Appendicular Skeleton

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Appendicular Skeleton

• Development and Evolution

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Major Functions

• Aquatic
• Stabilize swimming
• Yaw, pitch, and roll
• Semi-aquatic
• Pivot points, maneuverability
• Land and water
• Swimming on land
• Terrestrial
• Mobility
• walking
• Support

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Basic Elements of Fins

• Pterygiophores
• Basals
• Radials
• Rays
• Elasmobranchs
• Ceratotrichia-Keratin
• Actinopterygians
• Lepidotrichia-ossified cartilage
• Actinotrichia-Keratin

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Major Regions of Limbs

• Stylopod
• Humerus, Femur
• Zeugopod
• Radius, ulna, tibia, fibula
• Autopod
• Manus, pes

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Major bones of Pectoral Girdle

• Endochondral Bones
• Fish
• Scapulacoracoid
• Tetrapods
• Scapula
• Procoracoid
• Coracoid

• Dermal Bones
• Clavicle
• Interclavicle
• Cleithrum
• Supracleithrum
• Postcleithrum
• Posttemporal

Presence of Dermal Bone suggests evolutionary
history Loss of Dermal, expansion of
Endochondral during tetrapod Evolution All
distal limb bones (stylopod, zeugopod and
autopod) are Endochrondral, they articulate with
Scapulacoracoid or Scapula
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Note size of clavicle. Yellowdermal,
pinkendochondral
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Major Bones of Pelvic Girdle

• All endochondral
• Appears to have evolved by fusion of basal
  elements
• Single element in sharks
• Puboischiac Bar
• Single Element in Crossopterygians
• Pelvic girdle, embedded in body muscle
• Tetrapods
• Ischium, Ilium, pubis, connected to vertebral
  column

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Expansion of Basals across midline leads to
puboischiac bar Embedded in muscle, not to spinal
column
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Evolution of Appendicular System

• Gill Arch
• Questions unaddressed
• Origin of Pelvic Girdle
• Presence of Dermal Bone in Pectoral girdle
• Differing embryologies of Gill arches and
  Pectoral girdle
• Fin-Fold
• Supported by embryology
• Shark embryos developed long lateral ridge
• Dermal shields to Dermal bone of Pectoral girdle
• Elements of long lateral region of somites are
  able to be induced to form limb, therefore
  competent
• Presence of Posterior Hox genes in anterior limb
  field

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AER

• AER (ectoderm)
• Apical ectodermal ridge
• Expresses FGF(s)
• ZPA (mesoderm)
• Zone of polarizing activity
• Expresses SHH

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Fin-to-Limb Transition

• Development of Zeugopod
• Expansion of chondrogenic condensations
• Brancing to Preaxial and Postaxial domains
• Development of Autopod
• All appear to come from Postaxial domain

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Model for the origin of the Zeugopod
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Major features of limb development

• There is a HOX limb code
• Both Pectoral and Pelvic limbs express posterior
  Hox genes
• Elements or Limb development are broadly
  conserved
• Requires cellular communication between epidermis
  (Apical Epidermal Ridge) and underlying mesoderm

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Fin-to-Limb Transition

• Development of Zeugopod
• Expansion of chondrogenic condensations
• Branching to Preaxial and Postaxial domains
• Development of Autopod
• All appear to come from Postaxial domain

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Origin of Autopod Note shift in Preaxial vs
postaxial Development of distal elements (green)
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