S1) Embryology of the Musculoskeletal System Flashcards

1
Q

When do the limb buds appear?

A

The limb buds first appear as small projections on the lateral body wall during the fourth week of development

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2
Q

Describe the structure of the limb buds

A
  • The buds consist of a mass of mesenchyme covered by a layer of ectoderm
  • At the tip of the bud the ectodermal cells divide to form an apical ectodermal ridge
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3
Q

Which limb develops first?

A

Upper limb buds develops a few days before the lower limb buds

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4
Q

What drives limb elongation?

A

Proliferation of mesenchyme (induced by AER)

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5
Q

As the limbs elongate, they rotate.

Describe the rotation of the upper and lower limbs

A
  • The upper limb rotates laterally through 90° so extensor muscles lie on the posterior aspects of the limb
  • The lower limb rotates medially through almost 90° so extensor muscles lie on the anterior aspects of the limb
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6
Q

Identify the three spatial axes in limb development

A
  • Proximal-distal axis

- Dorsal-ventral axis

- Anterior-posterior axis

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7
Q

Identify the controllers of axial specification in limb development

A

- Zone of Polarising Activity controls anterior-posterior axis

- Apical Ectodermal Ridge controls proximal-distal axis

- Ectoderm controls dorsal-ventral axis

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8
Q

What does the AER do?

A

AER lies immediately above the mesenchyme and orchestrates limb development from proximal to distal then regresses

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9
Q

Describe how the proximal-distal axis is generated by the AER

A
  • AER exerts an inductive influence on mesenchyme so it remains undifferentiated
  • Proximal mesenchyme begins to differentiate into constituent tissues
  • AER induces development of the digits within the hand/foot plates
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10
Q

What is the Zone of Polarising Activity and what does it do?

A
  • ZPA is a signalling centre located at the posterior base of the limb bud
  • It controls maintains the AER and generates the asymmetry of the limbs (anterior – posterior axis determination)
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11
Q

What are the digital rays?

A
  • Mesenchyme condensations within plates
  • Cartilaginous models of the digital bones
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12
Q

Describe how apoptosis sculpts the digits of the upper and lower limbs

A
  • AER breaks up and is maintained only over the tips of the digital rays
  • Interdigital spaces are progressively sculpted by programmed cell death
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13
Q

Describe the innervation of the upper and lower limb buds

A
  • Upper limb buds appear opposite the caudal & cervical spinal segments
  • Lower limb buds appear opposite the lumbar & sacral spinal segments
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14
Q

Describe the development of limb compartments

A
  • Myogenic precursors migrate into limbs from somites (body segments)
  • They coalesce into 2 common muscle masses around the newly formed skeletal elements
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15
Q

Identify the two compartments in the limbs

A
  • Flexor compartment (ventral)
  • Extensor compartment (dorsal)
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16
Q

Identify and describe 3 underlying problems in morphogenesis

A
  • Malformation – intrinsic error in coordination of morphogenesis
  • Deformation – constriction bands
  • Disruption – external agent e.g. thalidomide
17
Q

What is syndactyly?

A

Syndactyly is the fusion of digits (connective tissue / bones)

18
Q

What is polydactyly?

A

Polydactyly is an autosomal recessive trait resulting in extra digits

19
Q

What is amelia?

A

Amelia is the complete absence of a limb

20
Q

What is meromelia?

A

Meromelia is the partial absence of one or more limb structures