1.Stem cells reside in specialized microenvironments that precisely regulate their behavior. For this reason they are best studied in their in vivo context. A new paper from our lab describes a method for long-term imaging & quantitative analysis of hematopoiesis in vivo in flies

2. In our paper, now out in @eLife, we use organ culture, genetically encoded markers of cell proliferation/differentiation, & automated quantitative image analysis to study the process of fly hematopoiesis at single cell resolution in its original context

3. Some of our key findings: first, we show for the first time that blood progenitors in the fly undergo symmetric cell divisions consistent w/ self-renewal. (for example in the video below we image the progenitor marker dome-MESO-GFP)

4. Second, we show that the timing of blood progenitor division is linked to cell size & is spatially oriented. With 90% of cell vision oriented along the plane defined by the heart (which, since we use a cylindrical coordinate system, we refer to as the ρ plane).

5. Third, using genetically encoded markers for stemness & differentiation we tracked the kinetics of differentiation in real time. Let's start w/ this movie, showing a cytoplasmic stemness marker (green) being turned off & a nuclear marker for mature blood cells being turned on.

6. This approach allowed us to identify 2 distinct modes (or trajectories) of differentiation from progenitor to mature blood cell. These are shown in the movies below. The green (stemness marker) & red (mature cell marker) levels of a single progenitor were sampled over time.

7. The 1st trajectory (which for various reasons we call "sigmoid") involves shutting down stemness marker & then turning on mature cell markers. The 2nd ("linear") trajectory starts w/ high levels of both stemness & mature cell markers & then turning off the stemness marker.

8. Following bacterial infection (orange bars in the figure below), flies increase hematopoiesis massively in order to produce infection fighting immune cells. We find that this is achieved by a large increase in the number of events of the 2nd, linear, differentiation pathway.

9. Overall, our results show that even subtle shifts in proliferation & differentiation kinetics can have a large impact, in aggregate, to transform blood progenitors from a quiescent to an activated state.

10. These & many other observations are found in the paper now online. This was a heroic effort on the part of @KevinYLHo1 & Rosalyn Carr, an amazingly talented undergrad who wrote all the image analysis code. Another grad student, Sasha Dvoskin made other key contributions

11. I will leave you w/ one last fun movie where we used the FUCCI system to follow a dividing progenitor as it transition through the cell cycle.