Lecture 9 (Second week of development)

Question 1

What are Epiblast cells?

Answer 1

Epiblast cells adjacent to the cytotrophoblast are called AMNIOBLASTS and together with the rest of the EPIBLAST line the amniotic cavity

Question 2

Describe the endometrial stroma adjacent to the implantation site

Answer 2

• Oedematous
• Highly vascular

Question 3

What is the function of the large tortuous glands?

Answer 3

Secrete glycogen and mucus

Question 4

What happens on Day 8?

Answer 4

• Blastocyst is partially embedded in the ENDOMETRIAL STROMA
• Trophoblast has differentiated into two layers in the area over the embryoblast
• Cells in the CYTOTROPHOBLAST divide and migrate into the SYNCYTIOTROPHOBLAST where they fuse and lose their individual cell membrane
• Cells of the inner cell mass/embryoblast differentiate into two layers

Question 5

What are the two layers that the cells of the inner cell mass/embryoblast differentiate into in Day 8?

Answer 5

• HYPOBLAST LAYER = a layer of small cuboidal cells adjacent to the blastocyst cavity
• EPIBLAST LAYER = a layer of high columnar cells adjacent to the amniotic cavity

Question 6

What are the two layers the Trophoblast differentiates into in day 8?

Answer 6

• CYTOTROPHOBLAST = inner layer of mononucleated cells
• SYNCTIOTROPHOBLAST = outer multinucleated zone without distinct cell boundaries

Question 7

What happens on Day 9?

Answer 7

• Blastocyst is more deeply embedded in ENDOMETRIUM and penetration defect is closed by a FIBRIN COAGULUM
• Vacuoles appear in SYNCYTIUM and fuse forming TROPHOBLASTIC LACUNAE (LACUNAR STAGE)
• EXOCOELOMIC (HEUSER) MEMBRANE with the HYPOBLAST forms the lining of the EXOCOELOMIC CAVITY called the PRIMITIVE YOLK SAC

Question 8

What happens on Day 11 & 12?

Answer 8

• Blastocyst is completely embedded in the endometrial
• Trophoblastic lacunae are in open connection with MATERNAL SINUSOIDS in the endometrial stroma -> maternal blood enters the lacunar system
• EXTRAEMBRYONIC MESODERM proliferates and fills the space between the EXOCOELOMIC MEMBRANE and the inner aspect of the trophoblast

Question 9

Describe how the Uteroplacental circulation is established

Answer 9

As the trophoblast continues to erode more and more sinusoids -> Maternal blood begins to flow through the trophoblastic system

Question 10

What is the Extraembryonic cavity/Chorionic cavity?

Answer 10

Large cavities that develop in the extra embryonic mesoderm

Question 11

Describe the Decidua reaction that occurs in Day 12

Answer 11

• Cells of the endometrium become POLYHEDRAL and loaded with glycogen and lipids
• Intercellular spaces are filled with extravasate
• Tissue is oedematous

Question 12

What happens on Day 13?

Answer 12

• Surface defect in the endometrium has healed
• Bleeding occurs at the implantation site due to increased blood flow into the lacunar spaces
• Secondary yolk sac/Definitive yolk sac is formed
• Connecting stalk from extraembryonic mesoderm for the UMBILICAL CORD

Question 13

In which day do Primary villi appear?

Answer 13

Day 13

Question 14

What is the Syncytiotrophoblast responsible for?

Answer 14

Hormone production including HUMAN CHORIONIC GONADOTROPIN (hCG) which serves as the basis for pregnancy testing

Question 15

How does the maternal body not reject the embryo despite it being “foreign” since 50% is the father’s genome?

Answer 15

There is a shift from cell-mediated immunity to humoral (antibody-mediated) immunity

[this makes the mother at increased risk for certain infections such as influenza]

Question 16

Where in the uterus can abnormal implantation sometimes occur?

Answer 16

Normally the human blastocyst implants along the ANTERIOR or POSTERIOR wall of the uterus but sometimes it implants close to the cervix -> Obstructs cervix -> Later in development the placenta causes severe, even life-threatening bleeding in the second part of pregnancy and during delivery

Question 17

What are Ectopic pregnancies?

Answer 17

Pregnancies when implantation takes place outside the uterus

Question 18

Where is the most common site for Ectopic pregnancies?

Answer 18

95% if pregnancies occur in the uterine tube and 70% of those in the AMPULLA

Question 19

What happens in ectopic pregnancies?

Answer 19

The embryo dies about the SECOND MONTH of gestation and may result in severe haemorrhaging in the mother

Question 20

Where does the blastocyst attach itself to in the abdominal cavity in Ectopic pregnancies?

Answer 20

• Peritoneal lining of the Rectouterine cavity (POUCH OF DOUGLAS)
• Peritoneal covering of the intestinal tract or to the OMENTUM

Question 21

What is a Hydatidiform mole?

Answer 21

The trophoblast develops and forms placental membranes although little or no embryonic tissue is present

[these moles secrete high levels of hCG and may produce benign or malignant tumours]

Question 22

Describe how large doses of progestins and/or oestrogens inhibit implantation

Answer 22

Large doses of progestins and/or oestrogens do not prevent fertilisation but prevent implantation of the blastocyst -> Endometrium progresses to the LUTEAL PHASE of the menstrual cycle as the zygote forms, undergoes cleavage and enters the uterus -> Large amount of oestrogen disturbs the normal balance between oestrogen and progesterone necessary for implantation

Question 23

How do Intrauterine devices (IUD) work?

Answer 23

• Interferes with implantation causing a LOCAL INFLAMMATORY RESPONSE
• Contain progesterone -> interferes with endometrium development prevention implantation
• Copper wire -> Toxic to sperms and causes uterine endothelial cells to produce substances toxic to sperms