Image Dataset for Cardiac cells for Mus musculus?
Sure help yourself with the broad institute image sets collection

Broad Bioimage Benchmark Collection - Annotated biological image sets for testing and validation:
https://data.broadinstitute.org/bbbc/image_sets.html

And the EMBL-EBI:
https://www.ebi.ac.uk/bioimage-archive/

No mouse cardica cells, but I am confident you can easily do transfer learning after training on those.