cell movements of early zebrafish development and important signaling pathways jamie gunnarson and...
TRANSCRIPT
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.