Nature of the air-breathing organs of the Indian fishes Channa, Amphipnous, Clarias and Saccobranchus as shown by electron microscopy

The structure of the air-breathing organs of the Indian fishes Channa punctatus, Channa Striatus, Amphipnous cuchia, Clarias batrachus and Saccobranchus fossilis has been investigated using electron microscopy. In all species the barrier separating the air from the blood consists of three main layers (epithelium, basal lamina and endothelium). The total thickness ranging from 0.78 μm in C. punctatus 1.6 μm in S. fossilis.
In Clarias and Saccobranchus the presence of pillar cells characteristic of gill secondary lamellae confirms evidence for the origin of these organs by modification of a typical gill structure.
In Amphipnous and two species of Channa , however, the evidence suggests that the accessory organs represent modified gills. The presence of valve-like structures between the afferent and efferent blood spaces of the vascular papillae gave the appearance of pillar cells under the light microscope.
The structure of these organs is correlated with physiological studies on the degree of their importance in the life of the animal and the degree of gill development     

Gross and fine structure of the pseudo branch of the climbing perch, Anabas testudineus (Bloch)

The structure of the glandular pseudobranch of the air-breathing fish Anabas testudineus is described on the basis of light and electron microscopy. It is shown that the pseudobranch has the same basic structure as a typical gill, although in this case it is not freely exposed to the water.
Individual secondary lamellae can be recognized in which well-defined, but narrow, blood channels are present between typical pillar cells. The epithelial layer is mainly represented by enlarged mitochondria-rich cells which constitute a large proportion of the whole organ. Mitochondria-rich cells contain an abundant endoplasmic reticulum which is in close contact with the mitochondria and becomes concentrated near the vascular borders of the cell and opens directly into the basement lamina.
The presence of numerous pinocytotic vesicles in the enlarged pillar cell flanges may transfer material to the blood channels. The precise nature and role of any materials released in this way remains to be investigated.     

Fine structure of the gills of some Indian air-breathing fishes

A detailed account is given of the structure of the gills of Clarias batrachus, Heteropneustes (= Saccobranchus) fossilis, Channa punctata, Monopterus (= Amphipnous) cuchia and Boleophthalmus boddaerti, based upon light and electron microscopy. In all five species the basic organization into primary and secondary lamellae is apparent but the latter are very much more modified in Monopterus. Three main layers separate the water and blood on the surface of the secondary lamellae. The outer epithelium is usually two layered but may be multilayered close to the origin of the secondary lamellae from the gill filament. The basement membrane is relatively thin and a middle dense layer containing collagen fibrils separates two clear layers. The pillar cells, so characteristic of secondary lamellae, are present in all except Monopterus and flanges from these cells surround the blood channels with the exception of the marginal channels. The latter are lined by endothelial cells which line all the blood channels of Monopterus. The overall thickness of the three layers comprising the water/blood barrier ranges from 1.5 to 13 microns. A number of modifications to this basic organization can be related to the degree of dependence of the different species on air-breathing. Boleophthalmus is the only species commonly found in brackish water and its secondary lamellae have well developed lymphoid spaces between two layers of the epithelium. Special densely-stained regions of the pillar cell flanges were also present in this fish and may have a supporting function.     

Ultrastructure of the chorion and its micropyle apparatus in the mature Fundulus heteroclitus (Walbaum) ovum

The ovum 'membranes' and the micropyle apparatus of mature, extruded ova of Fundulus heteroclitus were examined by scanning electron microscopy. The ovum is covered by a thin jelly coat comprised of a dense mat of 0.3-0.5 urn fibrils, except for a 50–100 um fibril-free zone surrounding the micropyle apparatus. The micropyle apparatus consists of a 50–100 um diameter funnel-shaped vestibule, at the bottom of which is a circular aperture 4–5 μ in diameter leading to the micropyle canal which traverses the entire chorion layer. The inner micropyle aperture, 2–3 um diameter, apposes the inner ovoplasm mass.
The chorion is the major protective coating of the ovum. It consists of a thin (0.4 μm) outer zone, a thicker (9–12 μ), lamellated inner zone with 4–10 lamellae, and sometimes an innermost crystalline zone, varying in thickness from 1–13 μm. The extreme variability in the structure of the lamellated and crystalline zones of the chorion suggests that generalizations concerning ovum membranes can be misleading.     

A light and electron microscope study of the gills of larval lampreys (Geotria australis) with particular reference to the water-blood pathway.

The gills of ammocoetes of the Southern Hemisphere lamprey Geotria australis have been studied using light and electron microscopy. Emphasis has been placed on describing the structures and vessels involved in gaseous exchange, and on providing quantitative data for the water-blood barrier, including diffusion distance, diffusing capacity and the relative volumes of the component tissues. Although lamprey gills lie inside rather than outside the branchial skeleton as in gnathostomatous fishes. the morphology and ultrastructure of the gill filaments and secondary lamellae of G. australis larvae are very similar to those of teleost fishes. The extensive blood spaces within the secondary lamellae are enclosed by pillar cell bodies and pillar cell flanges which support two layers of epithelial cells. The outer surfaces of the epithelial cells are ridged and covered in a flocculent material which probably represents mucus. Differences were observed in the components of the water-blood barrier at the distal edges and at the surface of the secondary lamellae. At the distal edge, the lining of the marginal channel consisted of an endothelial cell rather than the pillar cell flanges which line the blood spaces of other regions. Based on light micrograph measurements, these differences result in a reduction in the arithmetic mean thickness of the water-blood barrier from 3.62 μm over the pillar cells to 2.22 μm over the marginal channel. Using values for the water-blood barrier obtained from light micrographs, the arithmetic and harmonic mean diffusing capacities were calculated as 1.1046 and 1.7589 ml O₂min/mm Hg/Kg.     

On the micro-circulatory system of the gills of certain freshwater teleostean fishes

The micro-circulatory system of the lamellae is in the form of a network of more or less parallel blood channels. A row of pillar cells separates two contiguous blood channels. All the pillar cells are situated in the same plane between the upper and lower basement membranes to which they remain fixed in position by means of columns. Histochemical investigations show that the basement membrane as well as the columns are collagenous in nature. Though both basement membranes and the columns are PAS positive (magenta colour), reticulin is not present as they do not respond to silver techniques. The development of new blood channels in the micro-circulatory system of Channa striatus has been studied. They arise as buds from the wall of the pre-existing vessels. There is some evidence to show the possible transformation of the smooth muscle cells of the tunica media of blood vessels into the pillar cells of the micro-circulatory system. Various aspects of the physiology of the micro-circulatory system of the gills have been discussed.     

A comparative study of the ultrastructure of the water-blood pathway in the secondary lamellae of teleost and elasmobranch fishes — benthic forms

Summary The ultrastructure of the secondary lamellae has been examined in four species of free-swimming elasmobranchs, two species of Raia and five species of bony flatfishes. Microvilli which are present on the outer epithelial surface vary in form and size. It is suggested that a possible function of the microvilli is to anchor a surface mucus covering whose possible functions are discussed. Vesicles are found immediately beneath the microvilli in elasmobranchs but not in teleosts. Chloride and mucous cells are present on the secondary lamellae of all species and often have microvilli of greater length than on the other cells of the epithelium. Micropinocytotic vesicles are found on both sides of the basement membrane. The number of columns enclosed in the pillar cells varied from 5–11.The water-blood pathway showed variations in thickness not only between different species but also in different parts of individual secondary lamellae. A general trend in the mean total thickness was found, being greater in the swimming elasmobranchs (10.22 ) than in the rays (5.47 ) and bony flatfish (3.59 ). This trend is also seen in the greater length of microvilli and thickness of basement membrane in the elasmobranchs than in the teleosts.     

Endothelin redistributes blood flow through the lamellae of rainbow trout gills

The lamellae of the fish gill are the primary sites for oxygen uptake from the water. Here, only two very thin layers of cells separate the blood from the water. Therefore, energetically costly ion-fluxes will also occur between blood and water, and it has been hypothesised that the blood flow within the lamellae can be regulated through vasoconstriction, but evidence for this has been lacking. Through direct observations of the lamellae of rainbow trout (Oncorhynchus mykiss) in vivo, using epi-illumination microscopy, we show here that an endothelium-derived vasoactive peptide, endothelin-1 (ET-1, 0.2 μg kg⁻¹ or 1.0 μg kg⁻¹), is able to completely constrict the vascular sheet in the lamellae, probably by inducing contraction of pillar cells. This coincided with a dose-dependent increase in ventral aortic blood pressure (rising from 6.6 kPa to 12.0 kPa in response to the high ET-1 dose). However, blood continued to flow through the marginal channel that circumvents each lamella. Thus, ET-1 caused an intralamellar blood shift from the lamellar sheet towards the marginal channels. Vasoconstriction in the lamellae is likely to provide the fish with a mechanism for matching its respiratory surface area with its respiratory needs, thereby minimising ion-fluxes. Accepted: 8 September 1998     

Surface area of the respiratory organs of the Climbing perch, Anabas testudineus (Pisces: Anabantidae)

Measurements have been made of the surface area of the gills and accessory respiratory organs of Anabas in the weight range 1–120 g, and the data analysed with respect to body weight using logarithmic transformations. The slope of the regression line for total gill area (0–615) is less than that found in most fish, the number of secondary lamellae/mm decreased more rapidly with body weight than for most water-breathing species (h = -0.152). The gill area of Anabas is relatively small but when the area of the accessory organs is added, the total respiratory area is of the same order as inactive water-breathing fish. The regression coefficient for combined areas of labyrinthine organs and lining of the suprabranchial chambers (0.713) exceeds that for the gills and together with other evidence (including estimates of diffusing capacity from morphological measurements), indicates an increasing importance of air-breathing of larger specimens. The average surface area of the accessory organs available for 1 ml of air within the suprabranchial chambers was found to be 2226 mm².     

The structure of the gills of the elasmobranch,Scyliorhinus canicula (L.)

Summary The anatomy of the blood supply to the gills of the dogfish, Scyliorhinus canicula, is described. The anatomical basis for a counter-current exchange system at the respiratory surfaces is reported. Within the interbranchial septum there is a capillary network joining all the afferent branchial arterioles of the gill. The structure of the walls of the corpus cavernosum is found to be of smooth muscle cells supported by a basal lamina and connective tissue and lined by endothelial cells containing phagocytic vesicles. Both the capillary network and corpus cavernosum are suggested to function in smoothing the pressure pulses of the blood flow. Pre- and post-lamellar vessels and pre- and post-lamellar sphincters are described. The sphincters are thought to control the number of secondary lamellae physiologically in the respiratory circuit, and by retaining blood within nonperfused lamellae to act in conjunction with pillar cells (contracting in antagonism to the hydrostatic skeleton of the blood) to maintain the rigidity of secondary lamellae in the water current.Whorls of cells of unknown function are found within the interbranchial septum. In the epithelium lining the water channel large cells having a complexly branching plasma membrane and a very large central vacuole occurs. The cytoplasm lining the lumen contains numerous vacuoles each surrounded by a double membrane.This work formed part of a thesis submitted for the degree of Master of Science at the University of Bristol. I should like to thank Professor G.M. Hughes for the use of facilities in the Department of Zoology, University of Bristol.     

The morphology of the accessory air-breathing organs of the catfish, Clarias batrachus: A SEM Study

The scanning electron microscope (SEM) has been used to study the morphology of the accessory air-breathing organs of the catfish Clarias batrachus . Although the gross morphology of the dendritic organs, the fan organs and the membrane lining the supra-branchial chambers differ, the nature of the respiratory surfaces are similar. The gaseous exchange surfaces of all three organs consist of double rows of paired lamellae, a feature strongly indicative of their common origin from the gills. The surfaces of the epithelial cells from the respiratory organs were seen to have numerous small projections consisting of microvilli and short microridges. This is in contrast to the concentric whorls of micro-ridges on the surface of cells from the interlamella regions of these organs.     

Myocardial interstitial Cajal-like cells (ICLC) in caveolin-1 KO mice

We compared, by transmission electron microscopy (TEM), the ultrastructure of interstitial Cajal-like cells (ICLC) in normal mammalian myocardium versus caveolin-1 null mice. TEM showed that myocardial ICLCs of caveolin-1-deficient mice retain their main ultrastructural characteristics, for example, location among cardiomyocytes, close vicinity to nerves and/or blood capillaries, specialized cell-to-cell junctions, presence of 2–3 typical processes, which are very long (several tens of micrometres), but are very thin (0.1–0.2 μm) and moniliform. However, the most striking modification of myocardial ICLC in caveolin-1 KO mice was the absence of caveolae . Beyond this main observation, three other findings could be reported: (1) the absence of caveolae in capillary endothelium, (2) persistence of (some) caveolae at the level of cardiomyocte sarcolemma or vascular smooth muscle cell sarcolemma and (3) (un)expected ultrastructural modifications such as increased thickness of capillary basement membrane and increased autophagy of several cardiomyocytes.     

Gill microcirculation of the air-breathing climbing perch, Anabas testudineus (Bloch):relationships with the accessory respiratory organs and systemic circulation

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.     

Fine structure of the respiratory lamellae of teleostean gills

Summary The blood-water pathway in respiratory lamellae of teleostean gills consists of an epithelial layer one or two cells thick, a basal lamina and a thin layer of cytoplasm which lines the blood lacunae. This layer of cytoplasm is formed by flange-like extensions of the pillar cells. The resulting location of the pillar cell perikarya between the surfaces of the blood lacunae is probably of paramount importance for maintenance of the flattened form of the lamellae.Collagenous bundles traverse the pillar cells within tubes formed by infolding of the cellular surface. These bundles, which are oriented normal to the flattened aspect of the lamellae, no doubt provide further protection against distension or collapse of the blood spaces. A compartment filled with collagenous tissue is interposed between the basal lamina and the lining layer of the lacunae in some of the species studied.Regulation of blood flow to the respiratory surfaces is thought to result in part from contraction of the pillar cells. This contractility presumably resides in tracts of filaments which course through the cytoplasm of the pillar cells parallel to the collagenous bundles. Since nervous tissue has not been demonstrated within the gill lamellae it is possible that contraction of the pillar cells is under some form of hormonal control, although existence of local control mechanisms (e.g. self-stimulation of the cells as a result of anoxia) is not excluded.Within the limited number of species studied, the structure of the blood-water pathway does not appear to be correlated with the characteristics of the normal habitat of a particular species.This work was performed during the tenure of a post doctoral traineeship under USPHS Grant 5 T 1 GM-136 to the Department of Biological Structure, University of Washington.Particular thanks are due Dr. John H. Luft of the University of Washington for his advice and criticism while this work was in progress and to Drs. Douglas Kelly, James Koehler and Daniel Szollosi for critical assistance with the manuscipt.     

Eimeria procyonis sp. n., an Isospora sp., and a Redescription of E. nuttalli Yakimoff and Matikaschwili, 1932 (Protozoa: Eimeriidae) from the American Raccoon (Procyon lotor)

SYNOPSIS. Thirty-two of 48 raccoons examined were infected with a previously undescribed species of Eimeria which is herein named E. procyonis. Of the 32 infected animals, 10 also harbored E. nuttalli and 1 had Isospora sp. oocysts.
The ellipsoid to ovoid oocysts of E. procyonis measured 23.4 × 18.0 (16–29 × 13–24) μm; its sporocysts measured 12.1 × 9.3 (11.5–15 × 7–10) μm, each containing a slightly flattened substiedal body. The sporocyst residuum consisted of numerous scattered granules each ∼1 μm in diameter. The oocyst wall was double-layered. The outer layer appeared rough and pitted, measuring 1.5 μm, except at the micropyle where it was 1 μm thick.
The oocysts of the Isospora sp. measured 16.8 × 13.7 (16–18.5 × 12.5–15.5) μm. The wall consisted of a single layer ∼0.5 μm thick. The sporocysts measured 11.2 × 9.1 (9.5–11.5 × 8–10) μm, and each contained 4 elongate sporozoites. The oocysts of E. nuttalli measured 17.5 × 13.6 (12-21 × 11-15) μm, with a smooth single-layered wall approximately 0.7 μm thick. The sporocysts measured 12.2 × 7.1 (9-13 × 5.5–11) μm. Each sporocyst had a thin, dark, Stieda body and the sporocyst residuum consisted of many fine granules.     

Morphometries of the respiratory organs of the Indian green snake-headed fish, Channa punctata

Measurements of the dimensions of the different gills and the suprabranchial chambers have been made and the data analysed with respect to body weight using logarithmic transformations (Y = aW^b). The slope (b) for area of the total gill surface is 0–592 and for the supra-branchial chamber 0–696, and their combined respiratory surface: 0–623. The slope values for the surface areas of the 1st, 2nd, 3rd and the 4th gill arches were 0–595,0–578,0–614 and 0–572 respectively.
The slope for secondary lamellae/mm is –0138 and that for the bilateral surface area of an average-sized lamella 0–304.
These results indicate differences in growth patterns for the dimensions of the different gills. The growth-related decrease in the number of secondary lamellae/mm and size of an average secondary lamella together with evidence from "drowning" experiments and diffusing capacity calculation, suggest that this fish is better adapted for aquatic respiration than Anabas or Saccobranchus. The slopes for the total respiratory surface area and gill area seem to be comparatively low in this species.     

Distribution and morphology of the ampullary organs of the estuarine long-tailed catfish, <Emphasis Type="Italic">Euristhmus lepturus</Emphasis> (Plotosidae,Siluriformes)

Ampullary organs of Euristhmus lepturus occur in high densities along the head and in four parallel pathways along the trunk of the body. Large ampullary pores (125–130 μm) are easily distinguishable from other sensory epithelial pores due to the differences in size and the presence of a collar-like structure. Simple, singular ampullary organs of the head region consist of an ampullary pore connected to a long canal with a diameter of 115–175 μm before terminating as a simple ampulla with an external diameter of 390–480 μm. The ampullary canal is composed of 1–2 layers of flattened squamous epithelial cells, the basement membrane and an interlocking collagen sheath. The innermost cells lining the canal wall are adjoined via tight junctions and numerous desmosomes, as are those of the receptor and supportive cells. Canal wall tissue gives rise to a sensory epithelium containing between 242 and 285 total receptor cells, with an average diameter of 11.7 ± 5.3 μm, intermixed with medially nucleated supportive cells. Each receptor cell (21.38 ± 4.41 μm, height) has an apically positioned nucleus and a luminal surface covered with numerous microvilli. Neural terminals abut the basal region of receptor cells opposite multiple presynaptic bodies and dense mitochondria. Supportive cells extend from the ampullary lumen to the basement membrane, which is adjacent to the complex system of collagen fibres.     

Sphaerospora epinepheli N. Sp. (Myxosporea: Sphaerosporidae) Observed in Grouper (Epinephelus malabaricus)

Sphaerospora epinepheli n. sp. is described from grouper, Epinephelus malabaricus , in cage-cultured and wild fish collected from both coastal lines of southern Thailand. Subspherical to spherical spores and mono- or disporous pseudoplasmodia were observed in the lumen of kidney tubules. Pseudoplasmodia were round to elongate, size range 15.6–22.9 μm (length) × 8.4–21.6 μm (width). Spores were 7.8–10.0 μm (length) × 12.3–14.5 μm (thickness), and 7.0–9.5 μm (width) with two spherical polar capsules of equal size measuring 2.9–4.4 μm in diameter and containing polar filaments with six or seven windings. Two uninucleate sporoplasms showed iodine vacuoles. Blood stages, similar to C-blood protozoans observed from freshwater fish in Europe, were found from peripheral blood smears of grouper. Ultrastructural studies of blood stages showed a similar structure to unidentified mobile protozoans from the blood of carp. Electron dense bodies were observed in the cytoplasm of the primary cell blood stages. Infected proximal-tubular epithelial cells showed highly vacuolated cytoplasm and pycnotic nuclei.     

The morphology of the gills of the freshwater African crab Potamon niloticus (Crustacea: Brachyura: Potamonidae): A scanning and transmission electron microscopic study

The gills of the African freshwater crab Potamon niloticus -Ortmann have been investigated by scanning and transmission electron microscopy. Potamon has seven pairs of phyllobranchiate gills contained in the branchial chambers. From the central axis of the gills arise bilaterally situated thin flaps, the lamellae. The afferent branchial vessel (the epibranchial vessel) is located on the dorsal aspect of the gill arch and the efferent vessel (the hypobrancial vessel) on the ventral side. Between these two blood vessels, the blood percolates through the lamellar vascular channels where it is oxygenated. The lamellae consist of an epithelial cell layer covered by a thin cuticle which consists of tightly fused but distinct layers. The epithelial cells approach each other at regular intervals and fuse in the middle of the lamellar sinus delineating the vascular channels. Apical profuse membranous infoldings and numerous mitochondria characterize the epithelial cells, features typical of cells involved in active transport of macro- and micromolecules. In Potamon , however, there were no distinct gas exchange and osmoregulatory regions of the gills. On average, the cuticle was 0.78 μm thick while the epithelial cell was 6 μm. Cells that were morphologically similar to the renal glomerular podocytes of the vertebrates were observed in the efferent gill vessel of Potamon. These cells have been said to be phagocytic and may play an important defensive role in the crustaceans. Although basically the morphology of the gills of Potamon is similar to that of the other decapods, fine structural differences were evident as would be intuitively expected in a group of animals that has undergone such remarkable adaptive radiation.     

Observations on the physiology and integumentary structure of the Antarctic pycnogonid Decolopoda austratis

The giant Antarctic pycnogonid Decolopoda australis Eights takes up oxygen by diffusion across the integument, particularly of the legs. The circulatory system is feeble and haemolymph pressure changes are induced by leg movements during locomotion rather than by cardiac action. Heart rate is about30–40 beats min^-1 between 1 and 5 °C; it becomes irregular above 5 °C and ceases (reversibly) at6–7 °C.
The integumentary structure appears to facilitate gaseous exchange. Although the cuticle (c. 200 μm thick) is chitinous, it is perforated (over 35% of the internal surface) by circular/ ellipsoidal tissue-filled pits which are separated from the external environment by thin layers of chitinous cuticle no more than4–6 μm thick. The pits occur in all parts of the body and appendages (except arthrodial membranes). It is suggested that the primary function of the tissue within the pits is to form a route for gaseous diffusion, which bypasses the relatively thick impermeable chitinous layers of the rest of the cuticle. Calculations suggest that the pits reduce resistance to gaseous diffusion by about 90%.
To function as a respiratory surface, the general body surface has to be kept clean. The cleaning action of the ovigers appears to be effective, except in the regions between the contiguous lateral processes of the body where detrital material accumulates.