Frogs

Question 1

Why frogs?

Answer 1

• Reproduce quickly
• Vertebrate
  -Large eggs
• External Development
• Lots of eggs

Question 2

Why not frogs?

Answer 2

• 4 sets of chromosomes
• Not transparent
• Slow maturation

Question 3

Xenopus laevis

Answer 3

Most common frog species studied

Question 4

Sperm entry point (SEP)

Answer 4

The sperm enters anywhere in the animal pole. This determines the ventral side. The point opposite of SEP is the dorsal point. Grey crescent forms directly opposite SEP

Question 5

Cortical rotation and significance of Grey Cresenct

Answer 5

When sperm enters, the centrioles reorganize microtubules in the embryo. The microtubules facilitate the cortical rotation. The cortical rotation is the rotation of the cortical cytoplasm rotates relative to the inner cytoplasm. This shifts the pigmented animal pole and part of the transparent vegetal pole is exposed. This section is called the grey crescent.

Question 6

Grey Crescent

Answer 6

The grey crescent is the site where gastrulation initiates and future dorsal side

Question 7

Role of microtubules in axis formation

Answer 7

Organized by sperm centrioles. The cortical rotation is dependent on microtubule arrays

Question 8

Effects of yolk on cleavage

Answer 8

Mesolecithal, uneven yolk distribution leads to unequal holoblastic cleavage. Yolk acts as a physical barrier to the cleavage furrow, which slows down the cleavage. The animal hemisphere become densely packed with numerous small cells, while the vegetal hemisphere consist of fewer, larger macromeres. This makes a blastula with the blastocoel towards the animal pole.

Question 9

Epiboly

Answer 9

The thinning and spreading of the animal cap cells over the vetal hemisphere, powered by proliferation

Question 10

Invagination

Answer 10

The inward buckling of the outer epithelial layer at the region where the animal and vegetal hemispheres meet on the dorsal side. This becomes the dorsal blastopore lip

Question 11

Involution

Answer 11

The leading edge of invaginating cells crawls up onto the blastocoel roof. This brings the mesoderm and endoderm into the embryo

Question 12

Convergent extension

Answer 12

Cell intercalation elongates the mesoder, driving the extension of the embryo along the anterior-posterior axis

Question 13

Nieuwkoop center

Answer 13

When the cortical rotation translocates maternal mRNAs and proteins, including Wnt, GSK-3, Disheveled, and b-cat, there is a group of dorsal-most vegetal cells.
This center activates Xnrs and with VegT, creates a dorsal-to-ventral gradient of Xnrs across the endoderm

Question 14

Dorsal lip of blastopore (DLP)

Answer 14

Organizer - responsible for orchestrating the development of the embryo’s body plan
Nieuwkoop center induces the formation of the organizer
Organizer induces different structures along the a/p axis
Cells that involute first have anterior structures while cells that involute later have posterior structures

Question 15

Morpholino oligonucleotides (MOs)
Sy

Answer 15

Synthetic molecules used to inhibit gene expression, since traditional genetic manipulation techiques are limited. MOs provide and alternative LOF approach. They bind specifically to complementary sequences in target mRNAs and lock translation initiation.

Question 16

VegT

Answer 16

Depleting VegT using MOs results inembryos lacking endoderm and mesoderm. This shows VegT’s role as a transcription factor in activating genes for endoderm and mesoderm formation

Question 17

When does MBT occur

Answer 17

Occurs late in the twelfth cell cycle. Once a certain threshold of chromatin in the cytoplasm is reached, the MBT is triggered

Question 18

Spemann-Mangold Experiment & major conclusions

Answer 18

This experiment used embryos from two different species of newts. In the experiment, the DLP from one species was put into the ventral region of the other. The transplanted DLP tissue invaginated and diffferentiated into dorsal mesodermal structures, demonstrating self-determining nature. This lead to a secondary embryo. This coined the term organizer due to the DLPs ability to induce and organize the embryo.