week 11 Flashcards
the anterior-posterior and dorso-ventral axes of Drosophila are specified in a _____
syncytium
transcription factors form _______
morphogen gradients
in many arthropods anterior-most segments form in a ___ but remaining segments form ____
syncytium
sequentially
in invertebrates AP and DV polarity is established in a _____
cellular environment
in vertebrates the determination of axes occurs partially in ____
a cellular environment
Asymmetry is already established in the oocyte (egg), what are the 2 poles?
animal pole - dark - contains nucleus
vegetal pole - white
The first cleavage in a vertebrate embryo is parrallel with ____
animal - vegetal axis
Animal-vegetal axis broadly defines the anterior / posterior axis. What showed this? and what is it?
fate map studies
animal is anterior and vegetal is posteror
The oocyte contains cytoplasmic determinants. What are these?
these are maternal mRNAs and proteins deposited in the egg before fertilisation
Why are cytoplasmic determinants important?
because it is enough material to see the embryo through the first cleavages until its own genes begin to be expressed
where do most of the developmentally important maternal products end up?
in the vegetal hemisphere
the yolk
What does removal of the vegetal half of the embryo affect?
AP and DV polarity
Experiment by Oppenheimer 1936 showed what?
fish
remove yolk early you get a radially symmetrical hyperblastula with no AP or DV pattern
remove yolk later it is a normal embryo
remove only the most vegetal yolk there is some AP and DV character
What does the experiment by Oppenheimer indicate?
that a diffusible molecule in the yolk is required to pattern the embryo.
Signal removed with yolk there is no positional information. Small amount of signal remains there is a limited gradient and hence limited positional information. Gradient established key positional information is already given and hence can remove yolk late on.
What was Spemanns experiment in 1903?
ligature along the plane of first division in normal embryos.
Released the ligation so that one nucleus could move over and you get 2 normal embryos. so every single nuclei is capable of generating a complete salamander
The outcome of the ligature experiment?
Depends on the cytoplasmic regions included in each blastomere
what is the gray crescent?
after fertilisation it appears
why is the gray crescent important?
the part of that receives the grey crescent after ligature develops normally
What is the function of the grey crescent??
contains the blastopore lip
and the formation of a secondary axis occurs after dorsal lip tissue transplantation
the transplant induces host tissue to form a secondary embryo
the dorsal blastopore lip is known as ____
Spemann organiser
how does the spemman organiser fomr?
the endoderm secretes the protein Vg1 and induces the mesoderm
the dorsal most endoderm cells induce the Spemann organizer - the Nieuwkoop center
the mesodermal cells receive different signals why?
because the mesodermal cells have different fates depending on their position along the DV axis
what is the Niewkoop center?
it is the dorsalmost vegetal cells in the endoderm which signal for the Spemann organiser to form
What gives the cells of the Nieuwkoop center their special properties?
beta catenin
beta catenin accumulates where in the embryo?
in the nuclei on the dorsal side after fertilisation
what does injecting beta catenin do?
in the ventral side where it is not normally causes a secondary axis to be produced
which mechanisms are involved in localising beta catenin?
Wnt signalling
Explain the Wnt signalling pathway
in the absence of Wnt signalling beta catenin is degraded
if Wnt binds, Disheveled is activated by Frizzled
Dsh together with GBP inhibits GSK3 from binding beta catenin
beta catenin is freed and can enter the nucleus
What happens to Disheveled and GBP during cortical rotation? How?
re located
Dsh and GBP associate with kinesin at the vegetal pole of the unfertilised egg. After fertilisation these proteins are translocated dorsally via subcortical microtubule tracts.
the dorsal side forms where?
always opposite the sperm entry site
How is there dorsal enrichment of Dsh and GBP and inhibition of GSK3?
Dsh and GBP are released from kinesin and accumulate in future dorsal third of 1-cell embryo.
Dsh and GBP block GSK3. Beta catenin is not degraded on the dorsal side
What is the consequence of beta catenin localisation to the nucleus on the dorsal side of the embryo?
results in the induction of the organiser
A beta catenin /____ complex binds to the siamois and twin promoters
Tcf3
Tcf3 on its own ____ the expression of siamois and twin genes
represses
Tcf3 binding to siamois and twin promoters causes the activation of _____ a gene that is expressed in the _________
goosecoid
spemann organiser
What is the role of the organiser protein Goosecoid? (5)
activates the migration properties of the dorsal blastopore lip
autonomously determines the dorsal mesodermal fates of those cells expressing it
enables cells to recruit neighbouring cells into the dorsal axis
activates genes for brain formation in neighbouring tissues (mesoderm and ectoderm)
represses ventralising genes e.g. Wnt8
The organiser establishes a nodal gradient, explain
higher levels of beta catenin activate more nodal related genes than do lower concentrations
Nodal abbreviation
Xnr
beta catenin and which 2 proteins activate Xnr?
VegT and Vg1
Xnr is highest where?
in the dorsal region
The Xnr gradient specifies what?
The various mesodermal fates
what are BMPs?
bone morphogenetic proteins
what do BMPs do?
they are antagonists of the organiser
BMP4 is highest where?
in the ventral region
BMP gradient ___ to ___
Nodal gradient __ to ___
ventral to dorsal (high low)
dorsal to ventral (high low)
BMP signalling leads to what?
the expression of different genes at different concentrations
what does BMP activity pattern?
the mesoderm along the DV axis
BMP is expressed where?
throughout the marginal zone except the organiser
What inhibits BMP in the organiser?
Noggin and Chordin diffuse out to inhibit BMP binding to its receptor
most dorsal (high Nodal, high Chordin) makes ___ and ___
organiser
notochord
most ventral (low Nodal, low Chordin) makes __ and ___
blood
gut endoderm
How do planarians reproduce?
sexually
asexually - regeneration
What do planarians do in unfavourable environments?
shrink
how many days does it take for a planarian to regenerate?
17 days
Planarians have a population of ____ that can regenerate all tissues
neoblasts - they are pluripotent
What are involved in the development and regeneration of planarians?
morphogen gradients
The rate of head generation decreases towards ___
posterior
what is the main morphogen in control of planarian regeneration?
Wnt/ beta catenin gradient
what happens when beta catenin1 RNAi is injected into planarians?
if the levels of beta catenine are lowered then odd phenotypes are produced. See graphs.
high conc. of beta catenin = tail
low conc. of beta catenin = head
What is the polar coordinate model?
cells receive positional information along the anterior-posterior and dorso-ventral axes
how do you test the polar co-ordinate model?
cut a piece out and rotate it
define morphogen
a protein that works concentration dependent, it has to be there at a specific concentration, it does different things at different concentrations
the dorsal lip of the blastopore is _______ in the amphibian embryo
site of initiation of gastrulation
which 2 morphogens determine the fate of dorsal ventral? How?
BMP and Nodal
BMP is higher in the ventral
Nodal is higher in the dorsal
when you take a small slice of a planarian you get 2 heads instead of 2 complete planarians, why?
the morphogen gradient. need a certain size of tissue to have a proper gradient, so if the slice is too thin then that wont work because there is not a proper gradient due to size. Length to width proportions
In a planarian when you remove the head and then cut out a slice you get a 2 headed planarian why?
the information is missing so these cells dont have neighbouring cells and each of them will form a head they dont recognise there are other cells so they dont have the proper positional information
define regeneration
the reactivation of development in post embryonic life to restore missing tissue
what are the 4 methods of regeneration?
stem cell mediated regeneration
epimorphosis
morphallaxis
compensatory rengeneration
define stem cell mediated regeneration
stem cell can proliferate creating more stem cells (self renewal) e.g. regrowth of hair, replacement of blood cells
define epimorphosis
dedifferentiation of adult structures to form an undifferentiated mass of cells that then become respecified
e.g. regeneration of salamander limbs
define morphallaxis
regeneration occurs through repatterning of existing tissue with little new growth
define compensatory regeneration
differentiated cells divide but maintain their differentiated functions
how does a stem cell give rise to more differentiated cellular progeny?
by asymmetric cell divisions
what are adult stem cells?
populations of embryonic stem cells which have been retained
Hematopoetic stem cells are derived from 3 embryonic regions?
Placenta
Yolk sac
Aorta gonad mesonephros
____ form the stem cell niche by which the hematopoetic stem cell population is maintained throughout life
osteoblasts
Osteoblasts signal to the hematopoetic stem cells by activating 3 signal transduction pathways?
notch
Wnt
Receptor tyrosine kinase
_____ cause HSCs to differentiate along a particular pathway
cytokines
what is the cancer stem cell hypothesis?
certain cancers arise from adult stem cells.
Human myeloid leukemias and astrocytic brain tumours have both been linked to stem cells that have managed to enter new domains and retain their stem cell properties
what can the cancer stem cell knowledge be used for?
to cure cancer - differentiation therapy
how would differentiation therapy work?
if the cancerous cell is actually a stem cell then it might be controlled by the right mixture of paracrine factors
what was found from using differentiation therapy?
it was found that acute promyelocytic leukemia which causes neutrophils to become malignant can be cured by retinoic acid as it is able to induce the differnetiation of the leukemic cells into normal neutrophils
Epimorphic generation of salamander limbs, explain
after amputation epidermal cells move to wound and form the apical ectodermal cap.
Cells dedifferentiate and form a proliferating mass of cells beneath the cap which is called blastema
How do cells regain the ability to divide?
thrombin
a protease
is released when the amputation occurs
what happens when Thrombin comes into contact with blood serum?
an unknown factor is produced which causes differentiated cells to enter the cell cycle and dedifferentiate
___ are required for blastema cells to proliferate
nerves
A minimum number of nerve fibres must be present for regeneration to take place. ______ is in addition required for angiogenesis and pattern formation
fibroblast growth factor 2
Fibroblast growth factor 10 is also required for the proliferation of the blastema cells. What happens when beads of this are placed in the stump of a frog hind limb?
the hind limb will regenerate
Morphallic regeneration occurs in ___
hydra
Hydra does not show senescence it is an immortal organism. Explain
the cells in the body column proliferate and are continuously displaced towards the hypostome and the foot. the cells differentiate in response to positional information in the body column as they migrate and finally slough off.
if hydra is cut into pieces each piece will regenerate a complete hydra. recent publications show that _____ are involved in head regeneration
stem cells
Explain compensatory regeneration in the mammalian liver
the injury of the liver is sensed through the blood stream by Kupfer and Stellate cells. These cells secrete tumour necrosis factor alpha and hepatocyte growth factor among others. this leads to the activation of cyclin D and E and cell division in the hepatocyte. the 5 types of liver cells do not dedifferentiate but produce more of themselves
The CNS of adult birds and mammals is not capable of regeneration but the CNS can functionally compensate for injury, this can occur in 3 ways?
restitution
substitution
compensation
what is restitution?
depends on the outgrowth of axons and dendrites from undamaged area and form new synapses within the damaged system to replace those lost to injury or cell death
what is substitution?
involves the adoption of function by a related system that imperfectly repalces the failed or damaged system. Substitution is never complete
what is compensation
involves the recovery of function due to adaptation of the undamaged components of the normal system so as to minimise the effects of a partial loss of function
Therapeutic approaches to CNS injury use all 3 mechanisms, what are 2 examples?
rehabilitative medicine works to enhance the efficacy of any residual function using physical training.
pharmacological agents can increase the conduction velocity of axons and therefore support the functional compensation
Peripheral axons of mature nerves are surrounded by three layers of connective tissue?
epineurium
perineurium
endoneurium
what is the epineurium?
consists of fibroblasts, fat cells, small blood vessels and collagenous matrix, surrounds the entire nerve
what is the perineurium?
consists of a collagen matrix and specialised cells that form the blood brain barrier, surrounds individual nerve fascicles
what is the endoneurium
consists of a collagen layer, surrounds individual axons
The ability of the peripheral nerves to regenerate depends on the _____
severity of the injury
what is the process of regeneration in the CNS?
injury
inflammation, activation of Schwann cells
Sprouting
Traverse region of injury, navigate back to original target
Reestablish synaptic contact, remyelination
Activated Schwann cells (PNS glia) produce a highly growth promoting environment. Explain
these factors stimulate the axons to regenerate. If the injury is mild and the Schwann cells can fill the gap between the distal and proximal nerve, there is a good chance that the nerve will regenerate
what is thought about the glial scar?
it may present a physical barrier that can not be penetrated by axons. the ECM of the glial scar expresses a large number of highly interactive molecules that are crosslinked
the high density of the glial cells in the glial scar might also act as physical barrier. Neurotrophic factors expressed in the scar not only promote neural growth but also glial growth, therefore they might contribute to the density.
What could prevent axon outgrowth
high concentrations of cell adhesion molecules might prevent axon outgrowth
cell migration can only occur over a narrow range of matrix concentrations where cells adhere strongly enough to generate traction, without being so strongly adhered that they are unable to change position. It can be assumed that the same is true for axons. They have to attach, detach and re-attach in order to move along a substrate. Therefore, high concentrations of cell adhesion molecules in the glial scar could prevent axon outgrowth
3 reasons why CNS regeneration fails?
substrate: wrong concentration of CAMs
Axons: wrong receptors
Physical barrier - glial scar
