Cleavage and Gastrulation
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what is cleavage? what does it create?
how fast is the rate?
series of mitotic divisions where enormous volume of egg cytoplasm is divided into numerous smaller cells called blastomeres
rate of mitosis will never again be as fast
what are the consequences of cleavage?
- 1) reduces volume of cells
- 2) blastomeres acquire differences
what regulates the mitotic events? describe both components of the complex
- MPF (mitosis promoting factor) is cyclin B bound to cdk.
- when these are bound together then the cell goes into M phase (mitosis)
- cyclin b - accumulates during s phase, degrades when cell enters M phase
what is stored in the oocyte cytoplasm relating to cell division?
- maternal cyclin mRNA
- regulators of cyclin B
what happens in the mid blastula transition?
- embryo starts to transcribe its own stuff (it didnt need to before because it was all in the cytoplasm)
- add G1 and G2 (growth phases) into the cell cycle
what is mitosis like for early blastomeres?
only M and S phase
in xenopus, when and why is MPF at high levels and when does it decrease?
- MPF increases when cell enters meiosis 2 and remains high till sperm entry
- MPF stays high because of presence of CSF
- when sperm enters -Ca release causes decrease in CSF - so MPF degrades and cell returns to S phase
what is the end result of cleavage? and what is its purpose?
- a blastocoel is created - it is a fluid filled cavity and it
- 1) permits cell migration
- 2) inhibits premature cell - cell interaction
what are the 2 things that determine cleavage patters?
- 1) amount and distribution of yolk
- 2) cytoplasmic factors that regulate orientation of mitotic spindle
what are the 4 types of yolk and describe them? what are their types of cleavage?
- isolecithal - evenly distributed - holoblastic
- mesolecithal - asymmetrically distributed, but not extreme, more like a gradient - holoblastic
- telolecithal - extremely asymmetric, dense yolk through out most of cell - meroblastic
- centrolecithal - yolk is in the center of the egg - meroblastic
what are the diff types of cleaveages?
- holoblastic - cleavage furrow extends through entire egg
- meroblastic - incomplete cleavage
- - bilateral
- - discoidal
- - superficial
What is gastrulation?
what happens to the cells?
- highly coordinated cell and tissue movements whereby cells of the blastula are dramatically rearranged
- cells: 1) new positions 2) new neighbors 3) 3 germ layers established 4) sets the stage for cell-cell interactions
What are the 5 different types of cell movement during gastrulation?
- invagination - stick a finger in a balloon
- involution - when an outside cell sheet moves inward to cover the inner surface of the outer layer
- ingression - migration of individual cells into the embryo (they become mesenchymal)
- delamination - splitting of one cellular sheet into two
- epiboly - the expansion of one cell sheet covering over all the others, completely envelops the inside
vegetal pole? animal pole?
- vegetal pole = lots of yolk
- animal pole = little yolk
describe the cleavage events in the sea urchin:
- 8 cells - animal half and vegetal half each have 4 THEN:
- the animal half splits into 8 equally sized ones called mesomeres
- the vegetal pole splits into 4 large ones (macromeres) and 4 little ones called micromeres..
- you finally get a 120 cell blastula with an empty middle space called the blastocoel
in the book what colors correspond with which of the 3 germ layers?
- ectoderm - blue (mesomeres)
- mesoderm - red (micromeres)
- endoderm - yellow (macromeres)
explain the role of disheveled:
- GSK3B is bound to APC, when this complex binds to B cadherin - B catenin is targeted for destruction... B catenin is responsible to transcription of particular genes
- Disheveled (Dsh) prevents B catenin from being destroyed
what happens when you have different amounts of B catenin?
big pic of sea urchin gastrulation?
- alot - mesoderm
- a little - endoderm
- none - ectoderm
big pic: lots of dsh in the vegetal portion, thats why macromeres and micromeres grow there and eventually become the mesoderm and endoderm; ectoderm is in the animal portion where there is no B catenin bc no dsh
What does B catenin actually do?
goes into nucleus and binds LEF1 and certain transcription factors get turned on
what happens if no b catenin anywhere?
entirely b catenin?
- get a ball of ectoderm
- no ectoderm
How does the blastocoel form?
- blastomeres are attached to hyalin layer, which keeps the single cell layer
- tight junctions between blastomeres
- need to create an osmotic gradient to get water in there
a cleavage stage cell resulting from mitosis
what are the different ways organisms create osmotic gradient to get water into their blastocoel?
- sea urchins - pump proteins
- amphibians - pump Na+
- mammals - secrete fluid - cavitation
what does the late blastula of a sea urchin look like?
it is 1000 cells that form a hollow ball and the vegetal side is kind of flat
what is story of beginning of gastrulation?
- micromere descendents - change shape, lose adhesions to neighboring cells, and break away from apical layer and go into the blastocoel and become skeletogenic mesenchyme cells (movement = ingression)
- these s.m. cells begin extending and contracting long, thin processes called filopodia
how do the vegetal epithelial cells ingress inwards?
they lose affinity to hyalin layer and for each other and gain affinity for proteins along the blastocoel
what happens to mesenchymal cells after ingression?
FGF (fibroblast growth factor) and VEGF are factors secreted by the ectoderm triggering the mesenchymal cells to come congregate there and form ring along animal - vegetal axis
what happens to cells left behind on the vegetal pole?
these cells thicken and flatten to form a vegetal plate - swelling in inner lamina of hyalin layer, but outer lamina does not swell - so the actin cytoskeleton constricts so cells become more wedge shaped and INVAGINATION happens!
what is the invaginated region called?
the archenteron (primitive gut)
what is the opening of archenteron at the vegetal pole called?
what are the 2nd and 3rd stages of archenteron invagination?
- 2nd stage - convergent extension - wide tube gets longer and more narrow b/c cells rearrange
- 3rd stage - the cells at the tip extend filopodia that attach to the inner surface wall of the blastocoel, the filopodia then shorten pulling the archenteron that last little distance
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