3) Embyrological Limb Development

Question 0

Describe the appearance of the limb buds

Answer 0

• originate from activation of somatic lateral plate mesoderm to form limb skeleton
• somites provide the Myotomes for musculature
• appear at 4th week of development, late as they are non-essential to developing embryo
• upper buds appear 2 days before lower

Question 1

The MSK system originate from where in the embryo?

Answer 1

• somites

- lateral (somatic) plate mesoderm

Question 2

Describe the development of the limb buds

Answer 2

• made up of profilerating mesenchymal cells with an ectoderm covering
• extend ventrally at first
• mesenchymal core profilerates to extend the limb with a thickening of ectoderm at the apex aka AER
• AER is made up of cuboidal/columnar epithelia

Question 3

Describe the degrees of symmetry in limbs

Answer 3

1. Proximal-distal, by AER
2. Dorsal-ventral, dorsal skin vs palmar
3. Anterior-posterior, mirror images

Question 4

What is the function of the AER?

Answer 4

• important in outgrowth, p-d asymmetry and final appearance of paddles
• releases signalling molecules to surrounding mesenchymal cells that causes to remain undifferentiated
• when they are further from the AER they differenciate
• creates the dorsal-ventral boundary

Question 5

Describe the formation of digital rays

Answer 5

• Mesenchymal condensations within plates give rise to cartilaginous models of digital bones
• they later undergo endochondral ossification
• fusion of bones also occurs later e.g coxal bone from ilium, ischium and pubis

Question 6

Describe the formation of the digits

Answer 6

• via apoptosis
• AER breaks up and is only maintained over tips of the digital rays
• interdigital spaces are exposed and sculpted via apoptosis

Question 7

Describe the formation of the musculature

Answer 7

• myogenic precursors migrate into limbs from somites with their innervation
• they coalesce into 2 common masses (flexors and extensors)
• later individual muscles split from the common mass
• cervical innervates upper and lumbar the lower
• without innervation development stalls

Question 8

Why are the upper and lower limb extensor and flexor compartments orientated differently?

Answer 8

• due to rotation
• limbs extend ventrally at first
• during elongation they rotate oppositely
• upper rotates laterally and lower rotates medially

Question 9

Describe digital defects

Answer 9

SYNDACTYLY, fusion of digits (CT and or bone)

POLYDACTYLY, extra digits, genetic AR

Question 10

Describe whole limb defects and possible causative agent

Answer 10

AMELIA complete loss
MEROMELIA partial loss
Tetragens, thalidomide, alcohol, drugs, infection (rubella)

Question 11

What happens to the notochord?

Answer 11

• becomes nucleus pulposus

- surrounded by annulus fibrosus