Embryology Week 2

Question 1

What happens to the cavities in weeks 4-6?

Answer 1

• The amniotic cavity expands and presses against the extraembryonic cavity
• Extraembryonic cavity reduces in size
• Yolk sac is being squashed up against the connecting stalk, making it thinner

Question 2

What happens to the cavities in weeks 6-8?

Answer 2

• Extraembryonic cavity almost disappeared
• Yolk sac almost completely inside connecting stalk

Question 3

Formation of the placenta

Answer 3

• Villi project from chorionic plate/extraembryonic mesoderm
  > Stem villi reach the cytotrophoblast shell
  > Free villi branch of the stem villi into the intervillous space
• Maternal blood has filled the intervillous space

Question 4

Parts of the endometrium (from week 8)

Answer 4

• Decidua basalis = endometrium of the placenta
• Decidua capsularis = endometrium surrounding the ‘bump’ of the embryo
• Decidua parietalis = endometrium in rest of uterus

Question 5

Parts of the chorion (from week 8)

Answer 5

• Chorion frondosum = part contributing to placenta
• Chorion laeve = smooth part on other side of embryo

Question 6

What layers fuse together by the end of week 12?

Answer 6

• Amnion
• Chorion laeve
• Decidua capsularis
• Decidua parietalis

Question 7

How are the thoracic and abdominal cavities initially formed?

Answer 7

• Septum transversum pushes in from ventral body wall
• 2 pleuroperitoneal folds push in from the dorsal side
• Eventually all 3 meet and fuse

Question 8

2 points where endoderm and ectoderm directly touch

Answer 8

• Buccopharyngeal membrane (cranial)
• Cloacal membrane (caudal)

Question 9

What happens in the foregut at 28 days?

Answer 9

• Buccopharyngeal membrane breaks down, forming stomodaeum
• Respiratory diverticulum buds ventrally
• Stomach begins to form as the gut tube swelling
• Liver buds ventrally
• Pancreas buds ventrally and dorsally

Question 10

What happens in the hindgut at 28 days?

Answer 10

• Urorectal septum grows to divide the cloaca
  > Dorsal portion
  > Ventral portion = urogenital sinus

Question 11

What happens in the foregut at 35 days?

Answer 11

• Pancreas, liver and lung buds develop more
• Stomach develops more
• Thyroid buds ventrally

Question 12

What happens in the midgut at 35 days?

Answer 12

• Primary intestinal loops extends ventrally into umbilical cord
  > Upper cephalic limb
  > Lower caudal limb
• Vitelline duct connects to apex of the loop
• Superior mesenteric artery branches down middle of the loop to supply midgut
• 2 limbs twist anticlockwise

Question 13

What mesenteries are found in the gut tube and where?

Answer 13

• Dorsal mesentery (all)
• Ventral mesogastrium (stomach)

Question 14

Aortic branches supplying parts of gut tube

Answer 14

• Coeliac artery (foregut)
• Superior mesenteric artery (midgut)
• Inferior mesenteric artery (hindgut)

Question 15

What does the liver separate the ventral mesogastrium into?

Answer 15

• Falciform ligament (liver to ventral wall)
• Lesser omentum (liver to stomach)

Question 16

Where does the spleen develop?

Answer 16

Dorsal mesogastrium

Question 17

How is the greater omentum formed?

Answer 17

• Stomach curves into shape
• Dorsal mesogastrium pulled down

Question 18

How is the lesser sac formed?

Answer 18

• Stomach rotates into position
• Liver pulled to its right and spleen to its left
• Cavity behind stomach is lesser sac

Question 19

What ligaments attach the spleen?

Answer 19

• Gastrosplenic ligament (spleen to stomach)
• Lienorenal ligament (spleen to body wall)

Question 20

How does the pancreas begin to develop?

Answer 20

• Ventral bud swings around gut tube
• Dorsal bud reconnects to ventral bud instead of gut tube