Timm Schroeder, German Research Center for Environmental Health

Much can be learned from observing. But stem cells are tough to see in their natural settings, where they lurk deep within tissues and are hard to pick out from surrounding cells. Seeing them requires broad skills in hardware, software and stem cell biology.1 Timm Schroeder of the Helmholtz Zentrum München Institute of Stem Cell Research in Munich explains.

(Note: This is part of a series of interviews conducted to accompany Nature Insight Regenerative Medicine

The imaging technologies seem so different from each other.

The way that you have to label and apply yourself varies a lot between different technologies, but the goal is always the same: to have high resolution temporally and spatially.

What can be done now that couldn't be done five or ten years ago?

Deep-tissue imaging with confocal microscopy is now much, much more accessible. Now you watch how lymphocytes interact in lymph nodes, or watch blood stem cells homing to the bone marrow. This has just become available in the last couple of years.

Also, fluorescent molecular tomographies that are now emerging are at the stage where they can be applied at least to mouse. You rotate them [mice] under the microscope and you can get optical sections from different areas. With multifocal microscopy, you can't look that deep into tissues, certainly not into a whole leg. With tomography, you can look into the living leg with reasonable resolution. [We] don't yet have cellular resolution, but we might be looking at that in the near future.

What can't be done now but could be done in five or ten years?

Fluorescent technology with close to cellular resolution I expect to be possible in the next couple of years. One major problem is immobilization and anaesthesia of the specimens. If you want to look at mammals, a lot of the biological questions we have are for processes that take days. As soon as we have movement of the organism it's hard to follow [the cells].

What can continuous observation reveal that snapshots can't?

One question is, where does blood come from during embryonal development? For instance, can one cell give rise to separate lineages or are there two short-lived progenitors? These are questions that are difficult to answer if we only look now and then because we have a lot of cell movement. Migration from outside your field of view would lead to cells appearing out of nowhere. You don't know where they come from, but you could assume that they came from some progenitors within your field of view.

You write that you're surprised skin hasn't been used in more imaging studies.

If you think about what is most difficult for imaging, it's always access to the tissue. And skin is always on the surface of the organism, so you don't have to go deep. Skin also has a clean 3D [three-dimensional] organisation, so if you know where the cells are, you know relatively well what they are. For blood and bone marrow, which are liquid tissues, that is very difficult.

Skin has a very high turnover and high regenerative activity. It's an important classical system. I could imagine that in the imaging field it will be one of the pioneering organ systems where we learn concepts that we try to transfer to other systems.

You list multiple challenges to applying imaging to stem cells: identifying cells, tracking cells, processing data. Which are the most tractable?

A problem with stem cells is you ultimately have to look in vivo. For that, you need novel markers and dyes and labels that give you strong, detectable signals and are biocompatible, and ideally [you need to be] able to encode them genetically. This is an area where I'd expect a lot of improvement.

Least tractable?

One very tough problem is identifying individual cells and looking at them over long periods of time. With stem cells, we usually don't have really good prospective markers, single molecules that tell us whether something is a stem cell or not. Even in the stem cell field, people underestimate how difficult that is.

We have a multitude of markers, and that makes imaging hard because you have to look at many colours at once. Also, to really prove that you're working with a stem cell, you define it functionally by its behaviour and its progeny and self-renewal. To know if a cell is a stem cell you need to know its future; you can't just look at it in the moment.

You write that overcoming challenges will likely require multimodal imaging, combining several different techniques.

The imaging systems all have strengths and weakness. MRI [magnetic resonance imaging] is a really good technology to [use to] look at large volumes of undisturbed homeostatic tissues without specific staining. You can recognize organs and anatomical sites, but it's really hard to get high enough resolution either temporally or spatially to recognize individual cells. Then we have fluorescent technology or the multifocal imaging, where you can really only observe small volumes, a hundred cells or a thousand cells. If you could somehow combine those two, the MRI could give you the ability to know where you are in the animal, and with the other technology you could really see the cells.

It's one thing when something can be done theoretically in a specialized lab and another when it's really applicable for someone with a biological question. Imaging technology is really hard to transfer; it's not something that can be put in an envelope and sent somewhere. It's real machinery, and it has to be operated by the people in those labs. Right now you have to be good at cells, hardware and labels.

What imaging systems might work in humans?

For the clinically compatible labels, MRI, PET [positron emission tomography], CT [computed tomography], you don't have good biocompatible dyes that can be safely applied. The fluorescent technologies that give you cellular resolution cannot be applied in human systems. You'd want to ask your biological question in a very specific way in a model organism, then ask your question using biocompatible MRI technology that doesn't give you that kind of resolution but might be enough to confirm your hypothesis.

Why does using imaging technologies to follow blood interest you?

A problem with the stem cells is you ultimately have to look in vivo. For that, you need novel markers and dyes and labels that give you strong detectable signals and are biocompatible, and ideally [you need to be] able to encode them genetically., Timm Schroeder, German Research Center for Environmental Health

Blood is a very complex system; it's a liquid organ, [and] it's hard to keep track of all the cells. Despite the very intense research done, people come to very different conclusions from very similar data. Bone marrow is hard to access; the cells move around a lot — they are not statically organized in strict three-dimensional patterns. And people fight about very basic questions. Looking at cells continuously with fine resolution will be very, very challenging, but hopefully very, very helpful.