lec 21 (19 and 20 not for exam)- establishing germ layers Flashcards
what are the three germ layers of an embryo?
endoderm- gut, liver, lungs
mesoderm- skeleton, muscle, kidney, heart, blood
ectoderm-epidermis of skin, nervous system
what do the lower third cells and adjacent cells give rise to in the xenopus blastula? what do the cells in the other pole give rise to?
-lower region gives rise to endoderm and the adjacent region gives rise to mesoderm
-cell in the other pole give rise to ectoderm
do the presumptive endoderm and mesoderm cells move inside the blastula during gastrulation? what do they go through to travel into the middle?
yes, they move through the dorsal lip of the blastopore while the ectoderm covers the outside
what do cells from the animal pole form in xenopus?
ectodermal cells
what do vegetal pole cells form in xenopus?
endodermal cells
what causes the ectoderm and endoderm to become specified before fertilization?
maternal factors
are signals from other regions of the embryo needed for specification of ectoderm and endoderm?
no
what forms mesoderm cells in the blastula?
animal cap cells that interact with vegetal cells
how is the animal half of the germ layer induced?
-the zygotic transcription factor Foxl1e is expressed, it specifies the development of animal cells into ectoderm
-ectodermin, which is a determinant, is also found in the animal cap and it is a ubiquitin ligase that stops nodal (coming from vegetal pole) from being expressed in the animal pole, thereby limiting mesoderm formation to the equatorial region
how is the vegetal half of the germ layer induced?
-the transcription factor VegT specifies endoderm and results in the production of nodal proteins
-mesoderm is induced in a band around the middle of the blastula in response to nodal signals from the endoderm, nodal also prevents expression of Foxl1e in the vegetal region
how does the nodal pathway work and how does ectodermin block it?
-nodal is a ligand that binds to serine/threonine kinase receptors, the receptors form two heterodimer subunits known as type 1 and type 2, type 2 becomes activated upon ligand binding to receptor, causing it to phosphorylated type 1, type 1 then phosphorylates signalling proteins called Smads
-when ectodermin is synthesized in cells, it targets Smad 4 and degrades it, preventing the signalling pathway from functioning
what is required for animal cap cells to form mesoderm after being removed or isolated?
that the animal cap cells received an inducing signal which activates the mesodermal specific genes for at least 2 hours, anytime between 4-11 hours after fertilization
how does the embryo always activate the mesoderm specific genes at the same time, no matter when the induced signal is received?
because the embryo tracks time
how does the embryo track time according to the hourglass egg-timer model?
a decrease in a certain molecules concentration gets to a certain threshold that triggers the developmental event
what certain molecules and its concentration level acts as the trigger for the event in the egg timer model?
as DNA increases in the embryo, repressor stays constant, causing a decreasing ratio of repressor to DNA, eventually the ratio of repressor to DNA becomes so low that the repressor cant bind to all the available sites on the DNA and the repression is lifted, triggering transcription
when extra amounts of DNA are added into the embryo, what happens to the embryo according to the egg-timer model?
transcription occurs earlier
what regions does the mesoderm pattern into?
as the embryo develops, the mesoderm becomes patterned into a number of regions along the dorsal-ventral axis, these regions give rise to the notochord, somites, kidneys, and blood
combining animal cap cells with dorsal-ventral portions induces what?
notochord and muscle developemnt
combining animal cap cells with ventral-vegetal tissues induces what?
blood and associated tissues
is the mesoderm patterned by different signals emanating from different regions of the blastula?
yes
how is the band of prospective mesoderm formed?
-vegetal region produces signals that induce the mesoderm and establish the Spemann-Mangold Organizer on the dorsal side
-signals from the ventral side pattern the ventral mesoderm
-the organizer emits a set of signals to counteract the ventral signals, limiting its range and enables thee adjacent mesoderm to turn into somites
-this causes the formation of a band of prospective mesoderm
what criteria must a signalling molecules that is an inducer of mesoderm meet?
- the signal and its receptor must both be present in the right concentration, place, and time in the embryo
- the appropriate cells must be able to respond to the signal
- blocking the response must prevent induction
what experiments help make sure candidate signalling molecules are in fact able to induce mesoderm?
- Animal cap cells are cultured in isolation and candidate protein signalling molecules are applied
- inject mRNA encoding the suspected inducers into animal cells of the early blastula then culture these cells in isolation to see if the form mesodermal derivatives
explain how TGF-B signals, VegT and B catenin are all involved in mesoderm induction?
-cortical rotation causes dorsal determinants to be at the dorsal side, this includes Wnt signals
-VegT is not moved and stays present throughout the vegetal region
-B-catenin is present in the nuclei of cells on the dorsal side
-VegT activates transcription of nodal related genes (Xnr proteins) in the vegetal cells of the blastula, these nodal signals induce mesoderm
-B-catenin on the dorsal side also stimulates nodal gene expression, causing high nodal concentration in the region where Spemann organizer forms and decreasing levels towards the ventral side, at high concentrations the organizer is specified
