Fertilisation, Implantation and Bilaminar Disc Formation

Question 1

Where is the optimal site for fertilisation?

Answer 1

The ampulla

Question 2

How long does it take for spermatozoa to reach and how long are they viable?

Answer 2

Reach ampulla 5-7hrs after sex and viable for several days

Question 3

When does fertilisation usually occur?

Answer 3

12-24hrs after ovulation

Question 4

Outline the key events during fertilisation

Answer 4

1. Sperm penetrates the corona radiata via membrane-bound enzymes in plasma membrane of its head and binds to ZP3 receptors on zona pellucida
2. Binding to ZP3 causes acrosome rxn, releasing enzymes onto zona pellucida.
3. Acrosomal enzymes digest zona pellucida making path through plasma membrane to ovum. Once reached they fuse.
4. Sperm head with DNA enters ovum’s cytoplasm.
5. Sperm stimulates release of enzymes stored in cortical granules in ovum, which stop ZP3 & harden zona pellucida to block polyspermy.

Question 5

Give summary of 1st week of human development

Answer 5

1. 12-25hrs after sex fertilisation occurs
2. Once sperm enters the oocyte goes through 2 meiotic divisions and the 2 nuclei fuse
3. 2 cell stage after 30hrs
4. 4 cell stages after 40hrs
5. Morula at 3 days (just when entering uterus)
6. Blastocyst on day 4.5-5
7. Attaches to endometrium on day 5.5-6

Question 6

What is the inner cell mass called and what does it become?

Answer 6

Called the embryoblast and become the hypoblast (inner aspect & the epiblast (outer aspect)

Question 7

What it the outer cell mass called and what does it become?

Answer 7

Called the trophoblast and becomes cytotrophoblast (inner aspect) & the syncytiotrophoblast (outer apsect)

Question 8

When does the formation of bilaminar germ discs occur?

Answer 8

By day 8

Question 9

What happens on day 8 of blastocyst development?

Answer 9

Syncytiotrophoblast grows into endmetrial stroma and bilaminar germ discs form.

Question 10

What happens on day 9 of blastocyst development?

Answer 10

1. Known as lacunar stage cos of formation of trophoblastic lacunae.
2. Fibrin coagulum froms sealing off blastocyst from uterine cavity ie its fully embedded in uterine wall
3. Exocoelomic cavity (primative york sac) & exocoelomic (Heuser’s) membrane form

Question 11

What happens on day 11-12 of blastocyst development?

Answer 11

1. Trophoblastic lacunae enlarge and eat into maternal sinusoids causing them to fill with blood creating the uteroplacental circuation
2. Primative yolk sac enlarges
3. Extraembryonic somatopleuric mesoderm, extraembryonic splanchnopleuric mesoderm & extraembryonic coelom (chorionic cavity) form.

Question 12

What happens on day 13 of blastocyst development?

Answer 12

1. Even more blood enters trophoblastic lacunae
2. Formation of connecting stalk (to become unbilical cord), secondary yolk sac & primary villi which spread out into endometrial stroma
3. Bilaminar disc will gastrulate to form 3 germ layers