Implantation, Attatchment And Invasion

Question 0

Blastocyst formation

Answer 0

Asymmetrical divisions leads to segregation and differentiation-> Outer layer-> trophoectoderm, epithelial wall of blastocyst-> forms placenta
Inner cell mass-> embryoblast-> forms embryo, eccentrically located
Zona pellucida prevents blastomeres falling part during early cleavage and prevents dizygotic twins sticking together
Morula enters uterine cavity-> fluid penetrates zona pellucida-> forms blastocyst cavity
Zona pellucida has disappeared allowing implantation to begin

Question 1

Transport of morula from ampulla to uterus

Answer 1

After the two cell stage-> series of mitotic divisions which incree cell number but not conceptus size, this happens as it travels along the Fallopian tube
Travel is facilitated by increase in ratio of progesterone to oestrogen in the luteal phase-> muscular contractions of Fallopian tube, relaxation of isthmic sphincter
Wafting of cilia

Question 2

Implantation

Answer 2

Occurs during the luteal phase if endometrium-> glands are secretory->provide nutrition for continued growth of trophoblast
Trophoblast cells begin to penetrate the endometrium over the embryonic pole

Question 3

Pituitary-ovarian axis

Answer 3

Maternal recognition of the pregnancy required-> stop cyclic FSH
Implanting trophoblasts secrete hCG.
hCG carried to ovary-> binds LH receptors on corpus luteum-> supresses lutenization-> prolonged corpus luteum life
Corpus luteum-> progesterone (2-6 weeks or pregnancy) till placenta takes over
Progesterone prevents further ovulation and maintains luteal phase of endometrium

Question 4

Short range cross talk during attachment

Answer 4

Blastocyst:
Dissolution of zona pellucida
Adhesion between trophoblast and endometrial cells
Endometrium:
Stromal reaction-> controlled by progesterone but oestrogen required
Initiated by attachment 
Loss of glycocalyx
Loss of anionic charge of microvilli
Thinning of mucin coat
Increase vascular permeability
Spreads away from attachment site

Question 5

Decidualisation of endometrium

Answer 5

During invasion->continued decidual response forms
Decidua basalis-> endometrium under conceptus
Decidua capsularis-> superficial, overlying conceptus
Decidual parietalis-> remaining uterine mucosa
Allows invasion, helps nutrition, protects from maternal immune system, she’d at brith

Question 6

Molecular language of attachment

Endometrium

Answer 6

Glands(oestrogen)-> leukaemia inhibitory factor-> increases receptivity and decidualisation (come hither message)
Epithelial cells-> heparin binding ECF-like growth factors and heparin sulphate protecoglycans-> proceeds dissolution of zona pellucida
Cell surface mucin is down regulated as this is a ‘go away’ message, would cause miscarriage ever wise

Question 7

Molecular language of attachment

Trophoblasts

Answer 7

Heparin sulphate protecoglycans
Bind to receptors in endometrium
EGF also binds
Blastocysts without EGF receptors die

Question 8

Formation of synctiotrophoblast

Answer 8

8th day, blastocyst partially embedded
Trophoblast differentiates in to two layers
Inner single layer of cytotrophoblasts (stem cells)
Outer multinucleated layer without distinct cell boundaries-> syncytiotrophoblast
Synctiotrophoblast lack proliferate capability so are maintained by continual fusion if undying cytotrophoblasts

Question 9

Molecular language of invasion, trophoblast

Answer 9

Synctiotrophoblast-> numerous finger like projections in to endometrium
Release proteases-> allow blastocyst to burrow in to endometrium

Question 10

Molecular language of invasion, endometrium

Answer 10

Histamine, prostaglandins, TGF-beta, PDGF, EG,IGF-> pro inflammatory response
VEGF, proliferin
Loosening of epithelial cells

Question 11

Invasion

Answer 11

Burrowing and expanding of syncytial processes-> further degradation of decidual tissue
Nutrients provided for yolk sac by-> secretions from glands and degradation of decidual tissue
Historophic nutrition, first 10 weeks

Question 12

Lacunae formation

Answer 12

Day 9
Synctiotrophoblast digests endometrial tissue
Small vacuoles in synctiotrophoblast form-> fuse-> lacunae
Day 11-12
Blastocyst now completly embedded
Synctiotrophoblast burrows deeper-> erodes endothelial lining of maternal capillaries (sinusoids)
Lacunae become continuos with sinusoids-> fill with maternal blood
More and more sinusoids eroded-> maternal blood flows through trophoblasts system-> uteroplacental circulation

Question 13

Cytotrophoblasts differentiation

Answer 13

Proliferate and fuse-> synctiotrophoblast-> projections in to lacunae-> branch-> free floating chorionic villi
Columns of cells that remain as discrete cells-> protrude deep in to endometrium-> anchoring cells-> anchoring villi
Migrate into endometrium up to first grid of myometrium
Invade endometrial maternal blood vessels-> extra villus trophoblasts