Lecture 5: Cleavage Flashcards
Describe events that occur following fertilization and prior to implantation as the embryo travels down the oviduct.
zygote. ..
1. becomes metabolically active
2. begins to undergo cleavage
3. transported down the uterine tube to the uterus
4. journey takes several days
5. zygote loses the zona pellucida prior to implantation
Describe the process of cleavage in the mammalian embryo, including the formation of the blastocyst.
- mitosis
- cell division and compaction when the blastomeres adhere via gap junctions (E cadherins and Ca2+ dependent CAMS)
- zona pellucida intact
- water is transported to the morula (4 days after fertilization, Na/ATP transporters
- embryo is now a blastocyst
Describe and compare trophoblast cells and cells of the inner cell mass. what is their fate? How do these cells determine polarity of the embryo?
trophoblast is the outer layer of cells; and the inner mass cells are internal.
The inner mass cells from the embryo and the trophoblast forms the placenta and other structures
Compare genetic control of cleavage between mammalian and non-mammalian embryos.
non-mammalian: genes transcribed from the mom and gene products will not appear until after blastulation
mammalian: maternal gene products are produced but degraded by the 2 cell stage of development
by the 4 cell stage, most transcription embryonic genome is in control.
Describe the methylation cycle in mammalian gametes and embryos.
- DNA of mature eggs and sperm is highly methylated
- from fertilization to the early morula the parental DNA is demethylated
- remethylated until late blastocyst state
- methylation levels fall after primordial germ cells
- remethylation occurs during gametogenesis and may lead to maternal and paternal imprinting
Describe the role of the genes Cdx-2, Oct-4, Nanog and Sox2 in early differentiation of the embryo.
cdx-2: trophoblast cell differentiation
Oct-4: expressed in morula cells; maintenance of undifferentiated state; required to proceed to the 2-cell stage; keeps inner cells from differentiating into the trophoblast
Nano: maintains integrity; keeps inner cells from differentiating into the endoderm
Sox2: controls regulation of genes involved in differentiation; in 8 cell stage
Compare the inside-out hypothesis and the cell polarity model in the polarization of the blastomeres at the 8-16 cell stage.
inside out- fate of the blastomere is determined by its position in the embryo, not from intrinsic properties
cell polarity: depends on the plane of the cell division during cleavage.
What happens if the cleavage plane is parallel to the outer surface of the embryo?
outer daughter cell becomes the trophoblast cell (polar)
inner daughter cell becomes the inner mass cell (apolar)
What happens if the cleavage plane is perpendicular to the outer surface of the embryo?
both daughter cells become trophoblast cells
Define genomic imprinting and explain how it is related to methylation of DNA.
differential gene expression depending on which parent the chromosome is inherited from
differences in DNA methylation can lead to different disease
Compare Prader-Willi and Angelman Syndromes.
both prader will and angel man syndrome are due to deletions in chromosome 15 but prader will is from the father and angel man is from the mother
prader will results in short stature, overweight bc appetite and mental retardation
angelman results in frequent laughter, large mouth, seizure, and uncontrolled muscle movements
What is the purpose of X-inactivation; how does it relate to dosage compensation; and
how does it relate to the Xist gene?
brought about with the action of Xist; which removes acetyl groups from histone proteins and the methylation of histones and alternation of histone composition
WHY?: provide equal expression of X-chromosome products in both males and females
What are Barr bodies? What do they tell us about a person’s genome?
Barr bodies signify an inactive X chromosome
Describe X-chromosome inactivation and reactivation throughout the mammalian life cycle.
Random inactivation can cause variations in the phenotype of the mammal; in cats in can cause a variety of fur colors
Define “regulation” and explain how it related to embryonic development.
refers to the ability of the embryo to compensate for removal of structures for addition of structures
Describe the modes of monozygotic twinning, conjoined twins (including types) and parasitic twins.
monozygotic: cleavage of an early embryo, with each half growing separately
conjoined: fusion of the babies
