The structure of the gas bladder of the spotted gar, Lepisosteus oculatus

Investigating the internal system of defense of Gastropoda Aplysia depilans (Gmelin, 1791): Focus on hemocytes

The internal defense system of mollusks represents an efficient protection against pathogens and parasites, involving several biological immune processes, such as phagocytosis, encapsulation, cytotoxicity, and antigenic recognition of self/non-self. Mollusks possess professional, migratory, and circulating cells that play a key role in the defense of the organism, the hemocytes. Several studies have been performed on hemocytes from different mollusks, but, to date, these cells are still scarcely explored. Different hemocyte populations have been found, according to the presence or absence of granules, size, and the species of mollusks studied. Our study aims to deepen the knowledge of the hemocytes of the gastropod Aplysia depilans using morphological techniques and light and confocal microscopy, testing Toll-like receptor 2, inducible nitric oxide synthetase, and nicotinic acetylcholine receptor alpha 7 subunit. Our results show two hemocyte populations distinguishable by size, and presence/absence of granules in the cytoplasm, strongly positive for the antibodies tested, suggesting for the first time the presence of these receptors on the surface of sea hare hemocytes by immunohistochemistry. These data help in the understanding of the immune system of this gastropod, providing additional data for comprehending the evolution of the defense response in metazoan phylogenesis.

Morpho-structural adaptations of the integument in different aquatic organisms

The integument acts as a barrier to protect the body from harmful pathogenic infectious agents, parasites, UV rays, trauma, and germs. The integument of invertebrates and vertebrates are structurally different: while invertebrates usually have a simple monolayer epidermis frequently covered by mucus, cuticles, or mineralized structures, vertebrates possess a multilayered epidermis with several specialized cells. This study aims to describe by morphological, histological, and immunohistochemical analyses, the morpho-structural adaptations throughout evolution of the integument of gastropod Aplysia depilans (Gmelin, 1791), ascidian Styela plicata (Lesuer, 1823), myxine hagfish Eptatretus cirrhatus (Forster, 1801) and teleost Heteropneustes fossilis (Bloch, 1794) for the first time, with special reference to sensory epidermal cells. Different types of cells could be identified that varied according to the species; including mucous cells, serous glandular cells, clavate cells, club cells, thread cells, and support cells. In all integuments of the specimens analyzed, sensory solitary cells were identified in the epidermis, immunoreactive to serotonin and calbindin. Our study provided an essential comparison of integuments, adding new information about sensory epidermal cells phylogenetic conservation and on the structural changes that invertebrates and vertebrates have undergone during evolution.

Coelomocytes of the Oligochaeta earthworm Lumbricus terrestris (Linnaeus, 1758) as evolutionary key of defense: a morphological study

Metazoans have several mechanisms of internal defense for their survival. The internal defense system evolved alongside the organisms. Annelidae have circulating coelomocytes that perform functions comparable to the phagocytic immune cells of vertebrates. Several studies have shown that these cells are involved in phagocytosis, opsonization, and pathogen recognition processes. Like vertebrate macrophages, these circulating cells that permeate organs from the coelomic cavity capture or encapsulate pathogens, reactive oxygen species (ROS), and nitric oxide (NO). Furthermore, they produce a range of bioactive proteins involved in immune response and perform detoxification functions through their lysosomal system. Coelomocytes can also participate in lithic reactions against target cells and the release of antimicrobial peptides. Our study immunohistochemically identify coelomocytes of Lumbricus terrestris scattered in the epidermal and the connective layer below, both in the longitudinal and in the smooth muscle layer, immunoreactive for TLR2, CD14 and α-Tubulin for the first time. TLR2 and CD14 are not fully colocalized with each other, suggesting that these coelomocytes may belong to two distinct families. The expression of these immune molecules on Annelidae coelomocytes confirms their crucial role in the internal defense system of these Oligochaeta protostomes, suggesting a phylogenetic conservation of these receptors. These data could provide further insights into the understanding of the internal defense system of the Annelida and of the complex mechanisms of the immune system in vertebrates.

Expression of Antimicrobic Peptide Piscidin1 in Gills Mast Cells of Giant Mudskipper Periophthalmodon schlosseri (Pallas, 1770)

The amphibious teleost Giant mudskipper (Periophthalmodon schlosseri, Pallas 1770) inhabit muddy plains and Asian mangrove forests. It spends more than 90% of its life outside of the water, using its skin, gills, and buccal-pharyngeal cavity mucosa to breathe in oxygen from the surrounding air. All vertebrates have been found to have mast cells (MCs), which are part of the innate immune system. These cells are mostly found in the mucous membranes of the organs that come in contact with the outside environment. According to their morphology, MCs have distinctive cytoplasmic granules that are released during the degranulation process. Additionally, these cells have antimicrobial peptides (AMPs) that fight a variety of infections. Piscidins, hepcidins, defensins, cathelicidins, and histonic peptides are examples of fish AMPs. Confocal microscopy was used in this study to assess Piscidin1 expression in Giant Mudskipper branchial MCs. Our results demonstrated the presence of MCs in the gills is highly positive for Piscidin1. Additionally, colocalized MCs labeled with TLR2/5-HT and Piscidin1/5-HT supported our data. The expression of Piscidin1 in giant mudskipper MCs highlights the involvement of this peptide in the orchestration of teleost immunity, advancing the knowledge of the defense system of this fish.

Ependymal and Neural Stem Cells of Adult Molly Fish (Poecilia sphenops, Valenciennes, 1846) Brain: Histomorphometry, Immunohistochemical, and Ultrastructural Studies

This study was conducted on 16 adult specimens of molly fish (Poecilia sphenops) to investigate ependymal cells (ECs) and their role in neurogenesis using ultrastructural examination and immunohistochemistry. The ECs lined the ventral and lateral surfaces of the optic ventricle and their processes extended through the tectal laminae and ended at the surface of the tectum as a subpial end-foot. Two cell types of ECs were identified: cuboidal non-ciliated (5.68 ± 0.84/100 μm2) and columnar ciliated (EC3.22 ± 0.71/100 μm2). Immunohistochemical analysis revealed two types of GFAP immunoreactive cells: ECs and astrocytes. The ECs showed the expression of IL-1β, APG5, and Nfr2. Moreover, ECs showed immunostaining for myostatin, S100, and SOX9 in their cytoplasmic processes. The proliferative activity of the neighboring stem cells was also distinct. The most interesting finding in this study was the glia-neuron interaction, where the processes of ECs met the progenitor neuronal cells in the ependymal area of the ventricular wall. These cells showed bundles of intermediate filaments in their processes and basal poles and were connected by desmosomes, followed by gap junctions. Many membrane-bounded vesicles could be demonstrated on the surface of the ciliated ECs that contained neurosecretion. The abluminal and lateral cell surfaces of ECs showed pinocytotic activities with many coated vesicles, while their apical cytoplasm contained centrioles. The occurrence of stem cells in close position to the ECs, and the presence of bundles of generating axons in direct contact with these stem cells indicate the role of ECs in neurogenesis. The TEM results revealed the presence of neural stem cells in a close position to the ECs, in addition to the presence of bundles of generating axons in direct contact with these stem cells. The present study indicates the role of ECs in neurogenesis.

Confocal Characterization of Intestinal Dendritic Cells from Myxines to Teleosts

Dendritic cells (DCs) are antigen-presenting cells (APCs) that regulate the beginning of adaptive immune responses. The mechanisms of tolerance to antigens moving through the digestive tract are known to be regulated by intestinal DCs. Agnatha and Gnathostoma are descendants of a common ancestor. The Ostracoderms gave rise to Cyclostomes, whereas the Placoderms gave rise to Chondrichthyes. Sarcopterygii and Actinopterygii are two evolutionary lines of bony fishes. Brachiopterygii and Neopterygii descend from the Actinopterygii. From Neopterygii, Holostei and Teleostei evolved. Using immunohistochemistry with TLR-2, Langerin/CD207, and MHC II, this study aimed to characterize intestinal DCs, from myxines to teleosts. The findings reveal that DCs are positive for the antibodies tested, highlighting the presence of DCs and DC-like cells phylogenetically from myxines, for the first time, to teleosts. These findings may aid in improving the level of knowledge about the immune system’s evolution and these sentinel cells, which are crucial to the body’s defense.

Localization of vasoactive intestinal peptide and toll-like receptor 2 immunoreactive cells in endostyle of urochordate Styela plicata (Lesueur, 1823)

The endostyle is the first component of the ascidian digestive tract, it is shaped like a through and is located in the pharynx's ventral wall. This organ is divided longitudinally into nine zones that are parallel to each other. Each zone's cells are physically and functionally distinct. Support elements are found in zones 1, 3, and 5, while mucoproteins secreting elements related to the filtering function are found in zones 2, 4, and 6. Zones 7, 8, and 9, which are located in the lateral dorsal section of the endostyle, include cells with high iodine and peroxidase concentrations. Immunohistochemical technique using the following antibodies, Toll‐like receptor 2 (TLR‐2) and vasoactive intestinal peptide (VIP), and lectin histochemistry (WGA—wheat‐germagglutinin), were used in this investigation to define immune cells in the endostyle of Styela plicata (Lesueur, 1823). Our results demonstrate the presence of immune cells in the endostyle of S. plicata, highlighting that innate immune mechanisms are highly conserved in the phylogeny of the chordates.

Neuroepithelial cells (NECs) and mucous cells express a variety of neurotransmitters and neurotransmitter receptors in the gill and respiratory air-sac of the catfish Heteropneustes fossilis (Siluriformes, Heteropneustidae): a possible role in local immune defence

Heteropneustes fossilis is an air-breathing teleost inhabiting environments with very poor O₂ conditions, and so it has evolved to cope with hypoxia. In the gills and respiratory air-sac, the sites for O₂ sensing and the response to hypoxia rely on the expression of acetylcholine (Ach) acting via its nicotinic receptor (nAChR). This study examined the expression patterns of neuronal markers and some compounds in the NECs of the gills and respiratory air sac having an immunomodulatory function in mammalian lungs. Mucous cells, epithelial cells and neuroepithelial cells (NECs) were immunopositive to a variety of both neuronal markers (VAChT, nAChR, GABA-B-R1 receptor, GAD679) and the antimicrobial peptide piscidin, an evolutionary conserved humoral component of the mucosal immune system in fish. We speculate that Ach release via nAChR from mucous cells may be modulated by GABA production in the NECs and it is required for the induction of mucus production in both normoxic and hypoxic conditions. The presence of piscidin in mucous cells may act in synergy with the autocrine/paracrine signals of Ach and GABA binding to GABA B R1B receptor that may play a local immunomodulatory function in the mucous epithelia of the gills and the respiratory air sac. The potential role of the NECs in the immunobiological behaviour of the gill/air-sac is at moment a matter of speculation. The extent to which the NECs as such may participate is elusive at this stage and waits investigation.

The gas bladder of Pantodon buchholzi: Structure and relationships with the vertebrae

We report here on the histological and structural characteristics of the gas bladder, the vertebral morphology, and the bladder-vertebra relationships of the butterfly fish, Pantodon buchholzi. The bladder opens at the boundary between the pharynx and the esophagus by a middle slit. A pneumatic duct is absent. The bladder shows a dorsolateral wall that adapts to the anfractuosities of the coelomic cavity and a ventral wall in contact with the abdominal organs. The vertebral bodies are formed by an hourglass shaped autocentrum, and by an arcocentrum reduced to several longitudinal ridges. The transverse processes adopt the structure of a cage whose walls are formed by bone trabeculae of variable size and distribution pattern. The dorsolateral wall of the bladder is a membrane that covers the kidney, adapts to the irregular shape of the vertebrae, and invades the transverse processes at several points before extending laterally. However, invasion of the vertebral bodies, the presence of a labyrinth, or the formation of respiratory parenchyma were not observed. The luminal surface of this wall is a thin respiratory barrier containing a single epithelial cell type. In addition, the wall contains numerous eosinophils that may be implicated in immune defense. The bladder ventral wall is a membrane rich in collagen, vessels, smooth muscle, and nerves that lacks a respiratory barrier. Its luminal surface contains ciliated and nonciliated cells. The two cell types appear implicated in surfactant production.

Pulmonary Smooth Muscle in Vertebrates: A Comparative Review of Structure and Function

Although the airways of vertebrates are diverse in shape, complexity, and function, they all contain visceral smooth muscle. The morphology, function, and innervation of this tissue in airways is reviewed in actinopterygians, lungfish, amphibians, non-avian reptiles, birds, and mammals. Smooth muscle was likely involved in tension regulation ancestrally, and may serve to assist lung emptying in fishes and aquatic amphibians, as well as maintain internal lung structure. In certain non-avian reptiles and anurans antagonistic smooth muscle fibers may contribute to intrapulmonary gas mixing. In mammals and birds, smooth muscle regulates airway caliber, and may be important in controlling the distribution of ventilation at rest and exercise, or during thermoregulatory and vocal hyperventilation. Airway smooth muscle is controlled by the autonomic nervous system: cranial cholinergic innervation generally causes excitation, cranial non-adrenergic, non-cholinergic innervation causes inhibition, and spinal adrenergic (SA) input causes species-specific, often heterogeneous contractions and relaxations.

Lungs and gas bladders: Morphological insights

This paper summarizes the main morphological tracts exhibited by lungs and gas bladders in fishes. The origin and organ location, the presence of a glottal region, the inner architecture, the characteristics of the exchange barrier and the presence of pulmonary arteries have been reviewed in the two types of air-breathing organs. With the exception of the dorsal (bladders) or ventral (lungs) origin from the posterior pharynx, none of the morphological traits analyzed can be considered specific for either lungs or gas bladders. This is exemplified by analysis of the morphology of the lung of the Dipnoii and Polypteriformes and of the bladder of the Lepisosteiformes. All of them are obligate air-breathers and show a lung-like (pulmonoid) air-breathing organ. However, while the lungfish lung and the bladder of the Lepisosteiformes occupy a dorsal position and are highly trabeculated, the polypterid lung occupies a ventral position and shows a smooth inner surface. Structural and ultrastructural differences are also highlighted. Noticeably, a large part of the inner surface area of the lung of the Australian lungfish is covered by a ciliated epithelium. A restricted respiratory surface area may help to explain the incapability of this species to aestivate. The respiratory bladder of basal teleosts displays a more complex morphology than that observed in more primitive species. The bladder of basal teleosts may appear divided into respiratory and non-respiratory portions, exhibit intricate shapes, invade adjacent structures and gain additional functions. The increase in morphological and functional complexity appears to prelude the loss of the respiratory functions.

Functional morphology of the respiratory organs of the air-breathing fish with particular emphasis on the African catfishes, Clarias mossambicus and C. gariepinus

The evolution of air-breathing and transition from water to land were pivotal events that greatly determined the ecological diversification, the advances and the successes of animal life. During their relocation onto land, the so-called bimodal breathers were literally caught at the water-air interface. Among such animals are the diverse air-breathing bony fish. Such taxa, however, strictly do not constitute the so-called 'bridging animals', i.e., the inaugural animals that crossed from water to land, nor are they their direct progenitors. The pioneer transitional animals were the Devonian rhipidistian amphibians that possessed a primitive lung which acquired O₂ directly from air and discharged CO₂ back into the same. By having particular morphological and physiological adaptations for terrestrialness, the modern amphibious- and aquatic air-breathers are heuristic analogues of how and why animals relocated from water to land. It has generally been espoused that lack or dearth of O₂ in water, especially in the warm tropical one, was an elemental driver for adoption of air-breathing. There is, however, no direct causal relationship between the evolution of air-breathing and the shift onto land: the move onto land was a direct solution to the existing inimical respiratory conditions in water. This is evinced in the facts that: a) even after attaining capacity of air-breathing, an important preadaptation for life on land, some animals continued living in water while periodically accessing air, b) in the fish species that live in the well-oxygenated waters, e.g., torrential rivers, only few air-breathe and c) air-breathing has still evolved in freshwaters and seawaters, where levels of dissolved O₂ are sufficiently high. Here, the structure and function of the respiratory organs of the air-breathing fish are succinctly outlined. Two African catfishes, Clarias mossambicus and C. gariepinus are highlighted.

Insights into the evolution of polymodal chemoreceptors

Respiratory chemoreceptors in vertebrates are specialized cells that detect chemical changes in the environment or arterial blood supply and initiate autonomic responses, such as hyperventilation or changes in heart rate, to improve O₂ uptake and delivery to tissues. These chemoreceptors are sensitive to changes in O₂, CO₂ and/or H⁺. In fish and mammals, respiratory chemoreceptors may be additionally sensitive to ammonia, hypoglycemia, and numerous other stimuli. Thus, chemoreceptors that affect respiration respond to different types of stimuli (or modalities) and are considered to be "polymodal". This review discusses the polymodal nature of respiratory chemoreceptors in vertebrates with a particular emphasis on chemoreceptors of the carotid body and pulmonary epithelium in mammals, and on neuroepithelial cells in water- and air-breathing fish. A major goal will be to examine the evidence for putative polymodal chemoreceptors in fish within the context of studies on mammalian models, for which polymodal chemoreceptors are well described, in order to improve our understanding of the evolution of polymodal chemoreceptors in vertebrates, and to aid in future studies that aim to identify putative receptors in air- and water-breathing fish.

Air- breathing in fish: Air- breathing organs and control of respiration: Nerves and neurotransmitters in the air-breathing organs and the skin

In fishes, exploitation of aerial gas exchange has evolved independently many times, involving a variety of air-breathing organs. Indeed, air-breathing occurs in at least 49 known families of fish (Graham, 1997). Many amphibious vertebrates, at some stage of their development are actually trimodal breathers that use various combinations of respiratory surfaces to breath both water (skin and/or gill) and air (skin and/or lung). The present review examines the evolutionary implications of air-breathing organs in fishes and the morphology of the peripheral receptors and the neurotransmitter content of the cells involved in the control of air-breathing. Control of breathing, whether gill ventilation or air-breathing, is influenced by feedback from peripheral and/or central nervous system receptors that respond to changes in PO2, PCO2 and/or pH. Although the specific chemoreceptors mediating the respiratory reflexes have not been conclusively identified, studies in water-breathing teleosts have implicated the neuroepithelial cells (NECs) existing in gill tissues as the O2 sensitive chemoreceptors that initiate the cardiorespiratory reflexes in aquatic vertebrates. Some of the air-breathing fishes, such as Protopterus, Polypterus and Amia have been shown to have NECs in the gills and/or lungs, although the role of these receptors and their innervation in the control of breathing is not known. NECs have been also reported in the specialized respiratory epithelia of accessory respiratory organs (ARO's) of some catfish species and in the gill and skin of the mudskipper Periophthalmodon schlosseri. Unlike teleosts matching an O2-oriented ventilation to ambient O2 levels, lungfishes have central and peripheral H+/CO2 receptors that control the acid-base status of the blood.

Lung anatomy and histology of the extant coelacanth shed light on the loss of air-breathing during deep-water adaptation in actinistians

Lungs are specialized organs originated from the posterior pharyngeal cavity and considered as plesiomorphic for osteichthyans, as they are found in extant basal actinopterygians (i.e. Polypterus) and in all major groups of extant sarcopterygians. The presence of a vestigial lung in adult stages of the extant coelacanth Latimeria chalumnae is the result of allometric growth during ontogeny, in relation with long-time adaptation to deep water. Here, we present the first detailed histological and anatomical description of the lung of Latimeria chalumnae, providing new insights into its arrested differentiation in an air-breathing complex, mainly represented by the absence of pneumocytes and of compartmentalization in the latest ontogenetic stages.

Evolution of Shh endoderm enhancers during morphological transition from ventral lungs to dorsal gas bladder

Shh signalling plays a crucial role for endoderm development. A Shh endoderm enhancer, MACS1, is well conserved across terrestrial animals with lungs. Here, we first show that eliminating mouse MACS1 causes severe defects in laryngeal development, indicating that MACS1-directed Shh signalling is indispensable for respiratory organogenesis. Extensive phylogenetic analyses revealed that MACS1 emerged prior to the divergence of cartilaginous and bony fishes, and even euteleost fishes have a MACS1 orthologue. Meanwhile, ray-finned fishes evolved a novel conserved non-coding sequence in the neighbouring region. Transgenic assays showed that MACS1 drives reporter expression ventrally in laryngeal epithelium. This activity has been lost in the euteleost lineage, and instead, the conserved non-coding sequence of euteleosts acquired an enhancer activity to elicit dorsal epithelial expression in the posterior pharynx and oesophagus. These results implicate that evolution of these two enhancers is relevant to the morphological transition from ventral lungs to dorsal gas bladder.

Endoderm enhancer MACS1 of Sonic Hedgehog is conserved in animals with lungs. Here, the authors show that mouse without MACS1 has defective laryngeal development, and use phylogenetic analyses to show association of evolutionary lung-gas bladder transition with change of the enhancer.

Developmental cardiorespiratory physiology of the air-breathing tropical gar, Atractosteus tropicus

The physiological transition to aerial breathing in larval air-breathing fishes is poorly understood. We investigated gill ventilation frequency (f_G), heart rate (f_H), and air breathing frequency (f_AB) as a function of development, activity, hypoxia, and temperature in embryos/larvae from day (D) 2.5 to D30 posthatch of the tropical gar, Atractosteus tropicus, an obligate air breather. Gill ventilation at 28°C began at approximately D2, peaking at ∼75 beats/min on D5, before declining to ∼55 beats/min at D30. Heart beat began ∼36-48 h postfertilization and ∼1 day before hatching. f_H peaked between D3 and D10 at ∼140 beats/min, remaining at this level through D30. Air breathing started very early at D2.5 to D3.5 at 1-2 breaths/h, increasing to ∼30 breaths/h at D15 and D30. Forced activity at all stages resulted in a rapid but brief increase in both f_G and f_H, (but not f_AB), indicating that even in these early larval stages, reflex control existed over both ventilation and circulation prior to its increasing importance in older fishes. Acute progressive hypoxia increased f_G in D2.5-D10 larvae, but decreased f_G in older larvae (≥D15), possibly to prevent branchial O₂ loss into surrounding water. Temperature sensitivity of f_G and f_H measured at 20°C, 25°C, 28°C and 38°C was largely independent of development, with a Q₁₀ between 20°C and 38°C of ∼2.4 and ∼1.5 for f_G and f_H, respectively. The rapid onset of air breathing, coupled with both respiratory and cardiovascular reflexes as early as D2.5, indicates that larval A. tropicus develops "in the fast lane."

The Effect of Hypoxia and Hyperoxia on Growth and Expression of Hypoxia-Related Genes and Proteins in Spotted Gar Lepisosteus oculatus Larvae and Juveniles

We studied the molecular responses to different water oxygen levels in gills and swim bladder of spotted gar (Lepisosteus oculatus), a bimodal breather. Fish at swim-up stage were exposed for 71 days to normoxic, hypoxic, and hyperoxic water conditions. Then, all aquaria were switched to normoxic conditions for recovery until the end of the experiment (120 days). Fish were sampled at the beginning of the experiment, and then at 71 days of exposure and at 8 days of recovery. We first cloned three hypoxia-related genes, hypoxia-inducible factor 2α (HIF-2α), Na(+) /H(+) exchanger 1 (NHE-1), and NHE-3, and uploaded their cDNA sequences in the GeneBank database. We then used One Step Taqman® real-time PCR to quantify the mRNA copies of target genes in gills and swim bladder of fish exposed to different water O2 levels. We also determined the protein expression of HIF-2α and neuronal nitric oxide synthase (nNOS) in the swim bladder by using confocal immunofluorescence. Hypoxic stress for 71 days significantly increased the mRNA copies of HIF-2α and NHE-1 in gills and swim bladder, whereas normoxic recovery for 8 days decreased the HIF-2α mRNA copies to control values in both tissues. We did not found significant changes in mRNA copies of the NHE-3 gene in either gills or swim bladder in response to hypoxia and hyperoxia. Unlike in normoxic swim bladder, double immunohistochemical staining in hypoxic and hyperoxic swim bladder using nNOS/HIF-2α showed extensive bundles of HIF-2α-positive nerve fibers in the trabecular musculature associated with a few varicose nNOS immunoreactive nerve terminals.

Expression patterns and quantitative assessment of neurochemical markers in the lung of the gray bichir, Polypterus senegalus (Cuvier, 1829)

Anatomical and functional studies of the autonomic innervation and the putative oxygen receptors-the neuroepithelial (NEC)-like cells of the bichirs are lacking. The present paper describes the distribution of both NEC-like cells and the polymorphous granular cells (PGCs) that populate the mucociliated epithelium of the lung in the air breathing fish Polypterus senegalus. By using confocal immunohistochemistry we determined the coexpression of specific neurochemical markers. Colocalization studies showed that 5HT is coexpressed with calbindin and nNOS in the NEC-like cells and PGCs, and choline acetyltransferase (ChAT) is coexpressed with nNOS in both the two types of cells. Distribution of neurotransmitters (5HT, NO) and neurochemical marker ChAT is also investigated in the lung muscle. The role of these transmitters may be the autonomic control of circulation and respiration. However, the importance of these signals for the respiratory responses in the species studied is still not known. The present study also shows for the first time the simultaneous occurrence of piscidin 1 and 5HT in the PGCs. The function of these cells being equivalent to ones found in fish gill subepithelial parenchyma, is still not known. Due to the importance of piscidin 1 in local immune defense, more research is useful to understand a possible interaction of PGCs with immune response in the bichir lung.

Sensing and surviving hypoxia in vertebrates

Surviving hypoxia is one of the most critical challenges faced by vertebrates. Most species have adapted to changing levels of oxygen in their environment with specialized organs that sense hypoxia, while only few have been uniquely adapted to survive prolonged periods of anoxia. The goal of this review is to present the most recent research on oxygen sensing, adaptation to hypoxia, and mechanisms of anoxia tolerance in nonmammalian vertebrates. We discuss the respiratory structures in fish, including the skin, gills, and air-breathing organs, and recent evidence for chemosensory neuroepithelial cells (NECs) in these tissues that initiate reflex responses to hypoxia. The use of the zebrafish as a genetic and developmental model has allowed observation of the ontogenesis of respiratory and chemosensory systems, demonstration of a putative intracellular O2 sensor in chemoreceptors that may initiate transduction of the hypoxia signal, and investigation into the effects of extreme hypoxia on cardiorespiratory development. Other organisms, such as goldfish and freshwater turtles, display a high degree of anoxia tolerance, and these models are revealing important adaptations at the cellular level, such as the regulation of glutamatergic and GABAergic neurotransmission in defense of homeostasis in central neurons.