The Use of Animals in Human Stem Cell Research:
Past, Present, and Future
John D. Gearhart and Russell C. Addis
Man, unlike any other thing organic or inorganic in the
universe, grows beyond his work, walks up the stairs of
his concepts, emerges ahead of his accomplishments.
– John Steinbeck, The Grapes of Wrath
E
fforts to build on the knowledge gained through stem
cell research, and eventually reach the goal of providing effective and safe therapies through the use of
stem cell interventions, require the resolution of many challenges. Research in stem cell biology and regenerative medicine covers a broad spectrum of basic and translational
studies, in which progress is slow but steady and the use of
animals has been and remains paramount.
Animal-based research is at the center of both “proofof-principle” experiments for cell-based interventions and
the safety and efficacy tests required for FDA approval of
clinical trials. The challenge for regenerative medicine in
restoring the function of tissues and cells is that it takes
place under diverse circumstances—acute injury, progressive degeneration, inflammation, or surgical resection—any
of which can abolish the architectural arrangements or spatial relationships of cells and impede morphogenetic cues,
the patterning controls that are critical for a cell whose
identity and function are dependent on both spatial and
temporal contexts.
Background
Stem cells have the capacity both to self-renew and to differentiate into mature, specialized cells and thus serve as the
catalyst for regenerative medicine. A brief review of current
research illustrates the importance of animal research in this
area, and, as has been demonstrated many times in other biological studies, the use of different species provides critical
information about fundamental mechanisms of cell regulation and function that are essential for designing appropriate
cell-based interventions.
John D. Gearhart, PhD, is James W. Effron University Professor and
Director, and Russell C. Addis, PhD, is a research associate, both at the
Institute for Regenerative Medicine at the University of Pennsylvania in
Philadelphia.
Address correspondence and reprint requests to Dr. John D. Gearhart,
Institute for Regenerative Medicine, University of Pennsylvania, Biomedical
Research Building, 421 Curie Boulevard – Room 1150, Philadelphia, PA
19104-6058 or email [email protected].
Volume 51, Number 1
2010
Although stem cell research with hematopoietic lineages
and mesenchymal stem cells (isolated from bone marrow) began in the 1970s, 2 decades later the first reports on the derivation of human pluripotent stem cells (Shamblott et al. 1998;
Thomson et al. 1998) signaled a new era in stem cell biology
and added impetus to the isolation of stem cells from other
tissues and organs. Human embryonic stem (ES) cells are derived from the inner cell mass of developing blastocysts and
are capable of not only self-renewing indefinitely but also
generating all cell types, a property called pluripotency.
Over the past decade there has been a major effort to
identify and characterize stem cells from adult tissues and
organs. Adult stem cells remain in an undifferentiated, or
unspecialized, state and, as Garzón-Muvdi and QuiñonesHinojosa (2010) explain, reside in “niches,” highly ordered
clusters of cells that regulate the stem cells and are subject
to modulating signaling pathways. These pathways, in turn,
determine the fate of the stem cells—to renew or to differentiate. Differentiating cells can give rise to specialized
cell types of the tissue from which they came, but it is still
unclear whether they can give rise to other cell types.
Studies in developmental genetics can reveal candidate
stem cells in embryos and tissues, and genetic markers can
then be used to isolate these cells. The identification of stem
cells generally involves physically isolating cells from various sources based on a set of expressed genes and then determining their fates based on an operational test, in which the
cells differentiate either in a dish or after grafting to a site in
an animal or embryo. The test reveals the potency of a cell—
whether it is pluripotent (defined above; present in an embryo, fetus, or adult organism, but not in the trophoblast or
placenta), multipotent (giving rise to multiple differentiated
cell types, usually in a particular tissue or organ), or unipotent (specializing into one cell type)—and/or the plasticity of
the grafted cells (whether they are capable of becoming specialized cell types of different tissues).
Analysis of these populations at the molecular, biochemical, and physiologic levels helps to define their “stemness”
based on a variety of parameters and reveals cells that are not
homogeneous and are therefore subject to various interpretations. A set of cells that might commonly be referred to as a
stem cell population may actually contain a variety of cells
at different points along the continuum from fully undifferentiated to fully differentiated. This continuum raises the
intriguing possibility that a stem cell is not so much a discrete entity but rather a state (Lander 2009; Zipori 2004).
1
Recent and Prospective Developments
The recent development of cellular reprogramming—either
from somatic cells (such as fibroblasts) to ES-like cells
(called induced pluripotent stem, or iPS, cells) (Takahashi
et al. 2006), or from one differentiated cell type to another
through lineage reprogramming (Zhou et al. 2008)—provides
additional sources of stem and specialized cells for therapeutic interventions. Cells from these sources are subject to the
same concerns about safety and efficacy as previously identified stem cell populations.
Efforts are under way to develop procedures that produce both high-efficiency differentiation and cells that are
“authentic,” that is, the same as those produced through normal embryogenesis and developmental processes. Such efforts are clearly important for ensuring both efficacy and
safety for prospective clinical use.
Womack (2010) describes other recent highlights—and
challenges—in animal studies of stem cells and their
applications.
Proliferating Questions
A number of variables are associated with the introduction
of cells to rebuild tissues or replace missing, damaged, or
compromised cells. For example, what is the best source
and type of stem cell for a particular treatment? Joers and
Emborg (2010) describe the wide array of stem cell types—
neural, mesenchymal, embryonic, and hematopoietic, to name
a few—that have potential utility for cell-based therapy.
In addition to the decision about what type of cell to use,
stem cell–based studies both give rise to and attempt to address a proliferation of questions. What “developmental
stage” of a cell should be used? Should cells be pretreated to
ensure appropriate differentiation and function? How many
cells should be delivered? How and where should they be
delivered? Would it be best to engineer tissues before grafting? The challenges of tissue engineering vary greatly between tissue types. Gordeladze and colleagues (2010)
describe progress in engineering bone cells and the application of such engineering in animal models. Is it possible to
prevent cells from migrating away from the site of grafting
when desired, or to enable cells to migrate, or “home,” to a
specific target area if desired? And how can transplanted
cells be tracked? Glover and colleagues (2010) describe the
state of the art in finding and tracking grafted cells, two capacities that will be essential in human trials but that require
further work with animal models of diseases and injuries.
Perhaps a more fundamental question is, How appropriate are animal models of the human condition they are designed to represent? As Song and colleagues (2010) point out,
even the best rodent models of diabetes lack the autoimmune
2
component of the human type 2 disease. For a cell-based
therapy to be effective in a non-insulin-dependent diabetic
human, the transplanted cells must withstand the underlying
immunopathology. In addition to the destruction of grafted
cells by the pathogenic processes of specific diseases, what
outcome measures for efficacy and safety must be assessed?
Over what time frame should the grafted models be followed
to determine efficacy and safety? Clearly, the comparatively
short life span of rodents makes it difficult to make claims
about the longevity and effectiveness for humans of cellbased therapies tested in rodents alone. Should tests first be
done with intraspecific grafts to show proof of concept before
attempting interspecific grafts, recognizing intrinsic differences in tissue organization and among cells from different
species? What are the best strategies for minimizing hostversus-graft and graft-versus-host rejection?
The answers to these and other critical questions lie in
continuing extensive animal work to prepare for human
application.
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ILAR Journal