Limb development in a nutshell

Limb development timeline:

• Starts in: 4^th week (Upper limb initiated 3-4 days before and stays ahead of the lower limb)
• Finished by: 8^th week

Limb Buds initiation:

• Limb bud is mesenchyme covered by ectoderm (end of 4^th week)
• Source of limb components:
  1. Lateral plate mesoderm/LPM (somatic)→bone, connective tissue, vasculature
  2. Paraxial mesoderm (somite-derived)→muscle (myotome)
  3. Neural tube: motor and sensory neurons
  4. Surface ectoderm: epidermis
  5. Neural crest: melanocytes
• Structure  & formation of limbbuds:

1. 1. Form from somatic lateral plate mesoderm
   2. Edge of bud the apical ectodermal ridge (AER)→critical for limb formation
   3. Within the limb bud are the progress zone (high proliferation) and the mesenchymal mesoderm (stop dividing, differentiate into proper proximal/distal fate)

• Molecular limb embryology:

1. 1. Intermediate mesoderm→ FGF8 (mitogen)→ ↑ LPM to proliferate→ Induces Limb bud
   2. FGF8→initiates Apical Ectodermal Ridge (AER)
   3. AER releases FGF8→ stimulates growth in progress zone→ controls where the limb forms along the A/P axis of the body

• Limb axes: Limb axes are unusual: anterior (thumb), dorsal (back of the hand), distal (fingers)

• The pattern signaling of Shh, Wnt-7a, and FGF are codependent and integrated
• Forelimb/hindlimb:

1. 1. Leg expresses: Pitx1 and Tbx4 txn factors (controlled by Hox-c genes)
   2. Arm expresses: Tbx5 which inhibits Tbx4

• Digits formation:
  1. AER delineates where digits will form.
  2. ZPA & SHH determine which digits will form & where.
  3. The webbing of finger and toes removed by apoptosis induced by BMP
• Limb bud malformations often related to urogenital malformations: both involve intermediate mesoderm defects.

Bone Formation

• Cells differentiate first furthest from AER
• Endochondral ossification: Mesenchyme differentiates→ cartilage→ bone
• Lateral plate mesoderm condenses along long axis→ cartilaginous perichondrium→ replaced by bone
• Interzones between bones: differentiation into joint tissue

• Ossification:
  • Diaphyses ossified by birth (begins at 7^th week)
  • Epiphyses shortly thereafter
  • Continued bone growth occurs at the epiphyseal growth plate

Muscle formation

• Connective tissues dictate the pattern of muscle formation by passively guiding the direction of migration of muscle precursor cells
• 6^th week: connective tissue guides the migration of muscle cell precursors from somite to limb bud → forming dorsal/ventral condensations
• Mesodermal Condensations differentiate into →myoblasts (muscle precursors)
• Dorsal and ventral muscle masses differentiate

Rotation of limbs

• The upper limb rotates 90° laterally
• The lower limb rotates 90° medially

Limbs innervation

• Dorsal branches innervate dorsal muscle mass; ventral branches innervate ventral muscle mass
• Axon growth guidance by mesenchyme controls the final pattern of innervation
• Postganglionic sympathetic fibers use developing vasculature as a guide
• Sensory axons develop later (slower) via sympathetic and motor axons as a guide

Congenital limb defects (developmental defects):

Also read:

1. Duke Embryology – Limb Development
2. Anomalies of Limbs
3. Embryology.med.unsw.edu.au – Limb Development

Also Watch:

• Embryology -Limb Development from the Youtube channel “DocvTV”

• Introduction to Limb Development from the Youtube channel “Kate Lee“

• Apical Ectodermal Ridge, zone polarizing Activity from the Youtube channel “Nabil Ebraheim”

References:

1. Sadler TW. Langman’s Medical Embryology. LWW
2. Dudek. High-Yield Embryology. Wolter Kluwers
3. Jorde L, Carey J, Bamshad M. Medical Genetics. Elsevier
4. Saladin KS. Human Anatomy. McGraw-Hill Medical Publishing