01-16: Cleavage

Question 1

What are the 6 steps of cleavage in the Starfish?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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

Question 4

What happens to the zygote post-fertilization?

Answer 4

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

Question 5

What are the steps of cleavage in humans?

Answer 5

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

Question 6

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

Answer 6

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

Question 7

Is the ZP still intact during blastomere compaction?

Answer 7

YES because its still in the oviduct

Question 8

How is the morula formed?

Answer 8

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

Question 9

What is a blastocyst surrounded by?

Answer 9

ZP

Question 10

How does a blastocoel divide?

Answer 10

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

Question 11

When does blastomere polarization occur?

Answer 11

at the 8-16 cell stage

Question 12

What does blastomere polarization do?

Answer 12

o Creates recognizable apical and basal surfaces

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

Question 13

What are the 2 theories of Blastomere Polarity Establishment?

Answer 13

• Inside-out Hypothesis

- Cell polarity model

Question 14

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

Answer 14

position of the blastomere within the embryo

NOT determined by intrinsic properties

Question 15

What determines the embryo polarity in the cell polarity model?

Answer 15

plane of cell division during cleavage

Question 16

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

Answer 16

• Outer daughter cell&raquo_space; Trophoblast (POLAR)

- Inner daughter cell&raquo_space; ICM (APOLAR)

Question 17

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

Answer 17

• microvilli

- ezrin (microfilament stabilizing protein)

Question 18

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

Answer 18

BOTH daughter cells&raquo_space; Trophoblast (POLAR)

Question 19

What is the Trophoblast and what does it form?

Answer 19

• Outer layer of Blastocyst

- Forms Extraembryonic structures (Placenta)

Question 20

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

Answer 20

-Inner mass of cells of Blastocyst

• Forms Embryo Proper
• Forms some Extraembryonic structures (yolk sac)

Question 21

Where is the ICM located in the blastocyst?

Answer 21

at the embryonic pole of the blastocyst

Question 22

Where is the Abembryonic pole located?

Answer 22

opposite to the embryonic pole/ICM

Question 23

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

Answer 23

Through gene products transcribed from maternal genome

Question 24

When do the gene products transcribed from maternal genome appear?

Answer 24

after blastulation

Question 25

How is cleavage controlled in Mammalian Embryos?

Answer 25

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

Question 26

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

Answer 26

maternal genome products are generally degraded by 2-cell stage

Question 27

What is methylation?

Answer 27

addition of methyl groups to DNA molecules

Question 28

What does methylation do?

Answer 28

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

Question 29

When does Demethylation occur?

Answer 29

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

Question 30

When does Remethylation occur?

Answer 30

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

Question 31

What does Remethylation after gametogenesis lead to?

Answer 31

maternal/paternal imprinting

Question 32

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

Answer 32

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

Question 33

What has the max potency/potential?

Answer 33

zygote

Question 34

What happens to the potency as the zygote development proceeds?

Answer 34

potency becomes more linear (more differentiated)

Question 35

What are the 4 genes involved in differentiation?

Answer 35

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

Question 36

What is Cdx2 essential for?

Answer 36

trophoblast cell differentiation

Question 37

What is Cdz2 antagonistic toward?

Answer 37

Oct-4

Question 38

Where is Oct-4 expressed?

Answer 38

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

Question 39

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

Answer 39

Oct-4

Question 40

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

Answer 40

Oct-4

Question 41

What produces Nanog genes?

Answer 41

inner cells in the late Morula stage

Question 42

What does Nanog do?

Answer 42

maintains integrity of ICM along with Oct-4

Question 43

What happens to the inner cells without Nanog?

Answer 43

inner cells differentiate into ECTODERM

Question 44

What happens to the inner cells without Oct-4?

Answer 44

inner cells differentiate into TROPHOBLAST

Question 45

When is Sox2 gene first expressed?

Answer 45

in 8-cell stage

Question 46

What does Sox2 gene do?

Answer 46

Helps control regulation of genes involved in differentiation

Question 47

What gene does Sox2 work with?

Answer 47

Oct-4

Question 48

What is genomic imprinting?

Answer 48

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

Question 49

What are 2 examples of genomic imprinting?

Answer 49

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

Question 50

When does parental imprinting occur?

Answer 50

during gametogenesis

Question 51

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

Answer 51

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

Question 52

When is imprinting erased and reestablished?

Answer 52

with each round of gametogeneis

Question 53

What are the clinical manifestations of Prader-Willi?

Answer 53

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

Question 54

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

Answer 54

father **Deletion in long arm of chromosome 15

Question 55

What are the clinical manifestations of Angelman syndrome?

Answer 55

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

Question 56

Who is the mutation inherited from in Angelman syndrome?

Answer 56

mother **Deletion in long arm of chromosome 15

Question 57

When is methylation erased in mammals?

Answer 57

in germ cells of each generation

Question 58

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

Answer 58

because sperm and eggs undergo different levels of methylation

Question 59

When does X inactivation take place in humans?

Answer 59

during first few weeks of development

Question 60

What happens to the X chromosome after it is inactivated?

Answer 60

it remains inactivate in all descendants of the first cell

Question 61

What causes X inactivation?

Answer 61

the action of Xist (X Inactivation Specific Transcript)

Question 62

Where is Xist located?

Answer 62

on the X chromosome

Question 63

What does Xist produce?

Answer 63

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

Question 64

What does coating the X chromosome with Xist RNA cause?

Answer 64

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

Question 65

What region escapes X inactivation?

Answer 65

pseudoautosomal region

Question 66

What is the Lyon Hypothesis?

Answer 66

the Barr body = inactive X chromosome

Question 67

What is X-inactivation an example of?

Answer 67

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

Question 68

Why is Lyon Hypothesis not seen in males?

Answer 68

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

Question 69

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

Answer 69

0 = XO

Question 70

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

Answer 70

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

Question 71

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

Answer 71

2 = XXX

Question 72

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

Answer 72

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

Question 73

How is X-inactivation involved in Tortiseshell cats?

Answer 73

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

Question 74

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

Answer 74

Imprinting = Autosomes X-Chromosome inactivation = Sex Chromosomes

Question 75

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

Answer 75

regulation

Question 76

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

Answer 76

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

Question 77

What is the procedure for producing tetraparental embryos?

Answer 77

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

Question 78

What are types of twins?

Answer 78

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

Question 79

What are the modes of Monozygotic twinning?

Answer 79

- 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

Question 80

What are the 5 types of conjoined twins?

Answer 80

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