Stefan Kobel
Since stem cells have the unique ability to produce more of themselves (i.e. to "self-renew") and to generate specialized tissue cells, they are an ideal source of cells for regenerative medicine and in vitro tissue models. In order to fully exploit this potential, we have to better understand the mechanisms that regulate stem cell behavior at a single cell level. However, current in vitro methodologies to study single cells are limited by their throughput, do not afford to distinguish the fate of daughter cells or track the progeny of a single cell over long time periods. This thesis addresses these shortcomings by developing miniaturized devices to handle and analyze single live stem cells. To this end, a micro-structured platform was first established that consisted of miniaturized wells with diameters of 100 µm that were arranged in a regular grid. Single hematopoietic stem cells (HSCs) were then seeded into these "microwell arrays" and, since the HSCs could not leave the microwells, they could be tracked over many days, and their dynamics and proliferation rates could be studied. However, in order to analyze the daughter cells of these single HSCs individually, the daughter cells have to be physically separated by micromanipulation. For this purpose, a microdevice was developed to reliably manipulate single cells using hydrodynamic single cell trapping. This platform was thoroughly characterized using computational simulations combined with particle flow velocimetry and live-cell time-lapse microscopy. The optimization of design parameters towards increasing trapping efficiency ensured a nearly perfect efficiency of single cell trapping (97%) and a successful re-capture of daughter cells generated by dividing mother cells. Furthermore, it was demonstrated that cell trapping under very low flow is sufficiently gentle to enable long-term cell trapping in the microfluidic environment. To identify and track captured cells in these microfluidic single cell traps, automated image analysis tools were developed. A reliable and fast algorithm was generated to identify the borders of microchannels that were indicative for the position of single cell traps. These detected edges were used to individualize the single cell traps with a sub-pixel resolution such that it was possible to segment single HSCs on the chip by thresholding. Accordingly, single HSCs were discovered with efficiencies as high as >95%, allowing the automated tracking of microfluidic single cell trapping, as well as the detection of cell cycle phases of trapped single HSCs engineered with dual fluorescence reporter system marking G1 and S/G2-M. To actively micromanipulate single cells for downstream cell-fate analyses, on-chip valves were integrated on the microfluidic platform to precisely control the direction of medium flow in the microfluidic channels. This approach enabled a proof-of-concept study to demonstrate a more complex single cell manipulation: Cells were first captured in single cell traps and transferred into a culture chamber for clonal expansion. Subsequently, the progeny of these single cells was removed from the culture chamber and trapped in a series of single cells traps. Importantly, since these traps could be addressed individually, a reliable physical separation of the daughter cells was achieved. To enable individual analyses of these separated daughter cells, the last aim of this thesis was to interface a microfluidic chip with standard multititer plates in order to efficiently recover the cells or the cDNA of the single cell generated on-chip from extracted mRNA. For this purpose, the bottom of the chip was equipped with an array of access holes that matched the layout of a 1536-well plate. These access holes were linked to a single cell trap via an analysis chamber, where the HSCs were lysed and their mRNA was reverse transcribed using bead-coupled primers. In this vain, the obtained cDNA was coupled to these beads that could then removed from the chip into a 1536-well by centrifugation. These beads could then be analyzed using standard bench-top PCR machines. In conclusion, this thesis presented several microfluidic modules to handle and analyze single stem cells in high-throughput. In a proof-of-concept study, these modules were applied to automatically capture single cells, culture them, and subsequently separate the daughter cells for downstream cell-fate analyses, thereby greatly enhancing the throughput compared to manual micromanipulation. During the development of this system, it became apparent how crucial reliable and gentle cell handling is to perform complex experimental tasks with live cells on microfluidic chips. The further integration of such microfluidic modules into multi-functional lab-on-a-chip devices will open the door to experimental paradigms previously not imaginable and promises novel insights in stem cell biology.
EPFL2012
