How does the extracellular matrix affect the rigidity of an embedded spheroid?

Cellularized tissue and polymer networks can both transition from floppy to rigid as a function of their control parameters, and, yet, the two systems often mechanically interact, which could affect their respective rigidities. To study this interaction, we consider a vertex model with surface tension embedded in a spring network in two dimensions. We identify two regimes with different global spheroid shapes and governed by the pressure resulting from competition between surface tension and tension in the network. In the first regime, the tissue remains compact, while in the second, a cavitation-like instability leads to the emergence of gaps at the tissue-network interface. Intriguingly, compression of the tissue promotes fluidization, while tension promotes cellular alignment and rigidification with the mechanisms driving rigidification differing on either side of the instability.