M2. vertebrate development 2. axes and germ layers

Question 1

three main stages of vertebrate development

Answer 1

1. setting up the main body axes (A/P and D/V)
2. specification of the three germ layers (ectoderm, mesoderm and endoderm)
3. germ layer patterning (mesoderm and early nervous system)

Question 2

comparison of axes and fate maps

Answer 2

• xenopus egg has radial symmetry running from the animal to vegetal axis
• radial symmetry is broken by establishment of the dorsal side
• gastrulation results in an embryo with a distinct anterior (head) to posterior (tail) axis and dorsal to ventral axis

Question 3

animal-vegetal axis

Answer 3

• patterning of animal-vegetal axis under control of maternal factors deposited prior to fertilization
• mid-blastula transition

Question 4

mid-blastula transition

Answer 4

change from maternal to zygotic gene expression

Question 5

Vg-1 (vegetalizing fator -1 )

Answer 5

• transforming growth factor β (TGFβ) family member (ligand)
• localized to vegetal cortex
• protein translated upon fertilization
• early signal for mesoderm induction

Question 6

XWnt-11

Answer 6

• Wnt / Wingless secreted signalling protein
• mRNA localized to vegetal cytoplasm
• required for establishment of dorsal-ventral axis

Question 7

VegT

Answer 7

• T-box family of transcription factors
• mRNA localized to vegetal hemisphere
• required for endoderm and mesoderm specification

Question 8

establishment of dorsal side of embryo

Answer 8

sperm entry

cortical rotation

Question 9

sperm entry

Answer 9

• can occur anywhere in the animal hemisphere, breaks the radial symmetry of the egg
• future dorsal side opposite of site of sperm entry

Question 10

cortical rotation

Answer 10

• the cortex ( a gel like layer under the cell membrane) loosens from dense inner cytoplasm
• rotates counter clockwise 30 degrees in a direction away from the site of sperm entry (rotation due to actin polymerization stimulated by sperm entry)
• movement of proteins and mRNA from vegetal region to site opposite of sperm entry
• Wnt pathway components involved (green -XWnt-11 mRNA and Dishevelled protein; red -nuclear β-catenin)

Question 11

canonical Wn signalling pathway

Answer 11

Wnt: diffusible ligand

Frizzled: receptor

Dishevelled: adapter / co-receptor

APC / Axin / GSK-3β: β-catenin destruction complex

TCF /LEF1: transcription factor

β-catenin: transcriptional co-factor

Question 12

the spemann organizer is a head organizer

Answer 12

• Spemann organizer is located in the dorsal lip of the blastopore
• the organizer taken from an early gastrula staged embryo and grafted to the ventral region of another early gastrula staged embryo results in a second embryo with dorsal structures as well as a complete head, trunk and tail
• dorsal blastopore lip functions as a head organizer
• in the late gastrula embryo the dorsal blastopore lip can only induce tail formation
• the properties of the Spemann organizer change over time

Question 13

during gastrulation the antero-posterior axis is laid down perpendicular to the dorsal-ventral axis

Answer 13

• the first cells to enter the blastopore are dorsal mesoderm and endoderm and move towards the future head of the embryo
• the dorsal lip is not a uniform cell population, different cells occupy the lip at different times during gastrulation

Question 14

the dorsal and posterior region are regulated by Wnt signalling

Answer 14

-nuclear β-catenin has its highest concentration of the future dorsal region of the embryo

• dorsal / ventral axis patterning and anterior / posterior axis patterning are separated by time
• in the blastula high Wnt signalling establishes the dorsal region

-during gastrulation the longest duration of high Wnt signalling established the posterior region