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Histological Study of Respiratory Organ of Betta sp.


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Betta sp. is a freshwater ornamental fish which also known as a fighting fish. One of the fundamental organs to support fishes life is respiratory organ. Fighting fish is belongs to the suborder Anabantoidei which means labyrinth fishes group. The aim of the study was to know histology of the respiratory organs of Betta sp. Histological preparations were done using paraffin method, stained with Hematoxylin-Eosin (HE). The result showed that Betta sp. has a respiratory organ common fish i.e gills and additional respiratory organ structure namely labyrinth and pseudobranch that makes Betta sp can survive in a low volume of water. The gill is consists of gill arch, gill raker, gill fillament and gill lamellae. The labyrinth is consist of connective tissue and folded ephitelium. Pseudobranch according to some literature function as an additional respiratory. Functions attributed to the pseudobranch include; regulation of oxygen to the eyes, enzyme production for use in the gas bladder, osmoregulation, and many others.
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ISSN 2597-5250
Volume 2, 2019 | Pages: 181-184 EISSN 2598-232X
Histological Study of Respiratory Organ of Betta sp.
Nurul Safitri Apriliani1,2*, Hikmah Supriyati3, M. Ja’far Luthfi3
1Center for Integrative Zoology, 2Biology Department, 3Biology Education Department, Faculty of Science and Technology, UIN Sunan Kalijaga
Jl. Marsda Adisucipto No. 1 Yogyakarta 55281, Indonesia. Tel. + 62-274-540971, Fax. + 62-274-519739
Abstract. Betta sp. is a freshwater ornamental fish which also known as a fighting fish. One of the fundamental organs to support fishes
life is respiratory organ. Fighting fish is belongs to the suborder Anabantoidei which means labyrinth fishes group. The aim of the study
was to know histology of the respiratory organs of Betta sp. Histological preparations were done using paraffin method, stained with
Hematoxylin-Eosin (HE). The result showed that Betta sp. has a respiratory organ common fish i.e gills and additional respiratory organ
structure namely labyrinth and pseudobranch that makes Betta sp can survive in a low volume of water. The gill is consists of gill arch,
gill raker, gill fillament and gill lamellae. The labyrinth is consist of connective tissue and folded ephitelium. Pseudobranch according to
some literature function as an additional respiratory. Functions attributed to the pseudobranch include; regulation of oxygen to the eyes,
enzyme production for use in the gas bladder, osmoregulation, and many others.
Keywords: Betta sp., Fighting fish, Histology, Labyrinth, Pseudobranch, Respiration
Betta sp. is a freshwater fish that has a special
character, such as its aggressiveness to hold their
territory from their fellow. Betta sp. has high survivor
rates in a low volume of water with no air circulator
(aerator). The fundamental organ to support the fish in
that environment is respiratory system organ.
Respiration is a physiological process by which
organisms exchange gases (oxygen and carbon dioxide)
from the environment. It is taking place between blood
and water through the medium of respiratory. This
process is also known as an internal respiration process.
The organ who take a respiratory system work as well
is gill and other additional structure respiratory.
The gills of some fish species can compensate for
ambient oxygen changes by exhibiting morphological
and functional plasticity that give the gill the ability to
modify its structure. Previous studies reveal aquatic air-
breathing fish with accessory air-breathing organ (the
labyrinth organ).
Air-breathing fish are defined as fish that have the
ability to exchange gases directly from the aerial
environment, and these all have an accessory air-
breathing organ (Graham, 1997). They are classified
into amphibious and aquatic air-breathing fishes. The
accessory air-breathing organs are alternative gas
exchange organs that are found in many different
tissues including the labyrinth organ, skin, lungs,
respiratory gas bladders, digestive tracts and structures
derived from buccal, pharyngeal and branchial cavities
from several different phylogenetic lineages (Graham,
1997). The species in the Anabantoidei have the
labyrinth organ as an accessory air-breathing organ and
possess branchial and systemic circuits similar to a
double-circuit circulatory system (Munshi et al., 1986;
Olson et al., 1986).
The other characteristic of Betta sp. that enable
them to survive in the low water volume is a respiratory
modification in the form of an air chamber filled with
plates or leaflets of respiratory epithelium located
above the gills and functioning adjuncts to them in
obtaining oxygen. This adaption and variations of it are
not uncommon in organisms which have become
adapted to water which is frequently low in oxygen
Until recently the classification for the Betta was
Anabantidae. Anabantid fishes are found throughout
southeastern Asia as far north as northern China and
Korea and including the Philippines and the Malayan
archipelago to, and though, southern and western
Africa. The Asian and African forms are somewhat
distinct and the African forms are not known. Genus
Betta has Asiatic distribution. Some are widely
distributed, but according to Smith (1945) Betta
splendens appears to be found in the wild state only in
Siam (Thailand). It may be that they were widely
distributed artificially for possible mosquito control at
an early date, making it difficult to accurately
determine their natural range.
Labyrinth fish are endemic to freshwaters
of Asia and Africa. In Asia, they are found throughout
East, Southeast, and South Asia, especially but not
exclusively in the warm, slow-flowing, low-oxygen
waters. In Africa, significantly smaller numbers of
labyrinth fish can be found in the southern half of the
continent, with concentrate in the rainforest waters. The
characteristics of the fish habitats are indicators of the
size of the labyrinth organ, as the organ size is
negatively correlated with the level of oxygen in the
waters. Species native to low-oxygen waters are more
likely to have larger and more complex labyrinth
organs than species found in fast-flowing, oxygen-rich
waters (Pinter, 1986).
2: 181-184, 2019
This paper aimed to investigated histological and
functional changes in the respiratory organs (gills and
additional organs/labyrinth) of the aquatic air-breathing
fish Betta splendens. The purpose of this research is to
know the histology of gill as main respiratory organ
and to know histology of labyrinth and pseudobranch
as an additional respiratory organ of Betta splendens.
The animals used in this study were three Betta fishes
obtained from Market fish, Balirejo, Yogyakarta. The
material used were bouin, decal solution, toluene,
xylol, paraffin, ethanol, hematoxylin-eosin, glycerin,
and entellan. The equipment used in this study were
dissection kit, paraffin tub, flacon bottles, oven
paraffin, staining jar, cassettes, beaker 250 ml, and 50
ml, measuring cup 100ml, funnel, stirrer glass, iron
clamp, slide warmer, balance, rotary microtome,
microscope, glass slide, cover glass, and camera. The
organ was sacrificed and fixed with bouin’s solution.
Decalcifying tissue were done with decal solution for
histological process. Histological slides were made
using paraffin method with Hematoxylin-Eosin (HE)
Figure 1. Histology of head part of Betta sp. (G) Gill; (Lb) Labyrinth;
(psd) Pseudobranch; (Bc) Buccal cavity. HE staining. (4x10
Fish living in an environment that is considerably
different from that encountered by terrestrial
vertebrates. Compared with air, water is 800 times
more dense, 60 times more viscous, has 1/30 the
capacity to hold oxygen and oxygen diffuses at 1/8000
the rate. Thus this respiratory medium is more abrasive
and oxygen depleted than air (Gary, 2000).
Most teleosts use gills as the main respiratory
surface, although accessory respiratory structures also
occur (Genten et al., 2009). The result showed that the
histology of respiratory organ of fathead Betta sp. is
consist of gill, labyrinth, and pseudobranch. In the
aquatic environment, fish gills serve multiple functions,
such as gas exchange, ionic regulation, acid-base
balance, and nitrogen excretion, for the purpose of
maintaining homeostasis (Perry, 1998; Evans et al.,
The fish gill has become a multifunctional organ
designed to deal with the vagaries of an aquatic
medium. The gill is not only the primary site for
respiration, it is also the principal, and often exclusive,
site for osmoregulation, acid-base balance, and
metabolism of circulating hormones and perhaps
xenobiotics (Maetz, 1971; Hughes and Morgan, 1973;
Heisler, 1984; Payan et al., 1984). The anatomy of gill
tissue and its vasculature is as diverse and specialized
as the function it performs and the gill might be the
most highly differentiated of any vertebrate organ
(Gary, 2000).
Figure 2. Histology of fathead of Betta sp. (Sm) Striated muscle; (T)
Teeth; (Psd) Pseudobranch; (G) Gill; (Bc) Buccal cavity. HE staining.
(10x10 Magnification).
Teleost fish have eight-gill arches arranged in four
pairs on either side of the buccal cavity. An additional
primordial gill hemiarch, the pseudobranch, is also
present in most species, notable exceptions are the
catfish. Structurally, the pseudobranch resembles a
vestigial gill arch (Laurent and Dunel-Erb, 1984). It
contains filaments similar to those on other gill arches,
but they are generally covered by a relatively thick
epithelium that prevents direct communication with the
environment. Furthermore, the vascular supply to the
pseudobranch is derived from the post-branchial
(systemic) circulation (Gary, 2000).
Figure 3. Histology of gill of Betta sp. (Ga) Gill arch; (Gf) Gill
fillament; (GL) Gill lamellae; (HM) Hypaxial muscle; (Bc) Buccal
cavity; (Ty) Thymus; (Opr) Operculum. HE staining. (10x10
Nurul Safitri Apriliani, Histological Study of Respiratory Organ of Betta sp
Most fish (teleost) have four pairs of gill arches
extending from the floor to the roof of the buccal
cavity. Each of the four pairs is supported by a
cartilaginous and/or bony skeleton facilitating
movement of gills to favorable respiratory positions.
The gills are covered and protected by an operculum
(Genten et al., 2009).
The four pairs of branchial arches in fish consist of
many filaments and lamellae covered with epithelial
cells. Basal wall, mitochondria-rich cells, mucous cells,
and undifferentiated cells are the four major cell types
in the gill epithelia (Perry, 1997; Evans, 1999). MRCs
are generally distributed in the filaments and inter- and
basal-lamellar regions and are believed to be the site of
ionic extrusion in seawater fish and ionic uptake in
freshwater fish (Perry, 1998; Evans, 1999; Evans et al.,
2005; Hwang and Lee, 2007).
The inner surfaces of the gill arches carry one or
more rows of stiff strainers called gill rakers. They
serve to sort and aggregate particulate food material
and to position larger food items before the food is
passed into the esophagus and then into the stomach or
intestine. The rakers tend to be long, slender, and
tightly packed in planktivorous fishes and particle
feeders (such as anchovies, herrings, alewife and
certain scombrids) (Genten et al., 2009). Each gill raker
is composed of an osseous or cartilaginous lamella
supporting the pharyngeal pluristratified epithelium and
connective tissue.
The purpose of the structures of gill lamellae is to
provide a large surface area that supports respiratory
and excretory functions. The efficiency of exchange,
which in the case of oxygen is roughly 50-80%, is
largely a function of the countercurrent exchange
between blood and water (Genten et al., 2009).
Figure 4. Histology of labyrinth of Betta sp. (mv) Microvilli; (mc)
Mucous cell; (gc) Goblet cell. HE staining. (10x10 Magnification).
The labyrinth organ, a defining characteristic of fish
in the suborder Anabantoidei, is a much-folded
suprabranchial accessory breathing organ. It is formed
by vascularized expansion of the epibranchial bone of
the first-gill arch and used for respiration in the air
(Pinter, 1986). This states is same with Munshi and
Graham that Anabantoidei is aquatic-breathing fishes
and have a labyrinth organ protruding from the first-gill
arch on both sides of the branchial cavity to assist in
gas exchange (Munshi et al., 1986; Graham, 1997).
This organ allows labyrinth fish to
take oxygen directly from the air, instead of taking it
from the water in which they reside through use
of gills. The labyrinth organ helps the inhaled oxygen
to be absorbed into the bloodstream. As a result,
labyrinth fish can survive for a short period of time out
of water, as they can inhale the air around them,
provided they stay moist.
Labyrinth in fishes is not initially functional at
immature individual (or at early life). The development
of the organ is gradual and most labyrinth fish breathe
entirely with their gills and develop the labyrinth
organs later in life (Pinter, 1986).
Figure 5. Histology of Pseudobranch (psd) of Betta sp. (G) Gill; (HM)
Hypaxial muscle; (Opr) Operculum; (Br) Branchiostegal rays. HE
staining. (10x10 Magnification).
The pseudobranch is a red, gill-like structure
derived from the first-gill arch and located on the inner
surface of the operculum. It is composed of gill
lamellae, connective tissue, and blood vessels. The
lamellae consist of pseudobranchial cells on an
underlying basement membrane. This latter is applied
to a network of parallel blood capillaries which can be
supported by thin cartilaginous rods. The pseudobranch
has a direct vascular connection with the choroid of the
eye, which is composed of similar arrays of capillaries
(rete mirabile) alternating with rows of fibroblast-like
cells. The pseudobranch is not present in all teleosts.
Those fish which do not possess such structure (some
Siluridae, Ictaluridae, Notopteridae, Cobitidae,
Anguillidae) invariably also lack a choroid rete.
Although it is considered to have an endocrine and
regulatory function as well as a hyperoxygenation
function for the retinal blood supply, these are still to
be defined in full.
In teleosts, pseudobranch occurs bilaterally along
the interior of the opercula anterior to the first pair of
gill arches. While morphologically similar to the gills,
pseudobranchiae have a single row of filaments and
receive oxygen-rich blood from the first efferent
arteries (and are therefore unlikely to serve a
respiratory function). In Cyprinidae,
including Pimephales promelas, the pseudobranch is
2: 181-184, 2019
completely covered by the opercular epithelium.
Lamellae, lacking any contact with the external
medium, are fused to each other forming a "glandular
pseudobranch" (Laurent and Dunel-Erb, 1984). The
epithelium is comprised predominantly of
pseudobranchial cells which are morphologically
similar to chloride cells (but unique to the
pseudobranch). Functions attributed to the
pseudobranch include; larval respiration before
maturation of gill arches, regulation of oxygen to the
eyes, enzyme production for use in the gas bladder,
osmoregulation, and many others. In a review of
pseudobranch morphology and function, Laurent and
Dunel-Erb (1984) found none of these explanations
completely satisfactory, concluding that a likely
function "should be at least partly sensory." These
authors believe the pseudobranch, which is richly
innervated, functions primarily in maintaining blood
Other from being assisted with labyrinth organ,
Betta sp. also assisted by pseudobranch organ which
has the same function to support respiration process in
fishes. So the fishes can survive in lack of oxygen
(environment). Pseudobranch only assisted in some
species. In fact, Betta sp. is indeed have this
pseudobranch. In some literature, the explanation about
pseudobranch is still had the different meaning of the
function. Other literature reported that pseudobranch
functions to respiration, but the other explained that
pseudobranch function to take oxygen into blood vessel
to choroid eye.
Betta sp. as an ornamental fish has a special character,
besides as a fighting fish, it has an ability to live in the
low volume of water. It makes Betta sp. can survive in
low-oxygen environment, as it has an additional
respiratory organ (labyrinth, pseudobranch).
Histologically, labyrinth is consist of connective tissue
and folded epithelium. Pseudobranch according to
some literature function as an additional respiratory.
Functions attributed to the pseudobranch include;
regulation of oxygen to the eyes, enzyme production
for use in the gas bladder, osmoregulation, and many
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gill: dominant site of gas exchange, osmoregulation, acid-base
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Part B: Ion and Water Transfer (eds W.W. Hoar and D. J
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breathing Climbing perch, Anabas testudineus (Bloch). Am J
Anat 176:321-331.
Olson KR, Munshi JSD, Ghosh TK, Ojha J. 1986. Gill
microcirculation of the air-breathing climbing perch, Anabas
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Vol X Gills. Academic Press, New York
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of freshwater fishes. Ann Rev Physiol 59:325-47.
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Pinter, H. (1986). Labyrinth Fish. Barron's Educational Series, Inc
... Other physiological adjustments included various metabolic strategies which allowed them to withstand endogenous ammonia toxicity during emersion [36]. Meanwhile, histoanatomical solutions are associated with the presence of the respiratory epithelium not only in the gills but also in the gillderived AROs [25,37], or even in the posterior intestine [38]. This epithelium is usually single-layered, with abundant mucous cells, and is, overall, a delicate structure that is fixed to its basal lamina and underlying connective tissue (rich in collagen), both of which provide proper mechanical support and necessary elasticity of the whole membrane [39]. ...
Full-text available
Accessory respiratory organs (AROs) are a group of anatomical structures found in fish, which support the gills and skin in the process of oxygen uptake. AROs are found in many fish taxa and differ significantly, but in the suborder Anabantoidei, which has a labyrinth organ (LO), and the family Clariidae, which has a dendritic organ (DO), these structures are found in the suprabranchial cavity (SBC). In this study, the SBC walls, AROs, and gills were studied in anabantoid (Betta splendens, Ctenopoma acutirostre, Helostoma temminckii) and clariid (Clarias angolensis, Clarias batrachus) fishes. The histological structure of the investigated organs was partially similar, especially in relation to their connective tissue core; however, there were noticeable differences in the epithelial layer. There were no significant species-specific differences in the structure of the AROs within the two taxa, but the SBC walls had diversified structures, depending on the observed location. The observed differences between species suggest that the remarkable physiological and morphological plasticity of the five investigated species can be associated with structural variety within their AROs. Furthermore, based on the observed histology of the SBC walls, it is reasonable to conclude that this structure participates in the process of gas exchange, not only in clariid fish but also in anabantoids.
Full-text available
All fish species in the Anabantoidei suborder are aquatic air-breathing fish. These species have an accessory air-breathing organ, called the labyrinth organ, in the branchial cavity and can engulf air at the surface of the water to assist in gas exchange. It is therefore necessary to examine the extent of gill modification among anabantoid fish species and the potential trade-offs in their function. The experimental hypothesis that we aimed to test is whether anabantoid fishes have both morphological and functional variations in the gills among different species. We examined the gills of 12 species from three families and nine genera of Anabantoidei. Though the sizes of the fourth gill arch in three species of Trichogaster were reduced significantly, not all anabantoid species had morphological and functional variations in the gills. In these three species, the specific enzyme activity and relative protein abundance of Na(+)/K(+)-ATPase were significantly higher in the anterior gills as compared with the posterior gills and the labyrinth organ. The relative abundance of cytosolic carbonic anhydrase, an indicator of gas exchange, was found to be highest in the labyrinth organ. The phylogenetic distribution of the fourth gill's morphological differentiation suggests that these variations are lineage specific, which may imply a phylogenetic influence on gill morphology in anabantoid species.
Histology is the discipline of biology that involves the microscopic examination of thin ( 5 µm) stained tissue sections in order to study their structure and to correlate it with function. Histology is an important field of fish health supervision that can often detect early signs of disease not easily recognized on gross examination. New species of fishes are described each year and a final total may well exceed 30,000 species. This vast group far outnumbers all other kinds of vertebrates : approximately 60% of all vertebrate species are teleost fishes distributed into more than 500 families. From the economic point of view, nearly half of all fish eaten today is farmed, not caught. Currently, freshwater and marine fisheries capture 95 million tons of fish annually, of which 60 million tons are destined for human consumption. Globally, consumer demand for fish continues to climb, especially in affluent, developed nations. Overfishing has slashed stocks – especially of large predator species – to an all-time low, worldwide, according to new data. Therefore the only option for meeting future demand for fish is by farming them and aquaculture now accounts for more than 40% of the world fishmeal consumption. Fish health in fisheries is therefore an important concern and as it is not possible to diagnose fish disease purely on the basis of behaviour or physical changes, further investigations and tests are often crucial to arrive at a definite diagnosis. For instance, if fish are still alive, taking and preparing a skin scrape or gill biopsy, during which a small amount of mucus or tissue is carefully removed and examined under the microscope is a common practice. Fish death, unfortunately, is one of the main problems that novice aquarist and even some expert aquarist face. Thus, the reporting of normal histology of fish tissues and organs serves as a foundation upon which to gather and build our ichthyopathology knowledge base. Moreover, fish have become one of the major resources in biomedical research and in some countries their use in laboratories has outpaced that of the more conventional rodents. Numerous microscopic techniques are available for studying cells, tissues and organs. One of the most common of these is the examination of fixed dead tissues which can be stained with various dyes and viewed under the light microscope. It is beyond the scope of this text to cite every histological procedures. However, we will present in the following introduction the general principles of histology and histochemistry including the methods used here to demonstrate the various aspects of normal fish microanatomy. Our aim has been to present an extensive set of histological images devoted to fishes. Although several studies treat histological aspects in relation to pathology, no recent synthesis on the normal histology of fish is available. This atlas is designed for use by students, biologists, ichthyologists, fish farmers and veterinarians working in fisheries to those on academic staffs and, of course, to comparative histologists who want to learn more about the fish world. Moreover it was the authors’ belief that a bilingual atlas (English/French) should interest a large audience. All photomicrographs are original. Light microscopy has been used exclusively and illustrated with colored photomicrographs. Tissue and organ samples chosen to illustrate this work are selected from reared food fish, as well as from species in the aquarium and in the wild. The samples were fixed in Bouin’s fluid and embedded in paraffin. The sections were stained with Hematoxylin and Eosin (H-E), and Masson’s trichrome. The periodic and Schiff reagent (PAS) was used for staining polysaccharide in combination with hematoxylin and orange G. Some specific immunohistochemical and lectin cytochemical preparations are also shown when relevant.
The vascular organization and endothelial cell specialization of the air-breathing organs of Anabas testudineus were examined by light and scanning electron microscopy of fixed tissue and vascular corrosion replicas. The vessels supplying blood to the lining of paired suprabranchial chambers and the plicated labyrinthine organs within the chambers are tripartite, having a median artery and paired, lateral veins. Hundreds of respiratory islets, the functional units of gas exchange, cover the surfaces of both the chamber and labyrinthine organ. A median islet artery supplies the central aspect of each islet and gives rise to numerous short arterioles from which the transverse channels are formed. Transverse channels are parallel capillary-sized vessels that extend in two rows away from the medial arterioles and drain laterally into one of two lateral islet veins. Basally situated single rows of endothelial cells lining the transverse channels form thick, evaginated, tongue-like cytoplasmic processes that project freely into the lumen from the tissue side of the channel. Other thin, septate, cytoplasmic extensions of the same cells form valve-like septa that extend across the channel. Both the septa and tongue-like processes appear to direct the red blood cells to the epithelial side of the channel and thus decrease the diffusion distance between the air and red cell. A large sinusoidal space lies under the transverse channels and may support the channels and even elevate them during increased oxygen demand. The epithelium covering the transverse channels is smooth, which enhances air convection and minimizes unstirred layer effects. The epithelium between the channels contains microvilli that may serve to trap bacteria or particulates and to humidify the air chambers.
The general macrocirculation and branchial microcirculation of the air-breathing climbing perch, Anabas testudineus, was examined by light and scanning electron microscopy of vascular corrosion replicas. The ventral aorta arises from the heart as a short vessel that immediately bifurcates into a dorsal and a ventral branch. The ventral branch distributes blood to gill arches 1 and 2, the dorsal branch to arches 3 and 4. The vascular organization of arches 1 and 2 is similar to that described for aquatic breathing teleosts. The respiratory lamellae are well developed but lack a continuous inner marginal channel. The filaments contain an extensive nutritive and interlamellar network; the latter traverses the filament between, but in register with, the inner lamellar margins. Numerous small, tortuous vessels arise from the efferent filamental and branchial arteries and anastomose with each other to form the nutrient supply for the filament, adductor muscles, and arch supportive tissues. The efferent branchial arteries of arches 1 and 2 supply the accessory air-breathing organs. Arches 3 and 4 are modified to serve primarily as large-bore shunts between the dorsal branch of the ventral aorta and the dorsal aorta. In many filaments from arches 3 and 4, the respiratory lamellae are condensed and have only 1-3 large channels. In some instances in arch 4, shunt vessels arise from the afferent branchial artery and connect directly with the efferent filamental artery. The filamental nutrient and interlamellar systems are poorly developed or absent. The respiratory and systemic pathways in Anabas are arranged in parallel. Blood flows from the ventral branch of the ventral aorta, through gill arches 1 and 2, into the accessory respiratory organs, and then returns to the heart. Blood, after entering the dorsal branch of the ventral aorta, passes through gill arches 3 and 4 and proceeds to the systemic circulation. This arrangement optimizes oxygen delivery to the tissues and minimizes intravascular pressure in the branchial and air-breathing organs. The efficiency of this system is limited by the mixing of respiratory and systemic venous blood at the heart.
The gill lamellar epithelium is composed of two predominant cell types, pavement cells and mitochondria-rich chloride cells. The chloride cells play a vital role in ionic regulation because they are the sites of Ca2+ and Cl- uptake from water. Consequently, lamellar chloride cell proliferation occurs in response to ionoregulatory challenges so as to increase the ion-transporting capacity of the gill. It has been argued that such chloride cell proliferation might increase the thickness of the blood-to-water diffusion barrier and thereby impede gas diffusion. This review focuses on the potential negative consequences of chloride cell proliferation on gas transfer and possible compensatory mechanisms that might minimise the extent of respiratory impairment. Two approaches were used to evoke chloride cell proliferation in rainbow trout, hormone treatment (growth hormone/cortisol) and exposure to soft water. In all cases, chloride cell proliferation was associated with a pronounced thickening of the lamellar diffusion barrier. The thickening of the diffusion barrier was associated with a significant impairment of gas transfer. Subsequent studies revealed that several compensatory physiological responses occurred concurrently with the chloride cell proliferation to alleviate or reduce the detrimental consequences of the thickened diffusion barrier. These included hyperventilation, an increased affinity of haemoglobin-oxygen binding and earlier onset of catecholamine release during acute hypoxia.
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.
Compared to terrestrial animals, fish have to cope with more-challenging osmotic and ionic gradients from aquatic environments with diverse salinities, ion compositions, and pH values. Gills, a unique and highly studied organ in research on fish osmoregulation and ionoregulation, provide an excellent model to study the regulatory mechanisms of ion transport. The present review introduces and discusses some recent advances in relevant issues of teleost gill ion transport and functions of gill ionocytes. Based on accumulating evidence, a conclusive model of NaCl secretion in gills of euryhaline teleosts has been established. Interpretations of results of studies on freshwater fish gill Na+/Cl- uptake mechanisms are still being debated compared with those for NaCl secretion. Current models for Na+/Cl- uptake are proposed based on studies in traditionally used model species. Many reported inconsistencies are claimed to be due to differences among species, various experimental designs, or acclimation conditions. Having the benefit of advanced techniques in molecular/cellular biology, functional genomics, and model animals, several new notions have recently been raised concerning relevant issues of Na+/Cl- uptake pathways. Several new windows have been opened particularly in terms of molecular mechanisms of ionocyte differentiation and energy metabolite transport between gill cells during environmental challenge.
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