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Summary lectures 'Molecular principles of development'
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A summary of all the given lectures of Principles of Development in . The summary contains pictures from the slides and from the book.
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Principles of Development 6th Edition By Lewis Wolpert; Cheryll Tickle; Alfonso Martinez Arias 9780198800569 Chapter 1-14 Complete Guide .
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Principles of Development 6th Edition By Lewis Wolpert; Cheryll Tickle; Alfonso Martinez Arias 9780198800569 Chapter 1-14 Complete Guide .
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Principles of Development 6th Edition By Lewis Wolpert; Cheryll Tickle; Alfonso Martinez Arias 9780198800569 Chapter 1-14 Complete Guide .
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1 Basic concepts of development1.1 Introduction
why developmental biology?
- One of the most fascinating processes in biology
- Foundational in understanding of life
- Relations between disease, development and differentiation
- Potential off regenerative medicine: applications of stem cells
Identification of patterning genes: the power of forward genetics
Forward genetics = random mutagenesis, screen for phenotypes; search for the mutation with the
help of a visual marker. (different from reverse genetics = make mutation in gene of interest, then
analyse phenotypes).
Patterning genes, different types in embryos:
- Gap gene: loss results in a reduced number of segments.
- Pair-rule gene (even-skipped): loss allows only odd-numbered segments to develop.
- Segment polarity gene: loss leads to segments with similar head and tail ends.
The patterning genes allow the genes to become segmented.
Hox genes: important for patterning, deciding which part of the developing body the genes are
present in.
1.2 Overview of development, model system life cycles
1.2.1 Drosophila melanogaster
Life cycle: Fertilized egg -cleavage-> syncytial blastoderm (stage 3-4) -gastrulation (after 180 min)-> embryo (stage
13) -hatching (after 24h)-> larva (1st, 2nd and 3rd instar) -> pupa -metamorphosis-> adult fly => only takes 9 days!
A syncytium forms in early development. Sperm entry through micropyle (anterior). The drosophila
egg is elongated: the micropyle on one end, the germ line on the other
end. The drosophila egg has polarity, which is not the case in all species.
Syncytium: early in development the nuclei divide but the cells do not.
This leads to a ‘bag of cytoplasm’ = syncytium. The syncytium allows for
diffusion of proteins. At a certain moment the syncytium will cellularize;
First the nuclei are in the centre → move to the periphery → syncytial
blastoderm → cellular blastoderm with pole cells posterior
The pole cells will give rise to the germ line.
Gastrulation and segmentation: Gastrulation = major morphogenetic
process of development. Process where the ball of cells turns into an
organism that has polarity and axes. Gastrulation can be defined as the
main process in development. At the end of gastrulation there is obvious
morphological segmentation. Patterning is important for this
segmentation.
,1.2.2 Drosophila vs vertebrate development
Maternal axes and symmetry Syncytium and cleavages Early embryonic cell
division time
Drosophila Anterior-posterior (A-P), dorsal-ventral (D- Nuclear divisions, 9 minutes (genome, 140
V) syncytium MBp)
Zebrafish Animal-vegetal (radial symmetry) Meroblastic divisions, yolk 15 minutes (genome 1,700
syncytial layer MBp)
Xenopus Animal-vegetal (radial symmetry) Holoblastic divisions 25 minutes (genome 1,560
(frog) MBp)
Mammals No polarity (point-symmetry)
Mouse Holoblastic divisions 15-20 hours (genome
2,720 MBp)
There is a syncytium in drosophila. In zebrafish there is a syncytium underneath the embryo: the yolk
syncytial layer. This because zebrafish undergoes meroblastic divisions: no division through the
whole egg, but on top of the egg. Frogs and mice undergo holoblastic divisions: right through the
middle of the embryo. The variation in time is best explained by external vs intra-uterine
development. Zygotes that develop external, have to develop faster to survive.
1.3 Body axes, patterning
1.3.1 Embryonic patterning
Patterning = process of establishing positional information at the molecular level among
similar cells. It helps cells respond differently depending on where they are. The cells look
identical but start to differentiate in different ways.
Establishes body axes: Starts with polarity/symmetry-breaking by:
- Dorsal-Ventral (D-V) (back-belly) - Asymmetric cell divisions
- Anterior-posterior (A-P) (nose-tail) - Molecular gradients; a local source of
- Medial-lateral / left-right (L-R) something with diffusion (in
syncytium or interstitial space)
Patterning is not the same as differential gene expression. Patterning: Process which establishes
differential gene expression (among otherwise similar cells) that is directly related to position
within the embryo.
Maternal genes: set up the body axes AP and DV. Drosophila ~50 maternal-effect genes set up AP
and DV. The oocyte is already patterned before fertilization and are expressed in gradients. Some
RNAs are localized within the oocyte. When they are translated, they form a
local source and the proteins diffuse.
Bicoid (anterior) inhibits caudal translation from maternal mRNA (both
maternal). If you know the concentrations of bicoid and caudal, the
ratio, you know their location within A-P axes within the embryo.
Zygotic genes: As soon as the oocyte starts to express its own genes, it starts to
respond to the activity of maternal proteins:
➔ Gap genes, e.g. hunchback (hb)
➔ Pair-rule genes, e.g. even-skipped (eve) & fushi tarazu (ftz)
➔ Segmentation genes, e.g. wingless (wg)
➔ Selector genes, e.g. abdominal-A (abd-A)
The activity of these proteins will induce increased diversity: gap genes have
multiple domains of expression in contrast of maternal products.
There are molecular differences between cells before morphological differences arise.
,1.4 Germ layers, induction, fate, determination
Germ layers: basic cell types: Germ layer is not the same as germ line!
Different species (bilateria), same three germ layers that form in early embryonic development and
form these, all the cell types of the adult body are formed:
Endoderm Mesoderm Ectoderm
General Stomach, colon, liver, pancreas, urinary Muscle (smooth and Surface epidermis: hair,
bladder, part of urethra, the epithelial striated), bone, cartilage, nails, lens of the eye,
parts of trachea, lungs, part the pharynx, connective tissue, adipose sebaceous glands, cornea,
the thyroid, the parathyroid, intestines tissue, circulatory system, tooth enamel, anterior
genito-urinary system, pituitary, the epithelium of
serous membranes, and the mouth and nose
notochord Neural crest: peripheral
nervous system, adrenal
medulla, melanocytes, facial
cartilage, dentin of teeth
Neural tube: brain, spinal
cord, posterior pituitary,
motor neurons, retina
Vertebrates gut, liver, lungs skeleton, muscle, kidney, epidermis of skin, nervous
heart, blood system
insects gut muscle, heart, blood cuticle, nervous system
Fate maps tell what the progeny of cells will be. → Done experimentally;
Lineage tracing with injected fluorescent dye in a single blastomere. You can
trace what this cell will likely give rise to.
! Fate isn’t the same as specified nor determined:
- Fate: lineage tracing, prediction based on position
o ‘we know what the cells (most likely) will give rise to – but the
cells do not’
- Specified: ‘cells know what they will be, but can change their mind’
- Determined: ‘Cells know what they will be and will proceed no matter what’
If not determined cells can adapt to a new environment, fate versus determination:
- Transplantation of a tissue at an early stage → cell changes fate to new environment, fate
was not determined yet. ‘regulative’
- Transplantation of a tissue at a later stage → the cell won’t change its fate to its new
environment, the cells were already determined. ‘mosaic’
➔ Transplantation of different stages gives different products.
1.5 In situ hybridization
Detection of protein: immune histochemistry (with Ag) & Detection of RNA: in situ hybridization
Whole mount in situ hybridization (WISH) is performed in the whole embryo, working mechanism:
- Cloning a gene of interest in vitro → produce RNA probe + providing labelled nucleotides.
The labelled nucleotide (DIG labelling) can be followed through development. The RNA probe
has to be complementary to the target RNA. The probe can anneal to the RNA of interest in
the embryo.
- Embryo prep & probe hybridization: Fixation → protein digestion and post-fixation →
prehybridization → hybridization → excess probe removal → detection of DIG molecule.
, Basic concepts of development summary
- Drosophila development
o Syncytium, blastoderm, imaginal disc
o Role of RNA transport, regulation of translation and protein diffusion
o Maternal, gap, pair-rule, segmentation and selector genes
- Forward genetics
- Drosophila versus vertebrate life cycle
- Blastula, gastrula, neurula
- Patterning, body axes (A-P, D-V)
- Germ layers (endoderm, mesoderm, ectoderm)
- Fate, induction, determination, mosaic, regulative
2 Origin and specification of the germ layers in vertebrates
2.1 Very early development (frogs & zebrafish)
Start of development: cleavage division. The embryo does not grow in the beginning, but the cells
cleave. A type of embryonic cell division, increase number of cells without increase of cytoplasmic
volume. Large eggs: cleavage division observed by the eye.
The size of an oocyte differs greatly between species: Human oocyte diameter 0.1 mm (largest cell
human body, sperm = smallest), frog oocyte diameter 1.1 mm. Maternal contribution varies greatly.
2.1.1 Maternal to zygotic transition (MZT)
The early stages are largely driven by maternal gene products (RNAs, proteins, etc already present in
the oocyte). Oocyte is quite large thus quite a lot potential for the maternal products to contribute to
the earliest stages of development.
Overall rule to development seems to be remarkable similar between species: at the earliest
cleavages division/earliest phase of development after fertilisation, the embryonic/zygotic genome is
not transcriptionally active. The embryo does not transcribe its own genes! It is completely
dependent on maternal products.
➔ After a while the genes of the embryo itself are being described, this way it is taking control
over its own genetic program = maternal to zygotic transition (MZT):
o Maternal RNA degradation
o Zygotic genome activation (ZGA)
➔ The time were this occurs is variable, the amount of cleavage divisions is too: mice 24h, 2 cell
stage; drosophila 1.5h, 10 cycles of cell division
➔ In flies, fish and frogs (externally developing species) this is called Mid-blastula transition
(MBT) because it happens at the mid-blastula stage:
o New (embryonic) transcription
o Loss of cell cycle synchrony (waves of cell division going through the embryo)
o Cells become more motile → gastrulation about to happen
o Maternal RNA starts to be degraded
Multiple layers of regulation in MBT: There is a repressor of transcription present in the early zygote:
a repressor of the events of MBT. As development starts, the nucleus-to-cytoplasm ratio changes →
increase in the amount of zygotic DNA present. So, there is a change in the amount of repressor
compared to the amount of DNA. If the repressor is diluted enough, the zygotic genome can start to
be described.
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