A cell retention device includes a structured support with a plurality of circumferentially distributed ribs to retain the active filtering surface of a flexible, porous membrane filter medium. The filter medium surrounds the support in contact with the peaks of the ribs, thereby forming axial voids between the rib peaks. This arrangement imparts sufficient structural support over small regions of the filter medium to facilitate its use in a circular (or other rounded) configuration while providing sufficient channel volume to support high throughput of fluid sparse of cells.