DEVELOPMENTAL BIOLOGY
CHAPTER 1: HISTORY AND BASIC CONCEPTS
1. Fate & potency
2. Methods in developmental biology
3. Model systems in developmental biology
1. FATE & POTENCY
‘fate’= what the cell is going to become, Fate map: where every cell comes from
Many genes are conserved between organisms. Similarity in the beginning of development, that you
can study across species. Important key genes and their function are often conserved during early
developmental stages: gastrula-neurula.
Zygote → blastula/ blastocyst → gastrula → neurula
1. CELL DIVISION
Amount of yolk protein determines the type of cleavages
FROG
- Initially very fast differentiation
- From 1 to 37.000 cells (43h) (mouse and human: 1 to 4 cells in 48 hours)
- Frogs have to be independent very quickly. Eggs in the water, they have enemies and can be
eaten → a lot of pressure to develop fast
Embryonic cleavage vs. somatic divisions
➔ Somatic cell cycle (16 hours): G1-S-G2-M
➔ Cleavage cycle (30 minutes): S-M
➔ Complete cleavages (cells divide completely), upper part is less heavier so the cleavage
proceeds faster → size of the cells is not equal anymore = MESOLECITHAL CLEAVAGE
(holoblastic= complete)
ZEBRAFISH
- Embryo develops really as a disc on top of the yolk = discoidal cleavage ( Telolecithal )
- Incomplete cleavage = MEROBLASTIC CLEAVAGE
FRUITFLY
- Syncytium: nuclei will divide but membrane will not divide → lots of nuclei in 1 cell that will
be organized in the outer area of the embryo
- Yolk in the center of egg = CENTROLECITHAL CLEAVAGE
MAMMALIAN
- Rotational cleavage: hollow blastocyst
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, Cells in the cleavage stage are called blastomeres. Cleavage is controlled by maternal factors and
cytoplasmic volume does not increase. Rather, the zygote cytoplasm is divided into even smaller
cells. The embryo accomplishes cleavage by abolishing the gap periods of the cell cycle (G1 and G2
phase). The pattern of embryonic cleavage is determined by 2 major parameters:
1) The amount and distribution of yolk protein within the cytoplasm, which determine where
cleavage can occur and the relative sizes of the blastomeres
2) Factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of
its formation
When one pole of the egg is relatively yolk-free, cellular divisions occur there at a faster rate than
at the opposite pole. The yolk-rich pole = the vegetal pole. The yolk concentration in the animal
pole is relatively low. The zygote nucleus is frequently displaced toward the animal pole.
2. DIFFERENTIATION
Zygotes containing large accumulations of yolk undergo meroblastic cleavage, wherein only a portion
of the cytoplasm is cleaved. Telolecithal cleavage: cell divisions occur only in a small disc of the
cytoplasm that is yolk-free → discoidal cleavage.
Activation of the zygotic genome = when the transcription from the embryo’s own genes starts
Mouse: already at 2 cells stage
Frog: only after 4000 cells (that’s why the early cleavage stages happen so fast)
Human: 8-16 cells stage
2. DIFFERENTIATION
Restriction vs. potency
Restriction & potency are 2 opposing theories on how cells became differentiated.
Restriction= the cells either lost all genes except those relevant to their task
Potency= they kept the entire genome and selectively turned genes on or off → true
→Spemann: salamander embryo at 8 cells stage → made a ligation with a hair so that one nucleus was
isolated from the rest. One nucleus is developing separately → had still the potency to make
everything. Cells in the early embryo are all identical, they all can give rise to a complete embryo!
= mechanism that happens with monozygotic twins
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,Waddington’s landscape
Drew differentiation as a landscape, he saw cells as marbles, initially all the marbles of the fertilized
egg are at the top of a mountain (stem cells) and can go any direction downhill but sometimes has to
choose which valley it will enter (one valley is for becoming neurons i.e.). Cells going downhill are
more and more restricted in potency. Cells follow paths and may be predetermined.
Induced pluripotent stem cells: overexpressing transcription factors → able to reprogram the cell to
an earlier stage
Germ layers
= first things the embryo develops during gastrulation
- Ectoderm: epidermis, central nervous system
- Mesoderm: cardiovascular system, urogenital system, muscles, bone and cartilage
- Endoderm: gastrointestinal system and associated organs (pancreas, liver), lungs
Potency
TOTIPOTENT (able to form all the cells of the developing embryo)
→ PLURIPOTENT (initial cells of the developing embryo that will give rise to ecto-, meso- and
endoderm)
→ MULTIPOTENT (different cells belonging to the same germ layer)
Specification and determination
Specification= what the tissue becomes without supplementation
If we supplement it → with Activin (growth factor), we can still change the fate
If the cell type is determined, it will not revert and changes its fate any longer. Knowing whether the
tissue is determined or not? → transplantation experiments
Differentiated cells have specialized structures, transcript
and protein compositions, and functions (e.g. cardiomyocyte
vs neuron)
During commitment, a cell may look indifferent from its
neighbours, but its developmental fate may have become
restricted
Commitment: first specification + then determination
= differentiation
Specification: a cell or tissue is specified to a given fate (e.g.
muscle cell) when it is capable of differentiating
autonomously when placed in isolation in a neutral
environment. At specification, the cell identity is still labile
(can be altered in another environment, by other
neighbours)
Determination: a cell or tissue is determined when it is
capable of differentiating autonomously even when placed in
another environment (e.g. another region of the embryo)
Terminally differentiated cells are usually post-mitotic
Transcription factors are often the determining factors that turn on the differentiated cell type
features.
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,Differential gene expression:
1. Every somatic cell nucleus of an organism contains the complete genome established in the
fertilized egg (genomic equivalence)
2. Unused genes of differentiated cells are neither destroyed nor mutated
3. A small portion of the genome is expressed in each cell, and only a portion thereoff is specific
for that cell type
The characteristic proteome of a cell is the result of:
1. Differential gene expression
2. Selective pre-messenger RNA processing
3. Selective messenger RNA translation
4. Differential posttranslational protein modification (PTM) regulates function/localisation/turn-
over
Genomic equivalence and reprogramming
Our genome (DNA) is the same in all the cells, but have a different epigenetic status. But if we managed
to erase that epigenetic information, cells can be programmed. (Dolly the sheep)
3. GROWTH
Growth happens disproportional (head initially is much bigger than the body)
4. MIGRATION
Five types of cell migration during gastrulation:
5. PATTERNING
= the formation of a well-ordered spatial pattern of cells and tissues (i.e. skin, stripes of a zebra).
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, 6. MORPHOGENESIS
The process of making changes in form in the embryo. It involves coordinated cell growth, cell
migration and cell death. Morphogenesis usually co-occurs with patterning.
7. APOPTOSIS
Programmed cell dead is a critical and physiological part of a normal embryonic development. (fingers)
EARLY DEVELOPMENT IN MAMMALS
- Oocytes of mammals are amongst the smallest oocytes in animal kingdom
- Development inside another organism
- Need for supportive extraembryonic tissues (placenta, yolk sac, amnion)
4 and a half days to develop the blastocyst (8 cells stage) → division starts slowly
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, 2 rounds of differentiation!
1) Wave 1= cell division result into ‘inside’ and ‘outside’ cells
Outside cells: extraembryonic trophectoderm (TE) → polarized + asymmetric cell division
TE forms blastocoel cavity + polarized epithelium with intercellular junctions
Inside cells: pluripotent inner cell mass (ICM) → non-polarized + symmetric cell division
2) Wave 2= (cells enriched in Fgfr2 → inhibits repression of Gata6 by Nanog), cells of ICM
segregate into:
Extraembryonic primitive endoderm (PE)
→Gata4-, Gata6-, Oct4 high
Pluripotent epiblast (EPI)
→upregulation of Fgf4: Nanog- and Oct4 high
‘Fate decisions’ are influenced by heterogenecity at 4-cell stage (but not determined)
Cavitation:
- TE forms blastocoel cavity
- Cavitation initiated (32-cell stage) by diffusion of water across osmotic gradients and transport
of water through aquaporins
- Location of blastocoel cavity determines embryonic-abembryonic axis
- TE forms a polarized epithelium with intercellular junctions: permeability seal that allows for
cavity expansion
TF Gatekeepers of TE, EPI and PE fate:
EPI → primitive ectoderm
PE → primitive endoderm
TE → trophectoderm
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