Lecture 6: Cleavage and Implantation

Question 1

When does Cleavage occur?

Answer 1

first event ~24hrs after fertilization
- From unicellular zygote to Multicellular Blastocyst –> increases cell number in embryo exponentially
- not synchronous and no growth beyond 0.1mm diameter

Question 2

What are the events in Cleavage?

Answer 2

1. Division of Zygote to form embryo
2. Involves rapid mitotic divisions with no growth phases
3. Blastomeres form resulting in daughter cells
4. Until 8-cell stage, Blastomeres are totipotent –> important for twinning

Question 3

What happens in Twinning?

Answer 3

dizygotic = two eggs fertilized by 2 different spermatozoa (fraternal)
monozygotic = 1 fertilized egg and 1 spermatozoon (identical)
- Twinning = higher mortality in twins due to preterm delivery

Question 4

What are different types of conjoined twins?

Answer 4

Diprosopus - two faces on same side of single head (form of parapagus) –> <1%

Parapagus - side-by-side connection w/ shared pelvis and cephalic sharing –> 28%

Ischiopagus - conjoined pelvis (6-11%)

Heteropagus - one twin monopolizes placental blood at expense of other

Question 5

How does Compaction work?

Answer 5

• occurs when outer blastomeres adhere together more tightly via E-cadherin –> making cell lose individual identity

Question 6

What is the role of E-cadherin?

Answer 6

mediates cell adhesion between blastomeres through Ca2+-dependent binding
- E-caderin molecules from one blastomere associates homophilically with E-cadherin molecules on adjacent blastomere

Epithin - colocalizes with E-cadherin for the essential of compaction

Question 7

What is the interaction of Cadherin with the Embryo?

Answer 7

Fluorescent microscopy used to detect E-cadherin in embryo by using anti-cadherin antibodies
- blocked with antibodies = will not undergo compaction

Question 8

How does the formation of blastocyst occur?

Answer 8

as cell increase after compaction, Cavitation occurs; fluid secreting between inner blastomeres of blastocyst
- Na+ and H2O accumulates between inner blastomeres
- leads to formation of a blastocoele and division of cells into inner and outer cells mass

Question 9

Describe Molecularity of Cleavage and the transcription factors that are involved in the process.

Answer 9

transition from maternal to zygote-produced gene products –> polarization of individual blastomeres
- 2 cell types: trophoblast and ICM

Cdx-2 gene and Cdx-2
Oct-4 gene and Oct-4
Sox-2 gene and Sox-2
Nanog gene and Nanog

Question 10

What is the end goal for implantation of zygote to blastocyst?

Answer 10

• implantation of embryo via binding and penetration into the uterine endometrium (epithelium)

Question 10

How do human embryos hatch?

Answer 10

prior to implantation, the blastocyst secretes proteases to break through digested zona pellucida
- inability to hatch = infertility –> caused by altered ZP
–> absence of proteases
–> embryo lacking ZP protein leading to premature hatching

Question 11

What happens during cellular differentiation prior to and during implantation?

Answer 11

Formation of synctiotrophoblast:
- differentiation of trophobalast into cytotrophoblast; fusion results in multinucleated syncytiotrophoblast

Formation of Epiblast and Hypoblast:
- inner cell mass –> farms into bilaminar disk

Question 11

What is the sequence of events of Implantation?

Answer 11

1. Apposition and Adherence:
   - embryonic poles near to uterine endothelium
   - adherence
2. Penetration:
   - syncytiotrophoblast invades into uterine tissue, making contact with maternal blood vessels
3. Decidual reaction:
   - uterine tissue responds to invasion by setting up immunological barrier (decidua)

Question 11

What is an Ectopic Pregnancy?

Answer 11

implantation in an abnormal site

Tubual pregnancies = most common type in women with endometriosis, early surgery, or pelvis inflammatory disease
Sites include:
- ampullary portion
- fimbriated end
- uterotubal junction