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Cellular Motion Versus Tissue Motion in the Amniote Embryo

Charlie Little Department of Anatomy and Cell Biology University of Kansas Medical Center

Authors

Little, C.D., Rongish, B.J., Czirok, A, Zamir, E.A., Cheng Cui

Abstract

The long-range goal of our studies is to understand the forces that shape the embryos of warm-blooded animals, in this case birds. The conceptual framework underlying our studies is that by using computational time-lapse imaging we can record and measure BOTH individual cellular trajectories and long-range tissue motion in a live embryo. Tissue motion analysis is based on measuring the passive displacements of endogenous extracellular matrix (ECM) fibers. By subtracting tissue displacements (ECM motion) from the total cellular displacements it is possible to calculate the residual, or actual, motion of individual cells. Thus, our approach yields a measurement of relative motion, if any, between embryonic cells and the ECM scaffold upon which such cells move. We examine embryos during, arguably, the most critical stages of embryogenesis - gastrulation and formation of the vertebral axis. To visualize biological motion we use two-color scanning time-lapse microscopy. This fluorescence-based technique records the motion of tagged cells and their surrounding ECM fibers across large length scales (µm-cm). The resulting data are then subjected to computational analysis. The main results show: 1) The displacements of ECM filaments and cells are large (hundreds of micrometers) and generally directed towards the midline or vertebral axis of the embryo. 2) The vortex-like movements near the vertebrate "organizer" are clearly visible regardless of whether the observer is examining manually-traced cellular trajectories, algorithm-based virtual trajectories, or time-projected ECM immuno-fluorescence patterns. 3) The trajectories for both individual cells and the corresponding ECM particles are approximately equivalent - with respect to direction and total cumulative distance traveled. Thus, the time-lapse data reveal that the motion of the ECM is very similar to the cellular motion pattern in many regions of the embryos. While there is ample evidence for "independent" or autonomous cellular motility, most of what is commonly referred to as "cell migration" is, in fact, due to passive displacement driven by tissue deformations of cells + ECM fibers. The tissue-driven cellular displacements are substantial, i.e., motion occurs across tissue-level length scales (millimeters). These results have profound implications for our understanding of vertebral axis formation and gastrulation - in particular the relative contributions of cell-autonomous migration versus composite tissue motion (cells + ECM). Further, extracellular chemotactic morphogen gradients are widely reported to regulate these critical motion patterns; however, our data show that the extracellular milieu is characterized by constant motion - therefore any morphogen gradient, proposed to drive cellular "migration" is itself in motion. We conclude that models describing early embryonic cellular guidance mechanisms must take into account extracellular matrix motion.