How is cleavage in the sea urchin different from the starfish?
- no blastula
- unequal cleavage that results in three sizes of cells
What happens to the zygote after fertilization?
- zygote becomes metabolically active
- zygote begins to undergo cleavage
- zygote is transported down uterine tubes to uterus (several day journey)
- zygote loses ZP prior to implantation
- implantation
During cleavage is the ZP. Intact?
Yes
What are the cleavage events outlined in lecture?
- zygote undergo mitosis to form an 8-cell embryo
- cell division continues and compaction occurs
- water is transported into the ball of cells (morula - 16 cells) -> after this the embryo is known as the blastocyst
What happens in compaction?
- outer blastomeres adhere via gap junctions and appear to lose their individual identity
- involves E-cadherins and other calcium dependent CAMs
When does water transportation into the morula occur? What happens with this event? What does this process require?
- occurs 4 days after fertilization
- results in the formation of a blastocoel as a result of cavitation
- requires Na and K-ATPase transporters
What are some features of the blastocyst?
- surrounded by ZP
- large, eccentrically placed blastocoel
- two types of cells (outer: trophoblast, inner: ICM)
- blastocyst is polarized
Why is the blastocyst polarized?
- eccentric placement of blastocoel and ICM
- embryonic pole marks the pole of the blastocyst where the ICM is located
- a embryonic pole marks the opposite pole
What are the cleavage stages of a human?
- fertilized egg
- 2-cell stage
- 4-cell stage
- 11-cell stage morula
- blastocyst (trophoblast and ICM)
- later blastocyst
Outline the genetic control of cleavage in invertebrates and non-mammalian vertebrates.
-early control of cleavage is through gene products transcribed from the maternal genome and embryonic gene products often do not appear until after blastululation
Outline genetic control of cleavage in mammalian embryos.
- maternal gene products are produced but generally are degraded by the 2-cell stage of development
- by the 4-cell stage, most transcription is via the embryonic genome
What does methylation do?
Inactivates genes
DNA of mature eggs ANS sperm are highly methylated, where does demethylation occur?
-demethylation of maternal and paternal genomes occurs shortly after fertilization until the early morula
When does remethylation occur?
- remethylation of ICM occurs until late blastocyst stage
- methylation levels fall after primordial germ cells enter genital ridges
- remethylation occurs later during Gametogenesis and may lead to maternal/paternal imprinting
What is polarization?
-determination of whether cells are destined to become part of the ICM or the trophoblast
When does polarization occur?
-occurs at 8-16 cells stages creating recognizable apical and basal surfaces
What are the two theories of polarity?
- inside out hypothesis
- cell polarity model
What is the inside out hypothesis?
-fate of the blastomeres is determined by its position within the embryo, not from intrinsic properties
What is the cell polarity model?
-depends on plane of cell division during cleavage
+cleavage plane parallel to the outer surface of the embryo
+outer daughter cell -> trophoblast cells (polar)
1) cells pick up a patch of outer cell membrane containing microvilli and ezrin
+inner daughter cell -> ICM cell (a polar)
+cleavage plane perpendicular to outer surface of embryo
1) both daughter cells become trophoblast cells
What are the genes involved in differentiation?
- Cdx-2
- Oct-4
- Nanog
- Sox2
What does Cdx-2 do?
- essential for trophoblast cell differentiation
- antagonistic toward Oct-4
What does Oct-4 do?
- expressed in developing oocytes and zygote
- required to permit cleavage to proceed to 2-cell stage
- expressed in all morula cells
- may play a role in the maintenance of the undifferentiated state
- without Oct-4 inner cells differentiate into trophoblast
What does Nanog do?
- produced by inner cells in later morula stage
- maintains integrity of ICM along with Oct-4
- without Nanog inner cells differentiate into endoderm
What does Sox2 do?
- first expressed in 8-cell stage
- along with Oct-4 it helps to control regulation of genes involved in differentiation
