Fish, gill arches

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). 

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. 

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. 

Fish DR, Sawyers D, Allen PJ, Blackie JD, Lees AJ, Marsden CD. The effect of sleep on the dyskinetic movements of Parkinson s disease, Gilles de la Tourette syndrome, Huntington s disease, and torsion dystonia. Arch Neurol 1991 48 210-214. 

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