3.2 Frog Development Flashcards
Experiment: Hilde Mangold (took blastopore lip then put onto new species in ventral side(
-pigments are dif colours in dif species
-took from newt and onto future ventral region (if left alone would’ve been epidermis)
-capable of altering cells induced 2nd gastrulation site
-2nd archenteron and notochord
-now has endo and mesoderm fate
-results in conjoined twins
-Upon transplantation, the dorsal blastopore lip retains its
activity, and induces the surrounding tissues to form a second axis
-Mangold and Spemann called this “primary embryonic
induction”
-only area determined autonomously (determines date of other cells it comes into contact with)
dorsal blastopore lip called
Spemann organizer
Mangold/Speman Two effects were observed of DBLip on embryo:
[1] Induction of neural tissues from ectoderm that would normally have become epidermis.
[2] Dorsalization of ventral mesoderm leading to the formation of a second embryonic axis.
organizer that instructs both neuralization and dorsalization, and showed that cells can adapt their developmental fate according to their position when instructed by other cells
3 Morphological reference points for early
induction events
- Cortical rotation establishes the “Nieuwkoop centre” which appears essential to establish the dorsal-ventral axis of the embryo
- The Nieuwkoop centre seems to induce the formation of the blastopore immediately above it.
- The dorsal lip of the blastopore is an autonomous region which organizes the anterior-posterior axis of the embryo, as shown by classic transplantation experiments (Mangoldand Spemann).
What is β-catenin? Why is it necessary
- ß-Cat is orignially all over
- a transcription factor that accumulates in the dorsal region of the egg during cortical rotation
- β -catenin is synthesized in the egg by maternal mRNA and is uniformly distributed.
- necessary for forming the dorsal axis
- depletion leads to a lack of dorsal structures,
- injection into the ventral side of embryo leads to a secondary embryonic axis
GSK-3
- GSK-3 degrades β-catenin
- inhibits dorsal region formation
- glycogen synthase kinase-3 (GSK-3).
- made inactive by DSH WntII and GBP
Dishevelled
- Dsh
- in the article V.hem
- in future doesal side where N.center will be
Cortical rotation is crucial in the dorsal
enrichment of β-catenin
- During cortical rotation Dishevelled (Dsh) moves from the vegetal pole with a microtubule “monorail” within membrane vesicles to the dorsal region of the egg.
- DSH is released from its vesicles and (in concert with some other proteins) inactivates GSK-3
- β -catenin is degraded everywhere in the embryo except in the dorsal region because GSK-3 is inactivated by DSH (and some other proteins)
- Thus, in the dorsal region, β -catenin activates dorsal genes.
Wnt Signaling Pathway
- Wnt is a ligand
- Wnt –> Frizzled/LRP5/6 –> Disheveled –x GSK3 –x ß-cat –> Transcription
- Wnt binds and activated Dsh which binds to a protein complex and inactivates GSK-3 which no longer degrades ß-cat and then transcription can take place
The role of β -catenin in forma1on of Spemann organizer
- B-cat accumulated in dorsal side of embryo
- B-cat enters nucleus and binds with Tcf3 to form active transcription factor (TF)
- transcribes Siamois and Twin gene encoding proteins
- interact with Smad2 in the organizer and TGF-ß
- Smad2 TF that is activated by nodal, Vg1 in organizer region
- these 3 transcription factors activate organizer genes like noggin
fate of the other cells of the blastula
- animal cap cell
- vegetal cells
- mesodermal tisue
- Cells of the animal cap become ectoderm.
- Vegetal cells become endoderm.
- Mesodermal tissues (e.g. blood, mesenchyme, muscle, bone, notochord) develop in a frog embryo from an equatorial band of ectodermal cells which are induced by the underlying vegetal cells to become mesoderm, i.e. have their fate changed from ectodermal to mesodermal
-if you put animal with vegetal cap you’d have no ectoderm formation as it’d go straight to mesoderm induction
This induction event has some interes1ng features
- The signals are diffusible: they can act even through filter paper “sandwiches” that prevent cell-cell contact are present.
- Timing is crucial: cells are “competent to respond” for a narrow window of time only; animal cap loses the ability to respond about 11 hours aQer fertiliza:on (well before gastrulation).
- “specification” and “fate” maps of the frog blastula can be constructed as a result of these observations.
- The induction signals seem to vary depending on the source of the vegetal cells—it can be shown that there are “dorsal” signals, and “ventral” signals.
Specifica1on and fate maps
•specification maps show what explanted tissues become in cell culture (from relatively early induction events)
• fate maps show what the tissues actually
become in the embryo (from later interactions and induction events)
dorsal and central mesoderm vegetal cells become
- ventral mesoderm becomes blood and mesenchyme
- dorsal mesoderm becomes notochord and somites
regional induction of mesoderm in frogs
- on dorsal side, the dorsal inducing mesoderm signals released by vegetal cell of the N.center and induces the formation of the speeman center
- on the ventral side signal is released for marginal cells to become mesoderm
