STEM CELLS
Image Credit: Mesenchymal precursor cells
Division and differentiation
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All cells reproduce by dividing
Cells produced by mitosis are
clones
This is how unicellular organisms
reproduce asexually
Multicellular organisms are a bit
different
Released into the Public Domain by Jrockley
When the cells of an embryo
divides, initially they are all the
same
Later the cells start to
differentiate (specialise)
Image Credit: www.scienceclarified.com
© 2010 Paul Billiet ODWS
Differentiation and gene expression
All the somatic cells (not gametes) of a
multicellular organism contain the same
genetic information
 But cells in different tissues perform
different functions
 Cells in different tissues have quite
different forms
 Therefore, some genes are expressed
in a cell and not others
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© 2010 Paul Billiet ODWS
Hox genes
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Genes that control
development are a
group called Hox
genes
Surprisingly similar for
all animals
The sequence in which
they express seems to
create the differences
in development
Image Credit: Hox genes of fruit fly and mouse
© 2010 Paul Billiet ODWS
What causes differentiation?
Cells differentiate according to their
position in an embryo
 Transferring cells from one place to
another illustrates this
 The signals are called growth factors
 These factors are released by zones of
cells called organisers
 The surrounding cells develop along a
determined route
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Zebra fish embryology
© 2010 Paul Billiet ODWS
Early experiments
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Displace organiser
cells and the embryo
will develop an organ
in a different place
from usual
Or if a second
organiser is added an
additional organ will
develop
© 2010 Paul Billiet ODWS
Copyright© 2006 Nature Publishing Group
Nature Reviews
Molecular Cell Biology
Totipotent and pluripotent cells
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In simple animals (e.g.
sponges) the cells retain
their capacity to
regenerate into whole
new sponge
Totipotent
More complex animals
lose this capacity
Cells of the early embryo
are capable of turning
into any type of cell
(pluripotent)
Differentiated cells may
not be able to do this
Image Credit: Sea Sponge
© 2010 Paul Billiet ODWS
Stem cells
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Cells that can develop
into any other cell are
called stem cells
A few still exist in the
body of an adult
E.g. bone marrow has
blood stem cells
E.g. umbilical cords
have stem cells
© 2010 Paul Billiet ODWS
Bone marrow stem cells © David Darling
Image Credit: Blood stem cell differentiation
Making stem cells
Stem cells could be used to replace tissues
that are damaged or diseased
 E.g. cardiac muscle will not divide once it
has differentiated
 Stem cells stimulated to grow into cardiac
cells could replace the need for heart
transplants
 The problem of tissue typing and tissue
rejection still remains
 Implanted tissue could become cancerous
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© 2010 Paul Billiet ODWS
Cloned stem cells
If stem cells can be cloned from the cells
of a patient they can be used to generated
genetically identical tissues
 Therapeutic cloning
 Mammalian cells need to be set back to
the beginning of the cell cycle (G0)
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© 2010 Paul Billiet ODWS
Somatic cell nuclear transfer
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SCNT
Made famous by Dolly the sheep
Oocytes (unfertilised egg cells)
harvested
Nuclei removed
Somatic cells from animal to be
cloned fused with enucleate oocyte
Electric shock sets the cell cycle
to G0
Nuclear genome cloned but…
Mitchondrial genome comes from
animal which donated the oocyte
© 2010 Paul Billiet ODWS
Image Credit: Removing the
nucleus from an oocyte
Problems for SCNT in therapeutic
cloning
Human oocytes are few and difficult to
obtain
 A lot of oocytes needed to generate
successful cell lines
(304 oocytes from 14 macaques to
produce 2 cell lines)
 Ethical problem of embryo destruction
 Early embryo cells are harvested to
generate cloned tissues for
transplants/grafts
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© 2010 Paul Billiet ODWS
The answers?
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Fuse somatic cell with
enucleated oocyte of
another species of
animal
Umbilical cord stem
cells
Induced pluripotent
stem cells (iPS)
(Genetically modified
somatic cells
reprogrammed back
to its undifferentiated
state)
© 2010 Paul Billiet ODWS
Image Credit: Umbilical cord