Embryology Week 1

Question 1

Ovulation

Answer 1

• Follicle develops in ovary
• Follicle fills with fluid and bursts
• Egg released in cloud of cells
  > Corona radiata directly around egg
  > Cumulus oophorus outer layer

Question 2

Where does sperm meet the egg?

Answer 2

Ampulla of the oviduct

Question 3

How does the egg stop multiple sperm entering?

Answer 3

• Zona pellucida is sugary jelly surrounding egg
• Egg releases enzymes to harden it when sperm membrane fuses with it

Question 4

What happens after fertilisation?

Answer 4

• Genetic information of both gametes doubles up to make 2 pronuclei
• Pronuclei fuse to form 1 46 chromosome pair nucleus
• Egg divides to 2 cells in first 24h
• 16 cell morula after 3 days

Question 5

Formation of a blastocyst

Answer 5

• Morula becomes leaky and takes in fluid to form a fluid-filled cavity
• Zona pellucida dissolves leaving trophoblast outer shell and inner mass of embryoblast

Question 6

Implantation of blastocyst

Answer 6

• Blastocyst comes to anterior or posterior endometrium
• Finger-like projections from trophoblast burrow into endometrium (day 6)
• By day 7 blastocyst completely embedded

Question 7

What does the trophoblast divide into on day 8?

Answer 7

• Cytotrophoblast (inner cellular layer)
• Syncytiotrophoblast (outer multinucleate mass)

Question 8

What does the embryoblast divide into on day 8 and where are they located?

Answer 8

• Hypoblast (cuboidal cells facing original blastocyst cavity)
• Epiblast (taller cells lining new amniotic cavity)

Question 9

Formation of the yolk sac

Answer 9

On day 9 hypoblast cells migrate to re-line the blastocyst cavity, forming the yolk sac

Question 10

What form in the syncytiotrophoblast on day 9?

Answer 10

Gaps called lacunae

Question 11

What new tissue layer forms on day 11 and where is it found?

Answer 11

• Extraembryonic mesoderm
• Surround epiblast and hypoblast under the cytotrophoblast

Question 12

What happens to maternal capillaries on day 11?

Answer 12

Swell into sinusoids and enter the lacunae

Question 13

What cavity forms in the extraembryonic mesoderm by the end of week 2?

Answer 13

Extraembryonic/chorionic

Question 14

What is the connecting stalk

Answer 14

Extraembryonic mesoderm that connects the embryo to nutritional supply outside the extraembryonic cavity

Question 15

What 2 structures can be seen in the epiblast when gastrulation starts?

Answer 15

• Primitive streak
• Oropharyngeal membrane

Question 16

What happens at the primitive streak and what is formed?

Answer 16

• Cells from the epiblast fold in on themselves
• Form the intraembryonic mesoderm between the epi- and hypoblast

Question 17

What are the epiblast and hypoblast referred to as when the intraembryonic mesoderm has formed?

Answer 17

• E = ectoderm
• H = endoderm

Question 18

Where does the notochord form from?

Answer 18

Epiblast cells folding in from the primitive pit of the primitive streak

Question 19

Divisions of the mesoderm (from closest to furthest from the notochord)

Answer 19

• Paraxial mesoderm
• Intermediate mesoderm
• Lateral plate mesoderm

Question 20

What happens with the lateral plate mesoderm dividing?

Answer 20

• A cleft forms, so separate mesoderm is connected to the endoderm and ectoderm
• Endoderm = visceral mesoderm
• Ectoderm = parietal mesoderm

Question 21

What makes the ectoderm fold in to begin formation of the neural tube?

Answer 21

Proteins secreted when genes are switched on in the notochord

Question 22

What are the 2 openings in the neural tube and when do they close?

Answer 22

• Anterior neuropore (day 25)
• Posterior neuropore (day 28)

Question 23

What are somites?

Answer 23

Bulges either side of the neural tube made of the paraxial mesoderm

Question 24

How are neural crest cells formed?

Answer 24

• Begin at the top of the neural folds
• When the neural tube separates, they’re released either side of it

Question 25

What happens during embryonic folding?

Answer 25

• Ectoderm covers outside of the body
• Amniotic cavity pulled around embryo
• Connecting stalk and vitelline duct pushed together (origin of umbilical cord)