Shark fin fin As the fin grows the muscles on the ventral side of the animal

Shark fin fin as the fin grows the muscles on the

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Shark fin
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fin As the fin grows, the muscles on the ventral side of the animal (hypaxial muscles) split around the fin (each carrying its own nerve supply) on top = elevators below = depressors
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elevators depressors THIS BASIC PLAN IS STILL WITH US! Elevators/extensors Depressors/flexors
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LIMBS, THEREFORE, ARE MODIFIED FINS Bony fish ( Osteichthyes ) evolved 400 Ma including one group of lobe-finned fish ( Sarcopterygii ) with bones in their fins
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(check out Tiktaalik in the HMNH)
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THIS HISTORY ACCOUNTS FOR HOW LIMBS DEVELOP BEGIN AS A BUD IN LATERAL PLATE MESODERM (also muscle cells from myotome)
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BUD POSITION DETERMINED BY SIGNALING FROM SOMITES Hindlimb Forelimb e.g., pectoral fin and forelimb = boundary between cervical and thoracic somites (anterior boundary of Hoxc6)
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Limb bud HOW DOES IT GET PATTERNED & ORGANIZED? APICAL ECTODERMAL RIDGE ± organizing center ² - sends out signals that cause pattern of cell differentiation
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Progress zone: cells divide, elongating limb As in axial skeleton, Hox genes are expressed to pattern the limb (from proximal to distal) Hoxa9,d9 Hoxa9 -11 Hoxd9-11 APICAL ECTODERMAL RIDGE Hoxa9 -13 Hoxd9-13
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Progress zone APICAL ECTODERMAL RIDGE (AER) A clever experiment: The earlier you remove the AER, the less of the limb that develops AER AER AER
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OTHER GENES PATTERN OTHER AXES posterior vs anterior (shh) dorsal vs ventral (wnt7a) Just so you know You can learn about this in other classes! region cells that secrete sonic hedgehog
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PUTTING IT ALL TOGETHER Limbs grow out from body wall as modified fins with one set of muscles on top and another below, all of which are connected to the neural tube (spinal cord)
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ANTHER BIG DIFFERENCE IS THE HEAD! Most complex region of body
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Don ³ t worry: we won ³ t learn the whole embryology of the head! Except for a few key facts, principles: 1. Heads, like limbs, are evolutionary novelties. Created by using old cells, organs in a new way 2. Many distinct modules, that become highly integrated 3. Lots of opportunities for variation! and it starts with the central nervous system (CNS) brain and spinal cord
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Remember neurulation? During neurulation, the neural plate folds over itself into a neural tube
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As the mesoderm forms segments (somites) so does the neural tube Neural segments Mesodermal segments (Somites) Forebrain Midbrain Hindbrain Spinal cord
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Forebrain (prosencephalon) Midbrain (mesencephalon) Hindbrain (rhombencephalon) The various segments of the brain start to grow and flex Also parts differentiate into primordial sensory organs (eyes, olfactory cells, etc)
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  • Fall '10
  • AndrewA.Biewener,PeterT.Ellison,andDanielE.Lieberman
  • Developmental Biology, Neural tube, Mesoderm, basic body plan

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