Stem cell traits in long-term co-culture revealed by time-lapse imaging.

A deeper understanding of stem cell niche engagement and subsequent behaviors would be enhanced by technologies enabling the tracking of individual stem cells at the clonal level in long-term co-culture (LTC), which mimics the complexity of the bone marrow microenvironment in vivo. Here, we report the application of time-lapse imaging with intermittent fluorescence for tracking well-defined populations of GFP(+) murine hematopoietic stem cells (HSCs) using LTC for >5 weeks. Long-term (LT) and short-term (ST) repopulating HSCs and hematopoietic progenitor cells (HPCs) were compared. The transition from cobblestone areas (CAs) under the stromal cell mantle into dispersed migrating cells on top of the stroma (COS) were directly observed. The ST-HSC and LT-HSC were able to initiate multiple waves of CA formation and COS expansion beyond 2 and 4 weeks, respectively. Retrospective tracking of individual CA forming cell (CAFC) revealed a preference for residing under stroma before the first division and a longer interval before first division for LT-HSC. Inability to maintain quiescence in subsequent divisions was revealed. Our study represents an important starting point from which the LTC system can be augmented to provide a better in vitro model for bone marrow stem cell niches.