BIO 342
Comparative Vertebrate Anatomy
Lecture Notes 2 - Vertebrate Skeletal Systems


Bone formation begins with a blastema (any aggregation of embryonic mesenchymal cells which will differentiate into tissue such as muscle, cartilage, or bone). These cells then develop into either FIBROBLASTS or OSTEOBLASTS. Fibroblasts form collagen; osteoblasts form bone cells. Together, these form MEMBRANE BONE (bone deposited directed in a blastema).

Intramembranous ossification is the process of membrane bone formation. This process give rise to:

Endochondral ossification is the process in which bone is deposited in pre-existing cartilage, & such bone is called REPLACEMENT BONE.


Skeletal elements:



Dermal bone of fishes:

Also check:

1 = lamellar bone, 2 = spongy bone, 3 = dentin, 4 = enameloid, & 5 = fibrous plate (collagen)

Tetrapods - retain dermal elements in the skull, jaws, & pectoral girdle

Somatic skeleton = axial skeleton (vertebral column, ribs, sternum, & skull) + appendicular skeleton

 Vertebral column:

  Vertebrae - consist of a centrum (or body), 1 or 2 arches, plus various processes

Vertebral arches:

Vertebral processes:

Transverse processes - most common type of process; extend laterally from the base of a neural arch or centrum & separate the epaxial & hypaxial muscles

Diapophyses & parapophyses - articulate with ribs

Prezygapophyses (cranial zygapophyses) & postzygapophyses (caudal zygapophyses) - articulate with one another & limit flexion & torsion of the vertebral column

Vertebral columns:

Diplospondyly = 2 centra and 2 sets of arches per body segment; occurs in some fish (including sharks)

Agnathans - only skeletal elements associated with the notochord are paired, lateral neural cartilages

Vertebral columns of tetrapods


Cross-hatched areas = hypocentrum; black areas = pleurocentrum.

Ribs - may be long or short, cartilaginous or bony; articulate medially with vertebrae & extend into the body wall

Sternum - strictly a tetrapod structure &, primarily, an amniote structure.

Useful link:

Skeletal system - the axial skeleton

Lecture Notes - Skeletal System II

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