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Animal Development 2
Cell Fate and Differentiation
Today’s topics:
• Principles ofDevelopment
• Determination andDifferentiation
• Examples
– Fruit fly segmentation
– Muscles
– Snakes
27 March 2009
Organogenesis
Organogenesis
Human embryo, stage 14. Somites visible Neural tube and somites
Some General Principles of Development
• Cell division and cell death
• Movement and adhesion
• Cell-cell interactions and signalling
• Determination and differentiation
Apoptosis (programed cell death)
Cell death is
tightly regulated
and results fromspecific signals.
Cell movement and attachment
Only in animals. Plant cells can’t move.
2
Cell sorting &
adhesion
Figure 47.16 Change in cellular shape during morphogenesis
Fig. 47-24a
Dorsal lip ofblastopore
Pigmented gastrula(donor embryo)
EXPERIMENT
Nonpigmented gastrula(recipient embryo)
Cell-cell interactions
Fig. 47-24b
Primary structures:
Neural tube
Secondary(induced) embryo
Notochord
Primary embryo
RESULTS
Secondary structures:
Notochord (pigmented cells)
Neural tube (mostly nonpigmented cells)
Fig. 47-21b
(b) Cell lineage analysis in a tunicate
64-cell embryos
Larvae
Blastomeresinjected with dye
Cell fate
Fig. 47-22
Mouth
Zygote
Intestine
Nervous
system,outer skin,
muscula-ture
Intestine
Hatching
Eggs Vulva
Anus
1.2 mm
ANTERIOR POSTERIOR
Muscula-ture, gonads
10
0First cell division
Germ line(future
gametes)
Musculature
Outer skin,
nervous system
Tim
e a
fte
r fe
rtil
iza
tio
n (
ho
urs
)
3
C. elegans cell fate
Determination
and
Differentiation
Stem cells
DNA
OFF OFF
OFFmRNA
mRNA mRNA mRNA mRNA
Another
transcription
factor
MyoDMuscle cell(fully differentiated)
MyoD protein
(transcription factor)Myoblast
(determined)
Embryonic
precursor cell
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Other muscle-specific genesMaster control gene myoDNucleus
Determination. Signals from
other cells activate a master
regulatory gene, myoD,
1
Differentiation. MyoD
protein activates
other muscle-specific
transcription factors, which
in turn activate genes for
muscle proteins.
2
Determination and differentiation of muscle cells
Fig 18.16
The cell is now
ireversibly
determined
The cell is now fully
differentiated
A mutation in
bicoid leads
to tail
structures at
both ends
(bottom
larva).
Tail
Head
Normal larva
Tail Tail
Mutant larva (bicoid)
T1 T2T3
A1 A2 A3 A4 A5 A6A7
A8
A8A7 A6 A7
A8
Drosophila pattern formation
Translation of bicoid mRNAFertilization
Nurse cells Egg cell
bicoid mRNA
Developing
egg cell
Bicoid mRNA in mature
unfertilized egg
100 !m
Bicoid protein in
early embryo
Anterior end
(b) Gradients of bicoid mRNA and bicoid protein in normal egg and early embryo.
1
2
3
Hierarchy of Gene Activity in Early Drosophila Development
Maternal effect genes (egg-polarity genes)
Gap genes
Pair-rule genes
Segment polarity genes
Homeotic genes of the embryo
Other genes of the embryo
Segmentation genesof the embryo
4
Conserved from
flies to mammals
Homeotic genes
These are all transcription factors!
Fruit Fly
Mouse
The homeobox is relatively constant
because it has a precise job.Why snakes don’t have legs
http://www.wallpaperbase.com/wallpapers/animals/snakes/snake_2.jpg
http://www.flickr.com/photos/mark_leppin/3322493554/
Vestigial
Claw
Snakes evolved
from tetrapod
ancestor
Why snakes don’t have legs
Broader expression of hoxC6 in snakes results in loss of legsand more vertebrae.
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