The cause of this overpressure is probably due the contractile stroma surrounding the tumor. However, it is not clear why this mechanical stress is not screened by peripheral cells and why, on the contrary, is it diverges in the tumor core.
To answer the question we developed cell-like pressure sensors, hydrogel microbeads that, like sponges, are squeezed under pressure. These microbeads resemble in size, mechanical properties and surface state to cells. Thus, they are easily incorporated into a spherical aggregate of tumor cells, an in-vitro model of growing tumors.
We show an anomalous propagation of the mechanical stress, with a progressive increase towards the center of the aggregate. This effect, which is uncommon in passive materials, is explained by an anisotropic arrangement of the dividing cells : elongated tangentially, the outer cellular layers do not shrink under pressure, but transmit the pressure (amplified) to the subjacent layers.
Such pressure amplification might explains the reduction of proliferation in the center of a tumor and the appearance of a necrotic core.
The article has been published in Nature Communications