zwierzęta: Hagfish

Evidence: 100 • authority: 72

Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped jawless fish (occasionally called slime eels). Hagfish are the only known living animals that have a skull but no vertebral column; however, they do have rudimentary vertebrae. Hagfish are marine predators and scavengers that can defend themselves against other larger predators by releasing copious amounts of slime from mucous glands in their skin. Although their exact relationship to the only other living group of jawless fish, the lampreys, was initially the subject of controversy, genetic evidence suggests that hagfish and lampreys are more closely related to each other than to jawed vertebra…

Evidence: 68 • authority: 94

Vertebrae arise as segmental endoskeletal elements associated with the notochord, and are regarded as a vertebrate synapomorphy 1 , 2 , 3 , 4 , 5 . No such structures, nor the somite-derived vertebral primordial, the sclerotome, are present in non-vertebrate chordates such as amphioxus and the tunicates 6 , 7 . The vertebrae in gnathostomes, the jawed vertebrates, consist of dorsal and ventral elements 1 , 2 , 3 , 4 , 5 , 8 , 9 , 10 , of which only the dorsal elements are present in lampreys 11 , one of the two extant agnathan taxa (jawless fishes) 1 , 2 . Based on evidence that the vertebrae of the gnathostomes and lampreys are developmentally derived from the sclerotome 12 , 13 , it is thought that their developmental mechanisms share the same ancestral origin. However, another extant agnathan, the hagfish, has generally been thought to lack vertebrae; the axial supporting tissue of this animal has been described as consisting only of notochord and cartilaginous fin rays 1 , 2 , 3 , 4 , 14 , 15 , 16 , 17 . Because of this morphological trait, the hagfish was excluded from the Vertebrata in some textbooks 1 , 2 , 5 , although its vertebrate affinities, as observed in the brain and skull, are widely recognized 1 , 2 , 3 , 4 , 5 . This raises a number of questions; specifically, whether the hagfish truly lacks any form of vertebra-like structure throughout its body and life cycle, and whether hagfish somites in the embryonic trunk are dissimilar to those in lampreys and gnathostomes. Importantly, one of the early reports on hagfish anatomy, in fact, described segmental cartilaginous elements (putative vertebral elements) on the ventral aspect of the caudal notochord in Bdellostoma more than a century ago ( Supplementary Fig. S1 ) 18 . These cartilaginous structures have not been re-examined, and have largely been overlooked by subsequent researchers 16 , 17 . The hagfish, a group of extant jawless fish, are known to lack true vertebrae and, for this reason, have often been excluded from the group Vertebrata. However, it has yet to be conclusively shown whether hagfish lack all vertebra-like structures, and whether their somites follow developmental processes and patterning distinct from those in lampreys and gnathostomes. Here we report the presence of vertebra-like cartilages in the in-shore hagfish, Eptatretus burgeri . These elements arise as small nodules occupying anatomical positions comparable to those of gnathostome vertebrae. Examination of hagfish embryos suggests that the ventromedial portion of a somite transforms into mesenchymal cells that express cognates of Pax1/9 and Twist , strikingly similar to the pattern of sclerotome development in gnathostomes. We conclude that the vertebra-like elements in the hagfish are homologous to gnathostome vertebrae, implying that this animal underwent secondary reduction of vertebrae in most of the trunk. To examine possible developmental similarities in putative vertebral elements between the hagfish and gnathostome vertebrae, we observed their embryonic development. Although we previously succeeded in collecting several early-stage hagfish embryos 20 , these were insufficient to trace the developmental processes of the somites and their derivatives. Hence, we conducted long-term incubation of fertilized hagfish eggs (Methods), and obtained 42 fertilized eggs at various developmental stages ( Supplementary Table S1 ). We selected three different stages of hagfish embryo, designated the early-, middle-, and late-pharyngula stages, for analysis of somite development ( Fig. 3 ; Supplementary Table S1 ). We used these three embryos to conduct detailed histological observations and used in situ hybridization to detect expression of Twist and Pax1/9 homologues ( Figs 3 and 4a–l ; Supplementary Figs S2 and S3 ), known to be specifically expressed in the sclerotomes of some gnathostomes 12 .

Evidence: 61 • authority: 72

Vertebrates (), also called craniates, are animals with a vertebral column and a cranium. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebrata ( VUR-tə-BRAY-tə) with some 65,000 species, by far the largest ranked grouping in the phylum Chordata. The vertebrates include mammals, birds, amphibians, and various classes of fishes and reptiles. The two major living vertebrate subgroups are the jawless Cyclostomi (including hagfish and lampreys), and the jawed Gnathostomata. The jawed fish include both the cartilaginous fish and the bony fish. Bony fish include the lobe-finned fish, which gave rise to t…

Evidence: 54 • authority: 72

Fish anatomy is the study of the form or morphology of fish. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish. In practice, fish anatomy and fish physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, as might be observed on a dissecting table or under a microscope, and the latter dealing with how those components function together in living fish. The anatomy of fish is often shaped by the physical characteristics of water, the medium in which fish live. Water is much denser than air, holds a relatively small amount of…

Evidence: 47 • authority: 72

A chordate ( KOR-dayt) is a bilaterian animal belonging to the phylum Chordata ( kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa: a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates, and cephalochorda…

Evidence: 39 • authority: 80

The Transoral-Transmaxillary Access Route to the Anterior Vertebral Column and Skull Base

Evidence: 39 • authority: 80

The Transoral-Transmaxillary Access Route to the Anterior Vertebral Column and Skull Base

Evidence: 39 • authority: 80

Embryology of the Skull Base and Vertebral Column

Evidence: 39 • authority: 80

Chapter-11 Embryology: Skull and Vertebral Column

Evidence: 39 • authority: 92

Modularity and integration of the neural arch and vertebral centrum in primates.. The vertebral column consists of multiple homologous elements that have specialized within and between taxa and serve important functions in positional support and as protection for the central nervous system. The study of modularity and integration provides new insights into the evolution of complex structures such as the vertebral column. Patterns of modularity and integration may reflect underlying genetic-developmental patterns and facilitate evolution. Previous studies have identified mixed modularity patterns within and between elements across mammals generally, within primates and carnivorans. Here, we…

Evidence: 0 • authority: 69

Evidence: 0 • authority: 62