L21: Early Amphibian Development

Question 1

Animals share similar body axis and major developmental steps

Answer 1

1. egg fertilization
2. ealry embryo - cleavage (most animals), syncytium (fly)
3. gastrulation
4. neurulation (vertebrates)
5. organogenesis

Question 2

why do we study amphibian (frog) developmet?

Answer 2

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)

Question 3

fate map of frog blastula shows germ layer specification prior to gastrulation

Answer 3

animal pole at top = has oocyte nucleus, will become ectoderm

vegetal pole at bottom = has yolk, very dense, sperm cannot penetrate -> will become endoderm

Question 4

tissue transplant in early frog embryo produces two dorsal axis

Answer 4

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)

Question 5

fertilization (sperm entry into egg) triggers formation of…

Answer 5

gray crescent

Question 6

the first cleavage division…

Answer 6

bisects (cuts in half) the gray crescent

Question 7

the blastomeres created by the gray crescent are

Answer 7

identical

Question 8

gray crescent provides information necessary for —- formation

Answer 8

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?

Question 9

early embryonic development in frog
(what two maternal components are in the vegetal pole?)

Answer 9

• 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

Question 10

sperm entry triggers…

Answer 10

cortical rotation -> causes Wnt11 mRNA to move towards dorsal side

Question 11

what allows Wnt11 to move during cortical rotation?

Answer 11

motor protein Kinesin moves Wnt11 along the microtubules from the vegetal pole to the future dorsal side

Question 12

egg and latter on blastula contain spatially organized gradients of maternal fate determining factors (Wnt11 & VegT)

Answer 12

Question 13

what is a notochord?

Answer 13

transient structure located directly underneath the
neural tube and is a major signaling center during vertebrate
embryonic development.

Question 14

role of transcription factor VegT

Answer 14

autonomously specifies the endoderm fate

depleting VegT causes loss of endoderm and loss of mesoderm

mesoderm fate may be induced

Question 15

how do vegetal (endoderm) cells induce mesoderm formation?

Answer 15

mesoderm is induced by signals produced by the vegetal cells

Question 16

identification of mesodermal-inducing molecules

Answer 16

Nodal, a TGFBeta (transforming growth factor) family (paracrine) signaling molecule has mesoderm inducing activity

Question 17

how do TGFBs function through enzyme coupled receptors

Answer 17

TGFB receptors are serine/threonine kinases, not RTKs (receptor tyrosine kinases)

Question 18

nodal location + function

Answer 18

*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

Question 19

what makes one side dorsal?

Answer 19

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

Question 20

Wnt signaling leads to activation of …

Answer 20

B-catenin and all of downstream target genes

APC is Adenomatous Polypoaia Coli

Question 21

Evidence/reasoning behind Wnt11 being a dorsal determinant

Answer 21

lack of Wnt11 -> lack of dorsal side

Question 22

What area is considered the organizer?

Answer 22

organizer is where Wnt11 signaling combines with Nodal signling to induce dorsal mesoderm

Question 23

how does the organizer work?

Answer 23

*BMP4 is another TGFB family signaling molecule

• In absence of BMP4 signaling, ectoderm has a neural fate
• BMP4 signals cause ectoderm to become epidermis

Question 24

organizer produces inhibitors of BMP4 activity

Answer 24

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

Question 25

how do Noggin/Chordin work together to inhibit BMP4

Answer 25

Noggin sequesters BMP4, preventing its activity

Question 26

overall diagram of how the organizer functions in a frog cell

Answer 26