by Monya Baker
Real-time imaging reveals the previously unseen.
The blood-forming system is simultaneously the hardest and easiest to study. On the one hand, collecting cells can be as simple as a blood draw; on the other hand, haematopoietic stem cells do their real work — generating new cells — while they are tucked away in the bone marrow. Two studies published this month in Nature show how to get a sustained peek inside this niche, revealing the complex support system that guides haematopoietic stem cells to produce specialized progeny.
Researchers led by David Scadden and Charles Lin at Harvard University in Boston figured out a way to follow individual cells inside the bones of healthy mice1. This revealed an organized production system in which different cell populations were sequestered into areas according to their stage of differentiation. Individual blood-making and bone-making cells were enmeshed in their own networks of microvessels.
Although previous techniques are limited to long-term outcomes at the level of an individual organism, Scadden's team applied several types of technology to watch individual cells migrate and divide in real time. They used confocal and two-photon microscopy, which allowed them to see into the depths of the bone cavities within the plates of the skull, quantum dots, which allowed them to see blood vessels, and fluorescent reporter genes, which made bone-making cells visible. Then the researchers isolated haematopoietic stem (HS) cells and progenitors from blood, labelled them with dyes and retransplanted the cells.
Researchers led by Linheng Li of the Stowers Institute for Medical Research in Kansas City, Missouri, and Ricardo Feldman of the University of Maryland School of Medicine in Baltimore examined what happened as the blood system repaired itself after radiation2. They used both two-photon microscopy and immunostaining to figure out where labelled HS cells went after irradiation. They found that the cells often homed to the pre-osteoblast cells and found a zone within the bone cavity that normally serves to maintain HS cells. Indeed, the techniques allowed dividing stem cells to be visualized alongside their stable counterparts. By injecting transgenic mice with luciferase-encoding viruses that target stem cells, the investigators were able to both visualize long-term self-renewing HS cells in the bone cavity in live mice and distinguish the cells from short-lived HS cells in other locations, including the spleen.
Both Scadden and Li found that the niches of bone-making and blood-making cells overlap and can interact. This provides clues as to how damaged bone marrow triggers HS cells to increase in number. More broadly, the ability to focus on individual cells should allow researchers to piece together the many, many components of the networks that regulate how stem cells behave.
Source: Nature