L21: Early Amphibian Development Flashcards
Animals share similar body axis and major developmental steps
- egg fertilization
- ealry embryo - cleavage (most animals), syncytium (fly)
- gastrulation
- neurulation (vertebrates)
- organogenesis
why do we study amphibian (frog) developmet?
It is a vertebrate!
* Eggs and embryos are large and easy to
manipulate.
- Pioneering studies using Xenopus laevis led to
critical insight into mechanisms underlying the
establishment of the vertebrate body plan. - A great system for experimental embryology
(but not good for genetics)
fate map of frog blastula shows germ layer specification prior to gastrulation
animal pole at top = has oocyte nucleus, will become ectoderm
vegetal pole at bottom = has yolk, very dense, sperm cannot penetrate -> will become endoderm
tissue transplant in early frog embryo produces two dorsal axis
discovery led to understanding a region known as the organizer
the organizer:
* Can induce the formation of an entire secondary dorsal side!
* Is present in all vertebrates studied.
* Has been named the Spemann-Mangold organizer (SMO)
fertilization (sperm entry into egg) triggers formation of…
gray crescent
the first cleavage division…
bisects (cuts in half) the gray crescent
the blastomeres created by the gray crescent are
identical
gray crescent provides information necessary for —- formation
DORSAL
leads to questions…
* What are the molecules in the gray
crescent?
- How do they pattern the frog embryo
along the dorsal-ventral axis? - What is the relationship between the
gray crescent and the organizer?
early embryonic development in frog
(what two maternal components are in the vegetal pole?)
- Frog eggs are asymmetric along the animal-vegetal axis
- Some maternal components are asymmetrically localized
- VegT mRNA in the cytoplasm of the vegetal pole
- Wnt11 mRNA in the cortical cytoplasm of the vegetal pole
sperm entry triggers…
cortical rotation -> causes Wnt11 mRNA to move towards dorsal side
what allows Wnt11 to move during cortical rotation?
motor protein Kinesin moves Wnt11 along the microtubules from the vegetal pole to the future dorsal side
egg and latter on blastula contain spatially organized gradients of maternal fate determining factors (Wnt11 & VegT)
what is a notochord?
transient structure located directly underneath the
neural tube and is a major signaling center during vertebrate
embryonic development.
role of transcription factor VegT
autonomously specifies the endoderm fate
depleting VegT causes loss of endoderm and loss of mesoderm
mesoderm fate may be induced
how do vegetal (endoderm) cells induce mesoderm formation?
mesoderm is induced by signals produced by the vegetal cells
identification of mesodermal-inducing molecules
Nodal, a TGFBeta (transforming growth factor) family (paracrine) signaling molecule has mesoderm inducing activity
how do TGFBs function through enzyme coupled receptors
TGFB receptors are serine/threonine kinases, not RTKs (receptor tyrosine kinases)
nodal location + function
*Expressed in the vegetal cells, and
expression is activated by VegT
- Can diffuse away and activate a
signal transduction cascade in cells
above the vegetal cells to induce
mesodermal formation
what makes one side dorsal?
Wnt11 is a dorsal determinant
- A signaling molecule of the Wnt signal transduction pathway
- Moved to the future dorsal side (the opposite side of sperm entry) during cortical rotation
- Induces expression of target genes essential for dorsal fate
Wnt signaling leads to activation of …
B-catenin and all of downstream target genes
APC is Adenomatous Polypoaia Coli
Evidence/reasoning behind Wnt11 being a dorsal determinant
lack of Wnt11 -> lack of dorsal side
What area is considered the organizer?
organizer is where Wnt11 signaling combines with Nodal signling to induce dorsal mesoderm
how does the organizer work?
*BMP4 is another TGFB family signaling molecule
- In absence of BMP4 signaling, ectoderm has a neural fate
- BMP4 signals cause ectoderm to become epidermis
organizer produces inhibitors of BMP4 activity
BMP4 is made uniformly throughout the late
Xenopus blastula
- The organizer secretes BMP4 inhibitors Chordin
and Noggin forming a gradient from dorsal (high)
to ventral (low) - Chordin and Noggin sequester BMP4 preventing its
activity - This creates a gradient of BMP4 activity from
ventral (high) to dorsal (low) - BMP4 ventralizes the embryo
how do Noggin/Chordin work together to inhibit BMP4
Noggin sequesters BMP4, preventing its activity
overall diagram of how the organizer functions in a frog cell
