01-16: Cleavage Flashcards by L Dandridge

What are the 6 steps of cleavage in the Starfish?

Fertilized egg → First cleavage (2 cell stage) → Second cleavage (4 cell stage) → Third cleavage (8 cell stage = morula) → Early blastula → Late blastula

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What are the 6 stages of cleavage in the Sea Urchin?

Fertilized egg → First cleavage (2 cell stage) → Second cleavage (4 cell stage) → Early morula → Late morula → Blastula

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What does the unequal cleavage in the Sea Urchin result in?

Micromeres (small cells), Mesomeres (medium cells), Macromeres (large cells)

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What happens to the zygote post-fertilization?

1- Becomes metabolically active
2- Begins to undergo cleavage
3- Transported down the uterine tube to the uterus (several days)
4- Loses ZP
5- Implantation

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What are the steps of cleavage in humans?

Fertilized egg → First cleavage (2 cell stage) → Second cleavage (4 cell stage) → Morula (11 cell stage) → Blastocyst (Inner cell mass & Trophoblast) → Later Blastocyst

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How does the blastomere adhere to each other so that compaction can occur?

gap junctions, E-cadherins, other Ca+ dependent CAMs

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Is the ZP still intact during blastomere compaction?

YES because its still in the oviduct

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How is the morula formed?

(4 days after fertilization) Na+/K-ATPase transporters draw water into ball of cells → forms morula (16 cells) → cavitation → forms blastocoel

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What is a blastocyst surrounded by?

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How does a blastocoel divide?

Eccentrically placed/pushed towards one pole → causes blastocyst to be polarized

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When does blastomere polarization occur?

at the 8-16 cell stage

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What does blastomere polarization do?

o Creates recognizable apical and basal surfaces

o Determines whether cells destined to become ICM (embryo) or Trophoblastic (placenta)

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What are the 2 theories of Blastomere Polarity Establishment?

Inside-out Hypothesis

- Cell polarity model

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What determines the embryo polarity in the Inside-out hypothesis?

position of the blastomere within the embryo

NOT determined by intrinsic properties

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What determines the embryo polarity in the cell polarity model?

plane of cell division during cleavage

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What happens to the daughter cells if they divide PARALLEL to the cleavage plane?

Outer daughter cell&raquo_space; Trophoblast (POLAR)

- Inner daughter cell&raquo_space; ICM (APOLAR)

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What do the outer membrane cells of the outer daughter cells contain?

microvilli

- ezrin (microfilament stabilizing protein)

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What happens to the daughter cells if they divide PERPENDICULAR to the cleavage plane?

BOTH daughter cells&raquo_space; Trophoblast (POLAR)

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What is the Trophoblast and what does it form?

Outer layer of Blastocyst

- Forms Extraembryonic structures (Placenta)

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What is the Inner Cell Mass and what does it form?

-Inner mass of cells of Blastocyst

Forms Embryo Proper
Forms some Extraembryonic structures (yolk sac)

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Where is the ICM located in the blastocyst?

at the embryonic pole of the blastocyst

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Where is the Abembryonic pole located?

opposite to the embryonic pole/ICM

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How is cleavage controlled in Invertebrate and Non-Mammalian Vertebrate Embryos?

Through gene products transcribed from maternal genome

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When do the gene products transcribed from maternal genome appear?

after blastulation

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How is cleavage controlled in Mammalian Embryos?

Through gene transcription mostly by the embryonic genome (4 cell stage)

Why is Mammalian Embryo cleavage controlled by embryonic genome instead of maternal genome?

maternal genome products are generally degraded by 2-cell stage

What is methylation?

addition of methyl groups to DNA molecules

What does methylation do?

Decreases DNA transcription which inactivates genes (such as enhancers and promoters)

When does Demethylation occur?

Shortly after fertilization >> Early Morula | maternal/paternal genome

When does Remethylation occur?

- Early Morula >> Late Blastocyst state (ICM) | - Gemetogenesis

What does Remethylation after gametogenesis lead to?

maternal/paternal imprinting

After primordial germ cells enter the genital ridges, what happens to the Methylation levels?

Methylation levels fall >> Causes the DNA to become more active

What has the max potency/potential?

zygote

What happens to the potency as the zygote development proceeds?

potency becomes more linear (more differentiated)

What are the 4 genes involved in differentiation?

- Cdx-2 - Oct-4 - Nanog - Sox2

What is Cdx2 essential for?

trophoblast cell differentiation

What is Cdz2 antagonistic toward?

Oct-4

Where is Oct-4 expressed?

- in developing oocytes and zygote | - in all Morula cells

What gene is required to permit cleavage to proceed to the 2-cell stage?

Oct-4

Which gene may play a role in the maintenance of the undifferentiated state?

Oct-4

What produces Nanog genes?

inner cells in the late Morula stage

What does Nanog do?

maintains integrity of ICM along with Oct-4

What happens to the inner cells without Nanog?

inner cells differentiate into ECTODERM

What happens to the inner cells without Oct-4?

inner cells differentiate into TROPHOBLAST

When is Sox2 gene first expressed?

in 8-cell stage

What does Sox2 gene do?

Helps control regulation of genes involved in differentiation

What gene does Sox2 work with?

Oct-4

What is genomic imprinting?

differential gene expression depending on whether a chromosome is inherited from the male or the female parent

What are 2 examples of genomic imprinting?

- Prader-Willi (chromosome 15) - Angelman syndromes **Due to DNA methylation differences in sperm and eggs

When does parental imprinting occur?

during gametogenesis

What happens to the imprinted genes throughout the patient's life?

they are maintained throughout development and perhaps adulthood **permanent once they are an adult

When is imprinting erased and reestablished?

with each round of gametogeneis

What are the clinical manifestations of Prader-Willi?

- small hands and feet - short stature - poor sexual development - mental retardation - voracious appetites

Who is the mutation inherited from in Prader-Willi syndrome?

father **Deletion in long arm of chromosome 15

What are the clinical manifestations of Angelman syndrome?

- exhibit frequent laughter - uncontrolled muscle movement - large mouth - unusual seizures

Who is the mutation inherited from in Angelman syndrome?

mother **Deletion in long arm of chromosome 15

When is methylation erased in mammals?

in germ cells of each generation

Why are there differential expression of male and female alleles in offspring?

because sperm and eggs undergo different levels of methylation

When does X inactivation take place in humans?

during first few weeks of development

What happens to the X chromosome after it is inactivated?

it remains inactivate in all descendants of the first cell

What causes X inactivation?

the action of Xist (X Inactivation Specific Transcript)

Where is Xist located?

on the X chromosome

What does Xist produce?

an RNA molecule that: - Coats the X chromosome - Induces X inactivation

What does coating the X chromosome with Xist RNA cause?

- Removal of acetyl groups from histone proteins - Methylation of histones - Alteration of histone composition

What region escapes X inactivation?

pseudoautosomal region

What is the Lyon Hypothesis?

the Barr body = inactive X chromosome

What is X-inactivation an example of?

- Dosage compensation | - Provides equal expression of X chromosome products in both males and females

Why is Lyon Hypothesis not seen in males?

because they only have 1 X-chromosome ** unless the male has 2 X-chromosomes

How many Barr Bodies can occur in Turner's syndrome?

0 = XO

How many Barr Bodies can occur in Klinefleter's syndrome?

``` 1 = XXY 1 = XXYY 2 = XXXY 3 = XXXXY ```

How many Barr Bodies can occur in Triplo-X syndrome?

2 = XXX

How many Barr Bodies can occur in Poly-X females?

``` 3 = XXXX 4 = XXXXX ```

How is X-inactivation involved in Tortiseshell cats?

Random inactivation of X chromosomes early in embryonic development produces a patchy distribute for the expression of X-linked characteristics **Single X-linked locus determines orange color

What is the difference between imprinting and X-chromosome inactivation?

Imprinting = Autosomes X-Chromosome inactivation = Sex Chromosomes

What is the ability of an embryo to compensate for removal of structures or for addition of structures?

regulation

What are some of the experimental methods of properties of early embros?

- Chimeras/Mosaics - Fate mapping - Totipotency - Production of tetra/hexaparental embryos - Production of interspecies chimeras - Deletion or ablation experiments - Addition experiments - Transgenic embryos - KO experiments

What is the procedure for producing tetraparental embryos?

Cleavage stages of 2 different strains of mice >> Removal of ZP >> Fusion of the 2 embryos >> Implantation of embryos into a foster mother >> Chimeric offspring obtained from the implanted embryos

What are types of twins?

- Fraternal (dizygotic) - identical (monozygotic) - Conjoined & Parasitic

What are the modes of Monozygotic twinning?

- Cleavage of an early embryo, with each half developing as a completely separate embryo - Splitting of the ICM of a blastocyst and the formation of 2 embryos enclosed in a common trophoblast - If ICM doesn't completely separate, or if portions of the ICM secondarily rejoin, conjoined twins may result

What are the 5 types of conjoined twins?

- Cephalopagus (head to head) - Pygopagus (rump to rump) - Massive fusion of head and trunk - Cephalothoracopagus (head and thorax) - Thoracopagus (chest to chest)