Enrichment of tumor-initiating breast cancer cells within a mammosphere-culture microdevice.

We report for the first time a microdevice that enables the selective enrichment, culture, and identification of tumor-initiating cells on native polydimethylsiloxane (PDMS). For nearly a decade, researchers have identified tumor-initiating breast cancer cells within heterogeneous populations of breast cancer cells by utilizing low-attachment serum-free culture conditions, which lead to the formation of spheroidal colonies (mammospheres) that are enriched for tumor-initiating cells. However, the utility of this assay has been limited by difficulties in combining this culture-plate-based technique with other cellular and molecular analyses. Integrating the mammosphere technique into a microsystem can enable it to be combined directly with a number of functions, such as cell sorting, drug screens, and molecular assays. In this work, we demonstrate mammosphere culture within a PDMS microdevice. We first prove that a native hydrophobic PDMS surface is as effective as commercial low-attachment plates at selectively promoting the formation of mammospheres. We then experimentally assess the PDMS microdevice. Time-lapse images of mammosphere formation within the microdevice show that mammospheres form from single cells or small clusters of cells. Following formation of the mammospheres, it is desirable to evaluate the cells within the spheroids for enrichment of tumor initiating cells. To perform assays such as this (which require the loading and rinsing of reagents) without flushing the cells (which are in suspension) from the device, the culture chamber is separated from a reagent reservoir by a commercially available microporous membrane, and thus reagents are exchanged between the reservoir and the culture chamber by diffusion only. Using this capability, we verify that the mammospheres are enriched for tumor initiating cells by staining aldehyde dehydrogenase activity, a cancer stem cell marker. To the best of our knowledge, this is the first assay that enables the direct observation of tumor-initiating cells within a suspended mammosphere.