6M: Bilaminar Germ Disc

Question 1

Fertilization

Answer 1

• oocyte (plus corona radiata cells) is ovulated from the ovary and swept into the uterine tube
• spermatozoa (sperm) must undergo:
  
  1. capacitation
  2. acrosome reaction
• sperm can be viable in female reproductive tract for several days
• fertilization occurs 12-24 hours following ovulation (likely occuring in the ampulla)

Question 2

define:
a. capacitation
b. acrosome reaction

Answer 2

a. biochemical changes to sperm in the uterine tube (removal of glycoprotein coat and seminal plasma proteins & exposure of receptors on sperm acrosome cap)
- allows sperm to penetrate corona cells
b. leads to the penetration of the Zona Pellucida
- sperm bind to the ZP through ZP3 (ligand for receptor on sperm). That binding induces enzyme release from sperm acrosomal cap.

Question 3

define:
a. cortical reaction
b. zona reaction

Answer 3

a. contact leads to release of lysosomal enzyme from the cortical granules in the oocyte plasma membrane
b. ZP properties are altered to prevent polyspermy

Question 4

Development of zygote (up to 16 cell stage)

Answer 4

2 cell stage: 30 hpf
4 cell stage: 40 hpf
16 cell stage: 3 days pf (morula)
- ZP persists until early blastocyst stage (about 5 days pf)
- at 8-cell stage, compaction occurs (segregation of inner and outer cells). loss of totipotency

Question 5

Formation at 3 days

Answer 5

• morula is formed by 3 days with inner and outer cell masses
  inner: embryo proper, amnion, yolk sac
  outer: trophoblast
• enters uterine cavity at about 4 days

Question 6

Formation at 4.5-6 days

Answer 6

day 4.5-5: fluid penetration into intercellular spaces of the inner cell mass results in blastocoele cavity (creating the blastocyst)
day 5: blastocyst “hatches” from the ZP. Can’t insert into endometrium without hatching.
day 6: Implantation occurs by cell-cell and cell-ECM interactions

Question 7

Formation at day 7.5

Answer 7

both trophoblast and embryoblast differentiate into 2 layers
Embryoblast: 1. epiblast (with amnioblasts) 2. hypoblast
Trophoblast: 1. syncytiotrophblast: outer multinucleated cell layer. produces hCG which maintains CL 2. cytotrophoblast: inner mononucleated layer that divides and migrates into syncytiotrophoblast and fuse

Question 8

Formation at day 9

Answer 8

Deeper Embedment

• lacunae formation
• exocoelomic cavity formation (primitive yolk sac)

Question 9

Formation at day 12

Answer 9

• Syncytiotrophoblast erode maternal capillaries
• trophoblast lacunae connect to maternal sinusoids
• blastocyst starts to protrude into uterus lumen
• extraembryonic/chorionic cavity forms
• exocoelomic membrane splits to become extraembryonic splanchnopleuric membrane and somatopleuric membrane

Question 10

Formation at day 13

Answer 10

• secondary yolk sac forms
  
  • exocoelomic cysts pinch off
• primary villi form (initial stages of uteroplacental circulation)
  
  • this can lead to a little bit of bleeding
  • cytotrophoblast cells surrounded by
    syncytiotrophoblast
• connecting stalk forms (later to become the umbilical cord)