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Developmental Biology Intro > Cleavage > Flashcards

Cleavage Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

Do sea urchins have complete (holoblastic) or incomplete (meroblastic) cleavage?

A

complete/holoblastic cleavage

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2
Q

Does the Xenopus have complete (holoblastic) or incomplete (meroblastic) cleavage?

A

complete/holoblastic cleavage

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3
Q

Do chicks have complete (holoblastic) or incomplete (meroblastic) cleavage?

A

incomplete/meroblastic cleavage

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4
Q

What cleavage pattern (full description) does the sea urchin have?

A

Holoblastic radial cleavage

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5
Q

What cleavage pattern (full description) does the Xenopus have?

A

Holoblastic displaced radial cleavage

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6
Q

What cleavage pattern (full description) does the chick have?

A

Meroblastic discoidal cleavage

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7
Q

describe the yolk amount and distribution in sea urchin eggs?

A

Isolecithal - sparse yolk, distributed evenly around egg

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8
Q

describe the yolk amount and distribution in Xenopus eggs?

A

Mesolecithal - moderate amount of yolk, distributed mostly at vegetal pole

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9
Q

describe the yolk amount and distribution in chick eggs?

A

Telolecithal - huge amount of yolk. cleavage occurs in a small disc floating on top of the yolk in the animal pole

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10
Q

In cleavage, why does every cell division result in smaller and smaller cells?

A

Because the gap phases are skipped

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11
Q

What is the difference between cell specification and cell determination?

A

if cell fate is reversible (you experimentally can change its fate) - that cell is specified but not determined.

If cell fate is irreversible, even with experimental manipulation the fate is sealed, that is cell determination

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12
Q

Name the tiers we have at the 60 cell stage of the sea urchin embryo. For which tiers are the cell fate determined/locked in?

A

There are two animal tiers (an1 and an2) and two vegetal tiers (veg1 and veg2) with micromeres at the very vegetal end.
The micromere tiers are determined. All other cell tiers are specified but not yet determined.

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13
Q

if left alone, cells of the an1 and an2 form the _______ layer (germ layer) and cells of the veg1 and veg2 form the ______ layer (germ layer).

A

an1 and an2 = ectoderm

veg1 and veg2 = mesoderm and endoderm

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14
Q

What is autonomous specification?

A

When cell fate is specified due to the presence of a transcription factor in the cell.

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15
Q

What is conditional specification?

A

cell fate is specified by paracrine or juxtacrine factors released from neighbouring cells.

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16
Q

True or false. In the sea urchin embryo, the fate of a cell becoming a micromere is autonomously specified

A

true. The presence of the transcription factor beta-catenin determines whether a cell becomes a micromere in the sea urchin.

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17
Q

Beta-catenin gets degraded by every cell except the micromeres. Why?

A

The unfertilized egg has a maternally-inherited protein called disheveled (dsh).

dsh accumulates at the vegetal end of the embryo (where micromeres form). Dsh prevents the breakdown of beta-catenin, which acts as a transcription factor to specify the fate of the cells as micromeres.

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18
Q

Micromeres are also responsible for conditionally specifying cells neighbouring them. A factor released by micromeres causes the cells above them to become _______________

A

endomesoderm - specifically endoderm and secondary mesenchyme

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19
Q

Micromere specification is controlled by the wnt signalling transduction pathway. Describe the steps in the wnt signalling transduction pathway to specify a cell as micromere, and also how it works in non-micromere cells.

A

** Non-micromere cells**

wnt acts as a ligand for the frizzled receptor. This activates nothing, because there is no dsh in the cell.
The proteins GSK3 and Axin bind with APC to form a tri-protein complex called the “Degradation complex.”

The degradation complex breaks down beta-catenin by tagging it with ubiquitin, recruiting a proteasome to it. This depletes its levels. Beta-catenin is the transcription factor that causes a cell to become a micromere.

Because it’s been depleted, these cells do NOT become micromeres.

Micromeres

Wnt acts as a ligand for the frizzled receptor. This recruits dsh and activates dsh. Dsh recruits GSK3 and Axin, preventing them from forming the degradation complex with APC.

No degradation complex = elevated levels of beta-catenin in the cell. Beta-catenin acts as a transcription factor that controls gene expression to cause the cell to become a micromere.

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20
Q

what would happen in the sea urchin if the wnt pathway happened everywhere i.e. dsh was distributed equally around the egg instead of localized at the vegetal pole?

A

If you guessed you’d have a ball of micromeres, that is false. dsh and beta-catenin localization is not the only thing needed to create a micromere. For example, the cell must be at the right small size.

There appears to be many factors involved in creating a micromere.

In reality, the smallest cells will become micromeres, and the larger cells will become just endoderm

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21
Q

In the blastula stage of the sea urchin, what happens to the micromeres, i.e. what cell will they become?

A

They become primary mesenchymal cells, PMCs

22
Q

Yolk is localized where in the Xenopus egg?

A

the vegetal pole

23
Q

Why does cell cleavage in the Xenopus begin in the animal pole before the vegetal pole?

A

There’s more yolk in the vegetal pole, which slows down cell division

24
Q

True or false. In the Xenopus embryo, cells of the vegetal half are larger and fewer than the fastly dividing animal half.

A

true

25
Q

Why are cells of the animal pole in the Xenopus embryo darker in colour?

A

presence of pigment in the cortex cytoplasm of the animal pole.

26
Q

The 16-64 cell stage in the xenopus cleavage is often called the ______ (latin = mulberry) because the ball of cells look like a berry.

A

morula

27
Q

In the Xenopus, sperm will fertilize always at which pole of the embryo? And why?

A

the animal pole.

Certain proteins necessary for the interaction between sperm and egg are localized at the animal pole.

28
Q

Why is the point of entry of the sperm important in the Xenopus embryo?

A

Important for forming the initial dorso-ventral body-axis of the Xenopus embryo. THE INITIAL axis. It will change later, I think

29
Q

The point of entry of a sperm in the Xenopus embryo forms the transient _____ side, and 180 degrees from that is the transient ______ side.

A

The point of entry – transient ventral end of the Xenopus embryo
180° from the point of entry – the transient dorsal end of the Xenopus embryo

30
Q

Describe how the transient dorsal side of the Xenopus embryo is formed right after fertilization through Cortical Rotation?

A

Once sperm fuses with the egg, the centriole of the sperm forms a spherical microtubule tract between the cortex and the inner cytoplasm layers of the whole Xenopus egg.

This MT tract is used to rotate the cortex cytoplasm.

The cortex rotates 30 degrees towards the point of sperm entry, moving unpigmented cortex cytoplasm from the vegetal half to the animal half, forming an unpigmented “Gray crescent.”

The transient dorsal end is just below the gray crescent.

31
Q

Why can we observe cortical rotation in the Xenopus easily in lab?

A

The cortex is pigmented in the animal half of the embryo, making it easier to see the rotation.

32
Q

Compare and contrast dsh and beta-catenin localization in the Xenopus compared to the sea urchin.

A

The unfertilized Xenopus egg also initially has dsh localized at the vegetal half, but that changes. Cortical rotation moves dsh to the transient dorsal end of the embryo.

This means beta-catenin is not degraded at the transient dorsal end, and accumulates there. There, beta-catenin acts as a transcription factor for dorsal-specific genes, rather than forming micromeres like in the Xenopus.

Also, the wnt pathway ingredients were found throughout the sea urchin embryo, but in the Xenopus they are only found in the transient dorsal end, because they are all associated with vesicles. in fact, dsh is attached to these vesicles. These vesicles are what move to the dorsal end when cortical rotation occurs.

33
Q

What is the function of beta-catenin in the Xenopus?

A

To regulate gene transcription for dorsal-related genes.. This is unlike the sea urchin, where beta-catenin was associated with the formation of micromeres and endodermal & mesodermal cells

34
Q

Does the Xenopus also have a blastocoel, like the sea urchin?

A

Yes, but it is localized to the animal half, because of the large yolk at the vegetal half.

35
Q

In the Xenopus, cells of the transient dorsal side will have certain genes activated by beta-catenin, specifically two genes called _____ & ____. Their proteins act as transcription factors (with the aid of another protein smad2) for three genes that give rise to important proteins involved in gastrulation (they help form the organizer region) in Xenopus, named _____, _____ and _____.

A

Siamois & twin

Chordin, goosecoid and noggin.

36
Q

in the Xenopus, do bottle cells form before the blastopore or after?

A

before formation of the blastopore

37
Q

What are bottle cells in the Xenopus?

A

A group of cells on the transient dorsal side that are a key characteristic that gastrulation has begun. Bottle cells pinch the membrane inward (a transient invagination) causing the formation of the blastopore.

38
Q

In the Xenopus, Bottle cells will move inward toward the _____ pole, and create an opening (the blastopore), and the ridge dorsal to the blastopore named the ________________

A

animal pole

dorsal blastopore lip or the dorsal lip of the blastopore

39
Q

Which type of cells at the dorsal lip of the blastopore will start to involute inwards in the Xenopus?

A

mesodermal precursor cells - these cells will later become mesoderm.

40
Q

As the mesodermal precursor cells involute into the interior of the embryo in the Xenopus, where will they line?

A

They line the roof of the blastocoel, the entire wall of the blastocoel at the animal half

41
Q

As involution takes place in the Xenopus, we get the formation of the _______ and the displacement of the _______

A

formation of the archenteron

displacement of the blastocoel

42
Q

in the Xenopus, bottle cells pinch the membrane inwards because of a process called _____ __________ in which the apical end of the membrane of the cell is constricted

A

apical constriction

43
Q

What causes apical constriction in the bottle cells of the Xenopus embryo?

A

A concentration of actin and myosin at the apical end of the bottle cells constricts the apical end of the cell.

At the same time, the expression of cell tight junction component “cadherins” increases in bottle cells, to create more tight junctions in order to help neighbouring bottle cells constrict more.

44
Q

Cadherins of one cell will bind to _______ of the neighbouring cell at the extracellular domain between the cells. This is an example of a homophilic bond

A

another cadherin

45
Q

On the intracellular side of bottle cell tight junctions, the cadherins are attached to _______, which anchor the cadherins to the cell actin cytoskeleton

A

catenins - including beta-catenin, but also alpha and gamma catenin

46
Q

Are bottle cell formations necessary for gastrulation in the Xenopus?

A

No. Despite forming the blastopore, they are not needed for gastrulation to occur. Involution will still happen. They are simply a useful indicator that tells us gastrulation in the Xenopus has begun.

47
Q

What happens in the Xenopus after the bottle cells form the blastopore? What happens to bottle cells during this process?

A

mesodermal precursor cells at the dorsal lip of the blastopore begin to involute against the basal lamina of the epithelial cells forming the animal pole of the embryo, lining the roof of the blastocoel.

Bottle cells get dragged up with the mesodermal precursor cells during this process.

48
Q

What is causing mesodermal precursor cells in the Xenopus to involute inwards?

A

There’s extracellular matrix lining the basal surface of the ectodermal epithelial cells. Within this ECM, there’s a structural protein called fibronectin, whose receptor, like laminin in the sea urchin, is “integrin.”

Precursor mesodermal cells will increase expression of the fibronectin receptor “integrin”, so the interaction between these cells and the ECM of the basal surface of epithelial cells lining the roof of the blastocoel increases.

49
Q

At the same time as this involution, cells of the ectodermal layer of the animal pole undergo _____, until all the mesoderm and endoderm is internalized and only ectoderm covers the outer surface.

A

epiboly/extension

50
Q

True or false. In the Xenopus, the archenteron will begin to displace the blastocoel until the blastocoel is completely gone.

A

True

Developmental Biology Intro (8 decks)

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