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Cleavage/Gastrulation (wk 2) Flashcards

1
Q

What is cleavage?

A

Cell divisions immediately after fertilization

rapid mitotic divisions where the zygotic cytoplasm is divided into a number of smaller cells (blastomeres)

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2
Q

Describe the significance of the maternal genome

A

Immediately after fertilization, the maternal mRNAs are polyadenylated and translated= accumulate lots of maternal transcription factors

Important in implementing basic biosynthesis
Important in directing first mitotic divisions in the cleavage stage embryi
Important in specifying initial cell fate and patterning

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3
Q

Which part of the egg does cleavage start?

A

Animal pole (pigmented)

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4
Q

How is the egg activated?

A

After fertilisation

Brings the egg out of its suspended state so that cleavage can start

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5
Q

What is the exact moment cleavage starts

A

Following the completion of meiosis and the fusion of male and female pronuclei

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6
Q

What are the three major axes of the embryo?

A

anterior-posterior
Dorsal-ventral
Left-right

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7
Q

Describe cleavage and gastrulation of the frog in terms of a graph

A

Cleavage stage: first stage, really rapid cell growth

Gastrulation: less growth plateaus, slower rate of growth

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8
Q

What is the role of the protein cyclin?

A

When you inhibit protein synthesis in sea urchins, cell division does not occur
When you inhibit protein synthesis in sea urchins BEFORE prophase, no effect
Cyclin must be therefore made early on in the cell cycle and is critical for cell division

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9
Q

Distinguish between karyokinesis and cytokinesis

A 
Karyo= dividing the DNA
Cyto= cell division
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10
Q

Distinguish between mosaic (determinant) and regulative (inderteminant) cleavage

A

Mosaic: blastomeres are not totipotent, each blastomere has a different fate, not pre-determined

Regulative: each blastomere can regulate to produce a complete embryo, pre-determined

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11
Q

What type of cells are in the inner cell mass

A

embryonic stem cells

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12
Q

How does yolk affect cleavage?

A

Large quantities can slow down the process

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13
Q

T/F: the animal pole is generally yolk free and vegetal pole is yolk rich

A

True

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14
Q

What is an isolecithal egg?

A

Sparse equally spaced yolk e.g. humans

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15
Q

Distinguish between mesolecithal and telolecithal eggs

A 
Meso= moderate yolk, usually not equally spaced
Telo= dense yolk is distributed throughout most of the cell, only a small portion of the egg is free of yolk
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16
Q

Distinguish between holoblastic and meroblastic cleavage

A 
Holo= complete cleavage, furrow extends all the way down
Mero= incomplete cleavage, furrow can't extend all the way down due to the presence of yolk
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17
Q

What are the three types of holoblastic cleavage?

A

Based on orientation of the furrow

Radial
Rotational
Discoidal

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18
Q

Describe radial holoblastic cleavage

A

Type of holoblastic cleavage
Seen in sea urchins and frogs
Blastomeres have different fates- morphogenetic determinants are in different cytoplasmic regions of the egg
1/2 cleavage: animal to vegetal pole is meridional and perpendicular, equal
3/4 cleavage: equatorial, equal
5 cleavage: unequal, get macromeres and micromeres

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19
Q

Describe rotational cleavage

A

Type of holoblastic cleavage
First cleavage is equal
At the two cell stage, one blastomere divides in an miridial way and the other divides in an equatorial way (opposite pattern)

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20
Q

Describe discoidal cleavage

A

Type of holoblastic cleavage
The divisions are restricted to a disc-like region at the animal pole (blastodisc) = overlies the yolk cell
Embryo only develops on top of the yolk

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21
Q

At what stage in mammalian development is it termed a morula?

A

16 cell stage

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22
Q

What do the outside cells (e.g. trophoblast cells) become?

A

Trophoectoderm, placenta (chorion)

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23
Q

What occurs at the 64 cell stage?

A

There are two types of cells (inner and outer)
The first differentiation event in mammalian development

ICM and trophoblast cells are separate layers

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24
Q

Where does the egg hatch in mammalian development?

A

In the uterus

If it hatches earlier you get an ectopic pregnancy

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25
What happens to the maternal and zygotic genome?
Occurs during the Maternal to Zygotic Transition (MZT) Maternal transcripts are destabilised Zygotic genome is activated 2 stages: 1. Subset of the maternal transcript is eliminated 2. Transcription of the zygotic genome begins
26
T/F | The initial cell cycles are not synchronous and have gap phases
False Initial cell cycles are synchronous and lack gap phases They are fast and uninterrupted
27
T/F | The initial phase is driven by the maternal genome and then the zygotic genome takes over
True
28
T/F | In mammals, the zygotic genome takes 3 weeks to take over
False takes on quite early between 1 and 2 cell stage
29
T/F | implantation triggers some of the destabilisation of the control of maternal genome in mammals
False | fertilisation triggers some of the destabilization of the control of the maternal genome
30
When does the MZT occur in frogs?
During the mid blastula stage
31
Define what a blastula is
Ball of cells with a fluid filled space (blastocoel)
32
T/F | The blastocoel is offset to the animal pole in frogs due to slow cleavage at the vegetal pole
True
33
T/F | The blastula is not connected to the hyaline layer
False It is connected It serves as a major mechanical support for the blastula
34
What is the blastopore?
When gastrulation begins some invagination occurs to form the blastopore
35
T/F | Gastrulation begins at the animal pole
Fale | It begins at the vegetal pole
36
Define gastrulation
Process of highly coordinated cell and tissue movement whereby the blastula is transformed to a multilayered embryo with formation of the three germ layers: o ectoderm, o mesoderm o endoderm.
37
What are the 5 types of cell movement?
``` Invagination Involution Ingression Delamination Epiboly ```
38
Describe invagination
Infolding of a sheet Opening is called the blastopore (forms the anus) E.g. sea urchin endoderm
39
Describe involution
Inward movement of an expanding outer layer so that it spreads over the inner surface of the remaining external cells (inturning of a cell sheet over the basal surface of an outer layer) e.g. amphibian mesoderm
40
Describe ingression
Individual cells pop off and become mesenchymal cells They migrate independently e.g. sea urchin mesoderm, drosophila neuroblasts
41
Describe delamination
Splitting or migration of one sheet into two sheets e.g. mammalian and bird hypoblast formation
42
Describe epiboly
The expansion of one cell sheet over other cells
43
Why are sea urchins useful for the study of gastrulation?
Transparency (can see what's going on) Simple organisation Small number of constituent cells
44
Where does the archenteron form in sea urchin development?
At the vegetal pole
45
What marks the beginning of gastrulation in sea urchins?
Thickening and flattening of the vegetal pole to form the vegetal plate
46
What are the three stages of cell movement in sea urchin gastrulation?
Ingression (skeleton forming cells/mesodermal cells) Invagination (to form developing gut/endodermal cells) Elongation of the gut
47
Why do the primary mesenchyme cells in the sea urchin ingress?
They are specified to ingress Lose affinity for each other and the hyaline layer Gain affinity for the cells lining the blastocoel Microvilli that were anchoring them retract Lose adhesion with neighbouring cells Basal lamina is disrupted Microtubules for anchorage are disrupted
48
Primary mesenchyme cells in the sea urchin were initially _____ and then become _____ (shape)
Initially columnar | Become rounded
49
T/F: | The primary mesenchyme cells induce their neighbours to become endoderm after they migrate into the blastocoel
True
50
T/F: | Invagination during sea urchin development allows for the development of the skeleton
False Allows for the development of the archenteron (gut) Skeleton is formed by the primary mesenchyme cells
51
Describe the primary invagination of the sea urchin
Granules in the cortex of the vegetal plate release chondroitin sulphate into the hyaline layer Chondroitin sulphate absorbs water and causes swelling Together with the stiff surrounding hyaline layer this causes the plate to buckle/bend inwards
52
Describe the secondary invagination of the sea urchin
Secondary mesenchyme cells appear at the tip of the archenteron and extend long filopodia towards the animal pole They explore for the attachment site to reach the target (future mouth)
53
Describe the final stage of elongation of the sea urchin
Archenteron cells become elongated Involution allows for the recruitment of more cells Provides enough cells to have a sufficient number to form the differentiation of the sections of the gut
54
What is the role of beta-catenin in sea urchin gastrulation?
transcription factor activated by the Wnt pathway It specifies the micromeres Wnt8 is made by the micromeres and endoderm cells It acts on endoderm cells to invaginate at the vegetal plate Lots of beta catenin in the vegetal pole
55
What is the effect of adding LiCl to sea urchin embryos?
Vegetalises embryos More cells get beta catenin More transcription in the cells Embryo makes such a big gut that can't fit inside
56
What is the effect of beta catenin prevention in sea urchin embryos?
Stay as a permanent blastula | Can't make a gut without the signalling molecule
57
T/F: | The yolk is found at the animal pole
False | It is found at the vegetal pole
58
T/F: | In frogs, the vegetal hemisphere becomes the skeleton
False | It becomes the gut
59
Where is gastrulation initiated on the frog?
In the grey crescent- the marginal zone | This area is going to be the future dorsal side of the embryo
60
What initiates the presence of the grey crescent?
The sperm hits the egg which initiates rotation of cortical cytoplasm
61
What does the grey crescent contain?
Morphogenetic determinants | Rich in dorso-anterior forming determinants
62
What are the three main evens of gastrulation in the frog?
Invagination of bottle cells Involution of mesoderm Epiboly of ectoderm
63
Which part of the egg is most receptive to sperm?
The animal pole
64
What is the organiser in frogs?
The dorsal lip of the blastopore | Important for all animal development
65
In mammals, what does the trophectoderm form?
Chorion and placenta
66
In mammals, what does the inner cell mass give rise to?
All embryonic tissues
67
What theory explains why the mammalian embryo forms two distinct cell layers?
Blastomere position is important (dependent on whether they are in contact with the external environment or not) At the 8 cell stage, certain proteins are restricted to the apical region (side facing the outside of the embryo) These cells will inherit the apical region cytoplasm and will adopt a position outside= they become trophectoderm Contractility of the cells also plays a role - Daughter cells of two different contractility - Less contractile cells spread over the more contractile cells - More contractile cells become the inner cell mass
68
T/F: | In mammalian development, the more contractile cells become the trophectoderm
False | They become the inner cell mass
69
What is the role of transcription factor Yap?
Transcription factor Yap activates a gene expression program that instructs the cell to become trophectoderm at the 32 cell stage- giving rise to the placenta Highly contractile cells do not have nuclear Yap, adopt an inside position to become the ICM- giving rise to the embryo
70
What are the three types of cell movement involved in mammalian gastrulation?
Delamination Invagination Ingression
71
T/F: | The yolk sac stays outside the mammalian embryo
True
72
Describe invagination in mammalian gastrulation
Epithelial sheet bends inwards to form a raised groove on the dorsal surface of the epiblast = primitive streak/groove Primitive groove ends n Hensen's node
73
Describe ingression in mammalian gastrulation
Cells lose their E-cadherin and leave the epithelial sheet become freely migrating cells Become the mesoderm and endoderm cells
74
What does the endoderm form?
Lining of digestive and respiratory tracts
75
What does the mesoderm form?
Bone, muscle, other tissues

HISTO3003 (6 decks)

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