Developmental biology Flashcards
lecture 15-20
Developmental Biology is centred on a single
phenomenal fact:
A single cell, a fertilised egg cell, can give rise to a
complex multicellular organism.
what is a key word in the development of the embryo?
cell types are produced progressively
what are the model animals invertebrates?
1.The fruit fly Drosophila melanogaster
2.The nematode worm Caenorhabditis elegans
3.The sea urchin- Echinodermata
what are the model vertebrates?
The frog Xenopus laevis
The chicken Gallus gallus
The mouse Mus musculus
The zebrafish Danio rerio
The model plant
Arabidopsis thaliana
what is the C. eligans advantage?
Simplicity (<1000 cells)
Cell lineage, Fate maps
there genome is sequenced
why would someone pick a Mus musculus
Genome sequenced
Molecular techniques
Transgenic mice
The first steps to building a new organism
Descriptive embryology
fertilisation,
cleavage,
gastrulation,
neurolation,
orgoneogenisis
what is the cleavage stage?
Cleavage forms a hollow ball
or disk of cells –
blastula/blastoderm
Gastrulation
turns the blastula
into a three germ-layered
gastrula
Gastrulation is marked by …..
extensive cell movement
triploblastic animals
ectoderm, mesoderm and endoderm
diploblastic animals e.g.
cnidaria(jellyfish)
the primitive gut is called the
gastrula
Ectoderm
structures formed on the outside of the embryo: skin and nervous system
why is nervous system ectoderm?
the nervous system is formed on the outside and then internalises
Mesoderm
Structures formed in the middle : bone, blood, muscle and some organs such as
kidney
the Endoderm
Structures that form inside: i.e. Two tubes and associated organs : the respiratory
tract and the digestive tract. Respiratory and digestive systems and organs
Organogenesis
Interaction of germ
layers to form
organ systems
what allows for communication in cells?
The positioning of cell layers in the gastrula allows cells to interact in new
ways
Neurulation
– formation of the neural
tube- a special type of organogenesis –
it sets cells aside and forms the entire
nervous system
what is the first operating organ in new developing embryos?
The circulatory system is the first operational organ system in the
developing embryo, and the heart the first functional orga
3 Major inter-related processes are involved in building a complex organism
from a single fertilised egg cell what are they?
1.Cell Division
2.Cell Differentiation
3.Morphogenesis
to turn off and on a gene process
The “turn on” and “off” of genes in specific cells is
controlled by cell specific transcription factors
that bind to gene specific “enhancer” regulatory
elements, influencing stable binding of the
transcriptional machinery to the promoter
earliest changes do not involve visual changes what do they involve?
the set of
regulatory molecules present are changing
Morphogenesis
The emergence of shape and
structure in the body plan of the developing embryo
How do cells receive information about their
relative position?
Localisation of
cytoplasmic determinants of mRNA and ribosomes e.g. unequal distribution.
Induction ; adjacent cells/tissues
communicating with each other
what is the ability to respond to a cell signal called?
competence.
how was the drosophila developmental genes identified?
study of mutations
indeed there is a ordered
segmental pattern
what genes define the body plan?
maternal effect genes, - establish poles and axes
* segmentation genes
* homeotic
mutants of homeotic genes cause what?
Structures characteristic of a
particular part of the animal arise in the wrong place
Drosophila body plan genes were
found to encode
components of cell signalling pathways- that allow cells to
communicate (basis of induction) and
* transcription factors
hox genes do what?
They control the identity of
body parts
Patterning of the secondary field
A new set of positional cues have to
be established to pattern the
secondary field. e.g. the limb
Proximal-distal
shoulder to finger
Anterior-posterior =
thumb to small finger
dorsal to ventral
back to palm
what marks out the future skeleton?
Cartilage condenses within the mesenchyme(gelatinous tissue )
to mark out the future skeleton of the limb.
Organiser
A group of cells that influences the
development of surrounding tissue
what are the two organisers of the developing limb?
1.The Apical Ectodermal Ridge (AER)
2. The Zone of Polarising Activity (ZPA)
If the AER is removed
the limb ceases to grow
where is the zone of polarizing activity found?
located in the posterior
mesoderm of the developing limb bud.
Important conclusion of the organiser
experiment:
Organiser cells from the donor could change the
fate of the recipient cells and can set in motion a
chain of events leading to the production of a
new body plan
Cell Signaling: Only the cells that possess specific
receptor(s) on their cell surfaces can respond in a particular
way
- That some cells will not respond at all to a
signal (if they don’t have a receptor). - That two different cells can respond in
different ways to the same signal (if they
possess different receptors or receptor
complexes)
receptor to messenger
two part communication system
what is the sonic hedgehog genes receptor?
Patched/
Smoothened
what is the transcription factor for sonic the hedgehog?
Gli
what does sonic hedge hog gene do?
Positional info- neural
tube and limb
fibroblast growth factor does what?
Roles at all stages
* AER of limb bud
* Gastrulation
Maintaining mesoderm
the vulva develops from what?
six cells present on the ventral
surface of the second-stage larva
what initiates
a cascade of signals that
establishes the fate of the vulval
precursor cells.
the anchor cell
why is the sonic hedgehog gene called this way?
embryo is shorter (C) than normal (A)
and the ventral surface is completely
covered in denticles (bumps)–
vertebrate’s have three hedgehog genes
Ihh –Indian hedgehog
Dhh –Desert hedgehog
Shh – Sonic hedgehog
where is the sonic hedgehog gene translated
primary cilium
hedgehog in drosophila
Role in the establishment of para-segment
boundary
sonic hedgehog in mice
–neural tube,
developing limb buds
desert hedgehog
in testis development
what secretes sonic the hedgehog?
The ZPA
sonic hedgehog and zpa
Sonic hedgehog is a major active component of the ZPA.
It is required for pattern formation of the limb along the anterior-posterior axis.
sonic hedgehog gene (Shh) are
responsible for human limb malformations such as
Human preaxial
polydactyly (PPD)
AER secretes what?
fibroblast growth factor 8
if too much AER is added what happens?
the overgrowth of the limb
wingless and how many other wnt proteins
6
in mice and humans how many wnt genes?
In mouse and human- 19 Wnt genes
wnt signalling in mice cause what?
Patterning (generating positional information)
-regulating cell proliferation
-Limb development
-urogenital system development (kidney
development)
-stem cell contro
Wnt1 role
patterning of the
future mouse brain and spinal cord
A note about cancer
Cancer cells have usually accumulated mutations in a number of genes
before they become malignant
What type of genes when mutated can lead to cancer?
Proto-oncogenes (called oncogenes when mutated)
Tumour suppressors
what are proto-oncogenes?
They are regulators of cellular behaviour
colon cancer is involved with what pathway and what is affected?
Wnt signaling pathway
ACP
what is the wnt signal needed for in the gut?
In the gut:- needed for differentiation and regulation of
stem cell population for repair of gut lining.
sonic the hedgehog pathway can cause what if it is activated?
tumorigenesis (sporadic mutations or other
mechanisms
why is time important in the development of embryos?
differentiation is progressive and temporally coordinated
SPACE why is it important in the development pathways?
positional information and the concept of morphogens
what is the tell tail sign a gene is switched on?
the presence of mRNA
how can we test for mrna?
~Transcriptomics- e.g. RNA sequencing (see fig
19.12)
* RT-PCR amplification from mRNA (see fig 19.11)
* In situ hybridisation.
in situ hybridisation can help detect what?
Visualising precisely when and where
master control genes are expressed in the embryo
how does institute hybridisation work
the use of a probe that is coded into the gene sequence or the use of reporter genes
outline what a layer of information is
Each new signal and response is a layer of information that channels the cell on
toward its final differentiation fate.
Each layer influences how the cell will respond to the next layer
A great intro for an essay
This progressive nature is reflected in the fact that cells
become more and more restricted in their fate until
eventually a single fate has become irreversibly determined
when is the cells fate determined?
when it can no longer respond to
changing environmental signals and change its differentiation pathway
Specification
An early stage of commitment to cell fate . At this stage cell
commitment is still capable of being reversed. the cell has received
information but can still be further influenced by new information
Determination:
The
information received
by a cell has fixed its
fate and will allow no
further change.
Lateral inhibition:
a means of ensuring that all cells in a field of similar cells
exposed to the same neighbours do not undergo the same differentiation
pathway, at the same time
e.g. used to space out the differentiation of neurons in the developing
nervous system. eg. using an inhabitiry molecule
in the formation of the vulva, the cell colsest to the anchor cell produces what?
a secondary messenger
Cell lineage analysis
A method to physically mark a
cell early in development and
observe it and its descendent
cells at a later time .
For example using dye.
linkage example
Chick – Quail chimeras. Transplant quail tissue into the
chick embryo. Can distinguish the cells based on a
simple stain (quail cells more “dense”) e.g. used to map
the migration pathway and fate of Neural Crest cells
how did they find Green Fluorescent Protein GFP
form a jellyfish
A morphogen
a signal (chemical / molecule)
involved in pattern formation where cells have a
graded response depending on the level of the
signal they are exposed to
an example of a morphogen
Bicoid
Maternal effect genes –
i.e. the mother passes on Bicoid, high concentrations of Bicoid makes a head.
biophysical cues
But cues might also be generated by the
physical environment-
what biophysical cue example
lack of fetal movement creates weak boanes
why is fetal movement so important?
when embryos develop without muscle contractions, the cells of the developing
skeleton do not differentiate correctly and gene expression is changed
Hippo pathway
sensor of a cell’s environment, mutation which leads to tissue overgrowth: constitutively active Yap/Taz
-increase in liver size
yap does what
Yap protein elevated in regions of the developing humerus where
shape changes are occurring condyles(round part at the end of bones)
Stem cells are defined as
undifferentiated cells that are both self
renewing and can give rise to
differentiated cells when stimulated to
do so
Stem cells in bone marrow
can differentiate into all blood cell types
where are stem cells harvested form?
ESCs are derived
from the inner cell
mass of the
mammalian
blastocyst
Yamanaka, 2006 found out what?
that adult differentiated cells could
be reprogrammed to ESC-like cells by introducing the genes for 4
transcription factors associated with pluripotency
how to form a adult cell into a pulipotent cell?
Four “stem cell” master
regulatory genes were
introduced
what are organoids?
Stem cells can be cultured in vitro in 3D to produce ‘organoids’ –
structures that mimic aspects of tissues and organs
How might a cell respond to Positional Information
proliferation
differentiate
Change cell shape
Move
apoptosis
(all processes able to create cell shape )
change in cell shape examples
Localised contraction of particular cells (due to contraction of
cytoskeletal elements) can cause a whole sheet of cells to fold.
what carry’s out apoptosis
Caspases (e.g. Caspase 9) – carry out apoptosis. capasae 9. Are a family of proteolytic enzymes (proteases) that play a crucial role in apoptosis (programmed cell death)- forms fingers
Knockout of caspase-9 in brain
forms too many neurons
what is the homeobox
a 180bp sequence called the homeobox – that codes for a 60amino
The homeodomain in the protein forms a helix-turn-helix structure that directly binds
to specific regulatory sequences in target genes.
Binds to specific DNA sequences (homeobox motifs) to control the expression of target genes.
Regulates body plan development by turning genes on or off in specific cells
Colinearity:
Colinearity refers to the linear relationship between the order of Hox genes on a chromosome and their expression pattern along the body axis during development.
the difference between drosophila and vertibrates hox genes?
Vertebrate Hox genes are also
clustered,
but they have many clusters .
Name a hox gene variation in mice.
when a single
Hox gene is
mutated in mice
(Hoxc8) → An
extra thoracic
vertebra with ribs is
formed: vertebrae
being produced in
one position that
are appropriate to
another position
hiw many hox codes are there in humans
There are 39 Hox genes in the mouse and
the human, arranged in 4 clusters
What is meant by “elaboration “ of the body plan?
different combinations of master regulators (e.g. Hox) are active in different
parts of the embryo and at different times, leading to an elaboration of
positional information
features of hox genes
Features of Hox genes.
*They contain a sub-class of highly conserved homeobox, so they
encode transcription factors.
*They are involved in organising the body plan of an animal.
*They exist in clusters of similar genes in the genome (display
collinearity)
Genes of the Hox a
cluster are expressed in
overlapping domains along the P/D axis
Genes of the Hox d
cluster are expressed in
overlapping domains along an axis between
P/D and A/P
A mutation in
human HOXD13
causes
syndactyly
plants do not have hox they have
The A B and C genes encode transcription factors but are not Hox family genes –
they are MADS family transcription factors. So similar mechanism but
different family of conserved genes
Features of hox genes
They contain a sub-class of highly conserved homeobox→ they
encode transcription factors.
*They are involved in organising the body plan of an animal.
*They exist in clusters of similar genes in the genome
Evo-Devo
the study of developmental genes is impacting on our
understanding of evolution
Comparative studies
attempting to relate detailed molecular developmental observations in
different species with the organisation and structure of their body parts.
Relating molecules to structure
one thing that is crucial for development.
Developmental regulatory genes have been referred to as the genetic
“toolkit” for development the
toolkit that the embryo has to build its body plan
regulatory genes encode what?
Most encode either transcription factors or components of signalling
pathways.
types of gene regulatory differences (three)
- Change in number of genes (gene duplication).
- Change in the timing and spatial domain of expression (where
and when the regulators are switched on in the embryo) - Change in gene interaction
number of genes
- Invertebrates have
a single Hox cluster,
evoleved to Postulation: The cluster
duplicated c520 million years ago
(mya), allowing elaboration of
the vertebrate body plan.
then a third duplication for jaws etc
Change in spatial expression e.g. how snakes lost their limbs
expansion of Hox gene
expression domains along the
body axis giving “thoracic type”
(including HoxC-6) code at all
points
Also removes the “cue” to
make a forelimb (anterior
boundary of HoxC6
How could changes in spatial expression occur?
By changing the control regions (enhancers) driving expression of
the Hox genes in the embryo
makes spine gene
pitx1
Change in gene interactions
Pax6 is called the master regulator of eye development. (It is a homeobox gene
but not in the same class as Hox genes
Forced expression of mouse or zebrafish Pax6 in Drosophila also leads to
the formation of extra “ectopic” eyes.
but are they mouse or diphallia eyes?
Drosophila compound eyes
how do antherpods differ form grass hoppers
so the change has been in the activity of Ubx protein and its
interaction with target genes (loss of the hox gene)
ubx in fly’s suppresses the formation of extra limbs
Organogenesis is .
characterised by local interactions superimposed on
the information laid down with the basic body plan, to allow definition and
development of an organ
Neurulation
begins as cells from the dorsal
mesoderm form the notochord, a rod like structure
extending along the dorsal side of the embryo
Signaling molecules secreted by the notochord
cause the
ectoderm to thicken and form the neural plate
the neural folds fuse to enclose
the
neural tube
Neural crest cells are formed
by interaction between
surface and neural ectoderm
PNS
- -the sensory and motor system
-autonomic nervous system
Neurulation
signaling from the notochord – Shh is a key component. Shh
also influences the type of neuron that differentiates along the dorso-ventral
axis (working with other pathways)
Neurogenesis
is the formation of neurons : Remember that the choice of
forming a neuron or a glial cell is specified by the Delta-Notch signalling
pathway (lecture 3)
* Activation of Delta-Notch pathway = differentiation of neuron (lateral
inhibition of neighbouring cells
The somites
are mesoderm cells that form into blocks on
either side of the neural tube
somite’s eventually form
muscle blocks and vertibrates
