ABSTRACT
Oxygen (O2) is a gaseous molecule capable of accepting electrons and participating in oxidationreduction reactions to promote the production of metabolic energy within cells. It is this dependence on O2 for cellular energy that deŒnes aerobic organisms and is a characteristic of most eukaryotes. Complex multicellular organisms, such as Œshes, have evolved elaborate mechanisms through which uptake of O2 across the gills, and subsequent delivery to the body tissues, can be regulated so as to defend the animal against hypoxic or ischemic damage. O2 chemoreceptors, cells that display speciŒc responses to chemical change, detect environmental or arterial hypoxia (low O2) and initiate asuite of compensatory physiological adjustments, which may include increased frequency of gill ventilation (i.e., hyperventilation), increased ventilatory amplitude, decreased heart rate, vascular changes, and aquatic surface respiration (Perry et al., 2009; Milsom, 2012). These adaptations are fundamental for survival in environments with «uctuating O2 availability. Physiological responses to hypoxia are exquisitely exempliŒed in Œshes because they live in an aqueous environment where
4.1 Introduction .......................................................................................................................... 149 4.2 Respiratory Chemoreceptors ................................................................................................ 150
4.2.1Chemoreceptors DeŒned .......................................................................................... 150 4.2.2Distribution of Chemoreceptors in Fishes and Mammals ........................................ 150
4.3Gills ...................................................................................................................................... 151 4.3.1 Gill Organization and Anatomy ............................................................................... 151 4.3.2 Nerve Supply ............................................................................................................ 152 4.3.3Gill Neuroepithelial Cells ......................................................................................... 152
4.3.3.1Morphology ................................................................................................ 152 4.3.3.2Biochemistry .............................................................................................. 158 4.3.3.3 Innervation ................................................................................................. 158 4.3.3.4 Cellular Mechanisms of O2 Sensing .......................................................... 159 4.3.3.5 Cellular Mechanisms of CO2 Sensing........................................................ 164 4.3.3.6 Ammonia Sensing ...................................................................................... 164 4.3.3.7Models of Neurotransmission .................................................................... 164
4.4Extrabranchial Chemoreceptors ........................................................................................... 167 4.4.1Oropharyngeal Epithelium ....................................................................................... 167 4.4.2 Skin ........................................................................................................................... 167
4.5 Development ......................................................................................................................... 168 4.5.1Development of O2 Sensing in the ZebraŒsh Gill .................................................... 168 4.5.2 Extrabranchial to Branchial O2 Sensing ................................................................... 168 4.5.3 Renewal of O2 Chemoreceptors ................................................................................ 169
4.6 Conclusion ............................................................................................................................ 169 Acknowledgments .......................................................................................................................... 170 References ...................................................................................................................................... 170
the solubility of O2 is relatively low compared to that of air (Burleson et al., 1992). Thus, naturally occurring «uctuations in the partial pressure of O2 (PO2) will have profound physiological consequences, compared with air-breathing or terrestrial species.
