#### What is CellFIT?

CellFIT is a package of force inference equations and assessment tools that calculates the forces in cell edges based on their geometries. In this approach, cells in an image are segmented and equilibrium equations are constructed for each triple junction based solely on edge tensions and the angles at which edges approach each junction. Solving the resulting system of tension equations yields a set of relative edge tensions whose scaling must be determined from data external to the image.

#### A Video Explanation of CellFIT

To see a video explanation of CellFIT, click here.

#### The Theory behind CellFIT

Details of the CellFIT method are presented in the following papers:

Brodland, G.W, Veldhuis, J.H., Kim, S., Perrone, M., Mashburn, D., Hutson, M.S., 2014, "CellFIT: A Cellular Force-Inference Toolkit Using Curvilinear Cell Boundaries," PLoS ONE, Vol. 9, No. 6, e99116. doi: 10.1371/journal.pone.0099116

Veldhuis, J.H., Mashburn, D., Hutson, M.S., Brodland, G.W., 2015, "Practical aspects of the cellular force inference toolkit (CellFIT)," Methods in cell biology, Vol. 125, pp. 331-351. doi:10.1016/bs.mcb.2014.10.010. preprint pdf

CellFIT also supercedes an earlier force inference technique called Video Force Microscopy (VFM):

Brodland, G.W., Conte, V. Cranston, P.G., Veldhuis, J., Narasimhan, S., Hutson, M.S., Jacinto, A., Ulrich, F., Baum, B., and Miodownik, M., 2010, "Video Force Microscopy Reveals the Mechanics of Ventral Furrow Invagination in Drosophila," Proceedings of the National Academy of Sciences (PNAS), Vol 107, No. 51, pp. 22111-22116. doi: 10.1073/pnas.1006591107 (open access).

#### Practical considerations

CellFIT works on Planar (2D) data and is ideal for images of cells in monolayers. A 3D version that works on confocal image stacks is in preparation. Images should be taken normal to the surface of the cells. If cells are not in a flat plane (tilted by more than approximately 30 degrees) the images should be unwarped. CellFIT relies on being able to clearly see the angles at which edges approach junctions. Images should clearly show cell edges and be of sufficient resolution to accurately determine approach angles. More practical considerations are given in:

Veldhuis, J.H., Mashburn, D., Hutson, M.S., Brodland, G.W., 2015, "Practical aspects of the cellular force inference toolkit (CellFIT)," *Methods in cell biology*, Vol. 125, pp. 331-351. doi:10.1016/bs.mcb.2014.10.010. preprint pdf

#### Useful Links

CellFIT co-author M. Shane Hutson.

Seedwater Segmentation Software.

#### Validation

It is important to validate CellFIT results. It is possible to get meaningless results if cellFIT is incorrectly applied. Also, this version of CellFIT assumes that the only driving forces are edge tensions and may not provide intuitive results in systems where other types of driving forces are present.

#### Disclaimer

This software was written with care and tested on dozens of data sets. However, it is supplied "AS IS" without any warranties and support.

A Windows software version of CellFIT is available for download here.

#### CellFIT 3D

We recently developed a 3D version of CellFIT. In Veldhuis et al., it was used to determine the tensions acting in exterior and interior interfaces of 8-cell marine embryos. The estimated errors in the exterior tensions were smaller than the 10% uncertainty in companion aspiration experiments. In Neumann et al., CellFIT-3Ddemonstrated its potentialto predict the motility of cells based exclusively on their geometries.

The computer code that we used to implement CellFIT-3D consisted of several independent software modules, including ones for image processing, trace alignment, angle determination and force analysis. Regrettably, we did not have the resources to amalgamate these codes into a single package, nor did we have the time and data sets needed to validate the software so that it could be released to other users.

We hope that others will pick up the software and the science where we laid them down.

#### For More Information about CellFIT-2D

Contact our collaborator, Dr. M. Shane Hutson