Fine spatiotemporal activity in contracting myometrium revealed by motion‐corrected calcium imaging

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.

Single-cell mechanics and calcium signalling in organotypic slices of human myometrium

“…To remove these artifacts, the image sequences were imported into MATLAB (Mathworks, Natick, MA, USA) and processed using our motion-correction algorithm. A detailed description of the motion correction procedure has been given elsewhere ( Loftus et al, 2014 ). Briefly, the main steps of the algorithm comprise: (i) landmark identification by band-pass filtering the image to emphasise the bright blobs always present in the images as potential landmarks; (ii) tracking the motion of each landmark between frames for the entire image stack and removing outlier landmarks; and (iii) extrapolating the motion of landmarks to all neighbouring pixels to yield a complete description of the tissue motion.…”

“…However, the spatio-temporal resolution of [Ca 2+ ] i dynamics is limited due to the motion artifacts occurring in contracting tissue ( Bru-Mercier et al, 2012 ). Recently, we developed an image processing algorithm for off-line correction of motion artifacts that allows extraction of [Ca 2+ ] i dynamics on a cellular level within multicellular tissue slices ( Loftus et al, 2014 ). Here, we used this processing algorithm to investigate the [Ca 2+ ] i dynamics in control and oxytocin stimulated slices of human myometrium on a cellular level.…”

Progress in understanding electro-mechanical signalling in the myometrium

“…Our findings have recently been verified in a paper using motion‐corrected calcium imaging (Loftus et al . ). For another, mathematical approach to this area, see the recent paper by (Young & Barendse ) .…”

Activity-dependent self-regulation of viscous length scales in biological systems

“…It is known that the activity of the motor molecules, like myosin and kinesin, change the property of an assembly of filamentlike molecules and motor molecules. A number of recent studies have explored the effect of activity on the properties of such systems [7][8][9][10][11][12][13][14][15][16], for example, myosin II activity softens suspended cells [17], changes the viscoelastic behavior of an actomyosin network [18] and controls the dynamics in mouse oocytes [19], bacterial cytoplasm (devoid of myosin) gets fluidized by metabolic activity [20], kinesin motors lead to spontaneous flow in an assembly of microtubules [21] etc. The existing theories of actomyosin cortex have mostly focused on the quiescent state and treated the system as an elastic network [10,13,22,23].…”