Implantation and Week 2

Question 1

What does a trophoblast cells differentiate (outer crust) into on day 7?

Answer 1

Cytotrophoblast
Syncytiotrophoblasts

Question 2

What does an embryoblast differentiate into on day 7?

Answer 2

Embryoblast cells within the blastocyst differentiate into 2 layers forming bilaminar embryonic disc
-Hypoblast layer
- Epiblast layer

Question 3

What are cytotrphoblast?

Answer 3

continue to act as a crust and maintain good cellular boundaries

Question 4

What are syncytiotrophoblasts?

Answer 4

Lose defined cell boundaries and form a syncytium, produces proteins to modify the maternal immune system and prevent rejection, lack of cell membranes prevents identification of stray proteins and prevents immune system reaching epiblast

Question 5

What is a synctium?

Answer 5

Fluidy pool with lots of nuclei

Question 6

What is the significance of becoming bilaminar?

Answer 6

• First stage of organisation of the body plan
• Establishes the dorsal/ ventral axis of the embryo
• The epiblast forms on the dorsal surface of the embryo and the hypoblast forms on the ventral surface

Question 7

Formation of the amniotic cavity

Answer 7

• A small fluid filled cavity in the epiblast layer
• Occurs within the epiblast layer

Question 8

Full implantation of the Blastocyst

Answer 8

• Syncytiotrophoblast continue to move through the endometrium of the uterus
• They reach the uterine glands and the capillaries within the endometrium and spaces within the syncytiotrophblast form- called trophblastic lacuna (lakes)

Question 9

What do the trophblastic lacuna do during the full implantation of the blastocyst?

Answer 9

• The lakes eventually allow for the diffusion of maternal blood products to leak from the capillaries and supply the episode and hypoblast cells
• Migration of hypoblast cells to encase the blastocyst cavity
• Hypoblast cells are beginning to migrate around the edge of the blastocyte cavity and form a Hausers membrane

Question 10

Uteroplacental circulation (10-11)

Answer 10

• The migrated hypoblast cells formed a membrane which encircles the primary yolk sac
• Amniotic cavity has increased in size
• Extraembryonic mesoderm has formed (important for development of placenta & UC)
• Syncytiotrophoblast erodes walls of maternal capillaries= uteroplacenta circulation

Question 11

What happens to the yolk sac days 10-12?

Answer 11

• A new layer of connective tissue derived from the yolk sac cells forms between the primary yolk sac and cytotrophoblast- extraembryonic mesoderm
• Develops to cover the amniotic cavity too (extraembryonic mesoderm completely surrounds)

Question 12

What is anything chronic?

Answer 12

Placenta

Question 13

Formation of the chronic cavity (12-13)

Answer 13

• Cavity forms in the extraembryonic mesoderm
• 3rd cavity within the embryonic formation

Question 14

Formation of the definitive yolk sac

Answer 14

Simultaneously, a second wave of hypoblast cells migrate to form a secondary (definitive) yolk sac

Question 15

What happens at the end of week 2?

Answer 15

Amniotic cavity and secondary yolk sac are suspended in the chorionic cavity by a connecting stalk

Question 16

The role of Human Chorionic Gonadotrophin

Answer 16

• Secreted by synctiotrophoblasts
• Signals ovary to increase progesterone

Question 17

Role of hCG in pregnancy testing

Answer 17

Syncytiotrophoblast secretes a hormone- hCG
Helps maintain endometrium and may play a role in maternal immunotolerance
Good indicator of pregnancy

Question 18

How common is abnormal implantation?

Answer 18

occurs in 2% of pregnancies and account for 9% of pregnancy related deaths for the mother

Question 19

What is Placenta Previa

Answer 19

Can cause severe bleeding in late pregnancy
Planned C section

Question 20

Tubal implantation

Answer 20

The majority of ectopic pregnancies occur in the uterine tube (80% in the ampulla)
Uterine tube- cannot support pregnancy= rupture

Question 21

Abdominal implantation

Answer 21

There is a gap between the ovary and uterine tube therefore fertilised egg can enter the abdomen
Within the mesentery of the abdomen due to fimbriae of uterus failing to pick up oocyte

Question 22

Lithopaedion

Answer 22

• Can occur when an abdominal pregnancy remains undiagnosed and the foetus dies
• The foetus is too large to be reabsorbed, instead it calcifies
  This is thought to protect mother from the necrotic tissue of the foetus

Question 23

Formation of a complete hydatidiform mole

Answer 23

• Fertilisation of enucleated oocyte by 2 haploid sperm
• Development of the trophoblast without any embryonic tissue
• Due to fertilisation of an empty egg lacking a nucleus
• Suggests that paternal genes favour the formation of the trophoblast at the expense of the embryo- parental imprinting

Question 24

What is the result of a complete hydatidiform mole

Answer 24

• Syncytiotrophoblasts produce hCG
• However no embryoblasts= no embryo
• High hCG= choriocarcinoma (cancer of placental tissue)- 15-20%
• Medical or surgical management

Question 25

Formation of a partial hydatidiform mole

Answer 25

• Normal oocyte fertilised with 2 haploid sperm
• Results in a triploid zygote
• Development of the trophoblast without any embryonic tissue± malformed embryo (not compatible with life)

Question 26

What is the result of a partial hydradiform mole?

Answer 26

• Results in moderately high hCG production
• Normally spontaneously miscarry
• Moderately high risk of choriocarcinoma