MSK Session 3 (Lecture 3.1)

Question 1

On which side of the developing embryo do the limb buds appear? In what order?

Answer 1

1) Ventro-lateral body wall

2) Upper limb > Lower limb

Question 2

When do limb buds appear?

Answer 2

End of 4th week.

Question 3

What makes up the core and lining of the limb bud?

Answer 3

Core of proliferating mesenchymal cells with an ectoderm covering

Question 4

Where does the limb bud come from?

Answer 4

Activation of mesenchyme within lateral mesoderm derived from the somatic layer of the lateral plate.

Question 5

What does the somatic mesoderm and the somites give rise to?

Answer 5

1) Limb skeleton

2) Musculature

Question 6

What is the AER?

Answer 6

Apical ectodermal ridge

This is a thickened ectoderm covering at the apex of the limb bud

Question 7

Describe the 3 degrees of asymmetry.

Answer 7

1) Proximal - Distal axis e.g. shoulder to fingertips
2) Dorso - Ventral axis e.g. front and back
3) Anterior - Posterior acis e.g. L&R hands mirror images

Question 8

What is the AER important for?

Answer 8

1) Limb bud outgrowth
2) Limb development proximal to distal
3) Appearance of paddles
(Then regresses)

Question 9

How is the P-D axis generated?

Answer 9

1) AER exerts an inductive influence on the immediately underlying mesenchyme to remain undifferentiated and for it to keep growing.
2) The proximal mesenchyme begins to differentiate into constituent tissues e.g. bone as no signal is inhibiting its differentiation
3) AER induces development of the digits within the hand/foot plates

Question 10

How is the D-V axis generated?

Answer 10

AER marks boundary between dorsal & ventral limb ectoderm (which exerts dorsalising and ventralising influences over mesenchyme core)

Question 11

What is the ZPA and its importance?

Answer 11

Zone of polarising acivity

Generation of asymmetry in the limbs (A-P axis). Has a signalling centre located at the posterior base of the limb bud which controls both patterning and maintains the AER.

Question 12

What are digital rays and where are they found?

Answer 12

Mesenchyme condensations within the hand/foot plates. Form the cartilaginous models of digital bones.

Question 13

Where is the AER maintained as it breaks up?

Answer 13

Maintained over tips of the digital rays with the interdigital spaces sculpted by programmed cell death.

Question 14

Which limb is affected more by defects?

Answer 14

Upper limb

Question 15

Describe syndactyly and polydactyly.

Answer 15

Syndactyly - Fusion of digits, may involve CT or bones

Polydactyly - extra digits, recessive trait

Question 16

What is amelia and meromelia?

Answer 16

Amelia - Complete absence of a limb

Meromelia - partial absence of one or more limb structures

Question 17

What migrates and where to start the development of the musculature in limbs?

What do they form?

Answer 17

Myogenic precursors migrate into limbs from somites

Coalesce into 2 common muscle masses around the newly formed skeletal elements (ventral=flexor, dorsal = extensor)

Question 18

What differentiates (muscle related) the U/L from the L/L?

Answer 18

U/L : flexor is anterior, extensor is posterior

L/L : flexor is posterior, extensor is anterior

Question 19

What does rotation of the limb do and how?

Answer 19

Limbs extend ventrally at first but rotate with the U/L rotating laterally and the L/L rotating medially. Brings right muscle compartments in the right place.

Question 20

What is opposite the upper and lower limb buds?

Answer 20

Upper - caudal cervical spinal segments

Lower - Lumbar & sacral spinal segments

Question 21

What do preaxial and postaxial refer to?

Answer 21

Preaxial (thumb and big toe)
Postaxial (little finger and toe)

Axis along AER.