Cell Movements of Early
Zebrafish Development and
Important Signaling
Pathways
Jamie Gunnarson and Jena Arne
l images are taken from S. Gilbert, “Developmental Biology, 8th edition, Sinauer and Associates
Embryogenesis
Early development of
zebrafish from the onecell zygote to
vertebrate embryo
Fertilization
200 μm
Poleo G A et al. Biol Reprod 2001;65:961-966
©2001 by Society for the Study of Reproduction
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.
Cleavage Stage
The 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 Stage
Mid-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 Stage
continued
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 Stage
Fate map
Text
The mixing of the
cells finally allows
for fate mapping
during lateblastula.
Blastula Stage
Late Blastula
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.
Ventral
Nodal
Dorsal
Vegetal Pole
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.
Gastrulation
Embryonic 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.
Gastrulation
Mesendoderm 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).
Gastrulation
D/V Axis Formation by BMP Inhibitors
Animal Pole
Ventral
BMP
Vegetal Pole
Dorsal
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.
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 dorsalventral 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 dorsalventral 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 anteriorposterior axis.