![]() Vertebral arches can be further defined as either neural arches (na) that project dorsally around the spinal cord or haemal arches (ha) that project ventrally. Vertebrae consist of a centrum (c) and vertebral arches (va). Vertebrae from animals of different vertebrate lineages show remarkable diversity in their three-dimensional shape, as illustrated by images of a human thoracic vertebra (A), a dog cervical vertebra (B), a chicken thoracic vertebra (C), and a sea bass thoracic vertebra (D). Its widespread acceptance throughout much of the 20th century has probably resulted in the shoehorning of experimental findings to fit a hypothesis that now seems to have lost any usefulness.ĭiversity in the structure of vertebrae between different vertebrate lineages. Nonetheless, the hypothesis has not completely disappeared, particularly from the palaeontology literature, and has caused a persistent confusion in terminology. More recent work reviewed here provides no evidence for Gadowian organization of the centrum within teleosts. ![]() It was eventually dismissed, or found not useful, for one vertebrate group after another – tetrapods ( Williams, 1959), the acanthodians ( Miles, 1970) and osteichthyans ( Schaeffer, 1967), and particularly the dipnoans (lungfish) ( Arratia et al., 2001). However, this view of a vertebra, appealing in the way it proposes a basic uniformity among diverse vertebrates, is worrisome because of its remarkable idealism, and was challenged repeatedly during its long history. Alternating with these two along the body axis, and completing the segmental composition of a single centrum, are interdorsals and interventrals. The dorsal ‘basidorsals’ and ventral ‘basiventrals’ form the vertebral arches, respectively the neural arches (enveloping spinal cord) and haemal arches (enveloping axial blood vessels), as well as the regions of the centra growing out (usually perichordally) from the arch bases. The ‘Gadowian’ hypothesis proposed that, across all vertebrates, four bilaterally paired primordia called ‘arcualia’ give rise to different parts of the vertebra. By clarifying the evolutionary relationship between centra and arches, and their varying modes of skeletal mineralization, we can better appreciate the detailed mechanisms that regulate and diversify vertebral patterning.Įarly work by Gadow (1896) and by Gadow and Abbott (1895) led Gadow to a hypothesis that markedly influenced understanding of vertebral development and terminology. We argue that centra are homologous among jawed vertebrates, and review evidence in teleosts that the notochord plays an instructive role in segmental patterning, alongside the somites, and contributes to mineralization. Vertebral arch elements were present in early stem vertebrates, whereas centra arose later. Here, we assemble evidence from ichthyologists, palaeontologists and developmental biologists to consider these issues. ![]() Particular controversy surrounds whether vertebral component structures are homologous across vertebrates, how somite and vertebral patterning are connected, and the developmental origin of vertebral bone-mineralizing cells. Despite being a defining feature of the vertebrates, much remains to be understood about vertebral development and evolution. The segmented vertebral column comprises a repeat series of vertebrae, each consisting of two key components: the vertebral body (or centrum) and the vertebral arches. ![]()
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