The unique ability of embryonic stem cells to make copies of themselves and to differentiate into the myriad cell types that make up the body’s tissues and organs has been described in almost magical terms in the popular press. But the action of some stem cells is quite familiar, and occurs right before our eyes. Clustered under every strand of our hair lie hair follicles, in which stem cells regulate the continuous growth of hair. Though the adult stem cells in hair follicles don’t have the potential of embryonic stem cells to take on nearly any cell fate, their role in regeneration makes them an exciting research tool for understanding basic stem cell biology.
Embedded just beneath the surface of the skin and frequently regenerating, the hair follicle is an ideal “mini-organ” for observing tissue regeneration. The follicle is a highly specialized and dynamic stem cell niche in which cell signaling cues originating both inside and outside the follicle and cell-cell interactions within it regulate the timing and location of stem cell divisions. Researchers have gotten a glimpse of these processes by examining skin samples taken at sequential time points during the hair follicle regeneration cycle under the microscope. But since these samples are only static “snapshots,” this technique may lead scientists to miss important steps of the regeneration process, creating the possibility that what is seen through the microscope does not reflect the dynamic biology of follicles in living animals.
School of Medicine researchers have now developed a non-invasive, high-resolution imaging approach that allows them to observe stem cell regeneration in the hair follicle in real time in live mice. Their findings, published in the journal Nature on July 26, have provided new insights that ill allow researchers to tackle questions related to both tissue regeneration and the role of stem cells in cancer and other diseases. “Because we can follow the same cells over time, we can really learn the true behavior rather than inferring it by static analysis,” says Valentina Greco, Ph.D., assistant professor of genetics and dermatology and senior author of the study. With this new imaging approach, says Panteleimon Rompolas, Ph.D., a postdoctoral fellow in Greco’s lab and lead author of the Nature paper, gaps in our knowledge of the basic steps of regeneration can be filled, and cell-cell interactions and signals can be mapped in detail.
The team used a two-photon laser scanning microscope that makes it possible to produce high-resolution images of tissue in living animals at precise depths up to 100 microns (one-tenth of a millimeter). Such penetration is achieved by the use of low-energy near-infrared light, which scatters less in tissue than the light used in traditional light microscopy. Another advantage of this approach is that the low-energy light inflicts little damage on tissue, says coauthor Ann M. Haberman, Ph.D., assistant professor of laboratory medicine and director of the medical school’s In Vivo Imaging Facility. These attributes of two-photon imaging mean that the microscopes can observe events as they would occur naturally.
In the new study, the researchers used mice engineered to express a fluorescent protein in the skin’s epithelial cells, which allowed them to label and visualize epithelial cells within hair follicles. A series of optical sections of these glowing cells were collected with the two-photon microscope over several hours, generating three-dimensional views of living hair follicles in which dynamic cell behaviors could be followed over time.
With the new technique, the researchers made two major observations that would have been impossible using static methods. First, they found that during the period of growth, when hair follicles rapidly extend underneath the skin, epithelial cells not only proliferate, but also migrate downwards. Second, they confirmed a long-standing suspicion that the mesenchyme, a cluster of cells at the base of the hair follicle, is a crucial signaling center that dictates follicle growth; in follicles in which the team removed the mesenchyme with a laser, regeneration halted. “Not only does the study answer pre-existing questions, but the observations also raise new ones about how organizations of cells and their migrations and divisions are controlled,” says Terry Lechler, Ph.D., assistant professor of cell biology at Duke University, who was not involved in the study. “This work opens a whole new tool for analysis of hair follicle morphogenesis.”
The study also has implications for other stem cell niches. “The hair follicle is just a paradigm representing what happens in other tissues,” says Greco, a member of the Yale Stem Cell Center. Using this study as a stepping stone, Greco’s team next plans to manipulate genes in the hair follicle, including over-expressing signaling molecules or knocking them out, to see how the regeneration process is affected. The idea is to eventually determine the function of each gene in regulating hair follicle stem cell biology. “While there are studies that have identified tissue regeneration signals, we still need to figure out how these signals interact with each other in the regeneration process, and in which behaviors they play roles.” Such knowledge, Greco says, would better enable scientists to harness stem cells for therapeutic purposes.
“We’re just now identifying the basis for future work,” Greco says. “It was a risky project, but one with high reward. We now have a new way of looking at things that could not have been explored before. I think the best is yet to come.”
Giving creative researchers freedom to pursue ideas
Solving the mysteries of disease requires creative, innovative ideas from the best minds in medical research. Creativity can’t be programmed to occur on a tight schedule or within a specific budget, yet that is precisely how most research grants are administered. Today’s tight budgets and risk-averse grant committees favor research awards that provide funds to build on what is already known—not what is novel or unexpected.
Private support for endowed professorships, like the Henry J. and Joan W. Binder Professorship described in this issue, can provide established researchers with secure, flexible funding to pursue new ideas . . . to think creatively . . . to discover new treatments. Additionally, endowed professorships ensure that a donor’s name and particular interests are advanced in perpetuity.
Gifts from donors can also spark new investigations by junior faculty like those who will be supported through the new Psychiatry Research Scholars Program covered in this issue.
Yale School of Medicine seeks donors who are not satisfied with a conservative approach to research, and who wish to participate in pushing the boundaries of knowledge. For more information on endowing a professorship or establishing a new research program, please contact Jancy Houck, associate vice president for development and director of medical development at (203) 436-8560 or firstname.lastname@example.org.
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