I. INTRODUCTION:

Primitive fishes were the first vertebrates to exploit atmospheric respiratory gases, in addition to gases dissolved in their aquatic milieu, prior to the colonization of the terrestrial habitats by amphibians. The ability to extract oxygen directly from the atmosphere enabled ancient fish to survive in hypoxic environments. Extant air-breathing fish are now the subjects of many studies coming from diverse laboratories since they are considered physiological models to study the evolutionary transition from gill to air-breathing ventilation. A consequence of this transition is the addition of accessory respiratory organs (ARO’s) that necessitate changes in both the general circulatory system and the microcirculation of the respiratory epithelia, thus providing indication of the evolution associated with adaptation to the terrestrial habitats (Olsson et al., 1995).

The major shifts in the integration of organ systems have coincided with the evolutionary transitions from aquatic to aerial respiration and from aquatic to terrestrial life. In freshwater fish, respiration, ion and water regulation and acid-balance reside mainly within the gills. By contrast, in mammals, gas exchange and respiratory acid-base regulation are lung functions whereas ion and water regulation, nitrogen excretion, and metabolic acid-base regulation depend on the kidney. In larval amphibians, excretion, osmoregulation, and respiration are bronchially mediated. However, the post-metamorphic amphibians show an intermediate position between fish and mammals in terms of kidney function, while auxiliary organs such as the skin and urinary bladder may be involved in respiratory and osmotic functions (Graham, 1997).

The ancient fish lineages are viewed as the archetypes for the physiological adaptations to amphibious life (Hedrick and Katz, 2016). Consequently, much research is now addressed on the basic metabolic and physiological modifications that have occurred during the transition to air-breathing. The main aspect of this transition is the reduction of the gill blood flow associated with air-breathing since it compromises basic gill functions such as CO₂ removal, osmotic regulation, acid-base balance and nitrogen excretion. Another important aspect is the control of air-breathing. Air-breathing fishes must be able to sense and to respond to changes in external and internal partial pressures of respiratory gases (O₂ and CO₂) via chemoreceptors, as well as to sense changes in the volume of the air-breathing organ via mechanoreceptors (Hedrick and Katz, 2016). Some aspects related to nitrogen excretion in air-breathing fishes remain somewhat obscure since air-breathing interrupts or reduces branchial function. Data obtained in the amphibious fishes, the mudskippers, revealed that the gills, skin and urinary tracts were all involved in nitrogen excretion (Graham, 1997) and, that, during forced emersion, they switched from ammonotelism to ureotelism.

The primary focus in this review will be the presumptive peripheral respiratory chemoreceptors of air-breathing fishes that were initially located in the gills of the teleosts (Zaccone et al., 2006; Jonz and Nurse, 2009; Jonz et al., 2016). Morphological and physiological studies of the peripheral O₂ sensing cells have been performed in a few number of air-breathing fishes, being compared to those of water-breathing fishes and mammals to study the evolution of O₂ chemoreception.

This review also focuses on the characteristics of the air-breathing that occurred in a group of teleosts that were secondarily adapted to aerial respiration. These include the air-breathing organs (ABO’s) and the aerial respiratory surfaces of the higher euteleosts that are present in the members of the Clariidae and Heteropneustidae families, grouped into the superfamily Claroidea (Sullivan et al., 2006). The gills and the skin of the amphibious fishes are also considered to be functional for aerial respiration. In mudskippers, the sensory system required to switch the site of gas exchange in emersed air-breathing species and in those having terrestrial habits, is not well characterized.

Extrabranchial sites of respiration in the mudskippers include the cutaneous surfaces, where rapid circulatory adjustments increase blood flow and facilitate O₂ transfer. According to Wright and Turko (2016), the cutaneous surfaces of amphibious fishes such as the rivulines and the mudskippers are primed for aerial respiration, and several plastic traits associated with locomotion, gas exchange, nitrogen excretion, ionoregulation and osmoregulation must be taken into account when explaining ABO specialization. The skin surfaces are also the histological site for the occurrence of putative oxygen receptor cells (the neuroepithelial cells, NECs) that show peculiar neurotransmitter profiles (Zaccone et al., 2017).

Mechanisms of air-breathing in primitive fishes and control of respiration:

The majority of air-breathing fishes employ a buccal force pumping mechanism to ventilate their lungs. The lungfish first aspirate air into the buccal cavity with the glottis closed preceding expiration from the lung. With expiration there is mixing of fresh air and lung gas in the buccal cavity. Subsequently, this mixed gas is pumped into the lung by contraction of the buccal musculature. The air-breathing activity in these fishes is affected by mechanoreceptors sensing the changes in pressure or wall tension. These receptors play a role in inhibiting inflation or promoting deflation (Hedrick and Katz, 2016). The mechanical act of breathing has changed from buccal force pump in air-breathing fishes and amphibians to sectional breathing based on contraction of the diaphragm and chest wall musculature. It is also likely that there are pacemaker cells or inhibitory interaction in pulmonary ventilatory control.

The ventilator control mechanisms in all the vertebrates involve three basic elements, namely, the peripheral sensory receptors (oxygen sensors and mechanoreceptors), the central nervous system and motor-neuron effectors. Unlike the unimodal control of respiration in fishes and mammals having both a continuous ventilatory pattern, in the ABO’s of the primitive actinopterygians, in view of their diversity, the coordination of the aquatic and aerial respiration played a critical role as well as the cardiorespiratory interactions. Ventilation is driven by sensory receptors that also include taste and olfactory receptors. The peripheral-receptor feedback occurs via the sensory afferent cranial nerve branches (Smatresk, 1994) that in fish connect the gill arches, the heart and the organs within the coelomic cavity. The ventilator control in air-breathing fishes is dominated by the peripheral chemoreceptors located in the gill arches, primarily in response to changes in PO₂ in contrast to terrestrial vertebrates where ventilation is primarily driven by central chemoreceptors in response to changes in PCO₂/pH (Jones and Milsom, 1982; Hedrick and Katz, 2016).

The lungs of primitive air-breathing fishes are known to have mechanoreceptors that sense changes in organ volume. These mechanoreceptors are generally characterized as Slowly-Adapting Receptors (SARs) or Rapidly-Adapting Receptors (RARs) based on their afferent firing patterns in response to inflation or deflation of these organs. Both SARs and RARs occur in Protopterus, Lepidosiren and Lepisosteus (DeLaney et al., 1983; Smatresk and Azizi, 1987). The primitive function of the ABO mechanoreceptors is to aid in buoyancy regulation (DeLaney et al., 1983), but future experiments are needed to distinguish the respiratory versus buoyancy relevant inputs of ABO mechanoreception. In bimodal breathers, the available evidence supports the existence of both water-sensing CO₂/pH chemoreceptors, presumably located in the gills or the orobranchial cavity, and of CO₂-sensitive pulmonary stretch receptors (Gilmour and Milsom, 2009). Also, in the ARO (air sac) of the bimodal breathers, such as the air-breathing catfish, Heteropneustes fossilis, there are no available data regarding the importance of mechanoreceptor input (probably located in vagal afferent of the cucullar muscle of the air sac) for the coordination of motor control over the respiratory muscle and structures along the aerial respiratory pathway.

The accessory respiratory organs (ARO’s) of the branchial region:

Fishes in the families Clariidae and Heteropneustidae include those with air-breathing organs derived from modified gills and branchial chambers. In Clariidae, the ARO’s are suprabranchial organs having a complex structural organization. They are comprised of a dendritic organ, a pair of highly vascularized suprabranchial chambers within which the respiratory trees are contained and fan-like structures guarding the entrance of the supra branchial chambers that help to take air. The suprabranchial organs, like the gills, are lined by thin outer epithelial layers with intercellular spaces separated by pilaster cells. The organs and the suprabranchial chambers are supplied by afferent and efferent blood vessels from the gill arches (Maina and Maloiy, 1986).

The Heteropneustidae family contains only two species: Heteropneustes fossilis and H. microps (Graham, 1997). In H. fossilis, a pair of tubular pneumatic air sacs act as ARO. These long tubular sacs arise as outgrowths from the branchial chamber and extend almost down to the tail between the body musculature, near the vertebral column. As in Clarias, a paired suprabranchial chamber forms above the gills and next to the cranium, and gill fans form at the top of each branchial arch. The walls of the posterior suprabranchial chamber and the air sacs are ensheathed by the cucullaris muscle that functions in ABO ventilation. The cucullaris muscle is innervated by a lateral branch of the occipito-spinal nerve originating from the posterior region of the auditory capsule, and encircles the entire respiratory sac (Munshi et al., 1986). The respiratory epithelium of the air sac is characterized by the alternating presence of highly vascularized respiratory islets and lanes of flattened cells. The respiratory islets originate from the gill lamellae and are smaller than those in the gills (Graham, 1997). The islets are essentially blood channels that are maintained opened by the pilaster cells. Mucus secreting cells are freely distributed across the ARO respiratory surface. There are striking similarities in circulatory specialization and AROs anatomy in Clarias and Heteropneustes. These catfish appeared in a common silurid ancestor and were later modified into an everted arborescent organ or an inverted air sac, respectively (Olson et al., 1995)

Nerve studies on the air-sac of H. fossilis were previously reported by Zaccone et al., (2002). Nerve supply of the air-sac is paralleled with that of the gill since the afferent branchial artery that runs into the main ridge of the air-sac produces a series of lateral blood vessels that supply blood to the lamellae of the respiratory epithelium.

Antibodies to met-enkephalin label nerves in the walls of the main efferent vessels of the respiratory air-sac, efferent lateral islet vessels and islet capillaries. A dense plexus of nitregic (nNOS positive) nerves is located in the submucosal layer, in the walls of the afferent and efferent arteries of the islets and in the cucullaris muscle. Numerous mucous cells display nNOS immunoreactivity, and a fine nerve plexus of nNOS positive nerve fibers is seen in close association with these cells. VIP antisera label nerve fibers in the connective tissue, and a perivascular nerve plexuses is observed in the vascular walls of afferent and efferent lateral vessels of the islets. VIP immunopositive nerve fibers are noticed in the musculature of the air-sac. Adrenergic innervation is very sparse.

Recent studies by Zaccone and Maina (unpublished observations) reported the presence of NECs in the gill fans and in the suprabranchial chamber, in areas of the filament-lamella structures, of the African catfish, Clarias gariepinus. The TH-nNOS immunopositive NECs were found touching the external medium, but others are seen at the base of the epithelium. The respiratory surfaces of the dendritic organ have a large surface area and contain pillar cells that are the structural entities of the vascularized membrane.

A dense VAChT innervations is seen running along the basement membrane that surrounds the vascular channels. They also bifurcate near the epithelial surface. VAChT extrabranchial innervation of the gill fans and dendritic organ may be correlated with the modulation of response to hypoxia. The importance of gill fans in aerial respiration derives from their well-developed gas-exchanging surface, ventilation and air retention (Graham, 1997).

CONCLUSIONS:

Much research remains to be accomplished regarding the cellular identification, distribution and neuro-physiological characterization of the peripheral chemoreceptor and their broader spectra of neurotransmitters that enabled to identify multiple populations of NECs. TH is now added as a neuronal marker for NECs in the gill and skin of the mudskipper including their sympathetic innervations. The role of afferent and efferent complex innervations and hypoxia sensitivity of the NECs, and the interaction of the chemo- and mechanoreceptors in the regulation of both aquatic and aerial gas exchange in air-breathing fishes, is still incomplete. NECs are distributed throughout the gill filaments and lamellae of all the gill arches in several teleost species, the air-breathing organs of primitive fishes and the gill fans and the suprabranchial chamber in the AROs of the air-breathing catfish.

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Received on 25.02.2020 Modified on 21.03.2020

Research J. Science and Tech. 2020; 12(2): 143-146.

DOI: 10.5958/2349-2988.2020.00018.2