Session 1-Development Of The Limbs

Question 1

When do the limb buds appear?

Answer 1

Towards the end of the 4th week

Question 2

Why does lower limb development lag by ~2 days behind upper limb?

Answer 2

The embryo develops in a head to tail order

Question 3

What is a limb bud?

Answer 3

Core of proliferating mesenchymal cells with an ectoderm covering

Question 4

True or false: the limb bud has a thickened mesoderm at the apex of the limb bud

Answer 4

FALSE - ectoderm, not mesoderm

Question 5

What are the three spatial axes in limb development?

Answer 5

1) Shoulder to fingertips (proximal-distal axis)
2) Palmar to dorsal surface (dorsal-ventral axis)
3) Side-to-side (anterior-posterior axis)

Question 6

What is the control of anterior-posterior axial specification?

Answer 6

Zone of polarising activity (ZPA)

Question 7

What is the controller of proximal-distal axial specification?

Answer 7

Apical ectodermal ridge (AER)

Question 8

What is the controller of dorsal-ventral axial specification?

Answer 8

Ectoderm

Question 9

What does the limb bud consist of?

Answer 9

Mesenchyme core (flexible mesoderm) with thickened ectoderm at the apex (apical ectodermal ridge)

Question 10

Complete the sentence:

The elongation of the limb bud is through the ______________ of the mesenchyme core.

Answer 10

Proliferation

Question 11

True or false: AER keeps the mesenchyme that is immediately underlying it undifferentiated

Answer 11

TRUE

Question 12

What is the final stage of limb bud development by AER?

Answer 12

Appearance of paddles

Question 13

Where is the ZPA located?

Answer 13

Posterior base of the limb bud

Question 14

What is the ZPA responsible for?

Answer 14

Generation of asymmetry in the limbs

Question 15

What does the ZPA control? (2)

Answer 15

Patterning and maintains the AER

Question 16

What progressively sculpts interdigital spaces?

Answer 16

Apoptosis

Question 17

When the AER breaks up, where is the only place where it is maintained?

Answer 17

Over the tips of the digital rays

Question 18

What is syndactyly?

Answer 18

Fusion of digits (may involve just CT or bones may be fused)

Question 19

What is polydactyly?

Answer 19

Extra digits, genetic recessive trait

Question 20

What is amelia?

Answer 20

Complete absence of limb

Question 21

What is meromelia?

Answer 21

Partial absence of one or more limb structures

Question 22

What is phocomelia?

Answer 22

Hands or feet are attached directly to the trunk

Question 23

What are the three underlying problems in morphogenesis?

Answer 23

1) malformation
2) deformation - constriction bands eg. Amniotic band syndrome (limb is truncated)
3) disruption by external agent eg. Thalidomide

Question 24

How does thalidomide cause an issue in morphogenesis?

Answer 24

Disrupts AER and stops elongation of limb bones

Question 25

What are the five stages of bone formation?

Answer 25

1) signals from AER to remain undifferentiated stop 2) lateral plate mesoderm condenses and differentiates 3) cartilage model forms 4) endochondral ossification 5) primary and secondary ossification centres

Question 26

Complete the sentences: Limbs extend __________ at first but, as they elongate, they rotate. The upper limb rotates __________ and the lower limb rotates ____________.

Answer 26

Ventrally Laterally Medially