STEM CELLS
Image Credit: Mesenchymal precursor cells
Division and differentiation All cells reproduce by dividing Cells produced by mitosis are
clones This is how unicellular organisms
reproduce asexually Multicellular organisms are a bit
different When the cells of an embryo
divides, initially they are all the same
Later the cells start to differentiate (specialise)
© 2010 Paul Billiet ODWS
Released into the Public Domain by Jrockley
Image Credit: www.scienceclarified.com
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
© 2010 Paul Billiet ODWS
Hox genes 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
Zebra fish embryology© 2010 Paul Billiet ODWS
Early experiments 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
Copyright© 2006 Nature Publishing GroupNature ReviewsMolecular Cell Biology © 2010 Paul Billiet ODWS
Totipotent and pluripotent cells 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 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
Bone marrow stem cells © David Darling
© 2010 Paul Billiet ODWS
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
© 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)
© 2010 Paul Billiet ODWS
Somatic cell nuclear transfer 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
Image Credit: Removing the nucleus from an oocyte
© 2010 Paul Billiet ODWS
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
© 2010 Paul Billiet ODWS
The answers? 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)
Image Credit: Umbilical cord
© 2010 Paul Billiet ODWS