Upper Limb Embryology

Upper Limb Embryology

Upper Limb development occurs during weeks 4-8 of gestation. This article details the mechanisms of development and critical milestones.

Summary Card


Overview
Upper limb development occurs during weeks 4-8 of gestation from the mesoderm and ectoderm.

Milestones
Limb bud appears on day 26 and differentiates into a limb by day 58.

Mechanisms
Upper limb embryogenesis is based on 3 axes, 3 signalling zones and 3 proteins.


Overview of Upper Limb Embryology

The upper limb develops during weeks 4-8 of gestation from mesoderm and ectoderm. More specifically,

  • 3 axis: proximodistal, anteroposterior, dorsoventral
  • 3 centres: apical ectodermal ridge, zone of polarizing activity, dorsal ectoderm
  • 3 proteins: fibroblast growth centre, sonic hedgehog protein, WNT7a.

These critical elements of embryological development are illustrated below.

Upper Limb Embryology

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Fun Fact: The lateral plate of the mesoderm forms bone, cartilage and tendon. The somatic mesoderm differentiates into myofibroblasts that muscle. 


Developmental Milestones


Key Point

These are general guidelines for the development milestones of limb embryogenesis.


The upper limb differentiates into a 'human-appearing' limb during 4-8 weeks of gestation. Here are some general timelines to consider

  • ~ Day 26: Limb bud is present
  • ~ Day 33: Paddle-shaped hand
  • ~ Day 36: Nerves appear
  • ~ Day 42: Webbed-shaped hand.
  • ~ Day 48: Palms move anteriorly and elbows flex
  • ~ Day 50: Digits separate
  • ~ Day 56: Digits ossify secondary to apoptosis.
  • ~ Day 58: Human limb

Upper limb embryology

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Tip: Tetramelia is the term used to describe the failure of limb induction. 

Formation of a Blood Supply

In the 4th week, the limb bud is supplied by a capillary network. During embryogenesis, the following occurs.

  • Subclavian-axillary brachial arterial axis is formed
  • Basilic-axillary-subclavian venous axis is formed
  • Brachial artery branches into interosseous and median arteries
  • Ulnar and radial arteries replaced median artery (now supplies median nerve)


Mechanisms of Development


Key Point

Upper limb embryology occurs via 3 axes, 3 signalling centres and 3 proteins.

Proximodistal Axis

  • Role: "distalises" the limb by differentiating limb from the shoulder to finger.
  • Signalling centre: Apical Ectodermal Ridge (AER)
  • Proteins: Fibroblast growth factors (FGFR 2, 4, 8) influence SHH expression.

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Fun Fact: AER is an ectodermal thickening at the leading bud edge. Its removal will result in a truncated limb.

Anteroposterior

  • Role: "ulnarises" the limb by controlling ulnar and radial growth
  • Signalling centre: Zone of Polarising Activity (ZPA)
  • Proteins: Sonic Hedgehog (SHH), which also influences FGFR expression.

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Fun Fact: ZPA contains mesenchymal cells in the posterior limb bud margin. Its removal will result in an ulnar longitudinal deficiency. 

Dorsoventral Axis

  • Role: "dorsalises" the limb by differentiating a dorsal and palmar surface.  
  • Signalling centre: Dorsal ectoderm
  • Proteins: WNT7A, which activates LMX1

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Fun Fact: WNT7A is restricted to the dorsum by the volar-acting en-1. Its removal will result in abnormal dorsalisation, such as nail-patella syndrome.

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