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1/9/2013
1
Topic 7: The Axial Skeleton
� What makes up the axial skeleton?
� Ribs and Sternum
� What is the morphology of these structures?
� What sorts of specializations have evolved?
� Vertebrae
� How do they develop?
� What are their forms and how have they evolved?
� Vertebral column
� What are the evolutionary trends in the regionalization
and complexity of the vertebral column?
� What are the functions of the vertebral column?
� What specializations have evolved?
Divisions of the Skeleton:
Liem et al. Fig. 8-15, 8-18; Dean 1895
� Endochondral
� _____________
� Sternum
� _____________
� Unpaired fins
� Dermal
� Gastralia
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� What are the functions of
a rib cage?
Sternum & Ribs
True Ribs
False Ribs
Floating Ribs
Liem et al. Fig. 8-7
� Ribs reduced to absent in
amphibians
� Lack a rib cage
� ______________ single element
� Associated with pectoral girdle
� Well-developed in amniotes
� Full rib cage
� Differentiate ___________ &
_______________
� Sternum articulates with ribs
� ________________ protect
abdomen in some
Sternum & Ribs
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Gastralia
© ucmp.berkeley.edu; plesiosauria.com
� Crocodylia
� Plesiosauria
� Protection of abdomen
Turtle Shell
Liem et al. Fig. 8-15; © Nova Scotia Museum
� _________ – top
� _________ – bottom
� Overlaid by
keratinized scutes
� Primarily dermal
� Ribs & vertebrae fuse to overlying plates
� What might be the
costs and benefits of
such a shell?
Turtle Shell - Development
Liem et al. Fig. 8-16
� Rib cage is external to the ________________
� Costal scerotomal cells move laterally, not ventrally
� Myotomal cells and
girdle sclerotome
move ventrally
Bird _______________
� Bony posterior
projection of ribs
� Overlaps one rib caudally
� Locks ribs together to
increase ___________
efficiency
� Some crocs have cartilaginous uncinate
processes
Liem et al. Fig. 8-17; © www.livescience.org
Vertebrae
� ______________
� Variable in morphology
� Form a vertebral column
© KA Stevens
� What kind of homology
do they demonstrate?
Vertebral Development
See Liem et al. Table 4-1
Paraxial Mesoderm
Dermatomes
Myotomes
Sclerotomes
Somites
Dermis
of Skin
Trunk
Muscles
Vertebrae
Segmentation
© R Anderson
Wolpert, 1998
�
Paraxial mesoderm
Somite
Dissociation
Resegmentation
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Vertebral Development:
Determination of Vertebral Number
________________________ Model
� Opposing Retinoic Acid and FGF/Wnt gradients set up a
posteriorly-moving determination front
� Expression of Lunatic Fringe is cyclic and anterior to posterior
� Each time Lunatic Fringe expression hits the determination
front a new somite boundary is formed
Gomez & Pourquie 2009; Gomez et al. 2008
Vertebral Development:
Determination of _________________
_____________ Evolution
� One Hox gene duplicated
� Cluster of 13 paralogues
� Hox cluster was duplicated
twice
� 4 paralogous clusters, or
52 Hox genes
� Some Hox genes have
been lost by mutation
� Fishes have 44 Hox genes
� Amniotes have 39 Hox
genes
Liem et al. 2001, Fig 4-41
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Vertebral Development:
Determination of Vertebral Identity
� Hox gene expression is ______
� Combination of Hox genes expressed determines vertebral identity
� e.g., Chicken HoxC expression
Modified from Burke et al. 1995
HoxC4
HoxC11
HoxC9
HoxC8
HoxC6
HoxC5
HoxC10
Vertebral Development
� Somite _____________
� Sclerotome condensation
� _________________
Chal & Pourquie 2009, Fig 3-6; © PJB
Ante
rior
Poste
rior
Vertebral Morphology
� Categorize where they
are concave� _______________
� Amphicoelous
� _______________
� Opisthocoelous
� _______________
� Who has acoelous vertebrae?
� How about
amphicoelous?
Kardong Fig. 8-4
Vertebral Morphology
� Petromyzontiformes
� _____________
� “Exposed” notochord
� Chondrichthyes
� _____________
� Encapsulated notochord
� Neural arch & canal
� Hemal arch & canal
Liem et al. Fig. 7-5A, 8-1A
Vertebral Morphology
� _______________
� Bone, amphicoelous vertebrae
� Undifferentiated
vertebrae
� Enclosed, continuous
notochord
Liem et al. Fig. 8-2A , 8-8B; A Morton
Salmon
Vertebral Morphology
� ____________ vertebrae
� Increased robustness
� Closer articulation
� Increased elaboration
Photo: M Taylor
• Opisthocoelous – many amphibians, some ‘reptiles’
• Procoelous – many ‘reptiles’• Zygapophyses
• Transverse processes• Neural & hemal spines
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Vertebral Morphology
� Bird vertebrae are more
differentiated
� Cervical
� Heterocoelous
� Saddle-shaped articulation
� _________________ along two axes, twisting
� Lumbar
� Fused to pelvis and sacral
vertebrae
� ____________________
� Eliminate ____________
© R Jayasena; Liem et al. Fig. 8-17B
Vertebral Morphology
� Mammals
� _____________ & differentiated
� Intervertebral disks
� Many processes –
What are they for?
© indyspinemed.com, Univ. Maryland; De Iullis & Pulera Fig. 7.8
Vertebral Morphology
� What is meant by vertebral
regionalization?
� What trends have we seen in complexity of vertebrae?
� What are the (competing)
functions of the vertebral column?
� ______________________
� ______________________
© indyspinemed.com
Vertebral Regionalization & Complexity
� Fish vertebrae are relatively undifferentiated
� Other vertebrates have
increasingly differentiated vertebrae
� Fish: Trunk, ___________
� Amphibians & “Reptiles”:
� ___________, trunk, sacral, caudal
� Mammals:
� Cervical, _____________,
lumbar, sacral, caudal
Kardong, Fig. 8-39
Vertebral Column Function
� Stability & Support� Control torsion
� Control bending
� Reduction in number of vertebrae
� What are some examples of this?
� ___________________________
� ___________________________
� ___________________________
Kardong, Fig. 8-34, 8-38; KA Stevens
Vertebral Column Function
� Movement & flexibility
� Bounding locomotion in mammals
� Lateral undulation in sprawlers
� Movement allowed in some planes, not others
� How might flexibility be increased?
Vertebrate Life 24-2; © KP Bergmann
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Human ___________________
� Humans have greater
lordosis than other apes
� Spinal loading
� Balanced COM
� Pregnancy shifts COM
_______________
� Lordosis is increased in response
Whitcome et al. 2007
Human spinal lordosis
� Females have greater lordosis than males
� What does this tell us?
Whitcome et al. 2007
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