1.2: Week 2- Bilaminar Germ Disc ✅

Question 1

Blastocyst composed of

Answer 1

Days 5-6

-ICM (inner cell mass) which forms the embryo- embryoblast

-outer layer, trophoblast- gives rise to extraembryonic structures incl. placenta

-fluid-filled cavity, blastocoel

Question 2

Day 8: implantation

Answer 2

Blastocyst is partially embedded in endometrium

Trophoblast differentiates into 2 layers, cytotrophoblast and syncytiotrophoblast

Question 3

Cytotrophoblast

Answer 3

Inner layer, mononucleated cells

Cells are stem cell-like

Gives rise to
-chorionic villi
-placenta
-syncytiotrophoblast

Question 4

Syncytiotrophoblast

Answer 4

Outer layer, multinucleated zone- syncytium

Comes from fusion of cytotrophoblast cells

Facilitates implantation

Establishes nutrient circulation from mother

Question 5

Chorionic villi

Answer 5

Villi that sprout from chorion
-provide max contact with maternal blood

Chorionic villus sampling can be used for prenatal diagnostics
(10-12 wks)

Question 6

Chorion

Answer 6

Outermost fetal membrane

Question 7

Day 8: bilaminar germ disc

Answer 7

Inner cell mass (embryoblast) differentiates into 2 layers
-hypoblast and epiblast

Endometrial stroma is highly vascularisée

Émergence of amniotic cavity

Question 8

Hypoblast

Answer 8

Small cuboidal cells

Adjacent to the blastocyst cavity

Question 9

Epiblast

Answer 9

High columnar cells

Adjacent to amniotic cavity

Question 10

Day 9

Answer 10

Lacunae stage

Blastocyst is more deeply embedded in endometrium

Vacuoles in syncytium fuse
-to form large lacunae

Question 11

Day 11 and 12

Answer 11

Establishment of uteroplacental circulation

Blastocyst completely embedded in endometrium

Lacunae spaces form a network and connect with maternal sinusoids

-maternal blood enters lacunae system

Question 12

Day 13

Answer 12

Cytotrophoblast cells penetrate into synciotrpphoblast
-forming primary villi

Hypoblast forms a new layer of cells
-that form a new cavity- secondary/definitive yolk sac

Extraembryonic mesoderms form the connecting stalk
-future umbilical cord

Question 13

Primary villi

Answer 13

Cellular columns

Surrounded by syncytium

Question 14

Abnormal blastocysts- Molar pregnancy

Answer 14

Hydatidiform Mole (HM)

Geostationary trophoblastic disease

Most abnormal blastocysts are aborted without producing signs of pregnancy

Rare cases (1:1,000) they implant-> form a HM

Single sperm (90%) or 2 spermatozoa (10%) combine w an oocyte with no DNA
-genotype usually 46XX or 46XY

Can form benign or malignant tumours- invasive mole, choriocarcinoma

Risk factors- maternal age (<20,>35), previous molar pregnancies