1. Induction: information from neighbors influences cell fate
inducer
responder

2. If induction tells cells
what to do,
how do they remember
these instructions
for a lifetime??
DETERMINATION

3. And usually there’s no turning back.
FATE: what a cell will become at a later time--its future!
DIFFERENTIATION: the actual elaboration of a cell’s fate
DETERMINATION: internal and self-perpetuating changes that distinguish a cell & its descendents from other cells and commit them to a specialized course
In other words, a cell is set to become a certain kind of cell before it actually becomes that type of cell.
And usually there’s no turning back.
This concept differs from fate in that for a time a cell’s fate can often be changed by changing environment; it differs from DIFFERENTIATION because this is overt cell specialization.

4. Determination can be assayed by transplantation experiments
Early
gastrula
Cells alter their fate
to match that of
neighbors
i.e.
not determined
Transplant
nervous system
to epidermis
and vice versa
Late
gastrula
Cells retain their
original fates,
regardless of
neighbors
i.e.
determined
(see also Figure 10.18)

5. Cloning: the ultimate transplant experiment Dolly (and Bonnie)
Figure 4.8
Dolly (and Bonnie)

6. The ultimate transplant experiment
Cloning:
The ultimate transplant experiment
Freddy, a cloned leopard frog (Rana pipiens)
Figure 4.5

7. How does
cloning work?

8. Cloning success drops dramatically as
the age of the nuclear donor increases
Figure
4.6

9. DETERMINATION
results from heritable changes in gene expression

10. All cells have the same genes yet different cells make different mRNAs
Non-base change modifications of DNA result in selective gene expression

11. Evidence for cytoplasmic contributions
to selective gene expression

12. DNA Methylation is used in mammals to keep unneeded genes inactiveCH 3 CH 3 CH 3
Bone-specific
genes are
methylated 3 CH 3 CH
Skin-specific
Gene
genes are not
expression
methylated CH 3 CH 3 CH 3
Muscle-specific
Skin cells
genes are
methylated 3 CH

13. Methylation of DNA
Figures\Chapter05\DevBio7e05181.jpg
Figure 5.19

14. DNA Methylation is used in mammals to keep unneeded genes inactiveCH 3 CH 3 CH 3
Bone-specific
genes are
methylated 3 CH 3 CH
Skin-specific
Gene
genes are not
expression
methylated CH 3 CH 3 CH 3
Muscle-specific
Skin cells
genes are
methylated 3 CH

15. Imprinting results from pre-programmed
differences in DNA methylation of selected genes in the sperm and egg
H19 function?

16. Genomic imprinting occurs in most mammals
* * *

17. Igf2r
Igf2
“A Silent Struggle”, New York Times, March 14, 2006

18. Imprinted genes affect fetal growth
Insulin growth factor Insulin growth factor 2
(Igf2) receptor (Igf2r)
growth
growth

19. What happens if dad’s copy of Igf2 is deleted?
Insulin growth factor Insulin growth factor 2
(Igf2) receptor (Igf2r)
growth
growth

20. copy of Igf2 isn’t imprinted?
What happens if mom’s
copy of Igf2 isn’t imprinted?
Beckwith-Weidemann syndrome
Insulin growth factor Insulin growth factor 2
(Igf2) receptor (Igf2r)
overgrowth
growth

21. What if mom’s copy of Igf2r is deleted?
Insulin growth factor Insulin growth factor 2
(Igf2) receptor (Igf2r)
growth
growth

22. What happens if mom’s copy of Igf2 is deleted?
Insulin growth factor Insulin growth factor 2
(Igf2) receptor (Igf2r)
growth
growth

23. DNA within cells is not naked!

24. DNA is wrapped around histone proteins as chromatin
Figure 5.1

25. Inactive genes are thought to be wrapped up in an inactive "chromatin conformation"
Bone-specific
genes are
wrapped up and
inaccessible
Skin-specific
mRNA
genes are not
Gene
wrapped up and
expression
so are accessible
Skin cells
Muscle-specific
genes are
wrapped up and
inaccessible

26. X chromosome inactivation:
Silencing one of the two X’s in female cells
Xist RNA is associated with the silenced
X chromosome
Read this in the text again

27. Post-translational modification of histones plays a key role
in regulating
gene expression
X chromosome
inactivation
(See also Figure 5.25)

28. myoD

29. By altering methylation, cells can be reprogrammed
Experiment #1
By altering methylation,
cells can be reprogrammed
Bone cells
Muscle cells
Normally fibroblast cells would become muscle cells?
"Fibroblast-like"
Fat cells
mouse cells
Treat with 5-azacytidine to reduce DNA methylation

30. Experiment #2
Chromatin transferred from myoblasts can transform untreated fat cells.
Transformation frequency suggests
only one or few of the genes have this ability

31. Ultimately a single gene was identified- myoD
-whose expression could convert
any other cell type into muscle cells

32. MyoD is expressed at the right place and time to be a master regulator of muscle development
Somites
Experiment #3

33. Does the myoD transcription factor bind DNA sites
Experiment #4
Does the myoD transcription factor bind DNA sites
within the promoters of the expected gene types?
Bone-specific genes lack
myoD binding sites
OFF
muscle-specific
mRNA
myoD ON
Muscle-specific genes have
myoD binding sites
myoD site
Fat-specific genes lack
myoD binding sites
OFF

34. Experiment #5
Scientists "knocked out" the myoD gene to test its predicted role in muscle development
Eliminate gene encoding myoD
Does mouse develop muscles?

35. !!

36. MyoD and related transcription factors
Experiment #6
MyoD and related transcription factors
work together to regulate muscle cell fate

37. How do cells remember their decisions?
Cellular memory and the engrailed gene

38. Remember that
Wingless is
expressed by
specific cells
within each segment

39. Engrailed is a transcription factor
expressed by cells posterior to those that express Wingless.
It defines the posterior boundary of each segment.
Anterior
Posterior Wg En

40. Here’s the real thing: engrailed

41. Mechanism #1:
cell-cell signaling
Engrailed expression requires Wingless signaling by the neighboring cells.

42. Mechanism #2: autoregulation
Later on, engrailed becomes autoregulatory
ie. it turns on its own synthesis
Segment
Segment
boundary
boundary en ON
More engrailed
en gene
protein

43. Mechanism #3: specialized chromatin proteins
In cells where engrailed is turned off, a set of specific chromatin proteins bind to it and wrap it up in a permanently inactive state.
Segment
Segment
boundary
boundary
OFF
en gene
Polycomb group proteins turn off engrailed in cells it’s not needed.
en gene
Permanently
OFF