Publications

Department Biological Physics

Research

Max-Planck-Institute
for the Physics of Complex Systems
Nöthnitzer Straße 38
01187 Dresden
Germany

The main focus of our research are theoretical approaches to understand dynamic processes in cells and tissues. Work on active cellular processes includes the study of cellular oscillations, cellular signaling and the cytoskeletal dynamics during cell division and cell motility. We furthermore study the biophysical basis of hearing. Finally, we investigate the biophysical properties and dynamics of tissues and epithelia. Based on the properties of individual cells and of cellular signaling systems, we are interested in the dynamics of developmental processes, for example wing development in the fruit fly.

- Physics of Cell Division

- Biophysics of Tissues and developmental processes
Cellular packings in epithelia
Cellular rearrangements during growth and development
Morphogen signaling and morphogen gradient formation

- Biophysics of hearing
Active mechanics of hair cells
Cochlear waves
Signal amplification by nonlinear oscillators

Compartments are distinct populations of cells in developing tissues. During development of the fly wing, anterior and posterior cell populations do not mix and are separated by a sharp and straight compartment boundary. Here we show that mechanical cell bond tension is increased along this boundary. This increase in bond tension is enough to guide cell rearrangements after division to maintain a stable interface between compartments.

We quantify the area and the shape of the developing fly wing as a function of time and show that the growth rate decays exponentially. We show that growth is anisotropic and that the shape changes during growth. This growth anisotropy is modulated by the gradient of the morphogen Dpp which is involved in patterning and growth control. The anisotropic growth can be described as the behavior of an active fluid in two dimensions with a source that is subject to and an anisotropic active stress due to cell division.

Many microorganisms are propelled by beating cilia and swim on helical trajectories because of the chiral asymmetry of the cilium and swimmer. Swimming is subject to fluctuations both in the environment as well as in signaling systems which steer the motion. We develop the stochastic differential geometry of chiral swimming and show that a generic mechanism permits reliable steering of swimmers in the presence of fluctuations and weak chemical cues.

P granules are small aggregates of proteins and RNA which specifically localize to germ-line cells during development. Here we show that these cellular structures are liquid drops that self-assemble in the cell by condensing from the multi-component cytoplasm. Localization in one half of the fertilized egg cell is achieved by establishing a gradient of supersaturation. Thus, the cytoplasm self-organizes in distinct domains using liquid-liquid phase separation.

The collective behavior of motor proteins on dynamic microtubules generates oscillations of the cell nucleus and a dynamic redistribution of motors in the cell. This phenomenon is studied by a combination of experiments and theory. It is of key importance for the recombinationof DNA during meiosis.

In the presence of concentration gradients, a colloidal particle is set in motion in a fluid. This motion also results in hydrodynamic flow perturbations. Motion induced by concentration gradients can be used to propel active particles. A generic description of these phenomena clarifies the underlying physical principles and the nature of the force balances which respect momentum conservation.

The segmented structure of all vertebrate organisms is generated by a dynamic process. In this process genetic cellular oscillations organize in collective spatiotemporal patterns that resemble traveling waves. These waves then freeze in a static periodic segmentation pattern. This pattern formation can be described by phase oscillators with a spatial frequency gradient that are coupled with time delays.

Sperm cells move in the plane on drifting circular trajectories. In the presence of fluctuations, these circles exhibit effective diffusion. This stochastic dance can be reliably steered by signaling systems which control the curvature of the swimming paths. This permits the sperm to find the source of a chemical attractant in a noisy environment.

Hair bundles are highly sensitive mechanosensors that detect sounds in the ears of all vertebrate animals. Hair bundles are active detectors which amplify stimuly by spontaneous oscillatory movements. The collective action of several hair bundles can substantially enhance this amplification and provide the amplifier with sharp frequency selectivity.

During growth, cells divide and have to push their neighbors to make room for newly produced daughter cells. In this process, cells rearrange and change their neighbors. We show that growing tissues can behave as viscoelastic fluids that are inherently active since cell division is associated with active stresses. If cell division is anisotropic, anisotropic active stresses lead to interesting cell flow profiles that can be described by a hydrodynamic theory.

Cilia are hair-like appendages of cells which are motile and drive propulsion in a fluid. We develop a theory of the three dimensional ciliar beat and show that in general helical beat patterns emerge from the self-organization of motors and filaments. Because of the chirality of the ciliar structure, the helical beat patterns have a preferred chirality. Such chiral helical beating of cilia controls the left-right symmetry breaking of embryos during the development of mammals.

Morphogen gradients control the formation of patterns of gene expression in developing tissues. Morphogens spread in the tissue from cell to cell, are internalized and degraded. Because of cell-to-cell variability, fluctuations in the morphogen profile occur. We develop a theory of gradient fluctuations and show that the precision of a morphogen gradient is highest at a certain distance to the source. We furthermore quantify the precision of the Dpp gradient in the developing fly wing.

We develop a vertex model to describe the morphologies of cell packings and cell shapes in growing epithelia. The ground state packing of identical cells is in many cases a perfect hexaginal lattice.  Cell division generates disorder which results in irregular cell packings. Biophyscal properties of cells determine the geometry and topology of the network of cell junctions. We compare our simulations to experimental data of cell packings in the developing fly wing. We quantify two biophysical parameters describing cell mechanical properties.