Limb Development - Anatomy & Physiology

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Introduction

The limbs develop from the lateral plate mesoderm. Limb development is highly conserved; in all land vertebrates there are only four limbs and they are always opposite each other with respect to the midline of the body. All vertebrate limbs have the same patterning of; stylopod - proximal part of the limb which produces the humerus or femur; zeugopod - intermediate part of the limb which produces the radius and ulna or tibia and fibula; autopod - distal part of the limb that produces the carpals and metacarpals or tarsals and metatarsals. Other animals also follow this limb pattern including the greatly modified bird's wing.

Limb Bud Formation

• The limbs begin as lateral proliferations of the mesenchymal lateral plate mesoderm covered by ectoderm.
  • The lateral plate mesoderm proliferates in a lateral direction along the entire midline, but then stops at the region of the interlimb (area between the limbs).
  • This produces small buds, which will continue to grow to eventually form the limbs.
• Proliferation of the lateral plate mesoderm cannot sustain entire limb growth; instead the apical ectodermal ridge (AER) maintains growth.
  • The AER develops as mesenchymal cells cause cells of the overlying ectoderm to change their morphology from cuboidal to columnar.
  • The ridge can be seen when looking at the limb bud in a distal to poximal direction.
• The AER maintains an area of rapidly proliferating cells beneath it, called the progress zone (PZ).
• The rapidly increasing number of cells drives the limb bud out distally, elongating the limb.
• Cells leave the PZ, and as they do so they differentiate.