Tracking growing tissue branches using BTrackMate Tissue Tracker

For quantification of branch morphogenesis present in mouse mammary glands we developed a skeletonization plugin in the imglib2 library of Fiji to track the growing branches. We further developed a seed pooling algorithm called VollSeg to seg- ment the cells inside the tissue in 3D in live cell imaging conditions followed by a tracking and track analysis notebooks using Napari with custom GUI widgets.


Introduction

This algorithm was developed to obtain growth rates of mouse mammary gland tissue imaged in 2D using transmitted light microscopy as a timelapse sequence. The algorithm required a segmentation method followed by a tracking methodology to track the growing end points of the tissue. To do so we developed a U-Net segmentation model to obtain a binary image, we then extended the
skeletonization implementation of imglib2 to obtain the skeleton end points. The end points of a skeleton always end before the actual tissue branch end, in order to ensure that the end point always ended at the actual tissue branch end we used a line drawn from the skeleton split point to the end point and obtained the intersection of that line with the boundary point extracted from the segmented image. Once we have such skeleton end points we repeat the process over time and use LAP/Kalman Tracker to link these points to obtain end point tracks for each growing tissue branch. Using these tracks we obtain growth rate of each track and finally average the growth rates of all the obtained tracks to obtain an average growth rate of the tissue. The developed Fiji plugin is a TrackMate detector called BTrack and is available after activating the Fiji update site BTrack from the update menu. TrackMate is fully interactive allowing for the users to remove or create new skeleton end points. The output of the Fiji plugin serves as an input to a jupyter notebook which takes in several of such experimental analysis and outputs a distribution plot of the growth rates coming from several such experiments.
We also created a T-test notebook using scipy stats  library functions of performing the T-test to compare the statistical significant difference between the results from several experiments.

VollSeg installation

For installation of vollseg please see the instructions here. vollseg is also available as a Napari plugin and can be used instead of using it with the Jupyter notebooks, for installing vollseg as a Napari plugin please see here.

BTrack installation

 For installing the Fiji plugin BTrack please visit it’s ImageJ wiki page and follow these instructions.

Tissue Segmentation

The tissue was imaged using transmitted light microscopy and its segmentation was challenging due to lack of background present in the images. To segment the tissue we created a training dataset of manually annotated tissue region using this Script and trained a U-Net model using this notebook. We used a kernel size of 7 as the spatial variation in transmitted light images is much lower as compared to the fluorescently labelled images. The segmentation model prediction can be applied either using our Colab notebook or local jupyter notebook.

Tissue branch tracking

Post segmentation we created a customized tool in Fiji called BTrack to track the growing ends of the tissue branches. The input to this plugin is the Raw image (to display the results of tracking) and the segmentation image (to perform the skeletonizaiton operation on). More details about the plugin can be found on its provided Fiji wiki page.

Tissue track analysis

Post tracking we perform the track analysis in python. We created tailored jupyter notebooks that take in the txt files generated by BTrack and plot the velocity of all the tracks, their statistical distribution and also computes the mean growth velocity for the whole experiment. After analyzing several experiments using the tool we can then evaluate statistical significant differences between the different populations using the standardized T-test approach using this notebook.

Software and data availability

Data is available from the corresponding authors upon request. The segmentation was performed using U-Net model that can be used by installing the python package VollSeg. The tracking was done using a custom GUI plugin available from the Fiji update site “MTrack” and shows up in Plugins > Tracking > BTrackMate Tissue Tracker. The results in the paper were generated using this jar. Following the recent updates to the codebase the upgraded tissue tracker is now a part of the popular Fiji plugin called TrackMate and is available from the Fiji update site “BTrack” and shows up as a TrackMate detector BTrack.

References

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