Visceral Arches

Visceral Arches Visceral arches are pieces of cartilages or bones that support the pharyngeal region of vertebrates and also help attach the jaws with the skull. There are typically 7 pairs of visceral arches in vertebrates which modify in different groups depending upon the presence or absence of gills and type of jaw suspension. Visceral arches are numbered 1-7, the first arch is known mandibular arch having two called palatoquadrate and meckel's cartilage. cartilaginous pieces Second visceral arch, called hyoid arch, consists of hyomandibular, ceratohyal and basihyal. Hyomandibula Palstoquadraste Meckel's cartilage Pharyngeal slits Pharyngobranchial Epibranchiat Ceratobranchial Hypobranchial Basibranchial Branchial arches Hyoid arch Mandibular arch FIGURE 7.5 Primitive splanchnocranium. Seven arches are shown. Up to five elements compose an arch on each side, beginning with the pharyngobranchial dorsally and in sequence to the basibranchials most ventrally. The first two complete arches are named: mandibular arch for the first and hyoid arch for the second that supports it. The characteristic five-arch elements are reduced to just two in the mandibular arch: the palatoquadrate and Meckel's cartilage. The large hyomandibula, derived from an epibranchial element, is the most prominent component of the next arch, the hyoid arch. Behind the hyoid arch are variable numbers of branchial arches 1. II, and so on. Labial cartilages are not included From 3 to 7th visceral arches are called branchial arches, since they support gills and typically consist of 4 pieces of cartilages, namely, pharyngobranchial, epibranchial, certobranchial and hypobranchial. FIGURE 7.6 Shark embryo, the dogfish Scyllium. Jaws appear to be in series with the branchial arches. The mandibular arch is first, followed by the hyoid and then several tiranchial arches. Such a position of the jaws, in series with the arches, is taken as evidence that the jaws derive from the most anterior branchial arch. Abbreviations: ceratohyal (Ch), hyomandibula (Hy). Meckel's cartilage (Mik), neural arch (Ne), occipital arch (Os), orbital cartilage (Oc), polar cartilage (Pc), palatoquadrate (Pq). trabecula (Tr). Labial cartilages are not included. Cyclostomes There is no resemblance with the typical pattern but cyclostomes possess a splanchnocranium in which usual cartilages are not identifiable. The whole pharyngeal skeleton fuses to form a branchial basket to support gills. Elasmobranchs They contain full set of visceral arches and three unpaired branchial cartilages called basibranchials. The arrangement is close to basic pattern as they have 5 pairs of functional gills and skeleton is all cartilaginous. Bony fishes Meckel's cartilage forms articular and becomes part of the lower jaw. Hyoid arch is modified for the movement of operculum and functioning of the lower jaw. Symplectic helps in jaw suspension. Last branchial arch shows sign of degeneration as the number of gills is reduced to 4 pairs. Amphibia Larval frogs have 6 visceral arches and the last 3 3 bear be gills. In urodeles having gills third, fourth and fifth epibranchials support gills while their basibranchials and ceratobranchials are reduced to two pairs. Hyomandibular modifies as columella of the middle ear cavity in frogs and toads. The air breathing hyobranchial apparatus of frogs and toads is made by the fusion of 2nd, 3rd and 4 visceral arches. Reptiles Quadrate and epipterygoid bones of the skull are modifications of pterygoquadrate, and articular of the lower jaw is a modified meckel's cartilage. Hyoid arch forms a small hyoid plate that also extends forward to support the tongue. One of two ceratobranchials may form the posterior cornu of the hyoid plate. Birds Modification is similar to reptiles except that there is only one cornu of the hyoid plate that is modified from the third visceral arch.