Preparation of embryos for Electron Microscopy of the Drosophila embryonic heart tube

The morphogenesis of the Drosophila embryonic heart tube has emerged as a valuable model system for studying cell migration, cell-cell adhesion and cell shape changes during embryonic development. One of the challenges faced in studying this structure is that the lumen of the heart tube, as well as the membrane features that are crucial to heart tube formation, are difficult to visualize in whole mount embryos, due to the small size of the heart tube and intra-lumenal space relative to the embryo. The use of transmission electron microscopy allows for higher magnification of these structures and gives the advantage of examining the embryos in cross section, which easily reveals the size and shape of the lumen. In this video, we detail the process for reliable fixation, embedding, and sectioning of late stage Drosophila embryos in order to visualize the heart tube lumen as well as important cellular structures including cell-cell junctions and the basement membrane

Repulsion by Slit and Roundabout prevents Shotgun/E-cadherin–mediated cell adhesion during Drosophila heart tube lumen formation

During Drosophila melanogaster heart development, a lumen forms between apical surfaces of contralateral cardioblasts (CBs). We show that Slit and its receptor Roundabout (Robo) are required at CB apical domains for lumen formation. Mislocalization of Slit outside the apical domain causes ectopic lumen formation and the mislocalization of cell junction proteins, E-cadherin (E-Cad) and Enabled, without disrupting overall CB cell polarity. Ectopic lumen formation is suppressed in robo mutants, which indicates robo's requirement for this process. Genetic evidence suggests that Robo and Shotgun (Shg)/E-Cad function together in modulating CB adhesion. robo and shg/E-Cad transheterozygotes have lumen defects. In robo loss-of-function or shg/E-Cad gain-of-function embryos, lumen formation is blocked because of inappropriate CB adhesion and an accumulation of E-Cad at the apical membrane. In contrast, shg/E-Cad loss-of-function or robo gain-of-function blocks lumen formation due to a loss of CB adhesion. Our data show that Slit and Robo pathways function in lumen formation as a repulsive signal to antagonize E-Cad–mediated cell adhesion

Biophysical basis for convergent evolution of two veil-forming microbes

Microbes living in stagnant water typically rely on chemical diffusion to draw nutrients from their environment. The sulfur-oxidizing bacterium Thiovulum majus and the ciliate Uronemella have independently evolved the ability to form a ‘veil’, a centimetre-scale mucous sheet on which cells organize to produce a macroscopic flow. This flow pulls nutrients through the community an order of magnitude faster than diffusion. To understand how natural selection led these microbes to evolve this collective behaviour, we connect the physical limitations acting on individual cells to the cell traits. We show how diffusion limitation and viscous dissipation have led individual T. majus and Uronemella cells to display two similar characteristics. Both of these cells exert a force of approximately 40 pN on the water and attach to boundaries by means of a mucous stalk. We show how the diffusion coefficient of oxygen in water and the viscosity of water define the force the cells must exert. We then show how the hydrodynamics of filter-feeding orient a microbe normal to the surface to which it attaches. Finally, we combine these results with new observations of veil formation and a review of veil dynamics to compare the collective dynamics of these microbes. We conclude that this convergent evolution is a reflection of similar physical limitations imposed by diffusion and viscosity acting on individual cells

Repulsion by Slit and Roundabout prevents Shotgun/E-cadherin–mediated cell adhesion during heart tube lumen formation-1

Robo (green). (A) A wild-type embryo showing the normal pattern of Slit. (B) embryo with one copy each of and ( GOF) in which Slit is no longer restricted to the CB apical domains. (C) Wild type pattern of Robo. (D) Robo is mislocalized to CB basal surfaces (arrow) in a GOF embryo. (E) EM of a wild-type embryo in XS showing the lumen (arrow) that forms between two CBs. (F) EM of a GOF embryo showing the two-lumen phenotype. Arrowheads indicate cell membranes. (G and H) Close up views of E and F. Arrowheads indicate the extracellular matrix. (I and J) XSs of embryos stained for Mef2, which labels CB and somatic muscle nuclei (brown). (I) The lumen is visible between CBs (arrowhead) in a wild-type embryo. (J) GOF embryo with two lumens (arrowheads). Bars: (A) 6 μm; (E) 2 μm; (G) 0.5 μm.<p><b>Copyright information:</b></p><p>Taken from "Repulsion by Slit and Roundabout prevents Shotgun/E-cadherin–mediated cell adhesion during heart tube lumen formation"</p><p></p><p>The Journal of Cell Biology 2008;182(2):241-248.</p><p>Published online 28 Jul 2008</p><p>PMCID:PMC2483515.</p><p></p

Lumen formation in the heart depends on specific sites of adhesion and de-adhesion between contralateral CBs

We propose that Slit/Robo signaling is required to maintain de-adhesion along the central apical domain of contralateral CBs. In wild-type embryos, CBs are specifically adhered at dorsal and ventral attachment points where E-Cad accumulates. Between these points, the apical membranes of the CBs are repelled from each other, allowing for the formation of a lumen. In LOF or GOF embryos, contralateral CBs remain adhered to each other, resulting in a block in lumen formation and an apical accumulation of E-Cad.<p><b>Copyright information:</b></p><p>Taken from "Repulsion by Slit and Roundabout prevents Shotgun/E-cadherin–mediated cell adhesion during heart tube lumen formation"</p><p></p><p>The Journal of Cell Biology 2008;182(2):241-248.</p><p>Published online 28 Jul 2008</p><p>PMCID:PMC2483515.</p><p></p