Abstract
Successful childbirth depends on the occurrence of precisely coordinated uterine contractions during labour. Calcium indicator fluorescence imaging is one of the main techniques for investigating the mechanisms governing this physiological process and its pathologies. The effective spatiotemporal resolution of calcium signals is, however, limited by the motion of contracting tissue: structures of interest in the order of microns can move over a hundred times their width during a contraction. The simultaneous changes in local intensity and tissue configuration make motion tracking a non-trivial problem in image analysis and confound many of the standard techniques. This paper presents a method that tracks local motion throughout the tissue and allows for the almost complete removal of motion artefacts. This provides a stabilized calcium signal down to a pixel resolution, which, for the data examined, is in the order of a few microns. As a byproduct of image stabilization, a complete kinematic description of the contraction–relaxation cycle is also obtained. This contains novel information about the mechanical response of the tissue, such as the identification of a characteristic length scale, in the order of 40–50 μm, below which tissue motion is homogeneous. Applied to our data, we illustrate that the method allows for analyses of calcium dynamics in contracting myometrium in unprecedented spatiotemporal detail. Additionally, we use the kinematics of tissue motion to compare calcium signals at the subcellular level and local contractile motion. The computer code used is provided in a freely modifiable form and has potential applicability to in vivo calcium imaging of neural tissue, as well as other smooth muscle tissue.
Highlights
During normal labour, uterine contractions are precisely timed and spatially coordinated
Our algorithm processes calcium indicator fluorescence imaging sequences of contracting myometrium to remove motion artefacts by tracking the movement of identifiable landmarks and extrapolating the data to obtain a complete description of the tissue-wide motion
We present an algorithm that tracks the motion of contracting myometrium during local [Ca2+]i intensity changes
Summary
Uterine contractions are precisely timed and spatially coordinated. The excitability and connectivity of the constituent myocytes increase, manifesting in the depolarization of the membrane potential of the myocytes (Parkington et al 1999) and an increase in the number of gap junction proteins (Garfield et al 1977, 1978; Miyoshi et al 1996; Miller et al 1989) This transformation provides the conditions necessary for the spontaneous generation of coordinated contractions during labour, but the precise mechanisms underlying this coordinated activity remain unclear. The [Ca2+]i measurements in this study reflect the averages of calcium indicator fluorescence from both the cell and the neighbouring region of tissue. The choice of cells from which calcium indicator fluorescence measurements could be taken was limited: measurements could be obtained only from cells positioned at least as far as their maximal displacement away from neighbouring cells
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