Essential Cell Biology 5th edition

Stem Cells and Tissue Renewal
717
regulators, including Oct4, Sox2, and Klf4. This treatment is sufficient
to permanently convert fibroblasts into cells with practically all the
properties of ES cells, including the ability to proliferate indefinitely, differentiate
in diverse ways, and, in the case of mouse cells, contribute
to the formation of any tissue. These ES-like cells are called induced
pluripotent stem cells (iPS cells). The drawbacks to this approach
include a low conversion rate—only a small proportion of the fibroblasts
make the switch to become iPS cells—and concerns over the safety
of implanting cells with such an abnormal developmental history into
humans. Much work remains to be done before this approach can be
used to treat human diseases effectively and ethically.
Better ways of producing human iPS cells are continually being developed.
In the meantime, human ES cells, and especially human iPS cells,
are proving to be valuable in other ways. They can be used to generate
large, homogeneous populations of differentiated human cells of specific
types in culture; these can be used to test for potential toxic or beneficial
effects of candidate drugs. Moreover, it is possible to generate iPS cells
from patients who suffer from a genetic disease and to use these iPS cells
to produce affected, differentiated cell types, which can then be studied
to learn more about the disease mechanism and to search for potential
treatments. An example is Timothy syndrome, a rare genetic disease
caused by mutations in a gene that encodes a specific type of Ca 2+ channel.
The altered channel fails to close properly after opening, leading to
multiple defects, including abnormal heart rhythm and, in some individuals,
autism. The iPS cells produced from such individuals have been
coaxed to differentiate in culture into neurons and heart muscle cells,
which are now being used to study the physiological consequences of
the Ca 2+ channel abnormality and to hunt for drugs that can correct the
defects.
In addition, experiments on pluripotent ES and iPS cells themselves are
providing insights into some of the many unsolved mysteries of developmental
and stem-cell biology, including the molecular mechanisms
that maintain pluripotency and those that restrict specific developmental
fates.
Mouse and Human Pluripotent Stem Cells Can Form
Organoids in Culture
Remarkably, under appropriate conditions, mouse or human ES cells
and iPS cells can proliferate, differentiate, and self-assemble in culture
to form miniature, three-dimensional organs called organoids, which
closely resemble the normal organ in its organization. An early striking
example is shown in Figure 20–41, where a developing eye-like structure
Figure 20–41 Cultured ES cells can give
rise to a three-dimensional organoid.
(A) Schematic drawing shows how, under
appropriate conditions, mouse or human
pluripotent cells in culture can proliferate,
differentiate, and self-assemble to form a
three-dimensional, eye-like structure (an
optic cup), which includes a multilayered
retina similar in organization to the one
that forms during normal eye development
in vivo. (B) Fluorescence micrograph of
an optic cup formed by human ES cells in
culture. The structure includes a developing
retina containing multiple layers of neural
cells (stained green) and an underlying layer
of pigmented epithelium, the apical surface
of which is stained red. All nuclei are stained
blue. (A, adapted from M. Eiraku and Y.
Sasai, Curr. Opin. Neurobiol. 22:768–777,
2012; B, adapted from T. Nakano et al., Cell
Stem Cell 10:771–785, 2012.)
multilayered
retina
pigmented
epithelial
layer
neural retina
aggregate of
cultured ES cells
hollow ball of
neural cells
budding of
optic vesicle
optic vesicle invaginates
to form optic cup
(A)
(B)
100 µm