Axial skeleton

Evolution of the Axial Skeleton

Overview of Axial Skeleton

  • Concentration of mesenchymal cells in the caudal part of sclerotome, which is a tissue in embryos that contributes to the axial skeleton.

  • Embryonic development of vertebrae involves:

    • Somite formation

    • Migration of sclerotomal cells to intervertebral locations

    • Formation of myotomes and associated structures

  • The central structure for early vertebrates is the notochord, serving as a precursor to the vertebral column.

  • Primordium of the centrum of vertebrae is established during early embryonic sections (refer to Figure 8-5).

Development of Vertebrae
Central Formation

  • Fishes:

    • Formation of vertebrae consists of different types according to species and embryonic development:

    • Chondrichthyans (cartilaginous fishes like sharks): Vertebrae develop through chondrification of mesenchyme cells at intersegmental intervals forming chordal centra.

    • Actinopterygians (bony fishes): Direct intersegmental deposition of bone forms perichordal centra around the notochord.

  • Amphibians:

    • Development of centra occurs from a continuous perichordal tube, thickening at intervertebral areas resulting in distinct cartilaginous bodies in adults.

    • Adult salamanders have distinct cartilaginous intervertebral bodies, while frogs typically ossify and fuse these bodies with ossified ends of centra.

  • Amniotes (including reptiles and mammals):

    • Differing development with no continuous perichordal tube; mesenchymal cells condense in the caudal half of each sclerotome to form vertebral centra.

    • Cranial sclerotome cells may not contribute to centra development, a debate persists among anatomists regarding their function.

Ribs and Transverse Processes

  • Definition of Ribs: Hard, rod-shaped structures, developing in myosepta beside vertebrae; initially cartilaginous, ossifying later in most vertebrates.

  • Reinforce the body wall and provide muscle attachments.

  • Types of Ribs:

    • Intermuscular Ribs (dorsal): Form between epaxial trunk muscles and hypaxial muscles, attaching to the lateral surface of a centrum.

    • Subperitoneal Ribs (ventral): Developed at the intersection of myosepta and lateral skeletogenous septum, attaching to ventral arch bases called basapophyses (transverse processes homologous to hemal arches).

Evolutionary Patterns

  • Various species exhibit unique combinations of rib types.

  • Chondrichthyans: Exhibit intermuscular ribs primarily while others exhibit subperitoneal or accessory ribs.

  • Ribs in tetrapods contribute to body weight support against gravity and lung ventilation; rib attachment patterns differ from those in fishes.

Sternum

  • Most terrestrial vertebrates, excluding turtles and snakes, possess a sternum.

  • Sternum Development: Arises from midventral connective tissue during embryogenesis and may ossify.

  • Functional Role: Supports rib attachments and aids in ventilation. Recent evolutionary history is uncertain, but it has not been found in basal tetrapods or fossil amphibians.

Evolution of the Axial Skeleton in Fishes
General Structure

  • The vertebral column undergoes significant modifications with environmental adaptations (water vs. land).

  • In water, the vertebral column primarily resists compression rather than bending forces.

  • The notochord persists as a buoyancy aid in hagfishes and lampreys, with some traces of vertebral structures in extinct jawless vertebrates.

Vertebral Composition in Fishes

  • Two main regions divided:

    1. Trunk Vertebrae: Formed in the body cavity, crucial for swimming.

    2. Caudal Vertebrae: Modified to encase blood vessels in the tail, contributing to locomotion.

  • Neural Elements in Fishes:

    • Chondrichthyans possess neural arches in a continuous form for flexibility; vertebral centra might be absent or present.

    • Advanced actinopterygians exhibit perichordal centra providing a more rigid skeletal structure to resist compressive forces during swimming.

Evolution of Vertebral Structures in Tetrapods
Transition from Water to Land

  • Transition characterized by adaptations for resisting gravity and torsional forces; vertebrae gradually evolved to better support body weight during locomotion.

  • These animals began to rely on limb thrust instead of axial undulations for movement.

  • Structural Changes:

    • The head gains mobility independent of the trunk, freeing it for exploration and feeding.

    • Trunk and head movements evolve, enhancing dexterity.

  • Rhachitomous Vertebrae in Early Tetrapods: Characterized by large intercentra and smaller pleurocentra, mirroring their fish ancestors but with increased strength.

Variation and Function of Ribs

  • Ribs of early tetrapods varying in structure, thickness, and function:

    • Later evolved to serve both as muscle attachment points and provide structural support.

    • Additional functions include protecting internal organs, helping ventilate the lungs (although varying importance across species).

  • Tichthyostegids: Example of early tetrapod with robust rib structure aiding in terrestrial adaptations.

  • Modern attitudes towards amphibian rib structures point towards reduced importance in respiratory functions, linked to small body size and other anatomical adaptations.

Evolution of the Axial Skeleton in Reptiles

  • Reptiloids developed stronger axial skeletons necessitating greater body support.

  • Pleurocentra larger than Intercentrum: A defining synapomorphy in the evolution of reptilomorphs leading to birds and mammals.

  • Rib Development: Ribs formed connections with a sternum enhancing lung ventilation and body support functions.

  • The regional differentiation observed in vertebrae of reptiles highlights a significant evolutionary step towards more specialized and efficient body mechanics. This differentiation includes distinct regions such as:

    • Cervical Vertebrae: Often reduced in number but specialized for increased head mobility, crucial for terrestrial feeding and predator avoidance.

    • Thoracic Vertebrae: Bear ribs that attach to the sternum, forming a rigid rib cage essential for protecting internal organs and facilitating costal ventilation of the lungs, a more efficient breathing mechanism than observed in amphibians.

    • Lumbar Vertebrae: Typically lack ribs (or have only short, floating ribs) and are adapted for transmitting forces from the hind limbs during locomotion, providing flexibility in the mid-body.

    • Sacral Vertebrae: A fused series (often two or more) forming a strong connection with the pelvic girdle, anchoring the hind limbs and supporting the body's weight against gravity.

    • Caudal Vertebrae: Form the tail, exhibiting considerable variation in number and structure depending on the species' specific adaptations (e.g., balance, defense, prehensile capabilities). This increased regionalization allowed for more complex patterns of movement, enhanced respiratory efficiency, and better support against the forces of gravity, contributing to their success in diverse terrestrial environments.