Limb DevelopmentLateral Mesoderm
Limb Morphogenesis
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Wikivet: Developmental Biology
Questions
- Somatic mesoderm
- Parietal parts of serous membranes
- Pleurae
- Pericardium
- Peritoneum
- Limb buds
- Tissues of the limbs
- Origins of tissues of the limb:
- Ectoderm: all skin and apical ridge
- Mesoderm: skeleton (via endochondral ossification of mesenchyme) and muscle (via differentiation of mesenchyme-myotome)
- Limb buds
- Takes cells from dependent segments
- Limbs grow outward from body wall (somatopleure) as limb buds
- Cartilage, muscles and related connective tissue all arise from somatic mesenchyme of the limb bud
Limb Morphogenesis
- Begins as a limb field
- An area of somatopleure committed to forming a limb
- Limb bud produced by
- Localised proliferation and condensation of mesenchyme
- Covered by ectoderm
- Apical ridge: condensation of ectoderm at limb tip
- Ridge is induced to form by underlying mesoderm
- Induces the mesoderm to grow and differentiate into a limb
- Limb bude elongates
- Develops into proximo-distal order
- The distal end of the limb bud (the footplate) is flattened like a paddle
- Proximal limb mesenchyme is undifferentiated until digits form and differentiation of the mesenchym to form chondrogeneic cells and bones of the scapula and pelvis
- Distal limb mesenchyme remains undifferentiated until digits are formed
- Apical ectodermal ridge signals limb development
- Multiple segments involved
- Ventral branches of spinal nerves
- Limb muscles from myotomes
- Limb bones from lateral plate mesoderm (inc hip bones)
- Mechanically, limb growth consists of:
- Elongation of a dorsoventrally flattened limb bud
- Ventroflexion of the proximal half of the limb
- Pronation of the distal half
- Local mesenchyme condenses to form cartilage models of limb bones
- Myotome cells migrate into the base of the limb forming etensor and flexor muscle masses that subsequently segregate into individual muscles
- Vessels and nerves grow into the limb
- Distal condensations of mesenchyme represent future digits (digital blastemal)
- Planned cell death shapes limb
- Separate digits are produced by interdigital necrotic zones (species with fwewer digits undergo futher degeneration and/or fusion of digits
- All species develop through 5 digits then reduce
- Animals with fewer digits develop the five digits forst but some are lost or regress to achieve the final animal form.
- These can be lost by distruction or fusion of blastemal
- The order of loss is sequentiona, 1, 5, 2, 4
- Vestigial digts include lost phalangeal portions of the digital blastemal
- Achondroplasia (dwarfism; Dachshund) — inherited, systemic premature ossification of physes of extremities.
- Arthrogryposis [Gr. gryposis = crooked) can result from malformed joints, denervation, abnormal muscle tension, or impaired mobility in utero.
- Polydactyly (extra digits); syndactyly (fused digits); brachydactyly (stumpy digits): Gr. dactylos = digit)
- Amelia (no limb); meromelia (absence of part of limb); micromelia (small limb): Gr. melos = limb)
- Note: phocomelia (seal limb) = absence of proximal segment(s) of limb was a consequence of pregnant women taking thalidomide in 1950s.
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Wikivet: Developmental Biology
Questions
- describe limb development
- list and describe some clinical considerations
- Describe distal limb development in large ungulating animals (eg horses) compared to cursorial animals and humans.