Gill arches

Disproportionately more is known about O2 chemoreception in mammals despite the fact that they make up only about 10% of known vertebrate species. There seems to be a general phylogenetic trend with 02-sensitive chemoreceptors having a more diffuse, intemal/extemal distribution in lower vertebrates to the more localized interior receptor groups in aortic and carotid bodies of mammals This concentration of chemoreceptor loci follows the phylogenetic trend seen in the internalization and reduction of gill arches. 

Figure 1 Schematic diagram to illustrate the distribution of 02-sensitive chemoreceptors in different vertebrate groups. VII, IX, and X refer to cranial nerves (facial, glossopharyngeal, and vagus nerves) while 2, 3, 4, 5, and 6 refer to arteries supplying the respective embryonic gill arches. While O2 chemoreceptors are found in the aortic bodies of mammals, they do not appear to serve a respiratory role. See text for further details.

Based on reflex studies, the hypoxic bradycardia exhibited by most teleost fish appears to be triggered by activation of externally oriented receptors that monitor aquatic O2 levels. These receptors are found largely on the first gill arch but have been reported on other gill arches in some species (see Refs. 13,65 for reviews). Not all fish follow this pattern, however. In the gar and the tambaqui they appear to be sensitive to both internal and external changes in Pq (79,85) while in the neotropical fish the traira they appear to only monitor the P02 of the blood (84). By contrast, the 02-sensitive receptors instrumental in producing the increases in ventilation frequency and amplitude in most teleost fish appear to monitor both the blood and the water (13,67,84,85). 

The only direct evidence for the presence of 02-sensitive ehemoreeeptors in frsh comes from three series of studies in trout and tuna (specifically, the pseudobranch and first gill arch) (14,52-54,62). The studies on both tuna and trout noted afferent activity arising from fibers that responded only, or preferentially, to changes in external (water) O2 stimulus levels and others that responded only, or preferentially, to changes in internal (perfusate/blood) O2 stimulus levels (Fig. 2)... 

Figure 2 ENG of activity from oxygen chemoreceptors in the first gill arch of skipjack tuna illustrating the effect of interrupting perfusion on afferent activity. The fiber depicted in the top panel is not sensitive to changes in the Pqj of the bathing solution hut does respond to changes in perfusion. The fiber illustrated in the bottom panel responds to changes in the P02 of the bathing solution hut is not very sensitive to changes in perfusion. (Reproduced from Ref 62.)...

Table 1 Summary of the Effects of Different Pharmacological Agents on 02-Sensitive Afferent Neural Activity Recorded from Cranial Nerve IX from the First Gill Arch of Rainbow Trout...

Burleson ML, Smatresk NJ, Milsom WK. Afferent inputs associated with cardio-ventilatory control in fish. In Hoar WS, Randall DJ, Farrel AF, eds. Fish Physiology, Vol. XII, Part B The Cardiovascular System. New York Academic Press, 1992. Burleson ML, Milsom WK. Sensory receptors in the first gill arch of rainbow trout. Respir Physiol 1993 93 97-110. 

Milsom WK, Brill RW. Oxygen sensitive afferent information arising Jfom the first gill arch of yellowfin tuna. Respir Physiol 1986 66 193-203. 

Branchial skeleton medial to gills, each gill arch being composed of five or less elements ... 

Following the interpretation proposed above, that the endoskeleton of galeaspid and osteostracan head-shield is essentially the neurocranium and shoulder girdle, and does not include gill-arch components, it may be assumed that the braincase of the... 

Past theories have identified the limits of ectoderm in the mouth as central to the determination of where teeth develop (Jollie 1968 cf. Smith and Coates 1998, fig. 8). However, this view takes no account of the widespread occurrence of denticulated plates deep within the pharynx of non-tetrapod vertebrates. These include a major, successful, group of teleosts which feed entirely as pharyngognaths, and have teeth only on the last gill arch. The model animal for developmental research in fish, the zebrafish, is one of these, and we predict that experimental data will show that this is an endodermally derived pattern. Moreover, one direction for future comparative research in all appropriate model taxa could be towards the identification of those genes which are expressed in, and give pattern and positional information to, the endoderm in relation to initiation of the dentition. Already, we know that endoderm is essential for tooth formation in amphibians, and snspect that it may be so for other gnathostomes. Finally, it shonld be possible to examine where the boundary forms between ectoderm and endoderm, and the role of this bonndary in setting up positional information for the dentition. 

Character 12 Supplementary rays of gill arches currently uninformative... 

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