The upper chamber is filled with the upper culture fluid medium, the lower chamber contains with the lower culture medium, and the upper and lower culture medium were separated by polycarbonate membrane. The cells are planted in the upper chamber. Due to the permeability of the polycarbonate membrane, the components in the lower culture medium could affect the cells in the upper chamber. This is used to study the how growth and movement of cells may be influenced by the components in the lower culture medium.
Reference:
https://www.protocols.io/view/look-for-cell-invasion-experiment-how-about-follow-br6nm9de.html