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Vertebrate development > Early embryo development - cleavage stages > Flashcards

Early embryo development - cleavage stages Flashcards

1
Q

Embryonic cleavage stages

A

= 1st divisions of embryo following fertilisation
-> mitotic cell cycles begin

Driven by maternal factors

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2
Q

Describe the divisions of the embryonic cleavage stages

A

No increase in overall volume

Short mitotic G1 & G2 phases
(cell growth)

Smaller daughter cells

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3
Q

Maternal factors

A

= factors inherited from oocyte
e.g. protein + mRNA

Asymmetrical inheritance
e.g. germ plasm

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4
Q

Blastomere

A

= embryo cell

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5
Q

Genesis of multicellularity

A

Follows fertilisation in developing embryo

  1. cleavage
  2. cell differentiation
  3. lineage allocation of embryonic cells
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6
Q

Cellular potency

A

= range of commitment options open to a given cell

e.g. pluripotent = give rise to all cell types

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7
Q

What happens to potency in blastomeres as time increases?

A

Potency decreases

-> different cells allocated different fates

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8
Q

Embryonic cleavage

- terminology

A

Cytokinesis
= cell division

Karyokinesis
= nucelar division

Cleavage furrow
= actomyosin ring that divides cells

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9
Q

2 types of embryonic cleavage

A

Holoblastic

Meroblastic

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10
Q

Holoblastic

A

Cleavage furrow completely separates
-> divides blastomeres
= complete cleavage

Sparse, uniform yolk distribution

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11
Q

Meroblastic

A

Only part of blastomere separated
= incomplete cleavage

Yolk impedes membrane formation

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12
Q

Sea urchin embryonic cleavage

A

Holoblastic divisions

- first 7 = stereotypic

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13
Q

Characteristics of sea urchin embryonic cleavage divisions

A

Divisions 1 + 2 = meridional
- pass through animal + vegetal poles

Division 3 = equatorial

Division 4 = cells of animal tier divide meridonally

Blastula = all cells same size

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14
Q

Sea urchin 60-cell embryo

A

Cell fates determined

2-step process in vegetal pole

Animal pole becomes ectoderm

Development through logic circuit of regulated TFs

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15
Q

2-step process in vegetal pole in sea urchin 60-cell embryo

A

Autonomous: maternal factors in large micrometer
-> becomes skeleton

Large micrometers then secrete paracrine factors
-> induce endoderm

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16
Q

Drosophila cleavage type

A

Meroblastic cleavage
= division plane doesn’t go completely through blastomere

Partial cleavage
= restricted to periphery

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17
Q

Drosophila cleavage stages

A

Karyokinesis occurs w/out cytokinesis
-> creates syncytium
(= many nuclei, 1 cytoplasm)

Cellular blastoderm = 6000 cells in <4hrs

18
Q

Drosophila cleavage

- mid-blastula transition

A

Embryonic gene transcription starts

  • initially inherits all maternal factors
  • > eventually needs to activate own genome
  • > switches on own TFs

Cycle 11: maternal mRNAs degraded

19
Q

Zebrafish cleavage type

A

Meroblastic cleavage
= cleavage doesn’t go all way through blastomere
- impeded by yolk

Cleavage restricted to thin region of animal pole
= blastodisc

20
Q

Zebrafish eggs

A

= teleoecithal

= mostly occupied by yolk

21
Q

Zebrafish

- mid-blastula transition

A

At cell cycle 10

= gene expression begins

22
Q

Xenopus cleavage type + symmetry type

A

Holoblastic cleavage
- egg has radial animal-vegetal symmetry

Mesolecithal
(=moderate yolk)
- displaced radial cleavage

23
Q

Xenopus eggs

A

Animal pole more heavily pigmented

region fertilised by sperm

24
Q

Fish embryo fate map

A

Polarised, non-uniform distribution of maternal factors
> pre-patterning
> arrangement of factors becomes 3 germ layers

25
Q

Xenopus cleavage stages

A

Animal pole cleavage impeded by yolk

-> 2nd division starts in animal region before 1st division is finished

26
Q

1st cleavage furrow imitation

A

Starts in animal pole

Initiates with 20micrometre wide contractile band
> actin filaments + myosin-II
> assembled just beneath surface

Surface initially forms periodic stress folds

27
Q

1st cleavage furrow

- filopodia

A

Filopodia interact w/ opposing cell surfaces across cleavage furrow
-> mediates blastomere-blastomere adhesion

28
Q

1st cleavage furrow

-SA needed

A

Extra 20% SA needed

29
Q

1st cleavage furrow

- motor

A

Actin-myosin motor

30
Q

Localisation of maternal factors

A

Pre-patterning in oocytes

If move factors from ectoderm to endoderm side
-> different features develop in wrong places

31
Q

Epigenetics

- DNA methylation

A

On 5-methylcytosine
- lots of methylation in promoter region
= switches gene off

32
Q

Paternal and maternal genome demethylation

A

Tet3 catalyses oxidation
- removes methyl groups

Compartmentalisation means the 2 genomes are demethylated at different times

33
Q

What does Tet3 do?

A

Oxidises 5mC to 5hmC/5fC/5caC

34
Q

Early embryo transcript levels

A

Maternal mRNA levels decline after fertilisation
-> mRNA degraded
= stops being egg + starts being embryo

35
Q

Embryonic gene activation

What happens in species like zebrafish or frogs?

A

Embryo transcription initiates:
> late 1-cell stage in mice
> 4-cell stage in humans

Mid-blastula transition
-> more cells but similar time-frame

36
Q

Lineage specification

A

= 1st embryonic differentiation

Cell fate is dictated in a pre-determined way in asymmetric distribution of maternal factors

37
Q

Lineage specification in mammals

A

Stochastic
= changes randomly over time

2 lineages established:
> trophectoderm
- gives rise to extra-embryonic tissue:
placenta + chorion

> inner cell mass
= gives rise to embryonic tissue

38
Q

What are the lineage specifications in mammals driven by?

A

Driven by interplay between TFs

Trophectoderm
= Tead4 (Hippo signalling pathway)
Cdx2

ICM
= Nanog, Oct4, Essrb, Sox2

39
Q

Molecules involved in lineage specification

- outside cells at 8-16 cell stage

A

Cdx2 levels increase

  • YAP dephosphorylated + enters nucleus with Tead4
  • > Forms co-transcriptional complex
  • > Cdx switches off Oct4, Nanog + Sox2

= Trophectoderm

40
Q

Molecules involved in lineage specification

- inside cells at 8-16 cell stage

A

Cdx2 levels decrease

  • > YAP is cytoplasmic
  • > Cdx2 increases Oct4, Nanog + Sox2

= Inner cell mass

Vertebrate development (12 decks)

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