Cleavage

Question 1

What happens with the zygote between fertlization and implantation of the embryo in the uterine lining?

Answer 1

Cleavage: which is cell division without overall growth/mass

First - 2-cell stage 
Second - 4-cell stage 
Third - 8-cell stage
At some point between these stages compaction occurs where you cant easily/or at all tell the cells apart
Zona pellucida intact
Forth- 16-cell morula 
Acculmulation of water via Na+K+ ATPase creates a blastocoel
Blastocyst- ~58 cells 
Still surrounded by zona pellucida

Question 2

What are the two layers of the blastocyst and what do they go on to form? How does this decide polarity of the embryo?

Answer 2

Outer layer of cells: trophoblast - placenta and other extraembyronic structures

Inner cell mass: forms embryo and some of the extraembyonic structures

The cell gets polarized because of the placement of the blatoceol and inner cell mass. The inner cell mass being the animal/embryonic pole, and the other side vegetal/abembryonic pole.

Question 3

what is X-inactivation? who does it happen in? when does it happen?

Answer 3

Random inaxtivation of one X chromosome in women

Takes place in first few week of development, when a cell becomes x-inactivated then each of its descending cells are inactivated in the same manner

Genere Xist - on X chromosome that induces X inactivation

Question 4

What happens when the chromosome is coated with Xist protein? Which part is not covered?

Answer 4

Removal of acetyl groups from histone proteins
Methylation of histones
Histone comp is altered

Pseudoautosomal region is not coated or inactivated - - because it pairs with the Y chromosome during meiosis/mitosis

Question 5

What is a barr body?

Answer 5

Inactive X chromosome

Question 6

What is dosage compensation?

Answer 6

Done through X-inactivation

It’s a way that regulates the expression of genes bewteen species so that women and men have the same level of expression of x-chromosome products

Question 7

Regulation

Answer 7

Refers to the ability of an embryo to compensate for the removal or additions of structures/cells.

Question 8

What are the major post fertilization events?

Answer 8

Zygote becomes metabolically active

Zygote undergoes cleavage

Zygote is trasported down uterine tube to uterus - several days

Loss of zone peelucida

Implantation

Question 9

How is cleavage controlled in invertabrate and non-mammalian embryos as opposed to mammalian embryos?

Answer 9

Cleave controls in invert and non-mammal embyros is largely due to traanscription of maternal DNA whereas the embryos gentics rarely get expressed until after blastulation

In mammals, right about the 2 cell stage maternal gene production decreases and degrades and by the 4 cell stage the embryo is expressing it’s own genes

So cleavage is controlled maternally in invert/nonmams and embryologically in mammals.

Question 10

What’s the methlyation cycle?

Answer 10

Eggs and Sperm DNA are highly methylated

Demethylation occurs shortlyafter fertilization
-until early morula

Inner cell mass gets remethlyated untillate blastocyst state

Demethylation occurs again when the primordial germ cells enter genital ridges

Remethylation occurs again during gametogenesis - maternal/paternal imprinting

Question 11

cdx-2

Answer 11

Gene essential for trophoblast cell differentiation

It’s antagonistic towards oct-4

Question 12

Oct-4

Answer 12

In developing oocytes and zygote, it’s requiredfor cleavage intothe 2-cell stage and expression in all morula cells

Question 13

Nanog

Answer 13

Produced by inner cell in the late morula stage and maintains the intergrity of the inner cell mass

–works with oct-4

Question 14

Without nanog inner cells would differenciate into _________________, without oct-4 inner cell would differentiate into ___________________

Answer 14

Endoderm

Trophoblast

Question 15

Sox2

Answer 15

Gene expressed first in the 8 cell stage

Works with oct-4 to control regulation of the genes involed in differentiation

Question 16

What’s the difference between the cell polarity model and inside-out hypothesis for explaining the polarization of embryo?

Answer 16

Inside out hypothesis says that the fate of a cell in a blastomere is dependent on it’s position and not form instrinsic properties.

The cell polarity model says that it depends on the plane of cell division during cleavage:

• if parallel to the embryo the the outer cel becomes trophosblast and the inner cell becomes inner cell mass
• if perpendicular then both daughter cells become trophoblast

Question 17

What is parental imprinting of the DNA?

Answer 17

parental imprinting is inherited methylation of the genome which silences certain genes.

The expression of a gene differs depending on if it was inherited by the mother or the father. Occurs during gametogenesis

Seen in prader-willi ad Angelman syndromes

Question 18

Prader-Willi syndrome

Answer 18

Small hands and feel
Short stature
Poor sexual development 
Mental retardation
Voracious appetites (obesity) 
Always inherited from Father 
-deletion in the long arm of chromosome 15

Question 19

Angelman Syndrome

Answer 19

Frequent laughter 
Uncontrolled muscle movement
Large mouth
Unusual seizures 
Always inherited from the Mother 
-deletion in long arm of chromosome 15

Question 20

How or when are X chromosomes activated or inactivated throughout the mammals life cycle?

Answer 20

Inactive at pronuclei fusion and zygote formation

Materal x chromosome is active at 2-cell, 4-cell and morula stages. Paternal is unknown at 2-cell but inactive at 4-cell and morula stages.

IN the blastocysts ICM both are active, only paternal is active in extraembyronic endoderm and trophectoderm

By embryonic stage either the maternal or paternal X chromosome is permantently inactivated (Xist factor)

Reactivates in the oocyte during meiosis

Question 21

what is bateson’s rule?

Answer 21

A phenomenon often seen in conjoined twins where the organs or the organ body plan layout are reversed

Duplicated structures that are joined during critical developmental stages one is the mirror image of the other

Question 22

Totipotency

Answer 22

The embryonic cells at certain stages that retain the ability to become any structure in the human body

Question 23

Fate mapping

Answer 23

Following the pathway one cell can differenciate

Question 24

Chimera/mosaic

Answer 24

Donor blastomere’s injecting into an embryo and becomes incorporated into the hosts development

Question 25

Transgenic embryos

Answer 25

Created by injecting foreign DNA into the pronuclei of zygotes

Question 26

Knock-out

Answer 26

Inactivation or silencing of genes (not the deletion of them)