Early Vert Development (6)

Question 1

When are the A/V and D/V axis determined?

Answer 1

• early on

Question 2

Where is there asymmetry in the adult body?

Answer 2

• with the positions of organs in the body
• ex. heart more towards the left side
• with chirality
• ex. coiling of tissue/organ in clockwise versus counterclockwise manner

Question 3

What is situs?

Answer 3

• position (in the body)

Question 4

What is situs solitus?

Answer 4

• the normal disposition of organs

Question 5

What is situs inversus?

Answer 5

• defect or abnormality that can happen with other organs

- organ is ‘flipped’ to wrong side

Question 6

What are some names of defects?

Answer 6

• situs inversus totalis
• left isomerism (polysplenia)
• right isomerism (asplenia)
• situs inversus abdominalis
• situs inversus thoracalis

Question 7

What is situs inversus totalis?

Answer 7

• all organs are ‘flipped’ or mirror imaged

Question 8

What is left isomerism and right isomerism?

Answer 8

• when right side mirrors the left side (like two left sides)
• same for right

Question 9

What is situs inversus abdominalis? Situs inversus thoracalis?

Answer 9

• the abdominal region is flipped

- the thoracic region is flipped (lung and heart)

Question 10

What are symptoms of laterality disorders in humans?

Answer 10

• viable but more vulnerable to certain diseases or conditions
• frequently associated with organ malformations and/or functional disorders

Question 11

What is situs ambiguus?

Answer 11

• anything deviating from situs solitus

Question 12

What conditions are associated with situs ambiguus?

Answer 12

• complex cardiovascular malformations
• anomalies of the spleen and gastrointestinal system
• over 80% of children present with complex congenital heart disease

Question 13

How is left right asymmetry established during development?

Answer 13

• correlative evidence underlying left right asymmetry trace back to the node
• nodal pit separates left and right side

Question 14

Where is nodal expressed?

Answer 14

• using in situ hybridization
• expressed “peri-nodal” (not in node but around it) at 7.5 days
• at 8 days, more pronounced on left side
• left right asymmetry of nodal

Question 15

What family is nodal in? What is it related to?

Answer 15

• TGF-B family

- related to Lefty which is also a TGF-B family member)

Question 16

What is KIF3B?

Answer 16

• microtubule motor protein (kinesin family member)

Question 17

What happens in a KIF3B mutant?

Answer 17

• in KIF3B null mice, lefty is no longer expressed asymmetrically on left side
• cilia are lost in the node

Question 18

What experiment was conducted to test if cilia in the node are important for left-right asymmetry?

Answer 18

• Nonaka
• visualized nodal cilia by dissecting embryo out and putting node on microscope slide with media
• used light microscopy to visualize
• see cilia moving in clockwise circles (when viewed from ventral side)

Question 19

What experiment was done to see if the cilia generate a current?

Answer 19

• use fluorescent beads to see where they travel
• travel from right to left
• generates/establishes asymmetry

Question 20

What experiment was conducted to see if cilia current contributes to left-right asymmetry?

Answer 20

• create an artificial current and use beads to see if overall current changes, then observe effects on asymmetry
• with fast current from left to right, the asymmetry is flipped

Question 21

It appears the flow is important for asymmetry, but what is the actual purpose of the flow?

Answer 21

• could be signalling molecule being released and transported
• could be physical/mechanical force to stimulate peripheral cilia (like hair cells)
• both would activate intracellular signalling cascades and gene transcription
• there are factors in the node but could be a combination of the two above

Question 22

What is Pitx2?

Answer 22

• transgene reporter expression within the heart

- used to visualize heart development

Question 23

What is CAC and TA?

Answer 23

• common atrium chamber
• truncus arteriosus
• examined for reversed or normal asymmetry

Question 24

What is the role of nodal in signalling left-right asymmetry?

Answer 24

• nodal loss led to left-right patterning defects
• sonic hedgehog activated the cerberus homologue caronte
• caronte inhibits BMP which stops BMP from inhibiting Nodal
• Nodal is able to activate transcription factor PITX2 (in the left side)

Question 25

What experiment was conducted to examine the role of Pitx2?

Answer 25

• injection of Pitx2 RNA to the right side at the 8-cell stage
• grow to tadpole and see effect of left-right asymmetry

Question 26

What was seen after Pitx2 overexpression?

Answer 26

• disrupts left-right asymmetry
• situs inversion of the heart
• situs inversion of the gut (chirality of coiling)
• also seen in chicks

Question 27

What is seen with pitx2 knockout mice?

Answer 27

• right pulmonary isomerism of lung

- systemic ventricle in midline and pulmonary ventricle to the right of normal position

Question 28

In summary, what is pitx2’s function?

Answer 28

• Pitx2 is both necessary and sufficient for influencing left-right asymmetry
• Pitx2 –> leftness

Question 29

What is Reiger syndrome?

Answer 29

• autosomal dominant human disorder caused by Pitx2 mutations in humans
• ocular anterior chamber anomalies
• dental hypoplasia
• mild craniofacial dysmorphism
• umbilical stump abnormalities

Question 30

Why doesn’t Reiger syndrome result in left-right defects?

Answer 30

• no complete loss of Pitx2 like in the experiments

- there is still one allele of the gene that is functional

Question 31

What occurs with human nodal mutations?

Answer 31

• certain mutations associated with situs ambiguus

- human nodal mutations: heterotaxy, visceral, 5, HTX5

Question 32

What mutations can lead to human left-right asymmetry disorders?

Answer 32

• nodal
• cripto
• lefty
• acvr2b
• points to early role of these genes in asymmetry