Cell Movements of Early Zebrafish Development and Important Signaling

Pathways

Jamie Gunnarson and Jena Arne

All images are taken from S. Gilbert, “Developmental Biology, 8th edition, Sinauer and Associates

Embryogenesis

Early development of

zebrafish from the one-cell zygote to

vertebrate embryo

Fertilization

Activated zebrafish embryo. Arrow indicates point of

sperm entry. Factors, such as beta-catenin

mRNA needed for dorsal-ventral axis

formation, are loaded into the egg.

200 μm

Poleo G A et al. Biol Reprod 2001;65:961-966

©2001 by Society for the Study of Reproduction

Cleavage StageThe fertilized egg

undergoes synchronous cleavage. This egg is telolecithal, meaning only a small region is free of yolk. This yolk-free region is termed

the blastodisc and divides by meroblastic,

discoidal cleavage.

Blastula StageMid-Blastula Transition

Blastula stages begin at the eighth division (128 cell

stage). The pulsing characteristic of synchronous

division stops and asynchronous division begins.

At this point transcription of the zygotic genome begins. Between the ninth and tenth division, the Yolk Syncytial

Layer (YSL) is formed by the fusion of cells with yolk.

Blastula Stagecontinued

The blastomeres begin to mix

randomly and mixing continues

through late gastrulation.

Dorsal YSL begins to express the

nodal signal Squint when nuclear beta-

catenin accumulates.

Blastula StageFate map

TextThe mixing of

the cells finally allows for fate

mapping during late-blastula.

Blastula StageLate Blastula

Ventral Dorsal

Vegetal Pole

Nodal

Animal Pole

All embryonic cells undergo epiboly driven by the expansion of the YSL. The outermost layer of blastomeres (the enveloping layer) surrounds the

yolk cell.

Marginal blastomeres express Znr proteins (squint and cyclops) creating a nodal

gradient causing different cell fate specification. Cells with very high nodal become prechordal plate, while cells with

decreased nodal become notochord.

GastrulationEmbryonic Shield Formation

Cells undergo dorsal convergence to form the embryonic shield. This structure

organizes gastrulation. The embryonic shield secretes nodal which sets up the dorsal-ventral gradient, in which high

nodal produces dorsal structures and low nodal produces ventral structures.

GastrulationMesendoderm Formation

Cells of the embryonic margin begin to involute, giving rise to

the mesendoderm. These specified cells involute to form

the inner cell layer (hypoblast).

Cells of the embryonic margin begin to involute, giving rise to

the mesendoderm. These specified cells involute to form

the inner cell layer (hypoblast).

GastrulationD/V Axis Formation by BMP Inhibitors

BMP is high on the ventral side and turned off on the dorsal side by the BMP inhibitors Noggin, Chordin, and Follistatin. Inhibitors

are expressed from the embryonic shield, and cell fates are specified

along the dorsal-ventral axis.

VentralDorsal

Vegetal Pole

Animal Pole

BMP

Gastrulation Anterior-Posterior Axis Formation

The anterior-posterior axis is formed by involution. The first cells to involute make anterior structures and the last

cells to involute make posterior structures.

Gastrulation Convergent Extension

Mesendoderm and ectoderm converge to

the dorsal side and extend along the

anterior-posterior axis.

Convergent extension, epiboly

and involution occur at the same time.

Courtesy of Dr. Jennifer Liang Spring 2010

Quiz

Question 1

What is the importance of loading beta-catenin mRNA into the Zebrafish embryo?

Answer

Beta-catenin mRNA is necessary for dorsal-ventral axis formation.

Question 2

What type of cleavage does the Zebrafish embryo undergo?

Answer

The zebrafish undergoes synchronous, meroblastic, discoidal cleavage.

Question 3

What do YSL and MBT stand for and how are they related?

Answer

YSL stands for Yolk Syncytial Layer and MBT stands for Mid-Blastula Transition. The YSL forms during MBT.

Question 4

Why is the mixing of the blastomeres important?

Answer

Without the mixing of blastomeres, fate mapping cannot take place.

Question 5

What is epiboly driven by?

Answer

Epiboly is driven by the expansion of the YSL and causes the enveloping layer to surround the yolk.

Question 6

Which structure secretes nodal and how does this signal affect dorsal-ventral axis formation?

Answer

The embyronic shield secretes nodal. Regions with high nodal produce dorsal structures and regions with low nodal produce ventral structures.

Question 7

Which cells involute to form the inner cell layer (hypoblast)?

Answer

Mesoderm and endoderm, termed mesendoderm cells, involute to form the inner cell layer and are derived from the embryonic margin.

Question 8

Explain the role of BMP inhibitors in dorsal-ventral axis formation.

Answer

The role of BMP inhibitors in dorsal-ventral axis formation is to turn off BMP signaling. This occurs on the dorsal side, thus causing BMP to accumulate on the ventral side, forming ventral structures.

Question 9

Which cell movement is responsible for anterior-posterior axis formation? Explain how this occurs.

Answer

Involution is responsible for anterior-posterior axis formation. The first cells to involute give rise to anterior structures and the last cells to involute give rise to posterior structures.

Question 10

What occurs during convergent extension?

Answer

During convergent extension all germ layers converge to the dorsal side of the developing embryo and extend along the anterior-posterior axis.