Lance A. Davidson

How we are shaped: The biomechanics of gastrulation

Although it is rarely considered so in modern developmental biology, morphogenesis is fundamentally a biomechanical process, and this is especially true of one of the first major morphogenic transformations in development, gastrulation. Cells bring about changes in embryonic form by generating patterned forces and by differentiating the tissue mechanical properties that harness these forces in specific ways. Therefore, biomechanics lies at the core of connecting the genetic and molecular basis of cell activities to the macroscopic tissue deformations that shape the embryo. Here we discuss what is known of the biomechanics of gastrulation, primarily in amphibians but also comparing similar morphogenic processes in teleost fish and amniotes, and selected events in several species invertebrates. Our goal is to review what is known and identify problems for further research.

Integrin α5β1 and fibronectin regulate polarized cell protrusions required for Xenopus convergence and extension

Summary Background Integrin recognition of fibronectin is required for normal gastrulation including the mediolateral cell intercalation behaviors that drive convergent extension and the elongation of the frog dorsal axis; however, the cellular and molecular mechanisms involved are unclear. Results We report that depletion of fibronectin with antisense morpholinos blocks both convergent extension and mediolateral protrusive behaviors in explant preparations. Both chronic depletion of fibronectin and acute disruptions of integrin α 5 β 1 binding to fibronectin increases the frequency and randomizes the orientation of polarized cellular protrusions, suggesting that integrin-fibronectin interactions normally repress frequent random protrusions in favor of fewer mediolaterally oriented ones. In the absence of integrin α 5 β 1 binding to fibronectin, convergence movements still occur but result in convergent thickening instead of convergent extension. Conclusions These findings support a role for integrin signaling in regulating the protrusive activity that drives axial extension. We hypothesize that the planar spatial arrangement of the fibrillar fibronectin matrix, which delineates tissue compartments within the embryo, is critical for promoting productive oriented protrusions in intercalating cells.

Planar Cell Polarity Genes Regulate Polarized Extracellular Matrix Deposition during Frog Gastrulation

The noncanonical wnt/planar cell polarity (PCP) pathway [1] regulates the mediolaterally (planarly) polarized cell protrusive activity and intercalation that drives the convergent extension movements of vertebrate gastrulation [2], yet the underlying mechanism is unknown. We report that perturbing expression of Xenopus PCP genes, Strabismus (Xstbm), Frizzled (Xfz7), and Prickle (Xpk), disrupts radially polarized fibronectin fibril assembly on mesodermal tissue surfaces, mediolaterally polarized motility, and intercalation. Polarized motility is restored in Xpk-perturbed explants but not in Xstbm- or Xfz7-perturbed explants cultured on fibronectin surfaces. The PCP complex, including Xpk, first regulates polarized surface assembly of the fibronectin matrix, which is necessary for mediolaterally polarized motility, and then, without Xpk, has an additional and necessary function in polarizing motility. These results show that the PCP complex regulates several cell polarities (radial, planar) and several processes (matrix deposition, motility), by indirect and direct mechanisms, and acts in several modes, either with all or a subset of its components, during vertebrate morphogenesis.

Assembly and remodeling of the fibrillar fibronectin extracellular matrix during gastrulation and neurulation in Xenopus laevis

Fibronectin, a major component of the extracellular matrix is critical for processes of cell traction and cell motility. Whole‐mount confocal imaging of the three‐dimensional architecture of the extracellular matrix is used to describe dynamic assembly and remodeling of fibronectin fibrils during gastrulation and neurulation in the early frog embryo. As previously reported, fibrils first appear under the prospective ectoderm. We describe here the first evidence for regulated assembly of fibrils along the somitic mesoderm/endoderm boundary as well as at the notochord/somitic mesoderm boundary and clearing of fibrils from the dorsal and ventral surfaces of the notochord that occurs over the course of a few hours. As gastrulation proceeds, fibrils are restored to the dorsal surface of the notochord, where the notochord contacts the prospective floor plate. As the neural folds form, fibrils are again remodeled as deep neural plate cells move medially. The process of neural tube closure leaves a region of the ectoderm overlying the neural crest transiently bare of fibrils. Fibrils are assembled surrounding the dorsal surface of the neural tube as the neural tube lumen is restored. Developmental Dynamics 231:888–895, 2004.

Live imaging of cell protrusive activity, and extracellular matrix assembly and remodeling during morphogenesis in the frog, Xenopus laevis

Cell motility and matrix assembly have traditionally been studied in isolation because of a lack of suitable model systems in which both can be observed simultaneously. With embryonic tissues from the gastrulating frog Xenopus laevis we observe stages of fibronectin fibrillogenesis coincident with protrusive activity in the overlying cells. Using live confocal time‐lapse images collected from Cy3‐tagged fibronectin and plasma membrane tethered green fluorescent protein, we describe the movement and the elaboration of a complex fibrillar network undergoing topological rearrangements of fibrils on the surface of an embryonic tissue. Discrete processes of annealing, polymerization, stretching, breaking, and recoiling are recorded. Elaboration and maintenance of the complex topology of the extracellular matrix appears to require filamentous actin. These findings support a mechanical‐model in which cell tractive forces elaborate the complex topological fibrillar network and are part of a homeostatic mechanism for the regulation of the extracellular matrix. Developmental Dynamics 237:2684–2692, 2008.

A biologically inspired programming model for self-healing systems

There is an increasing need for software systems to be able to adapt to changing conditions of resource variability, component malfunction and malicious intrusion. Such self-healing systems can prove extremely useful in situations where continuous service is critical or manual repair is not feasible. Human efforts to engineer self-healing systems have had limited success, but nature has developed extraordinary mechanisms for robustness and self-healing over billions of years. Natures programs are encoded in DNA and exhibit remarkable density and expressiveness. We argue that the software engineering community can learn a great deal about building systems from the broader concepts surrounding biological cell programs and the strategies they use to robustly accomplish complex tasks such as development, healing and regeneration. We present a cell-based programming model inspired from biology and speculate on biologically inspired strategies for producing robust, scalable and self-healing software systems.

The Xenopus Embryo as a Model System for Studies of Cell Migration

In this chapter, we describe procedures for the microsurgical removal of cells and tissues from early-stage embryos of the amphibian Xenopus laevis. Using simple culture conditions and artificial substrates, these preparations undergo a variety of quantifiable cellular behaviors that closely mimic cell migration in vivo. Two general methods are described. The first includes procedures for obtaining a dorsal marginal zone explant from early gastrulae in order to investigate the sheet-like extension and migration of the mesendoderm that spreads to cover the inner surface of the blastocoel roof in intact embryos. This preparation allows high-resolution analyses of cellular and subcellular events in a contiguous tissue preparation. The second describes methods for the isolation of cranial neural crest cells from tailbud stage embryos. Cranial neural crest tissue cultured in vitro on fibronectin will undergo segmentation and migrate as streams of cells as they do in the developing head. Each of these robust preparations provides an excellent example of the migratory events that are possible to observe in vitro using amphibian embryos.

Self-organization of vertebrate mesoderm based on simple boundary conditions.

Embryonic development requires cell movements whose coordination is robust and reproducible. A dramatic example is the primary body axis of vertebrates: despite perturbation, cells in prospective axial tissue coordinate their movements to make an elongated body axis. The spatial cues coordinating these movements are not known. We show here that cells deprived of preexisting spatial cues by physical dissociation and reaggregation nonetheless organize themselves into an axis. Activin‐induced cells that are reaggregated into a flat disc initially round up into a ball before elongating perpendicular to the disc. Manipulations of the geometry of the disc and immunofluorescence micrography reveal that the edge of the disc provides a circumferential alignment zone. This finding indicates that physical boundaries provide alignment cues and that circumferential “hoop stress” drives the axial extrusion in a manner resembling late‐involuting mesoderm of Xenopus and archenteron elongation in other deuterostome species such as sea urchins. Thus, a population of cells finds its own midline based on the form of the populations boundaries using an edge‐aligning mechanism. This process provides a remarkably simple organizing principle that contributes to the reliability of embryonic development as a whole. Developmental Dynamics 231:576–581, 2004.

CELL SEGREGATION, MIXING, AND TISSUE PATTERN IN THE SPINAL CORD OF THE XENOPUS LAEVIS NEURULA

Background: During Xenopus laevis neurulation, neural ectodermal cells of the spinal cord are patterned at the same time that they intercalate mediolaterally and radially, moving within and between two cell layers. Curious if these rearrangements disrupt early cell identities, we lineage‐traced cells in each layer from neural plate stages to the closed neural tube, and used in situ hybridization to assay gene expression in the moving cells. Results: Our biotin and fluorescent labeling of deep and superficial cells reveals that mediolateral intercalation does not disrupt cell cohorts; in other words, it is conservative. However, outside the midline notoplate, later radial intercalation does displace superficial cells dorsoventrally, radically disrupting cell cohorts. The tube roof is composed almost exclusively of superficial cells, including some displaced from ventral positions; gene expression in these displaced cells must now be surveyed further. Superficial cells also flank the tubes floor, which is, itself, almost exclusively composed of deep cells. Conclusions: Our data provide: (1) a fate map of superficial‐ and deep‐cell positions within the Xenopus neural tube, (2) the paths taken to these positions, and (3) preliminary evidence of re‐patterning in cells carried out of one environment and into another, during neural morphogenesis. Developmental Dynamics, 242:1134–1146, 2013.