Chapter 10 Part II: Sea Urchin Development- Gastrulation Flashcards
Before gastrulation, the cells destined tob e endoderm (gut) and mesoderm (skeleton and secondary mesenchymal structures like pigment cells etc) are still on the outside and need to be brought inside the embryo through gastrulation. What are the two main CONSECUTIVE processes?
1) move mesoderm underneath the outer epithelium 2) invagination of the gut tube.
Ingression: after the blastula hatches from its fertilization envelope (recall, animal pole produces hatching enzymes to allow the blastula to be free-swimming), the larger micromeres (forming skeleton mesenchyme/primary mesenchyme) undergo:
epithelial-mesenchymal transition, which involves the epithelial cells: 1) changing shape: thicken to form vegetal plate 2) losing adhesion to adjacent cells and hyaline layer due to cadherin degradation.
Ingression: How do larger micromeres lose adhesion to adjacent cells and hyaline layer?
they lose adhesion to adacent cells because of cadherin degradation. This downregulation of cadherin is controlled by TF snail. The gene for snail is activated by Alx1 TF, which was expressed because of PMAR1 excitation/HESC repression in double negative gate activities earlier. - there is an increase in affinity for basal lamina and extracellular matrix, even though there is a decrease in affinity for adjacent cells.
Ingression: After the large micromeres (on the blastula they are on the surface) undergo epithelial mesenchymal transition and lose adhesion to adjacent cells and the hyaline layer due to Snail induced cadherin downregulation, what happens?
1) the large micromeres then break away from the epithelium since they lost adhesion 2) the basement membrane gets remodelled, allowing for cells to enter the blastocoele as skeletogenic mesenchyme cells (large micromeres were specified earlier due to HESC+PMAR1 double negative gate)
Ingression: In order for the large micromeres to migrate through the basement membrane as skeletogenic mesenchyme cells, how does the basement membrane “soften” to allow cells to pass through?
the micromeres secrete PROTEASES that digest a hole in the membrane, just before the first mesenchymal cells are seen inside the blastocoele.
Ingression: after the large micromeres digest a hole through the basement membrane to enter as skeletogenic mesenchymal cells by secreting proteases, how is cell motility gained?
cell motility is gained by the micromeres by promoting cell movement. Cell motility is gained by transcription factor expression Fox2/3, which was activated by the HESC/PMAR1 GRN.
Once skeletal mesenchymal cells migrated through the blastocoele, how do they form the larval skeleton?
the mesenchyme cells extend filopodia that can sense environment for directional cues such as VEGF and FGF secreted by ventral basal lamina, allowing cells to aggregate and fuse near the ventral side of the larvae. The filapodia initiaite spicule formation later, which is the future skeleton of organism. The filopodia fuses into syncytial cables that will form the acis of the Ca2Co3 spicules of the larval skeleton rods.
the mesenchyme cells extend ____ that can sense environment for directional cues such as ___ and ___ secreted by ventral ___ ___, allowing cells to aggregate and fuse near the ventral side of the larvae. The filapodia initiaite ____ formation later, which is the future ____ of organism. The filopodia fuses into _____ cables that will form the axis of the ___ ____ of the larval skeleton rods.
the mesenchyme cells extend filopodia that can sense environment for directional cues such as VEGF and FGF secreted by ventral basal lamina, allowing cells to aggregate and fuse near the ventral side of the larvae. The filapodia initiaite spicule formation later, which is the future skeleton of organism. The filopodia fuses into syncytial cables that will form the acis of the Ca2Co3 spicules of the larval skeleton rods.
Invagination: After the skeletogenic mesechyme have already entered the blastocoele, how does invagination occur?
the vegetal plate cells (other vegetal cells) remain bound to one another and to the hyaline layer of the egg. They move to fill the gaps caused by the ingression of the skeletogenic mesenchyme. - this causes change of shape in the blastula, which causes INVOLUTION of the vegetal plate, and then invagination into the blastocoele.
The tip of the vegetal plate cells that are invaginating are aka
the archenteron. This tip forms the primitive gut.
the opening of the archenteron at the vegetal pole
blastopore.
Invagination:
the movement of the vegetal plate into the blastocoele is initiated by ___ ___ in the vegetal plate cells and in the extracellular matrix and basement membrane underlying them due to to ___ ____ ___ of the skeletogenic mesenchyme.
____ microfilaments collect the ___ ends of the vegetal cells, casuing the ends to constrict, forming ___ -shaped vegetal cells that pucker inwards. The hylaline layers at the vegetal plate buckles inward due to chnages in its composition, directed by the vegetal plate cells.
the movement of the vegetal plate into the blastocoele is initiated by SHAPE CHANGES in the vegetal plate cells and in the extracellular matrix and basement membrane underlying them due to to EPITHELIAL-MESENCHYMAL TRANSITION of the skeletogenic mesenchyme.
ACTIN microfilaments collect the APICAL ends of the vegetal cells, casuing the ends to constrict, forming BOTTLE -shaped vegetal cells that pucker inwards. The hylaline layers at the vegetal plate buckles inward due to chnages in its composition, directed by the vegetal plate cells.
While the skeletogenic mesenchyme ingresses into the blastocoele from the outer layer, what is the order of invagination through the blasopore?
1) non skeletogenic mesencymal cells are the first to invaginate, forming the tip of the archenteron, and formaing primitive musculature around the gut, pigment cells etc.
2) veg 2 cells become endodermal cells become foregut
3) veg 1/2 endodermal cells to become midgut
4) veg 1 endodermal cells to become hind gut and anus.
