Cleavage Flashcards
What happens with the zygote between fertlization and implantation of the embryo in the uterine lining?
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
What are the two layers of the blastocyst and what do they go on to form? How does this decide polarity of the embryo?
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.
what is X-inactivation? who does it happen in? when does it happen?
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
What happens when the chromosome is coated with Xist protein? Which part is not covered?
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
What is a barr body?
Inactive X chromosome
What is dosage compensation?
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
Regulation
Refers to the ability of an embryo to compensate for the removal or additions of structures/cells.
What are the major post fertilization events?
Zygote becomes metabolically active
Zygote undergoes cleavage
Zygote is trasported down uterine tube to uterus - several days
Loss of zone peelucida
Implantation
How is cleavage controlled in invertabrate and non-mammalian embryos as opposed to mammalian embryos?
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.
What’s the methlyation cycle?
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
cdx-2
Gene essential for trophoblast cell differentiation
It’s antagonistic towards oct-4
Oct-4
In developing oocytes and zygote, it’s requiredfor cleavage intothe 2-cell stage and expression in all morula cells
Nanog
Produced by inner cell in the late morula stage and maintains the intergrity of the inner cell mass
–works with oct-4
Without nanog inner cells would differenciate into _________________, without oct-4 inner cell would differentiate into ___________________
Endoderm
Trophoblast
Sox2
Gene expressed first in the 8 cell stage
Works with oct-4 to control regulation of the genes involed in differentiation
What’s the difference between the cell polarity model and inside-out hypothesis for explaining the polarization of embryo?
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
What is parental imprinting of the DNA?
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
Prader-Willi syndrome
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
Angelman Syndrome
Frequent laughter Uncontrolled muscle movement Large mouth Unusual seizures Always inherited from the Mother -deletion in long arm of chromosome 15
How or when are X chromosomes activated or inactivated throughout the mammals life cycle?
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
what is bateson’s rule?
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
Totipotency
The embryonic cells at certain stages that retain the ability to become any structure in the human body
Fate mapping
Following the pathway one cell can differenciate
Chimera/mosaic
Donor blastomere’s injecting into an embryo and becomes incorporated into the hosts development
Transgenic embryos
Created by injecting foreign DNA into the pronuclei of zygotes
Knock-out
Inactivation or silencing of genes (not the deletion of them)
