Cyclic Stress at mHz Frequencies Aligns Fibroblasts in Direction of Zero Strain

Recognition of external mechanical signals is vital for mammalian cells. Cyclic stretch, e.g. around blood vessels, is one such signal that induces cell reorientation from parallel to almost perpendicular to the direction of stretch. Here, we present quantitative analyses of both, cell and cytoskeletal reorientation of umbilical cord fibroblasts. Cyclic strain of preset amplitudes was applied at mHz frequencies. Elastomeric chambers were specifically designed and characterized to distinguish between zero strain and minimal stress directions and to allow accurate theoretical modeling. Reorientation was only induced when the applied stretch exceeded a specific amplitude, suggesting a non-linear response. However, on very soft substrates no mechanoresponse occurs even for high strain. For all stretch amplitudes, the angular distributions of reoriented cells are in very good agreement with a theory modeling stretched cells as active force dipoles. Cyclic stretch increases the number of stress fibers and the coupling to adhesions. We show that changes in cell shape follow cytoskeletal reorientation with a significant temporal delay. Our data identify the importance of environmental stiffness for cell reorientation, here in direction of zero strain. These in vitro experiments on cultured cells argue for the necessity of rather stiff environmental conditions to induce cellular reorientation in mammalian tissues

Extraktion physiologischer Koordinatensysteme von Pflanzenwurzeln und -blättern aus Bildsequenzen

Die vorliegende Arbeit beschreibt die Entwicklung eines Verfahrens zur Wuchsanalyse von Pflanzenblättern und -wurzeln in natürlich gegebenen Objektkoordinaten mit Bildverarbeitungsmethoden. Als Basis dient die bereits zur Wachstumskartierung verwendete Strukturtensormethode mit der örtlich aufgelöste Wachstumskarten in Bildkoordinaten gewonnen werden. DieWachstumsmessung in physiologischen Koordinaten hat den wesentlichen Vorteil, daß durch sie erstmals die Ergebnisse verschiedener Messungen vergleichbar gemacht werden und die Ergebnisse direkt interpretierbar sind. Sowohl für Blätter als auch für Wurzeln wird eine Methode zur Extraktion der Koordinatenachsen entwickelt. Die Mittellinie der Wurzel wird als physiologische Koordinatenachse mit einem auf aktiven Konturen basierenden Verfahren extrahiert. Bei Blättern stellen die Adern die Hauptachsen des physiologischen Koordinatensystems dar. Zu ihrer Suche wird ein Trackingalgorithmus verwendet, der auf Matchingmethoden basiert. Die Darstellung aller physiologischen Koordinatenachsen erfolgt in Form von B-Splines, an deren Positionen die mit der Strukturtensormethode berechneten Wachstumskarten abgetastet werden. Damit findet die Koordinatentransformation in das jeweilige physiologische Koordinatensystem statt. Der örtliche Fehler der Transformation liegt im Subpixelbereich und ist somit deutlich kleiner als die räumliche Auflösung der Wachstumskarten. Mit der entwickelten Methodik steht ein leistungsfähiges Werkzeug zur Wuchsanalyse von Pflanzenblättern und Wurzeln zur Verfügung

Robust vein extraction on plant

leaf image

Root Growth Measurements in Object

We show a framework for growth analysis of plant roots in object coordinates which is one requirement for the botanical evaluation of growth mechanisms in roots. The method presented here is appliable on long image sequences up to several days, it has no limit for the sequence length. First we estimate the displacement vector field with the structure tensor method. Thereafter we determine the physiological coordinates of the root by active contours. The contours are first fitted on the root boundary and yield the data for the calculation of the middle line as the object coordinate axis of the root. In the third step the displacement field is sampled at the position of the middle line and projected onto it

Root Growth Measurements in Object Coordinates

Abstract. We show a framework for growth analysis of plant roots in object coordinates which is one requirement for the botanical evaluation of growth mechanisms in roots. The method presented here is appliable on long image sequences up to several days, it has no limit for the sequence length. First we estimate the displacement vector field with the structure tensor method. Thereafter we determine the physiological coordinates of the root by active contours. The contours are first fitted on the root boundary and yield the data for the calculation of the middle line as the object coordinate axis of the root. In the third step the displacement field is sampled at the position of the middle line and projected onto it. The result is an array of tangential displacement vectors along the root which is used to compute the spatially resolved expansion rate of the root in physiological coordinates. Finally, the potential of the presented framework is demonstrated on synthetic and real data.

Cell Force Microscopy on Elastic Layers of Finite Thickness

Forces applied by cells to substrates can be measured using soft substrates with embedded displacement markers. Traction forces are retrieved from microscopic images by determining the displacements of these markers and fitting the generating forces. Here we show that using elastic films of 5–10-μm thickness one can improve the spatial resolution of the technique. To this end we derived explicit equations for the mechanical response of an elastic layer of finite thickness to point forces. Moreover, these equations allow highly accurate force measurements on eukaryotic cells on films where finite thickness effects are relevant (below ∼60 μm)