Chapter 11: Amphibian Development Flashcards
What determines dorsal-ventral polarity in am amphibian egg? How?
point of sperm entry. Where the sperm enters is the VENTRAL side of the embryo (belly), and 180 degrees from the point of sperm entry is the DORSAL side (spinal side)
What allows the cortical cytoplasm of the egg to rotate with respect to the inner cytoplasm?
the sperm centriole enters the egg and organizes microtubules of the egg into PARALLEL TRACKS in the VEGETAL cytoplasm, separating the outer cortical cytoplasm from the yolky internal cytoplasm.
T/F: Gastrulation happens at the region of point of sperm entry
false. gastrulation occurs on the grey crescent, which is at the opposite point of sperm entry (DORSAL)
In the zygote, the cortical cytoplasm rotates about ___ degrees with respect to the internal cytoplasm to expose a ___ ___. What cellular structure is required for this region to be exposed?
In the zygote, the cortical cytoplasm rotates about 30 degrees with respect to the internal cytoplasm to expose a GREY CRESCENT. MICROTUBULES are required for grey crescent to form.
Explain how an amphibian egg is already polarized before actual fertilization.
The amphibian egg contains different transcription factors, proteins, and mRNAs in different regions. There are two poles in an amphibian egg: 1) yolky vegetal 2) animal (not as much yolk). fertilization can occur any where on the animal pole, and this side becomes the ventral region of the frog. (fertilization takes place outside the female body ie she lays eggs)
what compounds allow sperm to bind to the egg in a frog?
specific glycoproteins within the vitelline envelope and cell membrane in animal hemisphere bind the sperm.
What kind of cleavage occurs in the frog embryo and explain
UNEQUAL, radial, holobloastic cleavage: cleavages being in the animal pole and extend to the vegetal pole, where they slow down due to yolk.
Explain the first few cleavages (types) during early frog embryogenesis
1st cleavage: meridional: bisects the grey crescent 2nd : meridional: 90 degrees to the first (*meridional: goes through animal pole and vegetal pole, producing identical cells) 3rd: asymmetric equatorial: displaced towards the animal pole. Divides the embryo into 4 ANIMAL MICROMERES AND 4 VEGETAL MACROMERES - as the cells divide, the animal region becomes packed with numerous small cells while the vegetal region contains a relatively small number of large, yolk laden micromeres.
A morula is 16-64 cells, afterwhich it becomes a ____. At what stage is the cavity visible, and what is its importance
blastula. At 128 cells, the blastocoele cavity is seen, and it is important because it keeps the above and below blastomeres from contacting and interacting with one another too early.
What is the animal cap
undifferentiatied portaion of cells from the ROOF of the blastocoele (animal pole) that will differentiate into ECTODERM
- IF THERE is no blastocoele cavity to separate dif layers, the animal cap cells will form mesoderm because of vegetal cell influence.
what germ layer does the animal cap form?
ectoderm.
What germ layer does the vegetal cell form
endoderm and mesoderm a bit.
In the blastula:
___ Cadherin keeps cells together, and the mRNAs for this protein are supplied by the ___
Animal cells, especially the cap form the ___
Vegetal cells form the ___
cells under the blastocoele become ____
cells oppositeto sperm entrybecome the ___ ___, ___ ___ and ___ ___
EP Cadherin keeps cells together, and the mRNAs for this protein are supplied by the OOCYTE/FEMALE
Animal cells, especially the cap form the ECTODERM
Vegetal cells form the ENDODERM
cells under the blastocoele become MESODERM
cells oppositeto sperm entrybecome the NEURAL ECTODERM, NOTOCHORD MESODERM, AND PHARYNGEAL ENDODERM
During the midblastual transition, what are two characteristics
1) blastomeres become more MOTILE
2) methylation of lyside 4 on histone3 to form TRIMETHYLATED LYSINE allows for active transcription. Chromatin thus becomes remodeled into euchromatin (loose structure) to allow for transcription factors to access the promoters of DNA and initiate transcription.
During mid-blastula transition, ____ induces vegetal cells to become:
VegT induces vegetal cells to become endoderm, which then secretes factors to induce the cells above them to become mesoderm.
Outline and briefly describe the 5 main steps of amphibian gastrulation
1) epiboly: proliferation and intercalation of cells. Thinning and spreading of animal cap cells over the vegetal hemispheres (to form epidermal and neural and general ectoderm)
2) vegetal rotatioN: vegetal cells press up against the inner blastocoele roof on dorsal side.
3) bottle cell formation (grey crescent) and invagination: apical constriction of bottle cells at dorsal blastopore lip to create anisotropic forces that encourage invagination
4) involution and cell migration: leading edge of invaginating cells crawl along the blasocoele roof
** cell involution is not passive. Before bottle cells form, internal cell rearragnements propel the cell of the dorsal floor of the blastocoele towards the animal cap.
5) convergence and extension: medial-to-lateral intercalation of cells on midline drives anterior-posterior axis extension.
Epiboly involves proliferation and intercalation and spreading of ectodermal cells around the rest of the embryo (even over the vegetal pole). How does intercalation occur? how do the ectodermal cells spread?
prior to epiboly, the aniaml cap ectoderm cells proliferates to be 3-4 layers thick. Due to intercalation, they become flattened to 2 layers, and this structure expands vegetally over the endoderm.
- deep cells extend projections radially towards surface layer of cells, and physically begin pushing their way through the layers as ectoderm expands outwards. This causes intercalation.
- fibrillar fibronectin allows for vegetal migration of the naimal cap cells and the enclosure of the embryo
During 2nd step of gastrulation (vegetal rotation and invagination of bottle cells) the mesoderm and ectoderm are moving inside the embryo/blastocoele as the ectoderm encases them. This infolding of the endodermal cells (vegetal) forms the ___ ___.
Cells invaginate to form the ___ ___ ___: on the dorsal side of the embryo. This blasopore is formed by ___ cells, which initiate the formation of the ___.
During vegetal rotation, cells on the floor of the blastocoele (vegetal cells) migrate upwards dorsally to form the ___ ___ right above the involuting cells that will form the mesoderm.
This infolding of the endodermal cells (vegetal) forms the PRIMITIVE GUT.
Cells invaginate to form the DORSAL BLASTOPORE LIP on the dorsal side of the embryo. This blasopore is formed by BOTTLE (GREY CRESCENT) cells, which initiate the formation of the ARCHENTERON.
*these bottle cells are epithelial cells that are bottle shaped; the body of the cell becomes displaced towards the inside, while a portion remains in contact with outside surface via a slender neck.
During vegetal rotation, cells on the floor of the blastocoele (vegetal cells) migrate upwards dorsally to form the PHARYNGEAL ENDODERM right above the involuting cells that will form the mesoderm. this ENDOMESODERM REGION migrates along basal surface of the blastocoele roof twards the anterior end (see 5 in image)
During involution, the involuting marginal zone consists of ___ and ___ ___, and spread over the internal surface (basal to ectoderm), following the pharyngeal endoderm.
During involution, the involuting marginal zone consists of MESODERM AND HEAD MESODERM, and spread over the internal surface (basal to ectoderm), following the pharyngeal endoderm (PE goes first because it didn’t involute; it just crawled up from vegetal pole during vegetal rotation).
as cells continue to migrate through the blastopore lip and crawl up and around to the anterior portion of the cell, the blastocoele cavity gets displaced towards the ventral aspect of the embryo.
- the blastopore continues developing, forming ___ and __- lips, where additional mesodermal and endodermal precursor cells pass.
With the formation of the ___ lip, the blastopore has formed a ring around the large endodermalcells (vegetal cells) that remain exposed on the vegetal surface. This lip is formed once the ___________________, leaving only the ___ ___ formed by remaining endodermal cells, which also eventually get internalized.
as cells continue to migrate through the blastopore lip and crawl up and around to the anterior portion of the cell, the blastocoele cavity gets displaced towards the ventral aspect of the embryo.
- the blastopore continues developing, forming LATERAL AND VENTRAL lips, where additional mesodermal and endodermal precursor cells pass.
With the formation of the VENTRAL lip, the blastopore has formed a ring around the large endodermalcells (vegetal cells) that remain exposed on the vegetal surface. This lip is formed once the crescent has moved around the entire blastula, leaving only the YOLK PLUG formed by remaining endodermal cells, which also eventually get internalized.
