Lect. 6 - Cleavage

Question 1

What events occur following fertilization and prior to implantation as the embryo travels down the oviduct?

Answer 1

zygotes become metabolically active, begins to undergo cleavage (mitotic activity), is transported down uterine tube, and loses ZP prior to implantation

Question 2

Describe the process of cleavage

Answer 2

zygote undergoes mitotic division to form an 8 cell embryo; cell division continues and compaction occurs; ZP is still in tact; water is transported into the ball of cells (morula 16) - 4 days after fertilization and results in the formation of blastocoel; embryo is referred to as a blastocyst at this stage (58 cells)

Question 3

In the blastocyst, what is the outer layer of cells and what is its fate?

Answer 3

trophoblast - forms extraembryonic structures including placenta

Question 4

In the blastocyst, what is the inner mass of cells and what is its fate?

Answer 4

Inner cell mass - forms embryo proper plus some extraembryonic structures

Question 5

Genetic control of cleavage in invertebrates and non-mammalian vertebrates

Answer 5

control through gene products transcribed from the MATERNAL GENOME and embryonic gene products often don’t appear until after blastulation

Question 6

genetic control of cleavage in mammalian embryos

Answer 6

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

Question 7

methylation

Answer 7

the addition of methyl groups to specific regions of DNA molecules; it inactivates genes, including enhancers and promotors

Question 8

Methylation cycle

Answer 8

• demethylation of maternal and paternal genomes occur shortly after fertilization until the early morula
• 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

Question 9

Cdx-2

Answer 9

essential for trophoblast cell differentiation antagonistic toward Oct-4

Question 10

Oct-4

Answer 10

expressed in developing ooctyes and zygotes; required to permit cleavage to proceed to 2-cell stage; expressed in all morula cells; may play a role in undifferentiated state; w/0 Oct-4 inner cells differentiate into trophoblast

Question 11

Nanog

Answer 11

produced by inner cells in late morula stage; maintains integrity of ICM along w/ Oct-4; w/0 Nanog inner cells differentiate into endoderm

Question 12

Sox2

Answer 12

first expressed in 8cell stage; along w/ Oct-4 it helps to control regulation of genes involved in differentiation

Question 13

inside-out hypothesis

Answer 13

fate of blastomere is determined by its position w/i the embryo, not from intrinsic properties

Question 14

cell polarity model

Answer 14

depends on plane of cell division during cleavage

Question 15

In the cell polarity model, what is the outcome when the cleavage plane is parallel to the outer surface of the embryo?

Answer 15

outer daughter cell -> trophoblast cell (polar): cells pick up a patch of outer cell membrane containing microvilli and exrin
inner daughter cell -> ICM cell (apolar)

Question 16

In the cell polarity model, what is the outcome when the cleavage plane is perpendicular to the outer surface of the emrbyo?

Answer 16

both daughter cells become trophoblast cells