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Topic 7: The Axial Skeleton
What makes up the axial skeleton?
Ribs and Sternum
Vertebrae
What is the morphology of these structures?
What sorts of specializations have evolved?
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?
Endochondral
Divisions of the Skeleton:
_____________
Sternum
_____________
Unpaired fins
Dermal
Gastralia
Liem et al. Fig. 8-15, 8-18; Dean 1895
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Sternum & Ribs
What are the functions of
a rib cage?
False Ribs
True Ribs
Floating Ribs
Sternum & Ribs
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
Liem et al. Fig. 8-7
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Gastralia
Turtle Shell
Crocodylia
Plesiosauria
Protection of abdomen
_________ – 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?
© ucmp.berkeley.edu; plesiosauria.com
Turtle Shell - Development
Liem et al. Fig. 8-15; © Nova Scotia Museum
Bird _______________
Rib cage is external to the ________________
Costal scerotomal cells move laterally, not ventrally
Myotomal
cells and
girdle
sclerotome
move
ventrally
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
Liem et al. Fig. 8-16
Vertebrae
______________
Variable in morphology
Form a vertebral column
Vertebral Development
Wolpert, 1998
What kind of homology
do they demonstrate?
Paraxial Mesoderm
Segmentation
Somites
Dissociation
Sclerotomes
Dermatomes
Myotomes
Somite
Resegmentation
Vertebrae
Paraxial mesoderm
© R Anderson
© KA Stevens
Dermis
of Skin
Trunk
Muscles
See Liem et al. Table 4-1
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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
Hox cluster was duplicated
twice
Cluster of 13 paralogues
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
e.g., Chicken HoxC expression
Somite _____________
Sclerotome condensation
_________________
Anterior
Vertebral Development
Hox gene expression is ______
Combination of Hox genes
expressed determines vertebral
identity
Posterior
HoxC11
HoxC10
HoxC9
HoxC8
HoxC6
HoxC5
HoxC4
Modified from Burke et al. 1995
Chal & Pourquie 2009, Fig 3-6; © PJB
Vertebral
Morphology
Categorize where they
are concave
_______________
Amphicoelous
_______________
Opisthocoelous
_______________
Vertebral Morphology
Petromyzontiformes
Who has acoelous
vertebrae?
Chondrichthyes
How about
amphicoelous?
_____________
“Exposed” notochord
_____________
Encapsulated notochord
Neural arch & canal
Hemal arch & canal
Kardong Fig. 8-4
Liem et al. Fig. 7-5A, 8-1A
Vertebral Morphology
Vertebral Morphology
_______________
Bone, amphicoelous
vertebrae
Undifferentiated
vertebrae
Enclosed, continuous
notochord
____________ vertebrae
Increased robustness
Closer articulation
Increased elaboration
• Opisthocoelous – many
amphibians, some ‘reptiles’
• Procoelous – many ‘reptiles’
• Zygapophyses
• Transverse processes
• Neural & hemal spines
Salmon
Liem et al. Fig. 8-2A , 8-8B; A Morton
Photo: M Taylor
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Vertebral Morphology
Vertebral Morphology
Bird vertebrae are more
differentiated
_____________ &
differentiated
Intervertebral disks
Many processes –
What are they for?
Cervical
Mammals
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 Regionalization
& Complexity
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?
Fish vertebrae are relatively
undifferentiated
Other vertebrates have
increasingly differentiated
vertebrae
Fish: Trunk, ___________
Amphibians & “Reptiles”:
______________________
______________________
© indyspinemed.com, Univ. Maryland; De Iullis & Pulera Fig. 7.8
___________, trunk, sacral,
caudal
Mammals:
Cervical, _____________,
lumbar, sacral, caudal
© indyspinemed.com
Vertebral Column Function
Stability & Support
Vertebral Column Function
Control torsion
Control bending
Reduction in number of vertebrae
What are some examples of this?
Kardong, Fig. 8-39
Movement & flexibility
___________________________
___________________________
___________________________
Kardong, Fig. 8-34, 8-38; KA Stevens
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 ___________________
Human spinal lordosis
Humans have greater
lordosis than other apes
Females have greater lordosis than males
What does this tell us?
Spinal loading
Balanced COM
♂ ♀ ♂♀
Pregnancy shifts COM
_______________
Lordosis is increased in
response
♂♀
♂♀
Whitcome et al. 2007
♂♀
Whitcome et al. 2007
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