Stem Cells Are Growing Up

It was at the start of the new millennium that scientists learned how to grow embryonic stem cells in the lab and nudge the cells to take on new identities. The research attracted headlines partly because of controversy around how the cells were obtained—typically from leftover embryos generated during in vitro fertilization treatment—and partly because they held promise for regenerative medicine, the idea of using stem cells to replace diseased cells or tissues in patients. Some advocates of the science made increasingly optimistic projections: With the right technology, it would soon be possible to grow whole organs in petri dishes. A patient’s worn-out liver, kidney, or heart could be swapped for a new one, like a set of tires.

“Claims were made about stem cells, particularly embryonic stem cells, that would clearly not be easy to fulfill,” says Brivanlou, who is Rockefeller’s Robert and Harriet Heilbrunn Professor.

Indeed, working with human embryonic stem cells was challenging from the start, and remained difficult even after 2006, when new technologies allowed scientists to create so-called induced pluripotent stem cells (iPSCs) by reprogramming somatic cells such as skin cells into an embryonic-like state. The iPSCs behaved similarly to stem cells derived from embryos and promised a limitless supply of pluripotent cells for research and medical applications, but they came with their own peculiarities and safety concerns. Scientists also developed methods to guide iPSCs or human embryonic stem cells to differentiate into slightly more mature cells called progenitors, but attempts to push them further, toward full specialization, kept failing. There were multiple challenges, starting with the fact that no developing cell operates in isolation—to function normally, it must exchange signals with nearby cells and sometimes connect with nerves, arteries, and nonliving material in the tissue. It’s a puzzle with so many pieces that its solution may still be decades away.

There were other hurdles, too. The creation of the first human embryonic stem cell line caused a cultural uproar so loud that it almost brought the science to a halt. Several European countries banned the creation of the cells, and in the United States, then president George W. Bush cut off the field’s access to government funding. Scientists had to secure private funding for work involving embryonic stem cells and, to comply with federal regulations, erect physical barriers in their labs to separate privately funded work on stem cells from government-funded research.

It was in such a cordoned-off corner that Brivanlou’s team began studying natural human embryos and deriving cell lines from them. They also developed a platform that made it possible to grow the embryos in petri dishes, even past day seven, when normal embryos need a uterus to survive. For the first time, they could watch the transformations taking place during the second week of human embryogenesis, up until around day 14. This is when a milestone called symmetry breaking occurs—characterized by the emergence of the body’s three axes (head to foot, front to back, left to right).

To observe later developmental stages, Brivanlou’s lab and Rockefeller physicist Eric Siggia created synthetic human embryos growing as self-organizing 3D cultures. These embryo models mimic the geometry of natural embryos and can be induced to undergo symmetry breaking, giving researchers a way to study the biological events occurring in the third week of development. They have also provided some of the most striking pictures of early human life ever seen.

In the 1970s, a series of botched experiments led Howard Green, a researcher at the Massachusetts Institute of Technology, to a surprise discovery. While unsuccessfully trying to replicate a rare cancer in a petri dish, Green noticed that the tumor-derived cells were forming structures resembling the outer layer of the skin, or epidermis. This “test-tube skin,” as he called it, was so similar to real skin that it could be used for skin grafting. In an oft-cited procedure in 1983, Green and others regenerated massive amounts of epidermis and saved the lives of two boys whose bodies were covered with third-degree burns. But Green didn’t know why his cultured cells could magically generate new tissue. What was going on inside that dish?

Eventually, he figured it out: Some of the cells had the capacity to divide over and over without specializing. Without these cells, now known as adult stem cells, no epidermal structures would grow.

Adult stem cells can be found in virtually all the body’s organs, where they live in localized homes called niches and self-generate to replenish the surrounding tissue. If it weren’t for adult stem cells, these organs would wither and we wouldn’t live long.

Elaine Fuchs was in graduate school when a lecture by Green upended her plans. Well on her way to becoming a microbiologist, she was so intrigued by the new discoveries that she switched gears and joined Green’s lab as a postdoc. Today, four decades later, she continues to study epidermal stem cells, which have guided her to numerous discoveries in far-ranging fields, from inflammation to cancer.

Fuchs realized early on that to understand the rejuvenating power of stem cells, she needed to study not only the cells themselves but also their niches, and the dynamic interactions taking place among stem cells, their neighbors, and their local environment. “A key discovery was that adult stem cells require other cells in their surroundings to be able to behave properly,” she says. “Many recent breakthroughs in the field—like organoid cultures, for example—are based on the knowledge that stem cells are not operating in a vacuum.”

Because stem cells continually exchange signals with their surroundings, Fuchs adds, all efforts to grow tissue in a dish begin with decoding the signals those cells use to communicate.

In the skin, specialized stem cells make the epidermis, hair follicles, and sweat glands; and in other organs like the intestines and lungs, different epithelial stem cell types regenerate similar absorptive layers. Bit by bit, Fuchs’s lab is piecing together what goes on in the niches of these different stem cells, pinpointing the specialized microenvironments that nourish and guide them, and how those environments change in injury, inflammation, and cancer.

“In the skin, there are 65 different cell types, and we believe a good number of them interact with stem cell niches in various ways and at different times,” Fuchs says. Neurons, fibroblasts, adipocytes, muscle cells, and immune cells all communicate with skin stem cells to do the right thing at the right time. And recently, the lab discovered that the lymphatic vasculature wrapping around the niches synchronizes the activities of stem cells across a tissue. The dynamic interplay between stem cells and their niche is at the heart of the skin’s routine maintenance and its ability to repair wounds, Fuchs says, and it also plays a role in many medical conditions—enough to keep her lab perpetually busy.

“How are stem cells suddenly called into action to regenerate tissue damaged by injury?” she says. “How do they defend against pathogens and cope with infections and inflammatory stress? And why do they sometimes malfunction?”

There are other circumstances in which adult stem cells trigger disease instead of healing and protecting their native tissue. One is metastatic squamous cell carcinoma, a highly aggressive type of skin cancer that Fuchs and her coworkers have studied for many years.

Like most of the body’s tissues, healthy skin epithelium contains many mutant cells, which is usually not a problem. When a mutant shows up, neighboring cells will keep it in check by curbing its proliferation, for example—and after a day or two, the cell will die. But if the mutant happens to be a stem cell, which is the case in squamous cell carcinoma, the scenario becomes far more dangerous. Because it can self-renew indefinitely, the stem cell will hold on to all the mutations it has gained in the past, building a vast repertoire of genetic errors and improving its chances of dodging the intrusive neighbors.

How does this cell dupe nearby cells, and how does it cut itself loose from its locale of origin to leak into the bloodstream and metastasize? “The epithelial stem cell has to manage to survive in order to give rise to cancer, and to do that it needs to change the way it communicates with other cells,” Fuchs says.

Recently, her team found that squamous cell carcinoma cells exit their native nook by giving self-serving instructions to neighboring epithelial stem cells that maintain the surrounding epithelium. “Epithelial stem cells normally control the tissue architecture,” Fuchs explains. “But when a mutant cell induces them to change their gene-expression program, they begin to lose control of the mechanical properties needed to keep the tissue supple and healthy.”

She and her coworkers found that the mutant cell can create an escape route by generating stiffer surrounding tissue. As a result, mechanical forces build up, allowing the mutant to break through and pierce into deeper layers of the skin, from where it can spread further throughout the body via capillaries. More benign skin cancers such as basal cell carcinomas, on the other hand, produce less rigid cell structures and are more likely to stay put.

The lab is now working to pinpoint which of the many mutations found in squamous cell carcinomas are responsible for hijacking the normal ability of epithelial stem cells to proliferate and heal wounds. “Figuring this out will get us much closer to understanding how the disease advances and develop effective interventions,” says Fuchs, the Rebecca C. Lancefield Professor.