Linheng Li is learning to think like an ecologist. His study subjects put down roots near sources of nourishment and depend on other living things in their environment to thrive. But Li doesn’t have mud on his boots. The ‘species’ he studies are stem cells and the ‘ecosystem’ is bone marrow. Within the anatomical forest of the marrow, Li’s stem cells occupy specific niches — a term borrowed from ecology. An organism’s ecological niche is a definition of where it lives, what it does, and how it interacts with its environment. Alter that environment, and the consequences for the organism can be dire. Conversely, if you take an organism and deposit it in an alien ecosystem, all hell can break loose.
At the Stowers Institute for Medical Research in Kansas City, Missouri, Li thinks in a similar way about stem cells. For example, in aplastic anaemia, stem cells are unable to produce sufficient blood cells, even though they look normal. Something about the cells’ microenvironment in the bone marrow may be awry, argues Li. “It’s like the soil being damaged,” he says. Similarly, just as an introduced species can run amok in its new environment, stem cells placed in the wrong tissue in the body might conceivably form a malignant tumour.
Clearly, you can’t hope to understand a woodland flower’s niche in the forest by examining a specimen grown in a pot. And biologists are realizing that they are missing an important part of the picture by studying stem cells in Petri dishes. “Thinking of stem cells in isolation can be productive,” says David Scadden, co-director of the Harvard Stem Cell Institute in Boston. “But it falsely simplifies what is a single component of a much larger, more complex system.”