Early embryo development - cleavage stages

Question 1

Embryonic cleavage stages

Answer 1

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

Driven by maternal factors

Question 2

Describe the divisions of the embryonic cleavage stages

Answer 2

No increase in overall volume

Short mitotic G1 & G2 phases
(cell growth)

Smaller daughter cells

Question 3

Maternal factors

Answer 3

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

Asymmetrical inheritance
e.g. germ plasm

Question 4

Blastomere

Answer 4

= embryo cell

Question 5

Genesis of multicellularity

Answer 5

Follows fertilisation in developing embryo

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

Question 6

Cellular potency

Answer 6

= range of commitment options open to a given cell

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

Question 7

What happens to potency in blastomeres as time increases?

Answer 7

Potency decreases

-> different cells allocated different fates

Question 8

Embryonic cleavage

- terminology

Answer 8

Cytokinesis
= cell division

Karyokinesis
= nucelar division

Cleavage furrow
= actomyosin ring that divides cells

Question 9

2 types of embryonic cleavage

Answer 9

Holoblastic

Meroblastic

Question 10

Holoblastic

Answer 10

Cleavage furrow completely separates
-> divides blastomeres
= complete cleavage

Sparse, uniform yolk distribution

Question 11

Meroblastic

Answer 11

Only part of blastomere separated
= incomplete cleavage

Yolk impedes membrane formation

Question 12

Sea urchin embryonic cleavage

Answer 12

Holoblastic divisions

- first 7 = stereotypic

Question 13

Characteristics of sea urchin embryonic cleavage divisions

Answer 13

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

Question 14

Sea urchin 60-cell embryo

Answer 14

Cell fates determined

2-step process in vegetal pole

Animal pole becomes ectoderm

Development through logic circuit of regulated TFs

Question 15

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

Answer 15

Autonomous: maternal factors in large micrometer
-> becomes skeleton

Large micrometers then secrete paracrine factors
-> induce endoderm

Question 16

Drosophila cleavage type

Answer 16

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

Partial cleavage
= restricted to periphery

Question 17

Drosophila cleavage stages

Answer 17

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

Cellular blastoderm = 6000 cells in <4hrs

Question 18

Drosophila cleavage

- mid-blastula transition

Answer 18

Embryonic gene transcription starts

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

Cycle 11: maternal mRNAs degraded

Question 19

Zebrafish cleavage type

Answer 19

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

Cleavage restricted to thin region of animal pole
= blastodisc

Question 20

Zebrafish eggs

Answer 20

= teleoecithal

= mostly occupied by yolk

Question 21

Zebrafish

- mid-blastula transition

Answer 21

At cell cycle 10

= gene expression begins

Question 22

Xenopus cleavage type + symmetry type

Answer 22

Holoblastic cleavage
- egg has radial animal-vegetal symmetry

Mesolecithal
(=moderate yolk)
- displaced radial cleavage

Question 23

Xenopus eggs

Answer 23

Animal pole more heavily pigmented

region fertilised by sperm

Question 24

Fish embryo fate map

Answer 24

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

Question 25

Xenopus cleavage stages

Answer 25

Animal pole cleavage impeded by yolk | -> 2nd division starts in animal region before 1st division is finished

Question 26

1st cleavage furrow imitation

Answer 26

Starts in animal pole Initiates with 20micrometre wide contractile band > actin filaments + myosin-II > assembled just beneath surface Surface initially forms periodic stress folds

Question 27

1st cleavage furrow | - filopodia

Answer 27

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

Question 28

1st cleavage furrow | -SA needed

Answer 28

Extra 20% SA needed

Question 29

1st cleavage furrow | - motor

Answer 29

Actin-myosin motor

Question 30

Localisation of maternal factors

Answer 30

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

Question 31

Epigenetics | - DNA methylation

Answer 31

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

Question 32

Paternal and maternal genome demethylation

Answer 32

Tet3 catalyses oxidation - removes methyl groups Compartmentalisation means the 2 genomes are demethylated at different times

Question 33

What does Tet3 do?

Answer 33

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

Question 34

Early embryo transcript levels

Answer 34

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

Question 35

Embryonic gene activation What happens in species like zebrafish or frogs?

Answer 35

Embryo transcription initiates: > late 1-cell stage in mice > 4-cell stage in humans Mid-blastula transition -> more cells but similar time-frame

Question 36

Lineage specification

Answer 36

= 1st embryonic differentiation Cell fate is dictated in a pre-determined way in asymmetric distribution of maternal factors

Question 37

Lineage specification in mammals

Answer 37

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

Question 38

What are the lineage specifications in mammals driven by?

Answer 38

Driven by interplay between TFs Trophectoderm = Tead4 (Hippo signalling pathway) Cdx2 ICM = Nanog, Oct4, Essrb, Sox2

Question 39

Molecules involved in lineage specification | - outside cells at 8-16 cell stage

Answer 39

Cdx2 levels increase - YAP dephosphorylated + enters nucleus with Tead4 - > Forms co-transcriptional complex - > Cdx switches off Oct4, Nanog + Sox2 = Trophectoderm

Question 40

Molecules involved in lineage specification | - inside cells at 8-16 cell stage

Answer 40

Cdx2 levels decrease - > YAP is cytoplasmic - > Cdx2 increases Oct4, Nanog + Sox2 = Inner cell mass