Download Other mechanism: MOSAIC development

Molecular mechanisms of Development
Dr. Robert Moss
Human body contains 50 TRILLION CELLS!
1.
Pattern formation/Morphogenesis
2.
Differentiation
1.
Determination
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Determinatin is a
step-wise process,
involving:
Communications
between cells;
Activation of
genetic
developmental
programs
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At 8 cell stage of human embryo, cells begin to
read positional cues, either by contact, or
morphogens.
 Cues influence expression of genes.
 In a chicken embryo, if take tissue at base of leg
bud [normally gives rise to thigh], transplant to tip
of identical-looking wing bud, tissue develops into
a TOE rather than THIGH!! Tissue already
determined to be leg, but took position
information from new position to decide on tissue.

– Cells determine fate both by reading their own gene
expression, and by reading environmental cues.
– Position info in leg and wing must be the same!
– Nature often re-uses systems, and genes!
Developmental
mechanisms:
morphogen gradients
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Regulative development mechanisms:
INDUCTION
 If separate animal and vegetal pole cells,
neither develops mesoderm! But if moved
together, some of the ectoderm becomes
mesoderm! What does that tell you?
 Induction seen in many places in
development; see that cells communicate
with eachother to decide what to become.
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Experiments show that
the notochord "induces"
any ectoderm above it to
become neural tube,
which will become
brain and spinal cord.
How can I demonstrate
this?
Neural tube later
induces formation of
vertebrae.
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EXPERIMENTAL DEVELOPMENTAL
BIOLOGY
 DROSOPHILA
MELANOGASTER:
–Life cycle
–Body plan
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
Homeotic genes:
Homeotic Genes
– “Master Control Genes”: Activate an entire developmental
program for a particular structure.
– determine identity of each segment/organ.
– Activates genetic program appropriate to organ.
– Overexpress “eyeless” throughout the body, get eyes developing
on wings, antenna, legs, etc.

Lepidopteran, with addition of two new genes, transforms
posterior pair of wings into “halteres”.
Network of crossregulation determines
position

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Gradients of ‘morphogens’ activate
particular genes in particular
regions.
Example: To express eve in the
leftmost two even bands, nuclei
must first express hb, and bicoid.
These bind enhancers near eve.
Balance between eve activators,
such as hb, and repressors, such
as Kr.
Bithorax & Antennapedia complexes

Order on chromosome matches expression pattern!
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Homeo Box: 180 base pairs
– 60 a.a. DNA binding domain;
regulators of transcription.
Found in all multicellular organisms!
Homeo box protein gradient in frog
limbs, could convey information about
position along limb.
Same homeo box gene involved in
limb development in mammals.
Must have evolved very early!
Mouse homeobox gene can substitute
for a flies!!
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Just as in flies,
in mice order of
genes reflects
order in body
plan!!
Must be some
significance to
order!!
“Major changes
between major
animal phyla
are correlated
with
duplications in
Hox genes or
an increase in
the number of
Hox genes”
Limb/digit formation

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Limb development stimulated by expression of FGF8
Internal mesodermal core stimulates formation of a thickened
ectoderm, “Apical Ectodermal Ridege” [AER]
AER stimulates proliferation and development of underlying
mesoderm to construct the proximal/distal axis of the limb
Grafting an additional AER creates additional limb.
»
Photos: http://www.scribd.com/doc/2473672/Limb-Development
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
Anterior/posterior gradient and digit formation:
– Zone of Polarizing Activity [ZPA]
» Sonic Hedge Hog, a homeotic gene first identified in flies!
»
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Photos: http://www.scribd.com/doc/2473672/Limb-Development
Primitive snakes, evolved from legged lizardes, and some retain
hindlimb vestiges.
Limbless lizards: AER regresses, and limbs do not appear. Only
requires changes in a few genes!
In mouse, eliminate FGF4 and 8, no limb development at all!
Thalidomide: Inhibits expression of Sonic hedge hog, which prevents
limb extension.
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Along trunk, co-expression of Hoxc6 and Hoxc8 seems to INHIBIT limb
development.
Change in expression of these two genes can create a dramatic change in
body plan!
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Mammalian Pattern Formation

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Cilia of blastomeres beat asymmetrically, due to their structure.
Probably determines axis.
At about 8 cell stage, homeobox transcription factors first show
distinction between “inner cell mass” and trophectoderm.
First ‘bilateral’ indicator: Nodal, expressed on left side only.
– If absent, randomization of orientation occurs.

Same hox genes used for A/P position of CNS in mammals as in flies!
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Epigenomics
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Remember, every cell in the body has the same set of genes!
So it’s the EXPRESSION of different genes in different cells that
determines pattern, differentiation, organ formation, cell behavior.
How do we get different expression in different cells?
Developmental signals/morphogens initiate changes, that are then
“heritable” as the cell divides.
“Epigenomics”.
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