Early Development in Fish and Birds
Cleavage
in Fish Eggs
•
Cleavage occurs in the
blastodisc, yolk-free cytoplasm at
the animal cap of the egg
•
Meroblastic cleavage
occurs only in the cytoplasm of the blastodisc - called discoidal
•
Calcium waves started at
fertilization stimulate contraction of actin - squeezes non-yolky cytoplasm
into the animal pole of the egg
•
Divisions are rapid -
about every 15 minutes
Discoidal
Cleavage in a Zebrafish
Cleavage
in Fish Eggs
•
First 12 divisions occur
synchronously - forms mound of cells atop a single large yolk cell
•
These cells are called
the blastoderm
•
At the 10th division -
mid-blastula transition - zygotic gene transcription begins
•
Now cells divisions slow
& cell movement becomes evident
Cells
Populations of the Fish Blastula
•
Yolk syncytial layer
(YSL) - formed at ninth or tenth cell cycle
–
This is when cells a the
vegetal edge of the blastoderm fuse with the underlying yolk cell
–
Get ring of nuclei in
the layer of yolk cell cytoplasm just beneath the blastoderm
–
Next some of these
nuclei move under the blastoderm and form the internal YSL as the blastoderm
expands
–
Others stay ahead of the
expanding blastoderm margin to from external YSL
–
YSL important in
determining some of the cell movements of gastrulation
Cells
Populations of the Fish Blastula
•
Enveloping layer (EVL) -
2nd cell population
–
Made of the most
superficial cells of the blastoderm
•
Will become periderm - an
extraembryonic protective covering that is sloughed off during later
development
•
Deep Cells - between the
EVL and YSL
–
Cells that give rise to
the embryo proper
•
Fate of blastoderm cells
not fixed until shortly before gastrulation begins
Fish
Blastula
Fate Map
of Deep Cells of the Fish Blastula
Gastrulation
in Fish Embryos
•
Epiboly of the
blastoderm cells over the yolk is the first gastrulation movements
–
Initially deep blastoderm cells move outwardly to
intercalate with more superficial cells
Gastrulation
in Fish Embryos
•
Next these cells move
over the surface of the yolk to envelope it completely
–
This movement is
provided by the autonomously expanding YSL “within” the animal pole
–
The EVL is dragged along
with it
•
Deep cell of the
blastoderm - fill in the space between the YSL and the EVL as epiboly continues
•
Expansion of the YSL is
dependent upon a network of microtubules in the YSL
Formation
of Germ Layers
•
After the blastoderm
cells have covered the yolk - the epibolizing cells thicken
–
Germ ring - superficial
layer - epiblast & inner layer called the hypobast
•
How the hypoblast is
formed is not known
•
Next cells of the
epiblast and hypoblast intercalate on the future dorsal side of the embryo -
forming the embryonic shield
–
This is equivalent to
the dorsal blastopore lip of amphibians
•
It can organize a
secondary embryonic axis when transplanted into a host embryo
Formation
of Hypoblast
Formation
of Germ Layers
• As cells undergo epiboly around the yolk - they are
also involuting at the margins and they are also converging anteriorly & dorsally
toward the embryonic shield
• Hypoblast cells of the embryonic shield converge and
extend anteriorly - eventually narrowing along the dorsal midline of the
hypoblast
Formation
of Germ Layers
•
This forms the
chordamesoderm (precursor of the notochord)
•
Cells adjacent to the
chordamesoderm - paraxial mesoderm cells (precursors of the mesodermal somites)
•
Simultaneously -
convergence and extension of the epiblast brings the presumptive neural cells
to the dorsal midline - form a neural keel
End of
Gastrulation
Axis
formation in Fish Embryos
• The embryonic shield is critical in establishing the
dorsal-ventral axis in fish
• It can convert lateral ad ventral mesoderm into dorsal
mesoderm
• It can also cause the ectoderm to become neural rather
than epidermal
– Shown from transplantation experiments
Axis
formation in Fish Embryos
• Two signaling centers supplying anterior-posterior
information in fish
• One located
at the border between neural and surface ectoderm
• Second located in the lateral mesoderm
Cleavage
in Bird Eggs
•
Egg is telolecithal like
fish
•
See discoidal
meroblastic cleavage
•
Cleavage only in the
blastodisc - small disc of cytoplasm 2-3 mm in diameter at the animal pole
•
First cleavage furrow
appears centrally in the blastodisc
•
Other cleavages follow
and create a single layered blastoderm
•
They do not extend into
the yolky cytoplasm
Discoidal
Cleavage in the Chick
Cleavage
in Bird Eggs
•
Later equatorial and
vertical cleavages divide the blastoderm into a tissue five to six cell layers
thick
•
Cells linked by tight
junctions
•
Between the blastoderm
and the yolk is space - subgerminal cavity
–
This is created when the
blastoderm cells absorb fluid from the albumin & secrete it between
themselves and the yolk
Cleavage
in Bird Eggs
•
At this stage - deep
cells of the blastoderm are shed & die
–
This leaves a one cell
thick area pellucida
•
This forms most of the
embryo
•
The peripheral ring of
blastoderm cells that have not shed there deep cells- the area opaca
•
Between the two is a
thin layer of cells - marginal zone or belt - some of these become very important in determining cell fate
in early chick development
Formation
of the Two Layer Blastoderm
Blastula
Stage in the Chick
•
When an chicken egg is
laid - blastoderm of 20,000 cells
•
Some area pellucida
cells have delaminated & migrated individually into the subgerminal
cavity to form - polyinvagination islands (primary hypoblast)
•
Many islands or cells
clusters (5-20 cells)
•
Next a sheet of cells
from the posterior margin called Koller’s sickle (a local
thickening) - migrates to join the above islands
–
This is secondary
hypoblast
Formation
of the Two Layer Blastoderm
The
Primitive Streak
•
Major structural
characteristic of avian, reptilian and mammalian gastrulation is the primitive
streak
•
First visible as
thickening of the epiblast at the posterior region of the embryo just
anteriorly to Koller’s sickle
• Caused by an ingression of endodermal precursors
from the epiblast into the blastocoel
• Also a migration of cells from the lateral region of
the posterior epiblast toward the center
•
As the above cells enter
the primitive streak - it elongates toward the future head region
Cell
Movements - Primitive Streak
The
Primitive Streak
•
At the same time -
secondary hypoblast continue to migrate anteriorly from the posterior margin of
the blastoderm
•
Eventually the primitive
streak extends 60-75% of the length of the area pelucida
•
The primitive streak
also defines the axes of the embryo
•
It extends from
posterior to anterior
•
Migrating cells enter through the dorsal side &
move to its ventral side
–
Also it separates the
left portion of the embryo from
the right
The
Primitive Streak
•
Those elements close to
the streak will be medial (central) structures
•
Those further from the
streak - will be lateral structures
•
As cells converge into the
streak - depression forms
–
Called primitive
groove - serves as an opening
through which migrating cells enter the blastocoel
–
Analogous to the
amphibian blastopore
The
Primitive Streak
•
At the anterior end -
local thickening - primitive knot or Hensen’s node
•
Center of the node -
funnel shaped depression - primitive pit
–
Passage way for cells to
enter the blastocoel
•
As soon as the primitive
streak is formed - epiblast cells begin to migrate through it into the
blastocoel
•
Thus the primitive
streak has a continually changing
cell population
Notochord
& Mesoderm Somite - Formation
Migration
Through the Primitive Streak
• First cells through Hensen’s node - become
pharyngeal endoderm of the foregut
• These cells now migrate anteriorly and
eventually displace the hypoblast cells
– This causes these cells to be confined to a region in
the anterior portion of the area pellucida - germinal crescent - the precursors of the germ cells
Migration
Through the Primitive Streak
•
Next cells through -
move anteriorly - but do not move as far ventrally as the presumptive foregut
endodermal cells
•
Remain between the
endoderm & the epiblast to form the head mesenchyme and the prechordal
plate mesoderm
–
Prechordal plate
induces formation of the forebrain
•
These early-ingressing
cells all move anteriorly -
pushing up the anterior midline region of the epiblast to form the head process
–
Thus the head
forms rostrally to Hensen’s node
Migration
of Endodermal & Mesodermal Cells
Migration
Through the Primitive Streak
• Next cells migrating through Hensen’s node
become chrodamesoderm (notochord) cells
– Induces formation of midbrain, hindbrain and spinal
cord
• These extend up to the presumptive midbrain - where
they meet the prechordal plate
Migration
Through the Primitive Streak
• Deep moving cells give rise to all the endodermal
organs of the embryo
– Also the extraembryonic membranes (hypoblast forms the
rest)
• Second migrating layer - spreads between the
endoderm and the epiblast - forms a loose layer of cells
– Generates mesodermal portions of the embryo and
extraembryonic membranes
Regression
of the Primitive Streak
• Even while mesodermal ingression continues - primitive
streak begins to regress
– Hensen’s node moves from near the center of the
area pellucida to a more posterior position
– Leaves behind the dorsal axis of the embryo and the
notochord
Regression
of the Primitive Streak
– Finally Hensen’s node regresses to its most
posterior position, forming the anal region
• The epiblast is now all ectoderm and the
presumptive mesoderm and endoderm have entered the embryo
– Anterior portion of the embryo is now more advanced
than the posterior portion - lasts for several days
Gastrulation
Regression
of the Primitive Streak
Epiboly
of the Ectoderm
•
Even while presumptive
mesoderm and endoderm are moving inward - the ectodermal precursors are
proliferating
•
The ectodermal cells
migrate to surround the yoke by epiboly
–
Similar to amphibians
–
Takes about four days to
complete
•
Epiboly requires
continuous production of new cells & migration of the presumptive
ectodermal cells along the underside of the vitelline membrane
pH in and
Na+ in Dorsal-Ventral Axis Formation
•
This axis is established
when the the cleaving cells of the blastoderm establish a barrier between the
basic (pH 9.5) albumin above the blastodisc and the acidic (pH 6.5) subgerminal
space below
•
Water and Na+
are transported from the albumin through the cells and into the subgerminal
space - get25mV difference in potential across the epiblast cell layer (+ at
the ventral side of the cells)
pH in and
Na+ in Dorsal-Ventral Axis Formation
• This establishes two sides of the epiblast - a side
facing the negative and basic albumin ( becomes dorsal)
• The other side faces the positive and acidic
subgerminal space (becomes ventral)
• This axis can be reversed by inverting the pH and
potential across the layer of cells
Gravity
in Anterior-Posterior Axis Formation
•
Conversion of a radially
symmetrical blastodisc into a bilaterally symmetrical structure is determined
by gravity
•
AS the ovum passes down
the reproductive tract it is rotated for about 20 hours in the shell gland
•
Spinning is at the rate
of about 10 - 12 revolutions per hour
•
The yolk shifts - such
that the lighter components lie beneath one side of the blastoderm
–
This tips up the end of
the blastoderm - it becomes the
posterior portion of the embryo
Specification
of Anterior-Posterior Axis
Left-Right
Axis formation
•
The paracrine factor
Nodal and the transcription factor Pixt2 regulate left-right axis formation
•
As the primitive streak
reaches its max length - sonic hedgehog gene ceases on the right side - due to
expression of activin
•
Through series of
interactions - nodal and lefty-2 are expressed on the left side of the embryo
•
This signals pixt2
expression on the left side of the developing organs