Transcript Developmental Biology

How does
a fertilized
egg become
an
animal?
Clam egg and sperm
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Developmental Biology is the study of a
PROCESS whereby a single cell (the fertilized egg)
divides and selectively activates expression of genes
to produce a complex organism composed of many
cell types.
Ex ovo omnia!
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What kinds of PROCESSES are required?
To form an embryo, the following (and more!) must occur:
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Gametes form and fuse (Reproduction)
Cells multiply (Growth)
Generation of Asymmetry
Axis Determination (Positional information)
- Anterior/Posterior (Head-Tail)
- Dorsal/Ventral (Back-Front)
- Left/Right
• Cells differentiate
• Structures are built from cells (Morphogenesis)
-Animal cells organize into sheets and move
-Plant cells form structures without moving
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Differentiation is a central idea of development:
All cells have the same DNA, but
DIFFERENT CELLS express DIFFERENT GENES
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Nature supports an
incredible diversity of plant
and animal body plans
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Yet all of these organisms share
conserved developmental mechanisms
that are evidence of their evolution
from a common ancestor.
Our challenge is to understand both this
diversity and this unity.
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Developmental Biology is studied
using the following TOOLS
1. Cell Biology
2. Genetics
3. Molecular Biology
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Let’s Review the Basics
1. The body is made of millions to billions of cells.
2. Cellular machinery is largely made up of proteins
3. Because of their different tasks, different cells
contain different proteins
4. Proteins are made up of chains of amino acids, and
these amino acids are "encoded" in the cell's DNA
1. Information flows from DNA to RNA to Protein
2. When one gene is mutated, one protein is affected
(usually disabled).
5. All cells have the same DNA but different cells
express different genes
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Development Occurs at an Unfamiliar Scale
If a cell was the size of a basketball (8 inches)
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• a mouse would be the size of Chapel Hill (10 miles)
• a gene would be about an inch long.
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Development Occurs at an Unfamiliar Scale
If a protein was the size of a Volvo (10 feet)
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• a cell would be the size of Chapel Hill (10 miles)
• a gene would be about 1.5 miles long but the strand of
DNA would only be a few feet wide.
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TESTS
Exam #3 March 31 (covers March 3-29)
FINAL May 5 (covers April 5-26)
NO make-up exams
Regrade requests must be submitted to your
TA within one week of exam
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Two Extreme Models for Differentiation
from the late 1800’s (neither is correct)
1. Mosaic development
2. Regulative development
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The Mosaic Development model proposes
that cells become progressively committed to specific cell fates
Roux’s landmark experiments with frog embryos:
 do cells have fixed identities that they can
maintain without influence from their neighbors?
Kill 2 cells with a hot needle
and allow the remaining
2 cells to develop
“YES”!
4-cell stage
Differential segregation
of genetic potential?
Only half
an embryo develops
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Roux’s landmark experiments
Figure 3.16. Destroying (but not removing) one cell of a
2-cell frog embryo results in the development of only half
the embryo.
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The Regulative Development model proposes that cells retain the
ability to adjust their fates in response to their cellular environment
Driesch’s experiments with sea urchin embryos:
 do cells have fixed identities that they can
maintain without influence from their neighbors?
“NO”!
Each cell regulated its development
to produce an entire embryo
(No differential segregation
of genetic potential)
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C. elegans
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How do cells know which genes to
activate as they go through development?
Most organisms use 2 sources of info
1. parents
2. neighbors
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Information from parents:
The Cell lineage
Mother cell
But what makes “red” different from “blue” in the first place?
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Information from parents:
Mother cell
Segregation of determinants
• mechanism to generate
asymmetry and subsequent
cellular differentiation
Unequal localization
of "determinants"
Cell division transfers
determinants to a single
daughter cell
• determinants are usually
proteins or mRNA.
• information (proteins/
RNA) can be passed on
uniformly, or can be
segregated to one of the
progeny cells.
Cells are now different.
Cell type A
Cell type B
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Information from neighbors:
Cell interactions
Mother cell
Cell division
Cell type A
Cell type B
• an alternative
mechanism to generate
asymmetry and
subsequent cellular
differentiation
• cell division places
daughter cells in different
environments
• different environments
lead to different cell fates
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Cells don’t have to be inside an animal to
communicate with each other
Examples
1. Yeast
2. Slime mold (Dictyostelium)
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Single yeast cells talk to each other when they want sex!
• 2 yeast cell types: "A" and "alpha"
"A"
cell
• only cells of different mating types can mate
HOW?
"alpha"
cell
"alpha"
factor
"alpha" factor
receptor
"A"
cell
each cell type makes a specific
signal (factor) and has receptors
only for the opposite signal
"alpha"
cell
"A" factor
receptor
"A" factor
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These cell-cell signals
lead the yeast cells
that receive them to
move together,
change shape
and ultimately fuse,
producing a diploid cell
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The slime mold develops into an animal
only when it (they?) gets hungry!
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The remarkable life cycle of a slime mold
Slug/Grex
cAMP signal
Figure 2.10
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Dictyostelium discoideum (slime mold) slug stage
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The Cells of the Grex Differentiate
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Conclusion:
Even cells of the most simple eukaryotic
organisms sense their environment,
migrate, adhere to each other,
differentiate, and interact
Now, on to more complicated ones!
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Breakthroughs in Modern Biology
1. All organisms share similar cellular
machinery
2. All animals use this machinery in
similar ways to direct embryonic
development
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Model Organisms in Developmental Biology
Plants Invertebrates Vertebrates
Why use model organisms?
What features do they have in common? 31