DEVELOPMENTAL BIOLOGY
Chapter 0: Introduction
Su#iciency versus Necessity
A. A cat is su#icient to cause a pit in the road.
MOVE-IT EXPERIMENT = take the cat and put her on a road without pits to observe what
happens.
B. A cat is necessary to cause a pit in the road.
BLOCK-IT/LOSE-IT EXPERIMENT = remove all cats and observe what happens over time
to a road without pits.
C. A cat could be needed to cause a pit in the road.
SHOW-IT/FIND-IT EXPERIMENT = to show correlation (verband) but not causality
(oorzakelijk verband).
Chapter 1: Fate and Potency
Development: a blend of di#erent processes
1. Cell division Zygote
2. Cell diTerentiation |
3. Growth Blastula/Blastocyst
4. Migration |
5. Patterning Gastrula
6. Morphogenesis |
7. Apoptosis Neurula
= these happen in diTerent places but somewhat simultaneously
, 1. Cell divisions
Embryonic cleavage VS. Somatic divisions
- Somatic cell cycle (16 hours) in somatic-/body cells
o G1 à Synthase à G2 à Mitosis
o The number of cells increase exponentially because of the variation in G1
and G2 cycles
o Duration of cycle can diTer between species
- Embryonic cleavages in early embryos (30 minutes)
o Synthase à Mitosis (no G1 and G2)
o Extremely short cell cycle
o Divisions can either happen fast or slow
• Fast: frogs, fruit fly, … (divisions happen faster when the embryo
isn’t inside of the mother).
• Slow: humans, … (takes longer because the eggs are protected by
the mother).
Patterns of embryonic cleavage
What determines the pattern of cleavage?
• Amount and distribution of yolk protein within the cytoplasm.
• Factors in egg cytoplasm influence angle and timing of mitotic spindle
(spoelfiguur).
Important distinction:
• Symmetric divisions
• Asymmetric divisions (yolk is heavy so it doesn’t
divide)
• Complete/holoblastic divisions (the entire egg divides
– no yolk present)
• Incomplete/meroblastic divisions (only part of the egg
divides – not the present yolk)
,Activation of the zygotic genome
In order for the divisions to begin, the DNA of the zygote must be activated à this process
can be stalled for a long time (up until the baby needs their own proteins).
Process in most species (not all!!)
- Maternal mRNAs are laid down in the oocyte during oogenesis à after fertilization
new proteins are made from the maternal mRNA.
- At one point the embryo needs to start its own transcription!
o How? One of the maternal proteins is an inhibitor of gene transcription à
by each cell division there’s a decay of pieces of the genes until the
repressor part is completely gone = genes can be activated.
= duration diTers between species because some embryos have to become independent
faster than others (predators, …)
2. Di#erentiation
DiTerentiation is the process by which an unspecialized cell becomes specialized into
one of the many cell types that make up the body.
Historical perspective: how do we know cells actually di#erentiate?
- Restriction: cells lose (irrelevant) genes during their development and
diTerentiation (this doesn’t happen).
VERSUS
- Potency: cells contain all the genes for every possible cell type AND can
selectively turn them on or o% (this does happen).
Potency – di#erent kind of cells
- Totipotent = the capacity of a cell to form all the cell types of an organism.
- Pluripotent = cells of the developing embryo that give rise to the ecto-, meso- and
endoderm.
- Multipotent = cells that can give rise to diTerent cell types belonging to the same
germ layer.
, Hans Spemann & Hilde Mangold – experiments to prove potency
- They took a strand of hair and tied it around a salamander embryo (in 16 cell
stage).
- One nucleus gets isolated from the others.
- Two independent embryos develop!
= one cell has the potency to complete a whole embryo = totipotent
Waddington’s epigenetic landscape
1. At the top: pluripotent undiTerentiated stem
cell/progenitor.
2. At the bottom: terminally diTerentiated cell.
à Fate choices occur at bifurcation points
(progenitor state).
à Cells follow paths that may be predetermined.
The first distinctions in an embryo - the 3 germ layers
When the embryo generates these layers, the cells within it become progressively laid
down (so more diTerentiated).
a) Ectoderm: epidermis & central nervous system
b) Mesoderm: cardiovascular & urogenital system, muscles, bone, cartilage, …
c) Endoderm: gastrointestinal system and associated organs, lungs, …
Chapter 0: Introduction
Su#iciency versus Necessity
A. A cat is su#icient to cause a pit in the road.
MOVE-IT EXPERIMENT = take the cat and put her on a road without pits to observe what
happens.
B. A cat is necessary to cause a pit in the road.
BLOCK-IT/LOSE-IT EXPERIMENT = remove all cats and observe what happens over time
to a road without pits.
C. A cat could be needed to cause a pit in the road.
SHOW-IT/FIND-IT EXPERIMENT = to show correlation (verband) but not causality
(oorzakelijk verband).
Chapter 1: Fate and Potency
Development: a blend of di#erent processes
1. Cell division Zygote
2. Cell diTerentiation |
3. Growth Blastula/Blastocyst
4. Migration |
5. Patterning Gastrula
6. Morphogenesis |
7. Apoptosis Neurula
= these happen in diTerent places but somewhat simultaneously
, 1. Cell divisions
Embryonic cleavage VS. Somatic divisions
- Somatic cell cycle (16 hours) in somatic-/body cells
o G1 à Synthase à G2 à Mitosis
o The number of cells increase exponentially because of the variation in G1
and G2 cycles
o Duration of cycle can diTer between species
- Embryonic cleavages in early embryos (30 minutes)
o Synthase à Mitosis (no G1 and G2)
o Extremely short cell cycle
o Divisions can either happen fast or slow
• Fast: frogs, fruit fly, … (divisions happen faster when the embryo
isn’t inside of the mother).
• Slow: humans, … (takes longer because the eggs are protected by
the mother).
Patterns of embryonic cleavage
What determines the pattern of cleavage?
• Amount and distribution of yolk protein within the cytoplasm.
• Factors in egg cytoplasm influence angle and timing of mitotic spindle
(spoelfiguur).
Important distinction:
• Symmetric divisions
• Asymmetric divisions (yolk is heavy so it doesn’t
divide)
• Complete/holoblastic divisions (the entire egg divides
– no yolk present)
• Incomplete/meroblastic divisions (only part of the egg
divides – not the present yolk)
,Activation of the zygotic genome
In order for the divisions to begin, the DNA of the zygote must be activated à this process
can be stalled for a long time (up until the baby needs their own proteins).
Process in most species (not all!!)
- Maternal mRNAs are laid down in the oocyte during oogenesis à after fertilization
new proteins are made from the maternal mRNA.
- At one point the embryo needs to start its own transcription!
o How? One of the maternal proteins is an inhibitor of gene transcription à
by each cell division there’s a decay of pieces of the genes until the
repressor part is completely gone = genes can be activated.
= duration diTers between species because some embryos have to become independent
faster than others (predators, …)
2. Di#erentiation
DiTerentiation is the process by which an unspecialized cell becomes specialized into
one of the many cell types that make up the body.
Historical perspective: how do we know cells actually di#erentiate?
- Restriction: cells lose (irrelevant) genes during their development and
diTerentiation (this doesn’t happen).
VERSUS
- Potency: cells contain all the genes for every possible cell type AND can
selectively turn them on or o% (this does happen).
Potency – di#erent kind of cells
- Totipotent = the capacity of a cell to form all the cell types of an organism.
- Pluripotent = cells of the developing embryo that give rise to the ecto-, meso- and
endoderm.
- Multipotent = cells that can give rise to diTerent cell types belonging to the same
germ layer.
, Hans Spemann & Hilde Mangold – experiments to prove potency
- They took a strand of hair and tied it around a salamander embryo (in 16 cell
stage).
- One nucleus gets isolated from the others.
- Two independent embryos develop!
= one cell has the potency to complete a whole embryo = totipotent
Waddington’s epigenetic landscape
1. At the top: pluripotent undiTerentiated stem
cell/progenitor.
2. At the bottom: terminally diTerentiated cell.
à Fate choices occur at bifurcation points
(progenitor state).
à Cells follow paths that may be predetermined.
The first distinctions in an embryo - the 3 germ layers
When the embryo generates these layers, the cells within it become progressively laid
down (so more diTerentiated).
a) Ectoderm: epidermis & central nervous system
b) Mesoderm: cardiovascular & urogenital system, muscles, bone, cartilage, …
c) Endoderm: gastrointestinal system and associated organs, lungs, …
