Chapter 8 Lecture

CHAPTER 8: The Skeletal System: Axial Skeleton

Components of Axial Skeleton
- Overview of structure and function.
Introduction
- Understanding basic elements and their role in vertebrate evolution.
Basic Components
- Notochord and vertebral column discussed in detail.
Embryonic Development / Phylogeny
- Evolutionary aspects of vertebral structures.
Form and Function
- Relationship between structure and mechanical function in vertebrates.

Structural Framework
- Two key components of the vertebrate body:
  - Notochord:
    - Rod of fibrous tissue providing support and sites for muscle attachment.
  - Vertebral Column:
    - Composed of cartilaginous/bony elements, replacing notochord in advanced vertebrates.
- Evidence in Evolution:
  - Protovertebrae seen in fossils of Haikouella and Haikouichthys.
- Modern Examples:
  - Hagfishes have notochord but lack vertebrae; Lampreys possess cartilaginous elements.
- Function of Vertebrae:
  - Initially for spinal cord protection, evolving for muscle attachment and locomotion.

Components of Vertebrae:
- Dorsal Arches:
  - Includes neural arches protecting the neural tube.
- Ventral Arches:
  - Hemal arches enveloping blood vessels.
- Formation of Centra:
  - Emergence of intercentrum and pleurocentrum as bases intersecting with notochord.

Structure:
- Each vertebral segment consists of arches and centra:
  - Dorsal (neural + interneural), Ventral (hemal + interhemal).
- Evolutionary Trends:
  - Displacement and specialization of the notochord in different vertebrate lineages.
- Differentiation within Axial Column:
  - Fish: anterior trunk and posterior caudal region.
  - Tetrapods: further differentiation into cervical and sacral regions.

Variety in Structure:
- Differences in types of centra between vertebrate groups (aspondyly, monospondyly, diplospondyly).
- Anatomical Relationships:
  - Aspidospondyly (separate) vs. holospondyly (fused) vertebrae.

Functional Importance of Centra:
- Classification based on motion capabilities (procoelous, amphicoelous) and adaptations in various taxa.
- Components:
  - Apophyses and processes aiding in rib articulation and vertebral function.

Functions of Ribs:
- Muscle attachment, body suspension, protection of visceral organs.
- Classification:
  - Differences in rib structure and articulation across fish and tetrapod species.

Structure and Function:
- Provides muscle attachment and secures true ribs, absent in fishes, evolved in tetrapods.
- Variations in sternal structure across amphibians and amniotes.

Definition & Function:
- Dermal elements, providing muscle attachment and support for abdominal organs.
Evolutionary Significance:
- Relation to ventral dermal scales in early vertebrates.

Formation of Vertebrae:
- Arising from mesoderm, with somites differentiating into vertebrae and associated structures.
- Complex developmental pathways with variations across groups.

Illustrated Phylogeny:
- Shows evolution of vertebrae and importance of notochord from early protovertebrates through various taxa.

Evolutionary History of Fishes:
- Different structures in placoderms, chondrichthyans, and ray-finned fish, outlining the shift from notochord to vertebral elements.

Mechanical Properties:
- Axial column behaves like elastic beams, optimizing for compression and resistance to torsional stress.

Vertebrae Formation:
- Process involves segmentation and differentiation of somites leading to specialized vertebrae structures for terrestrial locomotion.

Vertebral Adaptations:
- Features adapted for both aquatic and terrestrial life seen in early vertebrates’ skeletal systems reflecting changes in environmental challenges.

Mechanical Forces:
- Transition from aquatic to terrestrial life necessitated structural adaptations in vertebral formation to meet new mechanical demands.

Summary of the Notochord and Vertebral Column:
- Discusses evolutionary history, structural components, changes during development, and adaptation to terrestrial living.

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