The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system Nevertheless, substantial questions about the evolution of these mechanisms and control remain

https://people.clas.ufl.edu/devans/files/PRVGill.pdf

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

Textbook of histology

Human beings may affect the welfare of fish through fisheries, aquaculture and a number of other activities. There is no agreement on just how to we igh the concern for welfare of fish against the hum an interests involved, but ethical frameworks exist th at suggest how this might be approached. Different definitions of animal welfare focus on an animal's condition, on its subjective experience o f that condition and/or on whether it can lead a natu ral life. These provide different, legitimate, pers pectives, but the approach taken in this paper is to focus on welfare as the absence of suffering. An unresolved and controversial issue in discussion s about animal welfare is whether non-human animals exposed to adverse experiences such as physical injury or confinement experience what humans would call suffering. The neocortex, which in huma ns is an important part of the neural mechanism tha t generates the subjective experience of suffering, i s lacking in fish and non-mammalian animals, and it has been argued that its absence in fish indicates that fish cannot suffer. However, a strong alternative view is that complex animals with sophisticated behaviour, such as fish, probably have the capacity for suffer ing, though this may be different in degree and kind fro m the human experience of this state. Recent empirical studies support this view and show that painful stimuli are, at least, strongly avers ive to fish. Consequently, injury or experience of othe r harmful conditions is a cause for concern in term s of welfare of individual fish. There is also growing e vidence that fish can experience fear-like states a nd that they avoid situations in which they have experience d adverse conditions.

/pdf/current-issues-in-fish-welfare-3xfacgx1du.pdf

Current issues in fish welfare

The autonomic nervous system.

The vertebrate immune system is comprised of numerous distinct and interdependent components Every component has its own inherent protective value, and the final combination of them is likely to be related to an animal’s immunological history and evolutionary development Vertebrate immune system consists of both systemic and mucosal immune compartments, but it is the mucosal immune system which protects the body from the first encounter of pathogens According to anatomical location, the mucosa-associated lymphoid tissue, in teleost fish is subdivided into gut-, skin-, and gill-associated lymphoid tissue and most available studies focus on gut The purpose of this paper is to summarise the current knowledge of the immunological defences present in skin mucosa as a very important part of the fish immune system, serving as an anatomical and physiological barrier against external hazards Interest in defence mechanism of fish arises from a need to develop health management tools to support a growing finfish aquaculture industry, while at the same time addressing questions concerning origins and evolution of immunity in vertebrates Increased knowledge of fish mucosal immune system will facilitate the development of novel vaccination strategies in fish

/pdf/an-overview-of-the-immunological-defenses-in-fish-skin-563a4msfgs.pdf

An Overview of the Immunological Defenses in Fish Skin

Significant progress has been made in the last 30 years in our knowledge of cellular and physiological aspects of fish epidermis This review surveys the histology, histochemistry (including lectin- and immunohistochemistry) and function of the epidermis of various teleosts and cyclostomes in the light of the new data.

The epidermis is a multipurpose tissue, although the secretory function is dominant. Mucous secretions are produced by different types of epithelial cells, in particular those located in the superficial layer. Specific unicellular glands in the skin of gnathostome fish are the goblet cells, the sacciform cells and the club cells. In addition, lampreys and hagfish possess specific secretory cell types. Other specialized types are the holocrine venom cells which are aggregated in venom glands, the mitochondria-rich cells (ionocytes) and the photocytes, the latter grouped with other cell types to form photophores that are embedded in the skin.

The outer epidermal cells and the mucous goblet cells secrete the epithelial mucous layer, whose carbohydrate composition changes with stress and environmental conditions. The mucus contains neuronal and endothelial forms of nitric oxide synthase (NOS), the enzyme responsible for the production of nitric oxide (NO), a prominent vascular and neuronal messenger that regulates many epithelial functions.

The cytokeratins in the skin tissues have different distribution patterns in the various cell types, correlated with specific epithelial differentiation or, in hagfish, the modulation of the viscoelastic properties of skin mucus

Club cell and sacciform cell secretions are now considered as storehouses of biologically active substances. Their expression is paralleled with that present in glandular cells of the skin of other lower vertebrates. The fine structure and histochemical properties of club cells are clearly different from those of the venom cells in fish and amphibians.

Mitochondria-rich chloride cells (ionocytes), although primarily concentrated in the branchial epithelium, also occur in various areas of the skin. The skin chloride cells, like those in the gill, secrete monovalent ions in sea water and take up ions in fresh water.

Fish skin has a diffuse paraneuronal cell system responsible for the production of bioactive compounds that have possible regulating functions. Neuropeptides occur in the Merkel cells, the mechanoreceptors (neuromast hair cells) and the electroreceptors (sensory cells of the ampullary and tuberous organs) of the skin. Basal cells of the taste buds are the source of neuron-specific enolase, serotonin, bombesin and somatostatin. Photophores contain serotonin and adrenalin, which play a neuromodulatory role in the regulation of luminescence.

The functions of the skin secretions are primarily protective, as a result of their antimicrobial properties. Further analysis of the components of epidermal cells and unicellular glands of aquatic animals will disclose many protective factors

Structural, histochemical and functional aspects of the epidermis of fishes

Anatomical and histochemical studies have demonstrated that the bulk of autonomic neurotransmission in fish gill is attributed to cholinergic and adrenergic mechanisms (Nilsson. 1984. In: Hoar WS, Randall DJ, editors. Fish physiology, Vol. XA. Orlando: Academic Press. p 185-227; Donald. 1998. In: Evans DH, editor. The physiology of fishes, 2nd edition. Boca Raton: CRC Press. p 407-439). In many tissues, blockade of adrenergic and cholinergic transmission results in residual responses to nerve stimulation, which are termed NonAdrenergic, NonCholinergic (NANC). The discovery of nitric oxide (NO) has provided a basis for explaining many examples of NANC transmissions with accumulated physiological and pharmacological data indicating its function as a primary NANC transmitter. Little is known about the NANC neurotransmission, and studies on neuropeptides and NOS (Nitric Oxide Synthase) are very fragmentary in the gill and the air-breathing organs of fishes. Knowledge of the distribution of nerves and effects of perfusing agonists may help to understand the mechanisms of perfusion regulation in the gill (Olson. 2002. J Exp Zool 293:214-231). Air breathing as a mechanism for acquiring oxygen has evolved independently in several groups of fishes, necessitating modifications of the organs responsible for the exchange of gases. Aquatic hypoxia in freshwaters has been probably the more important selective force in the evolution of air breathing in vertebrates. Fishes respire with gills that are complex structures with many different effectors and potential control systems. Autonomic innervation of the gill has received considerable attention. An excellent review on branchial innervation includes Sundin and Nilsson's (2002. J Exp Zool 293:232-248) with an emphasis on the anatomy and basic functioning of afferent and efferent fibers of the branchial nerves. The chapters by Evans (2002. J Exp Zool 293:336-347) and Olson (2002) provide new challenges about a variety of neurocrine, endocrine, paracrine and autocrine signals that modulate gill perfusion and ionic transport. The development of the immunohistochemical techniques has led to a new phase of experimentation and to information mainly related to gills rather than air-breathing organs of fishes. During the last few years, identification of new molecules as autonomic neurotransmitters, monoamines and NO, and of their multiple roles as cotransmitters, has reshaped our knowledge of the mechanisms of autonomic regulation of various functions in the organs of teleosts (Donald, '98).NO acts as neurotransmitter and is widely distributed in the nerves and the neuroepithelial cells of the gill, the nerves of visceral muscles of the lung of polypterids, the vascular endothelial cells in the air sac of Heteropneustes fossilis and the respiratory epithelium in the swimbladder of the catfish Pangasius hypophthalmus. In addition, 5-HT, enkephalins and some neuropeptides, such as VIP and PACAP, seem to be NANC transmitter candidates in the fish gill and polypterid lung. The origin and function of NANC nerves in the lung of air-breathing fishes await investigation. Several mechanisms have developed in the Vertebrates to control the flow of blood to respiratory organs. These mechanisms include a local production of vasoactive substances, a release of endocrine hormones into the circulation and neuronal mechanisms. Air breathers may be expected to have different control mechanisms compared with fully aquatic fishes. Therefore, we need to know the distribution and function of autonomic nerves in the air-breathing organs of the fishes.

Neuropeptides and nitric oxide synthase in the gill and the air-breathing organs of fishes.

The neuro-endocrine cells of fish skin and respiratory surfaces, and their bioactive secretion as far as is known, are reviewed, and compared with similar elements in tetrapods, particularly amphibians. In the skin of teleost fish, immunohistochemistry has shown that Merkel cells react for serotonin, neuron-specific enolase and enkephalins. The pharmacology is not established in dipnoans or lampreys. In some teleosts, neuromasts react for substance P and leu-enkephalins; substance P is also reported from some ampullary organs (electroreceptors). Taste buds of teleosts may react for enkephalin and substance P. Basal cells of taste buds react for serotonin and neuron-specific enolase. Some unicellular skin glands of teleosts express bioactive compounds, including serotonin and some peptides; this ectopic expression is paralleled in amphibian skin glands. The dipnoan Protopterus has innervated pulmonary neuro-endocrine cells in the pneumatic duct region with dense-cored vesicles. In Polypterus and Amia the lungs have serotonin-positive neuro-endocrine cells that are apparently not innervated. In fish gills, a closed type of neuro-endocrine cell reacts for serotonin, an open type for enkephalins and some calcium-binding proteins (calbindin, calmodulin and S-100 protein). The functions of neuro-endocrine cells in fishes await investigation, but it is assumed they are regulatory.

Distribution patterns of the paraneuronal endocrine cells in the skin, gills and the airways of fishes as determined by immunohistochemical and histological methods

This chapter describes the distributional patterns of the neuroendocrine cells in the respiratory surfaces of fishes and their bioactive secretions which are compared with similar elements in higher vertebrates. The neuroendocrine cells in the airways of fishes differentiate as solitary and clustered cells, but the clusters are not converted into neuroepithelial bodies which are reported in terrestrial vertebrates. The dipnoan fish Protopterus has innervated neuroendocrine cells in the pneumatic duct region. In Polypterus and Amia the lungs have neuroendocrine cells that are apparently not innervated. Two types of neuroendocrine cells are found in the gill of teleost fishes. These cells are very different by their location, structure and immunohistochemistry. Advanced studies on functional morphology of neuroendocrine cells in fish airways are still necessary to increase our understanding of their multifunctional role in the gill area.

Paraneurons in the gills and airways of fishes.

Neuroendocrine (NE) cells in the gills of six fish species were identified by serotonin and peptide immunohistochemistry. They occur either as single cells or as cell clusters within the filamental epithelium. Two populations of NE cells were identified that exhibit an immunoreactivity either for serotonin alone or for met-enkephalin and leu-enkephalin co-distributed in the same cells. We demonstrate for the first time the occurrence of open-type endocrine cells in the fish gill epithelium and confirm the presence of only serotonin-immunopositive cells of the closed type. The NE cells of the species studied do not react positively to other neuroendocrine markers occurring in the mammalian lung. Great differences probably exist in the expression of these epitopes on antigenic structures among various animal species.



Although the physiological role of neuropeptides and serotonin in the fish gill NE cells is unknown, the presence of these molecular markers is of interest not only from an evolutionary aspect, but also in terms of a potential animal model to study the supposed functions of the airway neuroendocrine system in the vertebrates.

Giacomo Zaccone

Papers

Structural, histochemical and functional aspects of the epidermis of fishes

Neuropeptides and nitric oxide synthase in the gill and the air-breathing organs of fishes.

Distribution patterns of the paraneuronal endocrine cells in the skin, gills and the airways of fishes as determined by immunohistochemical and histological methods

Paraneurons in the gills and airways of fishes.

Immunocytochemical localization of serotonin and neuropeptides in the neuroendocrine paraneurons of teleost and lungfish gills