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Ch 11:Amphibians and Fish
Terms in this set (105)
-they don't form an amnion that allows embryonic development to take place on dry land
most frogs have _________ fertilization
-yolk is at the bottom (vegetal) end
-animal part is in the upper half with less yolk
sperm tends to bind to the egg at
-the animal hemisphere region
the point of sperm entry is important bc....
it influences dorsal-ventral polarity
-point of entry= ventral
-180* from entry= dorsal
band of inner gray cytoplasm that appears following a rotation of the cortical cytoplasm with respect to the internal cytoplasm
--gastrulation begins here
in gray crescent, which is opposite side of sperm entry (dorsal side)
unequal radial holoblastic cleavage in amphibians
-has a lot of yolk, and as cleavage begins the yolk is disproportionally apportioned in the vegetal hemisphere
-lots of yolk impedes cell division
-so animal region ends up with more numerous, smaller cells and vegetal region contains a a smaller number of large, yolk-laden macromeres
amphibian embryo containing 16-64 cells
at the 128 cell stage...
blastocoel becomes apparent and embryo is considered a blastula
the blastocoel prevents...
premature contact of the vegetal cells with the animal cap cells, keeping the animal cap cells undifferentiated
animal hemisphere cells will give rise to...
vegetal cells will give rise to...
cells beneath the blastocoel cavity give rise to...
cells opposite the point of sperm entry will become...
-pharyngeal (head) endoderm
important precondition for gastrulation
activation of the zygotic genome
(occurs late in the 12th cell cycle)
mid-blastula transition (MBT)
-different genes begin to be transcribed in different cells
-cell cycle acquires gap phases (G1, S, G2, M)
-blastomeres acquire the capacity become motile
The main purpose of gastrulation is...
-bring areas destined to form the endodermal organs inside the embryo
-surround embryo with cells capable of forming ectoderm
-place mesodermal cells in proper positions btwn endoderm and ectoderm
5 different behavior that occur during Xenopus laevis gastrulation:
2. Vegetal rotation
3. Bottle cell formation and invagination
4. involution and cell migration
5. convergence and extension
-diagram p. 330
-the thinning and spreading of the animal cap cells over the vegetal hemisphere
(pushing the ectoderm over the rest of the embryo)
-cell number increases and surface goes from 3-4 cell layers thick to 2 via radial intercalation
2. vegetal rotation
vegetal cells asymmetrically press up against the inner blastocoel roof on the dorsal side
-cells of the floor of the blastocoel move upward toward animal cap, spiraling in the direction of the dorsal side
-places prospective pharyngeal endoderm cells next to blastocoel and right above the cells that will give rise to mesoderm
combo of endodermal and mesodermal cells
3. bottle cell formation and invagination
localized apical (tip) constriction at the dorsal blastopore lip creates anisotropic forces (different magnitude/measurements in different directions) that foster invagination
-cells invaginate on dorsal side in the marginal zone to form blastopore (also called dorsal blastopore lip here), causes epithelial cells to look like bottles
-this invagination of outer epithelium allows for the infolding of the endoderm to form the embryonic gut
The invagination point where gastrulation begins
the region surrounding the equator of the blastula, where the animal and vegetal hemispheres meet (where gastrulation begins in amphibians)
primitive gut of an embryo
4. involution and cell migration
the leading edge of invaginating cells crawls up onto the blastocoel roof
behavior of a tissue turning inward and spreading over an internal surface
involuting marginal zone (IMZ)
-cells in amphibian gastrula that exhibit involution behavior
cleft of Brachet
region of ECM that separates the ectoderm from the involuting mesendoderm
precursor of the head mesoderm
transient (not permanent) mesodermal rod that plays an important role in inducing and patterning the nervous system
the large endodermal cells that remain exposed on the vegetal surface surrounded by the blastopore of the amphibian gastrulating embryo
-is eventually internalized
5. convergence and extension
the targeted medial-to-lateral intercalation/insertion of cells on the midline (convergence) drives anterior-posterior axis elongation (extension)
the cells that involute through the ventral and lateral blastopore lips during amphibian gastrulation and will form the heart, kidneys, bones, and parts of several other organs
2 major fxns of vegetal cells
1. to differentiate into endoderm
2. to induce the cells immediately above them to become mesoderm
which transcription factor is critical in generating both endodermal and mesodermal lineages?
by late blastula stage, the fundamental ______ ________ are becoming specified
the anterior-posterior, dorsal-ventral, and left-right axes are specified by events triggered at fertilization, but aren't realized until _________.
transcription factor that is opposite the point of sperm entry and species that region of the egg to become the dorsal region
mesoderm that involutes over the dorsal blastopore lip first...
will induce the above ectoderm to become anterior structures such as forebrain
mesoderm that involutes over the dorsal blastopore lip later...
signals ectoderm to form more posterior structures like hind brain and spinal cord
primary embryonic induction
process where the CNS forms thru interactions with the underlying mesoderm
(progeny of dorsal lip cells induce the dorsal axis and the neural tube)
only one tissue in the early gastrula has its fate autonomously determined....
the dorsal blastopore lip
the dorsal blastopore lip is derived from....
the gray crescent cytoplasm opposite the point of sperm entry
-can transplant this tissue and it will initiate gastrulation there too
another name the dorsal blastopore lip was originally called & why (2)
1) they induced the host's ventral tissues to change their fates and become a neural tube and dorsal mesoderm in transplantation
2)they organized the host and donor tissue into a secondary embryo
it is now known (about the "organizer") that the interaction of ectoderm and chordamesoderm...
are NOT sufficient enough to organize the entire embryo
-they rather initiate inductive events
two signals that form the "organizer"
1) the dorsal signal (tells them they are dorsal)
2)tells these cells that they are mesoderm
-these signals create a polarity good for creating dorsal-ventral polarity
the dorsal signal part one is....
the nieuwkoop center
the dorsal-most vegetal blasotmeres of the amphibian blastula
-formed as a consequence of the cortical rotation that's initiated by sperm entry
-it's an important signaling center on the dorsal side of the embryo
-one of it's main fxns is to induce the organizer
mesoderm arises from the marginal (equatorial) cells at the border between...
the animal and vegetal poles
the dorsal signal part 2 is...
the factor that forms the nieuwkoop center...
-becomes localized opposite the side of sperm entry
____ activates the beta-catenin protection pathway
Wnt paracrine factor
the dorsal signal part 3 is...
synergizing with vegetal signals
-nodal gradient (low ventral, high on dorsal) activates Smad2 transcription factor
cells of the organizer contribute to 4 cell types:
1) pharyngeal endoderm
2) head mesoderm (prechordal plate)
3) dorsal mesoderm (mainly the notochord0
4) the dorsal blastopore lip
4 major fxns of the organizer
1) the ability to self-differentiate into dorsal mesoderm
2) the ability to dorsalize the surrounding mesoderm into paraxial (somite-forming) mesoderm when it would otherwise form ventral mesoderm
3) the ability to dorsalize the ectoderm and induce formation of the neural tube
4) the ability to initiate the movements of gastrulation
it is actually the __________ and __________ that is induced to form by the organizer, not the neural tissue
epidermis & ventral mesoderm
(the organizer blocks molecules to allow the ectoderm to become neural tissue, but doesn't actually induce it to become neural tissue by itself)
BMPs induce ectoderm to become...
the nervous system forms from that region of ________ that is protected from epidermal induction by BMP inhibitors
the "default fate" of ectoderm is to become....
certain parts of the embryo induce the ectoderm to become epidermal tissue by secreting...
the organizer tissue acts by secreting molecules that block _________ protecting the ectoderm allowing it to become _______
BMPs, neural tissue
the dorsal-ventral axis across the animal kingdom is based on....
BMP and its inhibitors
the specification of the anterior-posterior axis is based on a gradient of...
the organizer tissue not only induces the neural tube, it also....
specifies regions of the neural tube
the cells of the organizer early on induce the formation of...
brains and heads
the cells of the organizer in later stages induce the formation of...
spinal cords and tails
specifying the left and right axis
the expression of a Nodal gene (Xnr1 in Xenopus frogs) in the mesoderm on the left side of the embryo
-cilia help drive this factor to the left side
-if somehow ends up on right side, organ formation sides are reversed
-if on both sides, organ organization is random
ciliated cells that form at dorsal blastopore lip in later stages of gastrulation
ZEBRA FISH DEVO
is meroblastic rather than holoblastic
only a small portion of its cytoplasm forms cells
one advantage of studying zebra fish eggs over frog eggs
they are transparent so you can look at them with a microscope
zebra fish eggs are also...
permeable to small molecules allowing us to test the effects of certain drugs by placing the drug in the water
the eggs of most bony fish are telolecithal, which means that...
most of the cytoplasm is occupied by yolk
Thin region of yolk free cytoplasm at the animal pole of the egg
-only place where cleavage can take place
doesn't completely divide the egg
discoidal meroblastic cleavage
when only the blastodisc becomes the embryo
_____ ions are critical for coordinating mitosis and movement of actin cytoskeleton
An embryonic cap of dividing cells resting on a large undivided yolk.
-mound of cells that sits at the animal pole on top of a large yolk cell
_______ are important roads along which cell fate specifying cytoplasmic determinants travel
yolk syncytial layer (YSL)
a cell population in zebrafish cleavage stage embryo when the cells at the vegetal edge of the blastoderm fuse with the underlying yolk cell, producing a ring of nuclei in the part of the yolk cell cytoplasm that sits just beneath the blastoderm
-important for directing some of the cell movements in gastrulation
YSL will not contribute cells or nuclei to the embryo, but is important for...
generating the fish organizer, pattering the mesoderm, and leading the epiboly of the ectoderm
YSL is formed at the _____ cell cycle
as blastoderm expands vegetal to surround the yolk cell, some of the yolk syncytial nuclei will move under the blastoderm to form______________, while others stay ahead of the blastoderm to form______________
enveloping layer (EVL)
-cell population distinguished at mid-blastula transition
-made up of the most superficial cells of the blastoderm
-is a protective covering that is slogged off after 2 weeks
-allows the embryo to develop in hypotonic water that would otherwise burst the cells
Gastrulation and the Formation of Germ Layers
all 3 layers of zebrafish blastoderm undergo....
the vegetal migration of the blastoderm margin is dependent on the epiboly of the....
YSL p.361 ???
a thickened ring of cells in the margin of the deep cells that appear once the blastoderm has covered about 1/2 of the yolk cell
-superficial layer= epiblast
-inner layer= hypoblast
as cells of the blastoderm undergo epiboly around the yolk, they are also internalizing cells at the blastoderm margin to form the.....
during normal Devo, epiboly and cell specification may be coordinated by the mechanical stress of....
A localized thickening on the future dorsal side of the fish embryo; functionally equivalent to the dorsal blastopore lip of amphibians.
(where epiblast and hypoblast come together)
paraxial mesodermal cells are precursors of the...
a band of neural precursors over axial and paraxial mesoderm, eventually develops a lumen to become the neural tube
Cells remaining in epiblast become ...
hypoblast ring moves over yolk cell eventually closing, which
internalizes the cells that will become mesoderm and endoderm
Dorsal-Ventral Axis Formation
-critical to establishing this is the...
like the frog blastopore lip, the embryonic shield forms the...
prechoral plate and the notochord
prechordal plate and notochord are responsible for inducing ectoderm to become....
dorsal blastopore lip (shield) will induce
ventral blastopore lip will induce
lateral blastopore lip will induce
trunk and posterior head structures
the blastopore lips use a ________ to form these structures
Nodal and BMP gradient
Left and Right Axis Formation
-the ways that different vertebrate classes accomplish the left-right asymmetry differ, but what's likely in all vertebrate classes to help be responsible for left-right axis is...
currents produced by motile cilia in the node
the nodal structure housing the cilia in zebrafish
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