Extrinsic regulation of fate choice in mouse haematopoietic stem cells The mechanisms regulating stem cell self-renewal, proliferation, and differentiation are still not fully understood. Improving our knowledge of these processes will not only provide greater insight into stem cell biology but will also have major implications in the understanding of cancer development, since numerous cancers can trace their origins back to single stem cells. It has previously been shown that variations in culture conditions can alter fate choice in haematopoietic stem cells (HSCs). For decades cytokines have been used to maintain and expand mouse and human HSCs in vitro, with a number of studies demonstrating that cytokines directly influence HSCs fate choice. In this thesis, I explored the extrinsic regulation of mouse HSCs fate choice using three different approaches:

1. Modulation of cytokine concentration
2. Establishment of minimal conditions to retain HSCs function in vitro
3. Development of 3D matrices to provide physical support beyond liquid culture

The first results chapter (3.1) identifies that the amount of Stem Cell Factor (SCF) signalling does not alter the number of functional HSCs retained, but may alter the degree of clonal expansion post transplantation. Chapter 3.2 demonstrates that minimal cell culture conditions depending solely on gp130 signalling can maintain HSCs as single cells for an extended period of time. These cells retain full functional repopulation potential but present with a myeloid differentiation bias. Finally, Chapter 3.3 represents a first proof-of-principle series of experiments showing that HSCs are better supported on soft substrates, implicating physical forces in influencing HSCs maintenance ex vivo. In conclusion, these findings further confirm that SCF is a key regulator of HSCs fate, but is not essential for the retention of HSCs function. The newly established minimal cell culture medium allows the specific investigation of various molecules affecting HSCs fate choice at the single cell level. Furthermore, it offers a new platform for studying exit from quiescence in a controlled manner over several days. This latter aspect could have major implications for the delivery of gene therapy and for HSCs expansion efforts in the future.