. Moreover, the epithelial sheet spreading over the sphere is mechanically stressed and this stress needs to be released. Here we show that during extraembryonic tissue (serosa) epiboly in the red flour beetle Tribolium castaneum , the non-proliferative serosa becomes regionalized into two distinct territories: a dorsal region under higher tension away from the leading edge with larger non-rearranging cells, and a more fluid ventral region under lower tension surrounding the leading edge with smaller cells undergoing cell intercalation. Our results suggest that fluidization of the leading edge is caused by a heterogeneous actomyosin cable that drives sequential eviction and intercalation of individual cells away from the serosa margin. Since this developmental solution utilized during epiboly resembles the mechanism of wound healing in other systems, we propose actomyosin cable-driven local tissue fluidization as a conserved morphogenetic module for closure of epithelial gaps.

During gastrulation, physical forces reshape the simple embryonic tissue to form a complex body plan of multicellular organisms ¹ . These forces often cause large-scale asymmetric movements of the embryonic tissue ^2,3 . In many embryos, the tissue undergoing gastrulation movements is surrounded by a rigid protective shell ^4,5 . While it is well recognized that gastrulation movements depend on forces generated by tissue-intrinsic contractility ^6,7 , it is not known if interactions between the tissue and the protective shell provide additional forces that impact gastrulation. Here we show that a particular part of the blastoderm tissue of the red flour beetle Tribolium castaneum tightly adheres in a temporally coordinated manner to the vitelline envelope surrounding the embryo. This attachment generates an additional force that counteracts the tissue-intrinsic contractile forces to create asymmetric tissue movements. Furthermore, this localized attachment is mediated by a specific integrin, and its knock-down leads to a gastrulation phenotype consistent with complete loss of attachment. Moreover, analysis of another integrin in the fruit fly Drosophila melanogaster suggests that gastrulation in this organism also relies on adhesion between the blastoderm and the vitelline. Together, our findings reveal a conserved mechanism whereby the spatiotemporal pattern of tissue adhesion to the vitelline envelope provides controllable counter-forces that shape gastrulation movements in insects.

During development, coordinated cell behaviors orchestrate tissue and organ morphogenesis. Detailed descriptions of cell lineages and behaviors provide a powerful framework to elucidate the mechanisms of morphogenesis. To study the cellular basis of limb development, we imaged transgenic fluorescently-labeled embryos from the crustacean Parhyale hawaiensis with multi-view light-sheet microscopy at high spatiotemporal resolution over several days of embryogenesis. The cell lineage of outgrowing thoracic limbs was reconstructed at single-cell resolution with new software called Massive Multi-view Tracker (MaMuT). In silico clonal analyses suggested that the early limb primordium becomes subdivided into anterior-posterior and dorsal-ventral compartments whose boundaries intersect at the distal tip of the growing limb. Limb-bud formation is associated with spatial modulation of cell proliferation, while limb elongation is also driven by preferential orientation of cell divisions along the proximal-distal growth axis. Cellular reconstructions were predictive of the expression patterns of limb development genes including the BMP morphogen Decapentaplegic.