Ricardo Olivares & Mariana Rojas

The first components of the dorsal vertebrae, were arches (neural and interneural) and ventral (haemal and interhemal) that relied on the notochord, the next step was the formation of two centers (intercentro and pleurocentro), which served to fix and support the arches. Many fish have dorsal and ventral ribs. In birds cervical ribs are reduced and vertebrae are fused, the first thoracic ribs are floating and the majority present processes allowing muscle fixation and reinforce the chest wall. Mammals have ribs on all thoracic vertebrae, the majority are true. The sternum is a structure of endochondral origin, fish, turtles, snakes and lizards have no sternum. Airborne fowl are provided with a large sternum prominent ventral keel. Development of the axial skeleton: The notochord and ventral neural tube express Sonic hedgehog (SHH) that induces the ventromedial portion of somite to become sclerotome and express the transcription factor Pax-1, which controls the formation of cartilage and bone that constitute the vertebrae. The expression pattern of Hox genes in vertebrates is what determines which type of vertebral structure is to be formed. The appendicular skeleton comprises the pectoral girdle consists of dermal and endochondral skeletal elements, holding the pectoral fin and pelvic girdle, consisting only endochondral elements that sustain pelvic fin. The forelimbs and hindlimbs of tetrapods are built on the same pattern, differing in three regions: autopod, zeugopod and stylopod. Appendicular skeletal development: Is formed from somatic lateral mesoderm and the apical ectodermal ridge. The lateral mesoderm receives somatic myotome and dermatome cells to form the outline member.

KEY WORDS: Skeleton; Vertebrae; Ribs; Sternum; Comparative anatomy; Comparative osteology.

OLIVARES, R.; ROJAS, M. Axial and Appendicular Skeleton of Vertebrates. Int. J. Morphol., 31(2):378-387, 2013.