Specification: Urchins + Tunicates + Frogs (lec 14) Flashcards
1) What is the major difference between DEUTEROSTOME and PROTOSTOME invertebrates?
2) What kind of invertebrates are urchins (echinoderms) and tunicates?
1) Deuterostome: blastopore —> anus, Protostome: blastopore —> mouth
2) Deuterostome
What kind of cleavage do sea urchins undergo?
Radial cleavage (isolecithal –> holoblastic)!
(T/F) The animal pole of the sea urchin produces ectoderm, while the vegetal pole produces micromeres.
True!
Match the following terms to their definitions:
1) Large micromeres
2) Small micromeres
A) Primordial germ cells
B) Primary mesenchyme cells
Large micromeres: Primary mesenchyme cells
Small micromeres: Primordial germ cells
1) What happens when you isolate the animal hemisphere of a sea urchin?
2) What happens when you mix the isolated animal hemisphere with isolated micromeres?
1) An isolated animal hemisphere becomes a ciliated ball of undifferentiated ectodermal cells (what it is normally supposed to be). It develops in an autonomous fashion.
2) When an isolated animal hemisphere is combined with isolated micromeres, a recognizable pluteus larva is formed. It develops in a conditional fashion; micromeres can secrete morphogens to cause a larva to form from cells that are normally autonomous!
What happens if micromeres are transplanted from the vegetal pole of an embryo into the animal pole of a host embryo?
The transplanted micromeres invaginate into the blastocoel to create a new set of SKELETOGENIC MESENCHYME CELLS, and they induce the animal pole cells next to them to BECOME vegetal pole endoderm cells.
The transplanted micromeres form SKELETAL RODS while the induced animal cells form a SECONDARY ARCHENTERON. Meanwhile, gastrulation proceeds normally from the original vegetal plate of the host.
*mirror duplication - what happens in the vegetal pole is happening in the animal pole!
Fill in the blanks regarding micromere specification of sea urchins.
______, a general transcription factor, and _______ from the maternal cytoplasm are concentrated at the _______ pole of the egg. These transcriptional regulators segregate to the micromeres and activate the ______ gene.
Otx; b-catenin; vegetal; Pmar1
What does Pmar1 do?
Pmar1 encodes a repressor of HesC which in turn encodes a repressor of several genes involved in micromere specification; DOUBLE NEGATIVE.
Genes encoding signaling proteins (e.g, Delta) are also under the control of HesC.
In which cells is the Pmar1 active and inactive?
In micromeres; Pmar1 is active (micromere specification and signaling genes are active)
In Veg2 cells; Pmar1 is inactive (HesC shuts down the SKELETOGENIC genes)
In cells with Notch; can respond to the Delta signal from the skeletogenic mesenchyme.
Differentiate double-negative gate circuits from feedforward circuit.
Double-negative gate circuits: A single gene encodes a repressor of an entire battery of genes. When this repressor gene is repressed, the battery of genes is expressed.
Feedforward circuit: gene product A activated both gene B and gene C, and gene B also activates gene C. Provides an efficient way to AMPLIFY a signal in one direction.
(T/F) Skeletogenic mesenchyme cells break through extracellular matrix.
True!
What happens to the circumference of the archenteron during early vs late gastrulation in sea urchin embryos?
There is cell rearrangement during EXTENSION (elongation + invagination) of the archenteron in sea urchin embryos.
Early gastrulation: early archenteron with 20-30 cells around its circumference.
Later gastrulation: archenteron has a circumference of 6-8 cells.
What kind of cleavage do tunicates undergo?
Bilateral cleavage (isolecithal –> holoblastic)!
Briefly describe the four steps of cytoplasmic rearrangement in the fertilized egg of tunicates.
1) Before fertilization, yellow cortical cytoplasm surrounds the gray inner cytoplasm
2) After sperm entry into the vegetal hemisphere of the oocyte, the yellow cortical cytoplasm and the clear cytoplasm derived from the breakdown of the oocyte nucleus contract vegetally TOWARD the sperm.
3) As the sperm PRONUCLEUS migrates ANIMALLY TOWARD the newly formed egg PRONUCLUES, the yellow and clear cytoplasms move with it.
4) The final position of the yellow cytoplasm marks the location where cells will give rise to the tail muscles.
Fill in the blanks regarding the gene network leading to notochord development in the early tunicate embryo:
____________ accumulation leads to the expression of the ________ gene, which helps specify the cells to become endoderm and secrete FGFs.
FGFs induce ________ expression in neighboring cells; cells that will become the _________.
The gene expressed ________ regulators of cellular activity such as ________, which regulates cell polarity leading to the _______ _____ of the notochord in the gastrula and neurula stages.
b-catenin; foxd
Brachyury; notochord
Activates; Prickle; convergent extension
*b-catenin —> foxD —> Fgfs —> Brachyury —> Prickle
What are the two steps of the two step process for specifying the marginal cells of the tunicate embryo?
The first step involves the cells’ acquisition (or nonacquisition) of the MACHO-1 transcription factor.
The second step involves the reception (or nonreception) of the FGF signal from the endoderm.
(T/F) If macho-1 is absent, the marginal cells become the nerve cord/notochord, and when it is present, the marginal cells become the mesenchyme and muscle.
True!
Where does gastrulation begin in frog eggs (ie what dictates the location of gastrulation)?
Gastrulation beings in the gray crescent - the region opposite to the point of sperm entry, where the greatest displacement of cytoplasm occurs.
*Point of sperm entry dictates the location of gastrulation.
(T/F) 80% into the first cleavage in a frog egg, the cortical cytoplasm rotates 50˚ relative to the internal cytoplasm.
False!
80% into the first cleavage, the cortical cytoplasm rotates 30˚ relative to the internal cytoplasm.
Which one of the statements is true regarding cleavage of a frog egg?
1) The second division begins in the animal region of the egg before the first division has divided the vegetal cytoplasm because the vegetal yolk impedes cleavage.
2) The third division is placed toward the vegetal pole.
3) As cleavage progresses, the animal hemisphere contains larger and fewer blastomeres than the vegetal hemisphere.
1!
The third division is placed toward the animal pole.
As cleavage progresses, the vegetal hemisphere contains larger and fewer blastomeres than the animal hemisphere.
What are the five key movements during Xenopus (frog) gastrulation?
1) Epiboly (flattening + spreading of epithelial cells to increase the amount of surface they cover)
2) Vegetal rotation
3) Invagination (into bottle cells)
4) Involution and migration (movement of cells inside the embryo as a coherent sheet)
5) Convergent extension (movement of cells toward an axis to extend the axis)
What are bottle cells?
Bottle cells are endodermal and contribute to the ARCHENTERON WALL during frog gastrulation (eventually lining the liver).
What drives the epiboly of the ectoderm layer toward the vegetal pole to enclose the endoderm in frog eggs?
Radial intercalation (squeezing together of cells leading to expansion)
How is radial intercalation occuring from chemotaxis to drive epiboly in frog eggs?
C3a is secreted from the superficial cells as an chemoattractant to C3aR+ deep cells, which guides deep cells to migrate RAPIDLY and OUTWARD, forcing the superficial layer to thin and expand.
This causes the ectoderm layer to enclose the endoderm in late gastrula.
