Embryology Week 3

Question 1

Development of the lungs

Answer 1

• Respiratory diverticulum develops into 2 primary bronchial buds
• Lobar bronchi develop from primary buds (2 on left, 3 on right)
• Lobar bronchi divide continuously until terminal bronchioles are formed (week 28)
• Respiratory bronchioles branch off terminal bronchioles
• Terminal sacs at ends of respiratory bronchioles are primitive alveoli

Question 2

What are the 2 horseshoe shapes of island cells to become blood vessels and where are they found?

Answer 2

• Endocardial tubes (outside)
• Dorsal aortae (inside)
• Both found in visceral lateral plate mesoderm

Question 3

What happens to the endocardial tubes on day 22?

Answer 3

• They have moved to the midline with embryonic folding, ventral to the gut tube
• They fuse

Question 4

Structures of the heart at day 23

Answer 4

• Sinus venosus
• Atrium
• Ventricle
• Bulbus cordis
• Aortic roots
• Aortic arches

Question 5

How does the heart twist in week 4?

Answer 5

• Bulbus cordis moves ventrally and right
• Atrium tucks up behind ventricle and left
• Called cardiac loop

Question 6

How many pairs of aortic arches are there when the heart twists and which will remain as the aorta eventually?

Answer 6

• Pair 1, 2, 3, 4, 6
• Left 4th will become aorta

Question 7

3 sections of divided bulbus cordis

Answer 7

• Truncus arteriosus (aorta + pulmonary trunk precursor)
• Conus cordis (outflow tract of both ventricles precursor)
• Primitive right ventricle

Question 8

What does the ventricle of the original heart tube become after twisting?

Answer 8

Primitive left ventricle

Question 9

What does the atrium of the original heart tube become after twisting?

Answer 9

Primitive right and left atria

Question 10

What brings blood from the veins into the primitive right atrium?

Answer 10

Sinoatrial orifice

Question 11

What grows upwards to begin dividing the 2 ventricles?

Answer 11

Muscular interventricular septum

Question 12

What carries blood from the atria to the ventricles?

Answer 12

Atrioventricular canal

Question 13

What growths occur in the atrioventricular canal and what do they do?

Answer 13

• Endocardial cushions
• Separate it into left and right canals

Question 14

How do the atria begin to divide?

Answer 14

• Septum primum grows down from left atrium ceiling (ostium primum under it allows blood flow)
• Holes begin to appear at top and connect to form ostium secundum when septum primum reaches the endocardial cushions
• Septum secundum grows down from right atrium ceiling
• Gap under it is foramen ovale to allow blood flow

Question 15

What growths separate the truncus arteriosus and conus cordis in 2?

Answer 15

Conotruncal ridges (become conotruncal septum)

Question 16

How are the ventricles finally separated?

Answer 16

Conotruncal septum fuses with muscular interventricular septum

Question 17

What 2 channels are formed by the conotruncal septum and where do they receive blood from?

Answer 17

• Pulmonary channel (right ventricle)
• Aortic channel (left ventricle)

Question 18

What veins does sinus venosus originally receive blood from?

Answer 18

• Common cardinal vein
• Umbilical vein
• Vitelline vein

Question 19

Development of the circulatory system

Answer 19

• Vitelline veins forms network called hepatic sinusoids
• Proximal parts of umbilical veins disappear and they drain into hepatic sinusoids
• Distal right umbilical vein disappears
• Ductus venosus carries blood from left umbilical past liver to right hepatocardiac channel
• Distal left vitelline vein disappears
• Right vitelline now called superior mesenteric vein
• Portal vein drains blood from liver to IVC

Question 20

Development of nephric tubules and glomeruli

Answer 20

• Blind-ended tubules develop in the intermediate mesoderm
• Branches of dorsal aorta form capillary knots at the nephric tubules

Question 21

Parts of the intermediate mesoderm (cranial to caudal)

Answer 21

• Pronephros
• Mesonephros
• Metanephros

Question 22

What connects the mesonephros to the cloaca?

Answer 22

Mesonephric duct

Question 23

What connects the metanephros to the mesonephric duct?

Answer 23

Ureteric bud

Question 24

Development of a kidney

Answer 24

• Ureteric bud connects to metanephros at renal pelvis
• Ureteric bud branches into metanephros forming major calyxes
• Minor calyxes branch off major calyxes
• Collecting tubules branch off minor calyxes
• Metanephric tissue condenses to form nephrons with a Bowman’s capsule at the end opposite the collecting tubule
• Tissue layers fuse between nephron and collecting tubule

Question 25

Movement of ureteric bud

Answer 25

- Eventually opens directly into urogenital sinus - Switches place with mesonephric duct to be above it

Question 26

What is the gonadal ridge and what happens to it?

Answer 26

- Block of tissue formed on mesonephros - Epithelium of it grows inwards, making sex cords - Primordial germ cells migrate into the sex cords

Question 27

How is sex determined?

Answer 27

- Sex-determining region of Y gene codes for the SRY protein - This wakes up SOX9 gene on chromosome 17 > Turned on in both males and females, but silenced in females - WNT4 gene silenced in male gonads but unleashed in female gonads

Question 28

Female gonadal development

Answer 28

- Sex cords replaced by cortical cords - Cortical cords form primordial follicles around germ cells - Germ cells start meiosis but don't complete process until they begin to form ova during ovulation

Question 29

Male gonadal development

Answer 29

- SRY expressed in testis cords - Cords remain solid until puberty, where they become hollow and form seminiferous tubules - Sertoli cells produce growth factor causing Leydig cells to differentiate

Question 30

What are paramesonephric ducts?

Answer 30

- Run next to mesonephric ducts - Open at top end into intraembryonic cavity

Question 31

Fate of (para)mesonephric ducts in females

Answer 31

- Mesonephric ducts regress - Paramesonephric ducts fuse at caudal end to form uterovaginal canal (forms upper vagina and uterus - eventually fuses with outgrowths of urogenital sinus which forms lower vagina) - Open ends of paramesonephric ducts become oviducts

Question 32

Fate of (para)mesonephric ducts in males

Answer 32

- Paramesonephric ducts regress - Mesonephric duct forms ductus deferens - Seminal vesicles bud of ductus deferens - Prostate and bulbourethral glands bud of urethra