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Stem Cell Basics
Introduction to Embryonic and
Adult Stem Cells
Nuclear Transfer
Cloning
Demonstrates
Nuclear Totipotency
A General Definition
• All stem cells—regardless of their
source—have three general properties:
– they are capable of dividing and renewing
themselves for long periods
– they are unspecialized
– they can give rise to specialized cell types.
The Process of Gastrulation Defines 3 Germ
Layers from Which All Tissues are Derived
Operational Definition of ES Cell
The plan of the experiment, spanning almost a decade, is diagrammed, starting at the upper left.
Grafted blastocysts form malignant teratocarcinomas in ectopic sites in the host mouse. After
dissociation, teratocarcinoma cells microinjected into genetically marked wild-type blastocysts
gave rise to offspring that were mosaics with chimeric tissues composed of cells derived from a
mixture of both the teratocarcinoma and blastocyst-derived cells
Defining Properties of an
Embryonic Stem (ES) Cell
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Derived from the inner cell mass/epiblast of the blastocyst.
Capable of undergoing an unlimited number of symmetrical divisions without
differentiating (long-term self-renewal).
Exhibit and maintain a stable, full (diploid), normal complement of chromosomes
(karyotype).
Pluripotent ES cells can give rise to differentiated cell types that are derived from all
three primary germ layers of the embryo (endoderm, mesoderm, and ectoderm).
Capable of integrating into all fetal tissues during development. (Mouse ES cells
maintained in culture for long periods can still generate any tissue when they are
reintroduced into an embryo to generate a chimeric animal.)
Capable of colonizing the germ line and giving rise to egg or sperm cells.
a Clonogenic, that is a single ES cell can give rise to a colony of genetically identical
cells, or clones, which have the same properties as the original cell.
Expresses the transcription factor Oct-4, which then activates or inhibits a host of
target genes and maintains ES cells in a proliferative, non-differentiating state.
Can be induced to continue proliferating or to differentiate.
Lacks the G1 checkpoint in the cell cycle. ES cells spend most of their time in the S
phase of the cell cycle, during which they synthesize DNA. Unlike differentiated
somatic cells, ES cells do not require any external stimulus to initiate DNA replication.
Do not show X inactivation. In every somatic cell of a female mammal, one of the two
X chromosomes becomes permanently inactivated. X inactivation does not occur in
undifferentiated ES cells
What Is an Adult Stem Cell?
• First, they can make identical copies of
themselves for long periods of time; this ability to
proliferate is referred to as long-term selfrenewal.
• Second, they can give rise to mature cell types
that have characteristic morphologies (shapes)
and specialized functions.
– Typically, stem cells generate an intermediate cell
type or types before they achieve their fully
differentiated state. The intermediate cell is called a
precursor or progenitor cell. Progenitor or precursor
cells in fetal or adult tissues are partly differentiated
cells that divide and give rise to differentiated cells.
Adult Stem Cells Vs. Progenitor Cells
Stem Cells From One Tissue Can
Differentiate Into Another Tissue
What Activates Stem Cells to
Differentiate
• In order to change its fate, a stem cell presumably
responds to key migration factors and growth factors.
– Factors released by damage may induce stem cells to home to a
particular tissue.
– Growth and differentiation factors within the tissue determine
which genes will be activated. These factors in turn alter the
pattern of genes expressed by a stem cell when it resides in a
new tissue.
– The molecular players involved in migration and differentiation of
stem cells are likely to differ depending on the tissue, degree of
injury, and stem cells involved. The factors that induce stem cells
to migrate may be tissue-specific or generic to injury.
What Maintains Stem Cells in The
Undifferentiated State
• Perhaps like the differentiated state, the stem cell state
requires active maintenance and is dependent on the
composition of proteins and the balance of those
proteins present in that cell at any given time.
• Given its plasticity, the stem cell state is dynamic since it
can respond to a variety of signals that dictate distinct
differentiated paths.
• Accordingly, stem cells may be protected or isolated
from cues such as differentiation signals at most times.
This precept raises the possibility that adult stem cells
are maintained in a nonresponsive state by a
constellation of as yet unidentified molecules until
needed, for example for tissue maintenance (skin and
blood) or in the repair of damage. (A “Stem Cell Niche”)
Two Conceptual Views of Stem
Cells
Important Questions to be
Answered
• What are the molecular correlates of the adult
stem cell state?
• Is there a “universal adult stem cell ~ to an ES
cell?
– Are all tissues capable of generating stem cells which
can contribute to other tissues
• What are the pathways followed from one
differentiated state to another (how plastic are
adult cells)?
• What are the “rules” that govern plasticity at the
molecular level (e.g. what changes in gene
expression are allowed and/or required)?