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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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