Biological microswimmers: From signaling to 3D beat to 3D swimming.

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Biological microswimmers are propelled by the beat of thin filaments protruding from the cell body - called cilia or flagella. The frequency and waveform of the beat is controlled by changes in intracellular messengers, in particular Ca2+. Microswimmers often undergo directed movement in the gradient of a chemical or physical stimulus, a process called e.g. chemotaxis or phototaxis, respectively. It is not known - perhaps with the exception of bacteria - how the stimulus function is transformed to a cellular response pattern, how this response controls the spatio-temporal pattern of the flagellar beat and, finally, how the beating waveform eventually leads to navigation in a stimulus gradient. Here, we want to study navigation of freely swimming, unrestricted sperm in a chemical gradient of the chemoattractant. We use sperm from the sea urchin Arbacia punctulata, humans, and mice. In particular, we want to employ (1) chemical tools (caged chemoattractants) to rapidly establish well-defined chemical gradients, (2) digital holographic microscopy to fully resolve in 3D the flagellar beat and record the 3D swimming path, and (3) simultaneously record signaling events, e.g. changes in Ca2+ concentration in single sperm cells, while navigating in a gradient of the chemoattractant. Thereby, we aim to provide fundamental insight into the logics of cellular locomotion and navigation, the cellular or even molecular underpinnings, and the physical principles. If successful, this armory of techniques will also allow to precisely track other microswimmers in different gradients.