Epithelial folding mediated by apical constriction converts flat epithelial sheets into multilayered, complex tissue structures and is employed throughout the development in most animals1. Little is known, however, how forces produced near the apical surface of the tissue are transmitted within individual cells to generate the global changes in cell shape that characterize tissue deformation. Here we apply particle tracking velocimetry in gastrulating Drosophila embryos to measure the movement of cytoplasm and plasma membrane during ventral furrow (VF) formation2, 3. We find that cytoplasmic redistribution during the lengthening phase of VF formation can be precisely described by viscous flows that quantitatively match the predictions of hydrodynamics. Cell membranes move with the ambient cytoplasm, with little resistance to or driving force on the flow. Strikingly, apical constriction produces similar flow patterns in mutant embryos that fail to form cells prior to gastrulation (“acellular” embryos), such that the global redistribution of cytoplasm mirrors the summed redistribution occurring in individual cells of wild type embryos. Our results suggest that during the lengthening phase of VF formation, hydrodynamic behavior of the cytoplasm provides the predominant mechanism transmitting apically generated forces deep into the tissue and that cell individualization is dispensable.

Many models of morphogenesis are forced to assume specific mechanical properties of cells, because the actual mechanical properties of living tissues are largely unknown. Here, we measure the rheology of epithelial cells in the cellularizing Drosophila embryo by injecting magnetic particles and studying their response to external actuation. We establish that, on timescales relevant to epithelial morphogenesis, the cytoplasm is predominantly viscous, whereas the cellular cortex is elastic. The timescale of elastic stress relaxation has a lower bound of 4 min, which is comparable to the time required for internalization of the ventral furrow during gastrulation. The cytoplasm was measured to be ∼10 3 -fold as viscous as water. We show that elasticity depends on the actin cytoskeleton and conclude by discussing how these results relate to existing mechanical models of morphogenesis.cell rheology | embryology | gastrulation

During Drosophila gastrulation, the ventral mesodermal cells constrict their apices, undergo a series of coordinated cell-shape changes to form a ventral furrow (VF) and are subsequently internalized. Although it has been well documented that apical constriction is necessary for VF formation, the mechanism by which apical constriction transmits forces throughout the bulk tissue of the cell remains poorly understood. In this work, we develop a computational vertex model to investigate the role of the passive mechanical properties of the cellular blastoderm during gastrulation. We introduce to our knowledge novel data that confirm that the volume of apically constricting cells is conserved throughout the entire course of invagination. We show that maintenance of this constant volume is sufficient to generate invagination as a passive response to apical constriction when it is combined with region-specific elasticities in the membranes surrounding individual cells. We find that the specific sequence of cell-shape changes during VF formation is critically controlled by the stiffness of the lateral and basal membrane surfaces. In particular, our model demonstrates that a transition in basal rigidity is sufficient to drive VF formation along the same sequence of cell-shape change that we observed in the actual embryo, with no active force generation required other than apical constriction.

The paper analyses John Steinbeck's travelogue "A Russian Journal" based on the writer's impressions of his visit to the USSR in 1947. Unlike many other American authors, whose representation of the Soviet Union, being ideologically biased, abounded in stereotypes of the Cold War, Steinbeck, in the spirit of New Journalism, placed special stress on objectivity and factualness of his "Russian text". Striving to achieve impartiality of imagological representation, he abstracted from considering the political regime and focused on ethnocultural traits of the people, their everyday life. The mixed genre, which includes travelogue and report features, provided an opportunity for Steinbeck to combine publicistic and belles-lettres discourse and to create an image of "the other", marked with the author's subjectivity. The view from the outside, an estranged vision of another culture, combined with the narrator's emotional involvement, enabled Steinbeck to execute an expressive picture of the Soviet life, the picture which has both a genuine sympathy for ordinary people who had survived the trauma of the war and hoped for a bright future, and irony and sarcasm towards the bureaucratic state system, Stalin's cult. The cycle of reports, written by Steinbeck, is distinguished by its rich imagery and emotive intensity; it demonstrates the author's skill in creating individualized characters and visualizing his impressions. Alongside with the artistic merits of "A Russian Journal", we should note Steinbeck's effort to overcome the stereotypes about Russia and to represent the Soviet hetero-image through the prism of universal human values, axiological constants, which are common for all mankind.