Comparative Anatomy : TOPIC SEVEN | LECTURE SLIDES |

Comparative Anatomy: Axial Skeleton

Topic Outline

  • 1. Arcualia and Vertebral Origins

  • 2. Vertebral Regionalization

  • 3. Axial Anatomy

    • 1. Centrum

    • 2. Intervertebral Disk

    • 3. Apophyses

    • 4. Ribs

    • 5. Sternum

    • 6. Gastralia

  • 4. Comparative Vertebral Development

    • 1. Primitive Gnathostomes

    • 2. Teleosts

    • 3. Tetrapods

1. Arcualia and Vertebral Origins

  • Defining Arcualia:

    • Arcualia are described as the most primitive axial skeletal structures found in vertebrates.

    • These structures are characterized as unmineralized segmented forms, possibly manifested in dorsal and/or ventral series.

    • It is important to note that the notochord is not classified as skeletal material.

  • Homology of Arcualia:

    • Arcualia are homologous to vertebrae in certain subgroups of Vertebrata but not across all.

    • For instance, vertebrae in tetrapods and teleost fish do not originate from arcualia.

  • Developmental Patterns in Primitive Fish:

    • In primitive fish and fish-like aquatic vertebrates, arcualia alternate between:

    • Basidorsals and interdorsals (dorsal arrangements)

    • Basiventrals and interventrals (ventral arrangements)

    • Each somite contributes a set of structures:

    • Interdorsal: from one somite

    • Interventral: from the same somite

    • Basidorsal and Basiventral: emerge from adjacent somites.

    • Neural arch and haemal arch develop from basidorsal and basiventral arcualia, respectively.

    • Centra Formation:

    • Centra are derived from the ventral arches; haemal arches form intercentra, while interhaemal arches yield pleurocentra.

2. Vertebral Regionalization

  • Concept of Regionalization:

    • Terrestrial vertebrates exhibit greater regionalization in the vertebral column compared to their aquatic counterparts.

    • Example: A fish typically has thoracic and caudal vertebrae, while a cat has cervical, thoracic, lumbar, sacral, and caudal vertebrae.

  • Synsacrum and Pygostyle:

    • The synsacrum is defined as a structure formed by fused thoracic, sacral, and caudal vertebrae.

    • It integrates with the innominate bones (left and right pelvis) to create the specialized avian pelvis.

    • The pygostyle represents a fused block of caudal vertebrae, distinctly separated from the synsacrum.

    • Questions arise regarding how the structure of the synsacrum and pygostyle may confer advantages for birds’ specific lifestyles.

3. Vertebral Anatomy

1. Centrum

  • Morphological Types of Centrum:

    • Aspondyly: No centrum is present.

    • Monospondyly: Presence of one centrum per body segment.

    • Diplospondyly: Comprises two centra (pleurocentrum and intercentrum) per body segment.

    • Polyspondyly: Found in organisms such as lungfish and holocephali, having more than two centra per segment.

    • In amniotes, the centra are classified as pleurocentra and intercentra contribute to intervertebral disks.

  • Centra Types by Organisms:

    • Acoelous: Observed in some reptiles and mammals.

    • Amphicoelous: Typical in certain fish species.

    • Procoelous: Characteristic of most extant reptiles and some amphibians.

    • Opisthocoelous: Found in certain amphibians, reptiles, and mammals.

    • Heterocoelous: Observed in retracting turtle necks and numerous birds.

2. Intervertebral Disk

  • Structure:

    • The intervertebral disk in amniotes comprises a cartilaginous disc known as the annulus.

    • The nucleus pulposus represents the remnant of the notochord.

3. Apophyses

  • Definition and Function:

    • Apophyses refer to processes on vertebrae that serve as sites of contact with other bones.

    • Example in Snakes:

    • Display two sets of zygapophyses: cranial and caudal zygapophyses.

    • Zygosphene is located cranially while zygantrum is found caudally.

    • It is essential to consider the function served by the zygapophyses in the context of additional sets in snakes.

4. Ribs

  • Roles of Ribs:

    • Ribs are crucial for several functions:

    • Provide muscle attachment sites.

    • Structure the thoracic respiratory pump in terrestrial vertebrates.

    • Help suspend and protect internal organs.

  • Embryonic Origin:

    • Ribs originate within the myosepta.

  • Turtle Shell Composition:

    • The carapace exhibits a composite nature primarily from expanded ribs, alongside vertebrae and dermal osteoderms.

    • The ventral plastron has been homologized with clavicle, interclavicle, and gastralia (dermal abdominal "ribs").

5. Sternum

  • Sternum Structure:

    • In many amphibians and reptiles, the sternum functions as a single element, while in mammals, it comprises a series of elements termed sternebrae.

    • Most ribs are divided into two sections:

    • Dorsal_section: Ossified “vertebral rib.”

    • Ventral_section: Costal cartilage or “sternal rib.”

    • Thoracic vertebrae are responsible for carrying ribs.

    • Pleurapophyses in the lumbar region of mammals signify a fusion between rib and transverse process.

    • Cervical ribs are present in birds and select early-limbed vertebrates.

6. Gastralia

  • Function of Gastralia:

    • Gastralia are defined as abdominal "ribs" specific to archosaurs, including birds and crocodylians and their evolutionary stem lineages.

    • Important clarification: Gastralia are not ribs and do not constitute endoskeleton.

  • Examples of Gastralia:

    • Gastralia of Tyrannosaurus rex.

    • Gastralia of Alligator mississippiensis.

4. Comparative Vertebral Development

1. Primitive Gnathostomes

  • Vertebral Development Process:

    • In primitive gnathostomes, each somite contributes a basidorsal, basiventral, interdorsal, and interventral, leading to diverse parts of the vertebral body:

    • Basidorsal: Forms neural arch.

    • Basiventral: Develops into intercentrum.

    • Interdorsal: Results in interneural structures.

    • Interventral: Leads to pleurocentrum development.

2. Tetrapods

  • Sclerotome Cell Migration:

    • In tetrapods, cells of the sclerotome migrate around the notochord; contributing the cranial end of one vertebra and the caudal end of another.

    • Clusters of sclerotome cells form perichordal rings.

    • Mesenchymal cells wrap around these rings, connecting them into a perichordal tube.

    • Mesenchyme extends dorsally, encircling the neural tube to progress towards forming the future neural arch.

3. Teleosts

  • Unique Developmental Mechanism:

    • In teleosts, segmented bony centra develop within the notochord sheath without forming a cartilaginous precursor.

    • Condensation of mesenchyme cells occurs in the myocommata to give rise to arches.

    • Sclerotome cells surround the developing centra and partake in the ossification process.

  • Homology Consideration:

    • Despite differences in vertebrae formation among primitive jawed vertebrates, teleosts, and tetrapods, there lies a need to view these structures as homologues to each other.

    • Observational examples of cartilage and bone development in zebrafish vertebrae showcased at 9, 17, and 23 days, highlighting bone development along the notochord in the absence of cartilage (indicated in dark blue).

References: Ota et al., 2011; Fleming et al., 2004