MSK

Question 1

What is a limb bud?

Answer 1

• structure that appears at end of 4th week
• have a core of proliferating mesenchymal cells with ectoderm covering
• elongation of limb bud is through proliferation of the mesenchymal core
• has thickened ectoderm at apex

Question 2

What are the 3 spatial axes?

Answer 2

1. Shoulder to fingertips (Proximal– distal axis)
2. Palmar surface to dorsal surface (Dorsal – ventral axis)
3. Side–to–side (Anterior – posterior axis)

Question 3

What controls the 3 spatial axes?

Answer 3

1. Proximal – distal axis: Apical Ectodermal Ridge (AER)
2. Dorsal – ventral axis: Ectoderm
3. Anterior – posterior axis: Zone of polarising activity (ZPA)

Question 4

What is the AER - Apical ectodermal ridge?

Answer 4

• at most distal end of limb bud and acts as a major signaling center to ensure maximum limb bud outgrowth
• form along proximal to distal axis

Question 5

What is the ZPA - Zone of Polarizing Activity?

Answer 5

• an area of limb bud mesenchyme that contains signals which instruct the developing limb bud to
• form along the anterior/posterior axis
• controls both patterning and maintains the AER

Question 6

How are hands and feet eventually formed?

Answer 6

• Digital rays are mesenchyme condensations within the plates (they’re cartilaginous models of the digital bones)
• Apoptosis of the tissues in between the digits occurs at the end to create interdigital spaces and shape single digits
• AER breaks up and is maintained only over the tips of the digital rays

Question 7

How does compartmentalization in development of limbs occur?

Answer 7

• myocytes (muscle cells) precursors enter the limb bud and arrange 2 muscle masses on either side of the bone
• dorsal (posterior) =extensor
• ventral (anterior) = flexor
• individual muscles split from the common masses

Question 8

Give examples of some limb defects.

Answer 8

• Syndactyly: Fusion of digits, may involve CT or bones may be fused
• Polydactyly: Extra digits (genetic recessive trait)
• Amelia: complete absence of a limb
• Meromelia: absence of one or more limb structures (e.g phocomelia which is where hands or feet are attached closer to the trunk (legs or arms) so are underdeveloped