The Vascularization of the Kidney of the Eel (Anguilla Anguilla - Osteichthyes) in the Freshwater Habitat

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Volume 7 Number 1 Article 30

2-14-1993

The Vascularization of the Kidney of the Eel (Anguilla anguilla - Osteichthyes) in the Freshwater Habitat

Hans Ditrich University of Vienna

Heinz Splechtna University of Vienna

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Recommended Citation Ditrich, Hans and Splechtna, Heinz (1993) "The Vascularization of the Kidney of the Eel (Anguilla anguilla - Osteichthyes) in the Freshwater Habitat," Scanning Microscopy: Vol. 7 : No. 1 , Article 30. Available at: https://digitalcommons.usu.edu/microscopy/vol7/iss1/30

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THE VASCULARIZATION OF THE KIDNEYOF THE EEL

Hans Ditrich* and Heinz Splechtna Dept. Anatomy and Morphology, Institute for Zoology University of Vienna, Althanstr. 14, A-1090 Vienna, Austria

(Received for publication November 25, 1992, and in revised form February 14, 1993)

Abstract Introduction

The renal vascular system of eels (Anguilla anguilla L.) The eel kidney has a highly •specialized structure, in freshwater has been investigated using light-, transmission although the order Anguilliformes is generally regarded as a electron-, and scanning electron microscopy of vascular lower teleost group by many taxonomists (see e.g., Gosline, corrosion casts and critical-point dried specimens. The mor 1971). Besides its physiological ability to adapt to fresh- and phology of the kidney exhibits a highly evolved, unpaired, seawater (e.g., Chester-Jones et al., 1969; Schmidt-Nielsen spindle-shaped form. The renal tissue extends partially into a and Renfro, 1975), the renal anatomy shows unique features recessus dorso-caudal to the anus. Renal glomeruli are com suggesting a highly evolved position. paratively large (100.8 µm +/-17 standard deviation) and well Most anatomical studies on the fish kidney are carried vascularized. They are arranged in grape-like clusters around out with light- and, less frequently, transmission electron the intrarenal arteries. Closely neighbouring glomeruli can be microscopy (for Refs. see Hickman and Trump, 1969). surrounded by incompletely separated Bowman's capsules Scanning electron microscope studies using vascular corro sharing the same renal tubule. The peritubular capillary sion casting on fish kidneys are relatively rare (for Refs. see plexus is mainly supplied from the right posterior cardinal Ditrich and Splechtna, 1990). The kidney of eels has already vein (renal portal vein) and is relatively dense (about 30 been studied using light microscopy (Grafflin, 1937) and vol.% - measured from corrosion casts). Conspicuous vas Neoprene casts (Mott 1950). However, recent physiological cular patterns consisting of a central vein and, at a distance of studies on the adaptive mechanisms of fish in fresh- and about 25 µm, a peripheral meshwork of arterioles are seawater (e.g., Borgatti et al., 1992; Talbot et al., 1992; frequently found. The nature and function of these structures, Trischitta et al., 1992) may raise interest in the anatomical however, remains to be studied. details that can be studied with the corrosion cast technique using the SEM. Additionally, the study of vascular specializations of a basically primitively structured excretory organ may offer the opportunity to understand functional constraints of evolutionary variability.

Materialand Methods

Silver eels (Anguilla anguilla - 12 animals) with a body length of 25 - 30 cm were obtained from a commercial fish breeding station (Scharfling, Austria). The animals were kept in a tank with running freshwater and fed a diet of beef heart and Tubifex worms. Before the preparations, the animals were anaesthetised by an admixture of ethyl-m-aminobenzoic acid (MS 222 - SANDOZ; 1: 10000 gig) to their water Key Words: Kidney, eel, corrosion casts, glomerulus, supply. The animals were then perfused with buffered saline scanning electron microscopy, blood vessels, teleosts, renal (pH 7.2; 330 mOsm; 20° C; - Holmes and Donaldson, 1969) anatomy, ultrastructure. using a peristaltic laboratory pump (PA-SF2, IKA, FRO) with a polyethylene-catheter tied into the Conus arteriosus. *Address for correspondence: The Sinus venosus was opened to allow drainage of the blood Hans Ditrich, Dept. Anatomy and Morphology, and saline. The exposed vessels, especially in the gills, were Institute for Zoology - University of Vienna; monitored with a light microscope during perfusion and Althanstr. 14, A-1090 Vienna, Austria injection to assure a natural filling-state of the vessels (i.e., Telephone No.: (0222) 31336/1239 no excessive dilation or collapse). The animals were either FAX.: co222) 31336noo 1.) injected supravitally with MERCOX (Jap. Vilene Co.)

279 H. Ditrich and H. Splechtna

Fig.I Lateral view of a cast kidney. The most cranial, paired extensions of the organ are not filled. A - aorta; L - left posterior cardinal vein; arrow - efferent renal vein; The approximate height of the anus is marked with an asterisk. Bar= 1 cm

Fig.2 Cast group of approximately thirteen glomeruli surrounding an intrarenal artery (A). Note the constrictions on the arteriolar surface (arrows) indicating muscular sphinc ters. An efferent arteriole is marked with an arrowhead. Bar= 100 µm

The bulk of the renal tissue forms a nearly spindle shaped, unpaired organ dorso-caudal to the anus. Thin, paired elongations extend cranially, although, the size and structure of these parts of the kidney do not indicate a considerable contribution to the excretory function. Histological sections reveal that only a small number of nephrons is present here and that the interstitial space is extensive. These parts were diluted with 25% methyl-methacrylate (eight animals) using consequently excluded from the following investigations, the a mechanical press and further processed as described pre term "kidney" hereafter being used only for the unpaired viously (Ditrich and Splechtna, 1986, 1987) or 2.) perfusion main mass of the organ. A considerable portion (more than fixed with 2% formaldehyde and 0.5% glutardialdehyde in one half) of the renal tissue extends into a recessus, i.e., an the saline used for flushing (10 min. - four animals). The extension caudal from the anus. latter specimens were quickly dissected, cut into 5mm blocks, The vascular supply of the kidney shows a highly postfixed in the same fixative (4°C, 12h), dehydrated with modified pattern when compared with other fish (Fig.l). The graded ethanol and either transferred into acetone and dorsal aorta runs through the haemal arcs of the vertebrae in critical-point (CP) dried for scanning electron microscopy the caudal portion of the kidney and gives rise to alternating (SEM) or embedded in Epon for light- (LM) and trans right and left intersegmental (intercostal) arteries. The latter mission electron microscopy (TEM). give off ventrolateral renal arteries. The venous supply of the Corrosion cast preparations were validated with light kidney (renal portal vein) is formed by the right posterior microscopy for completeness of filling, frozen in distilled cardinal vein. This vessel does not extend cranially beyond water, cut and air dried. Randomly selected cast glomeruli the renal tissue and becomes indistinguishable from the other from six animals were analysed. The measurements were superficial renal veins in the anterior portion of the kidney. carried out either with the line-width system on the micro The left V. cardinalis posterior apparently does not contribute scope directly (SEM) or on photographic prints (LM, TEM). to the renal portal system. It continues its course in a ventro A stereological test grid (Weibel, 1973) was used to estimate lateral position to the aorta and receives branches from the capillary densities in sectioned casts. For three-dimensional dorsal and lateral myotomes (segmental veins) alternating reconstruction of LM-sections, the "PC3D" software package with the intersegmental arteries. An unpaired efferent renal (Jandel Scientific Co.) was used. Numerical data were vein forms in the centre of the renal tissue. It leaves the evaluated with basic statistical methods (standard deviation, kidney at its anterior end and may therefore be regarded as a Student's t - test, ... ). (single) subcardinal vein or a posterior ca val vein.

280 Renal Vessels of the Eel

Fig.3A Section of two glomeruli with incompletely separated Bowman's capsules. The neck segment (N) of the renal tubule is common for both glomeruli. Note that the intrarenal artery (A) and the short afferent arteriole have a strongly developed Tunica muscularis. Section thickness = 1 µm; Toluidine-blue stain. Bar= 50 µm Fig.3B 3-D computer reconstruction of the lumen of Bowman's capsule and the neck segment of the same two glomeruli as above. The locations of the vascular poles of the glomeruli are marked with asterisks, indicating that they are well separated. Only the funnel-like initial neck segments are distinct for the two glomeruli; the rest of the segment is fused. Arrow indicates section shown in Fig. 3A. Section thickness = 1 µm; Section increment = 3 µm

The intrarenal arteries ramify in a tree-like pattern within the kidney. The glomerular afferent arteries are relatively Fig.4 Detail of Fig.2; The afferent arteriole shows elongated short. Thus the glomeruli form grape-like clusters around the endothelial imprints (arrow) and circular constrictions similar terminal branches of the renal arteries (Fig.2). Accordingly, to the intrarenal arteries. The glomerular capillaries, the glomeruli closely adjoin one another. Occasionally, however, show rounded nuclear patterns (arrowhead). The glomeruli with incompletely separate urinary spaces are structural change appears at the glomerular hilus, apparently present in LM sections (Fig.3A). Three-dimensional at the first branching within Bowman's capsule. Bar= 20 µm computer reconstructions reveal that although the capsules of Fig.5 Cast glomerulus showing branching glomerular Bowman are partly fused, two neck segments form initially capillaries. Numerous rounded endothelial imprints are and unite after a.very short distance (Fig.3B). The glomeruli, reproduced by the casting resin. E - efferent arteriole. on the other hand, were completely separate, each with its Bar= 20 µm own afferent and efferent arteriole. Double afferent arterioles to a single glomerulus have not been observed. Constrictions and endothelial imprints are conspicuous near the branching presence of vascular sphincters. A strong Tunica muscularis sites of cast glomerular afferent arterioles (Fig.4). The can be observed under the LM for intrarenal arteries and the numerous constrictions in the cast vessel's surface suggest the short afferent arterioles. The elongate (arterial type) nuclear

281 H. Ditrich and H. Splechtna

visceral epithelium exhibits a relatively simple structural differentiation. The podocytes show prominent perinuclear areas and comparatively blunt processes. The glomerular basal lamina is very heterogeneous in thickness (approx. 300nm-3µm) and shows no apparent trilaminar structure. The subendothelial area appears loosely fibrillar with occasional inclusions of cellular processes (intralamellar or mesangial cells) and electron-translucent regions in the TEM. The endo thelial cells form ridge-like cellular processes that extend from the rounded perinuclear area. The relatively flat areas between these processes show fenestrae (Fig.7) of varying size (approx. 30 - 150 nm). Endothelial imprints on the cast glomerular arterioles are round or slightly oval in outline (compare Figs. 4, 5), thus exhibiting a different nuclear imprint pattern than the afferent vessels. The statistically significant difference in size between the afferent (27 µm +/- 2.86 SD; n=15) and efferent glomerular arteriole (14.5 µm +/- l.4 SD; n=l 3) may suggest that a considerable proportion of the blood is removed by glomerular filtering. No con spicuous endothelial impressions suggesting possible constriction sites have been found on the efferent glomerular vessels. In LM-sections, the vascular walls appear thin and less muscular than in the afferent arterioles. The glomerular efferent arteriole fuses with the peritubular portal venous meshwork after a short distance. Direct connections between the arterial and the oeritubular vessels are infrequently found. The intrarenal pe;itubular vascularization is strongly de veloped (about 30 vol.% - measured from corrosion casts). Histological sections indicate a possible haematopoetic nature of parts of this vascular area. In transverse sections of whole cast kidneys, the peritubular vascularization was found approximately equal in size and density. The peritubular vessels unite in the middle of the kidney to form the renal Fig.6 TEM section through a glomerular capillary wall. The efferent vein. podocyte epithelium (P) shows an irregular arrangement of Highly conspicuous capillary networks (Fig.8) pass from pedicels and larger processes. The glomerular basement the kidney's centre towards the lateral periphery. There, these membrane appears loose and of variable thickness. Arrow vascular structures reach the surface and drain, occasionally indicates an intralamellar cell. E - capillary endothelium; uniting, towards the ventromedian margin of the kidney· Bar= 1 µm (Fig.9). Such a capillary plexus is formed by one to three Fig.7 SEM micrograph of CP-dried glomerular endothelium. longitudinally running vessels that are connected by a dense The endothelial cell's perinuclear region (E) gives off ridge vascular network. These vessels normally envelop a single, like processes. The endothelial pores are situated in the flatter thicker vessel. However, the distance between the central and areas between the thicker processes. Bar = 500 nm the peripheral vessels as well as the morphology of the latter indicate that these vascular patterns do not represent vasa vasorum of the central vessel; rather, they meander around a impressions become round or oval upon reaching the vascular cellular core of considerable thickness (about 20 to 30 µm). pole of the glomerulus, probably after the afferent arteriole Based on their endothelial impressions the "enveloping enters Bowman's capsule. capillaries" are supplied by arteries (Fig.10), while the central The glomerulus itself is comparatively large (100.8 µm vessels are veins. +/- 17.21 standard deviation (SD); n=62, six animals The topology of these structures suggests that they may measured) and consists of numerous branching arterioles be related to the urinary draining system of the caudal part of forming two to six glomerular lobes (Fig.5). Clefts which the kidney. However, these vessels could not be distinguished nearly separate the glomerulus as well as branching of the from others under the LM; it is therefore not yet possible to afferent arteriole resulting in two closely adjoining glomeruli, identify the cells being supplied by these networks beyond each having its own efferent arteriole, are common. doubt. The rich vascularization and distinct morphology of However, double efferent arterioles of single glomeruli have these structures merely allow the conclusion that some not been observed. transport processes may occur at these sites. The filtration barrier of the eel glomerulus is formed by the glomerular epithelium (podocytes), a loose basal lamina and the vascular endothelium (Fig.6). The glomerular

282 Renal Vessels of the Eel

Fig.8 Capillary network "enveloping" a central vessel at the ventro-lateral kidney surface. The outer network is supplied by arteries, while the inner vessel shows rounded (venous) endothelial imprints. Bar= 100 µm

Discussion

The kidney of eels is a highly evolved organ based on its morphology, physiology and internal structure. The nearly complete fusion of right and left kidney, the apparently exclusive supply of the renal portal system from the right posterior cardinal (caudal) vein and the caudal recessus are the most conspicuous specializations. Caudal extension of the kidney, however, is also found in other species from the order Anguilliformes (van den Brock et al., 1938; Fishelson, 1989). Strong prevalence of the right posterior cardinal vein in supplying the renal portal system and the reduction of this vessel in the more cranial parts of the animal has already been described by Mott (1950) using Neoprene casts. This author also described the origin of the unpaired efferent renal vein (which he termed the hepatic portal vein) with this method. A classification of fish kidney vascular systems based on the specific relationships of the renal afferent and efferent veins (Bertin, 1958; see also Hickman and Trump, Fig.9 Lateral view of a cast kidney showing arrangement of 1969) should, however, be substantiated by more material. capillary networks (arrow) at the surface. C - left cardinal The relatively large and complexly anastomosing net vein; Bar = 500 µm work of glomerular capillaries was first documented for eels in a detailed light-microscopic analysis (Grafflin, 1937). The Fig.IO Detail of the supply to a capillary network. Arterial large variations in glomerular size reported by Grafflin could type endothelial impressions are visible in the inset. not be statistically verified in the present investigation. Bar = 100 µm; Inset magnification: Bar= 400 µm

283 H. Ditrich and H. Splechtna

Glomerular asymmetry, a feature that is frequently found in vein) are involved in water exchange: they may provide a non-mammalians (compare Ditrich and Splechtna, 1990; see mechanism of additional (extraglomerular) fluid secretion in also Brown 1985) turned out to be slight in eels. The standard freshwater. Detailed histochemical and TEM analysis will be deviation of glomerular diameters (about +/- 17%) is also in required to study the transport processes in the involved cells. the normal range for lower vertebrates. The structure of the filtration barrier in eels resembles that of other teleosts in Acknowted2ements many aspects (comp. Hickman and Trump, 1969; Elger et al., 1984). The irregularly structured basal membrane and in This study was partly supported by FWF Project clusion of cellular (mesangial) elements may be a common P6353B. Special thanks are due to Dr. J. Weisgram, feature of lower vertebrates (Decker and Reale, 1991). Marianne Fliesser-Steiner and Sonja Ditrich-Hogler for their Grafflin (1937) provided light-microscopic evidence of technical assistance. two glomeruli sharing the same urinary space, but having separate neck segments. Our observations indicate that this References may result from close grouping of vascular tufts around a terminal artery, i.e., the short, branching afferent arterioles Anderson BG, Anderson WD (1976). Renal vasculature allow no complete detachment of Bowman's capsules. of the trout demonstrated by scanning electron microscopy, Whether this condition is the result of secondary fusion of compared with canine glomerular vessels. Am. J. Anat. ill, basically separately developing vascular tufts or whether a 443-458. primarily single vascular tuft separates later remains to be Bertin L (1958). Appareil Circulatoire. In: Traite de la studied ontogenetically. Judging from our material, which Zoologie, (Circulatory apparatus. In: Zoological Treaties) revealed the occurrence of two capsules of Bowman sharing Grasse PP (ed.) Vol.13(2), Libraires de l'Academie de the same renal tubule, the latter hypothesis is to be favoured . Medicine, Paris, 1399-1458. Freshwater teleost glomeruli are generally large and well Bonga WSE (1973). Morphometric analysis with the vascularized (Anderson and Anderson, 1976; Brown, 1985; light and electron microscope of the kidney of the anadro Endo, 1989). Their main function is to remove excess water mous three spined stickleback Gasterosteus aculeatus, form influx. A closely related permanent seawater inhabitant, the trachurus from freshwater and from seawater. Z. Zellforsch. Conger eel, has even significantly larger glomeruli than the ill, 563-588. Silver eel (Ditrich and Splechtna, 1990), although their renal Borgatti AR, Pagliarani A, Ventrella V (1992). Gill vascular systems are generally quite comparable. At the same (Na++K+)-ATPase involvement and regulation during time, the Conger eel also shows less glomeruli per volume Salmonid adaptation to salt water. Comp. Biochem. Physiol. unit of kidney tissue than freshwater eels. The general corre 102A, 637-643. lation of glomerular size with marine or freshwater habitat Broek AJP van den, Oordt OJ van, Hirsch GC (1938). (Marshall and Smith, 1930) must thus be modified for eels. Besondere Morphologie der Harnorgane der Wirbeltiere. In: One of the most conspicuous features of eels is their Handbuch der vergleichenden Anatomie der Wirbeltiere, ability to rapidly adapt to fresh- and seawater (Oide and (Special morphology of the urinary organs of vertebrates. In: Utida, 1968; Chester-Jones et al., 1969). This mechanism Handbook of comparative anatomy of vertebrates) Bolk L, probably involves the numerous arteriolar and preglomerular Goppert E, Kallius E, Lubosch W (eds.) Vol.5, Urban and sphincters that replicate as constrictions in the casts. While Schwarzenberg, Berlin, 711-841. the short-term reaction to increased ambient osmolarity may Brown A (1985). Renal microvasculature of the Rainbow involve intermittent glomerular perfusion and thus reduction Trout Sal mo gairdneri: Scanning electron microscopy of in the overall glomerular filtration rate similar to other corrosion casts of glomeruli. Anat. Rec. 213, 505-513. teleosts (e.g., Elger et al., 1984), the long-term adjustments to Chester-Jones I, Chan DKO, Rankin JC (1969). Renal brackish or seawater appear to be based on different function in the European eel (Anguilla anguilla L.): changes mechanisms. In most studied teleosts, this process involves in blood pressure and renal function of the freshwater eel reduction of the filtering surface and glomerular loss (Bonga, transferred to sea water. J. Endocrinol. ~. 9 - 19. 1973; Ruiter, 1980; Elger and Hentschel, 1982; Gray and Decker B, Reale H (1991). The glomerular filtration Brown, 1987; see also Hickman and Trump, 1969). barrier of the kidney in seven vertebrate classes. Comparative Additionally, ionic and water transport properties of other morphological and histochemical observations. Bas. Appl. organs, mainly the gills and the intestine, change (see e.g., Histochem. ~. 15-36. Borgatti et al., 1992, Talbot et al., 1992). Seawater adaptation Ditrich H, Splechtna H (1986). Functional aspects of in eels, on the other hand, is reported to decrease the net renal glomeruli based on scanning electron microscopy of tubular fluid secretion rather than glomerular filtration (Oide corrosion casts, with special emphasis on reptiles and birds. and Utida, 1968, Chester-Jones et al., 1969; Schmidt-Nielsen Scanning Electron Microsc. ~. 591-597. and Renfro, 1975). Consequently, the structural basis for Ditrich H, Splechtna H (1987). Scanning electron tubular fluid secretion (or reabsorption) of considerable microscopy of vascular corrosion casts in comparative studies volumes can be expected to exist even without being actually on renal vascular structure. Scanning Microsc. l, 1339-1347. required. The nature of this mechanism is not yet clear Ditrich H, Splechtna H (1990). Kidney structure (Schmidt-Nielsen and Renfro, 1975). It may be speculated investigations using scanning electron microscopy of that the special vascular arrangements we frequently en corrosion casts - a state of the art review. Scanning Microsc. countered (capillaries enveloping a central core containing a .4,943-956.

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Elger M, Hentschel H (1982). The glomerulus of a Discussionwith Reviewers stenohaline fresh-water teleost, Carassius auratus gibelio, adapted to saline water. Cell Tiss. Res. 220, 73-85. J.A. Brown, V.H. Gattone & A. Kikuta (paraphrased): The Elger M, Kaune R, Hentschel H (1984). Glomerular authors describe a unique vascular pattern of a surrounding intermittency in a freshwater teleost, Carassius auratus arteriole meshwork. Could they elaborate on the location, gibelio, after transfer to salt water. J. Comp. Physiol. B 154, origin, destination of the vessels as well as the cell types 225-231. associated with this unique structure? Endo M (1989). Study of glomerular vasculature in Authors: The topology of the enveloping arteries network - teleosts with the resin-replica method. Jap. J. Ichthyol. 36, central vein complex suggests that it belongs to the urinary 346-349. draining system of the posterior part of the kidney. However, Fishelson L (1989). Rhinomuraena - Fish with post the analysis of the associated cellular structures turned out to cloacal situated urogenital system. Ann. Soc. Roy. Zoo!. be very delicate. In order to be shure about the specific Belgique .l..12,104. location of the cells, 3-D reconstruction of semithin-sectioned Gosline WA (1971). Functional morphology and material is necessary. Then, in the TEM, the cells can be classification of Teleostean fishes. Hawaii Univ. Press, studied in detail as LM (and SEM of CP material) turned out Honolulu, 124-127. to be insufficient. So far, it can only be stated that the Grafflin AL (1937). Observations upon the structure of involved cell types resemble in some details those of collecting the nephron in the common eel. Am. J. Anat. fil., 21-62. ducts, but are different in others. A large number of questions, Gray CJ, Brown JA (1987). Glomerular ultrastructure of mainly of functional nature remains. Thus the information the trout, Salmo gairdneri: Effects of angiotensin II and given in this paper, based mainly on corrosion casts, is still adaptation to seawater. Cell Tiss. Res. 249, 437-442. preliminary in this respect. Hickman CP, Trump BJ (1969). The kidney. In: Fish Physiology, Hoar WS, Randall DJ (eds.) Vol.l, Academic A.P. Evan & D. Casellas: The authors suggest that the vascu lar cast constriction sites are sphincters. What evidence do the Press, New York, 91 - 239. authors have to suggest that these are actually regions of Holmes WN, Donaldson EM (1969). The body com vascular sphincters? partments and distribution of electrolytes. In: Fish Authors: The mentioned constrictions or narrowings in the Physiology, Hoar WS, Randall DJ (eds.) Vol.1, Academic cast vascular lurnina have been conspicuous only on terminal Press, New York, 62 - 65. arteries and afferent arterioles. Random distribution of similar Marshall EK, Smith HW (1930). The glomerular structures on other arteries has not been observed. LM reveals development of the vertebrate kidney in relation to habitat. that the Tunica muscularis of these vessels is strongly Biol. Bull . 135-152. .5.2., developed (comp. Fig.3A). No experiments have been made Mott JC (1950). The gross anatomy of the blood vascular to stimulate these structures. Therefore the interpretation of system of the Eel Anguilla anguilla. Proc.' Zoo!. Soc. Lond. these narrowings as sphincters is only based on their structural .11Q, 503-519. appearence (comp. Aharinejad et al. 1990). Oide H, Utida S (1968). Changes in intestinal absorption and renal excretion of water during adaptation to sea-water in D. Case!Jas: Have the authors observed multiple afferent or the Japanese eel. Marine Biol.1, 172-177. efferent arterioles as reported by Brown (1985) in Rainbow Ruiter, AJH de (1980). Changes in glomerular structure Trout? after sexual maturation and seawater adaptation in males of Authors: Multiple afferent or efferent arterioles have not been the euryhaline teleost Gasterosteus aculeatus L. Cell Tiss. found, however, such structures are reported to be rare Res. WQ, 1-20. (Murakami et al., 1971; Murakami, 1976) so it cannot be fully Schmidt-Nielsen B, Renfro JL (1975). Kidney function excluded that they may exist. in the American eel Anguilla rostrata. Am. J. Physiol. 228, 420 - 431. J.A. Brown & D. Casellas: The authors suggest that the small Talbot C, Stagg RM, Eddy FB (1992). Renal, respiratory efferent arteriole reflects a high filtration fraction, but there and ionic regulation in Atlantic salmon (Salmo salar L.) kelts appears to be little evidence for this hypothesis. The diameter following transfer from fresh water to seawater. J. Comp. of an arteriole does not necessarily reflect its blood flow and Physiol. ill, 358-364. concluding an effect on filtration is risky. Trischitta F, Denaro MG, Faggio C, Schettino T (1992). Authors: We could not find indications of possible regulatory Comparison of Cl- -absorption in the intestine of the seawater structures on the glomerular efferent arteriole - neither con and freshwater adapted eel Anguilla anguilla: Evidence for spicuous endothelial (e.g., Fretschner et al., 1990) or muscular the presence of an Na-K-Cl cotransport system on the imprints on the casts, nor a corresponding development of the luminal membrane of the enterocyte. J. Exp. Zoo!. ID, 245- vascular wall in the LM. The standard deviation of the 253. diameter (approx.+/- 10%) indicates that it is relatively Weibel, ER (1973). Stereological techniques for constant in size. Assuming a generally low blood pressure, the electron microscopic morphometry. In: Principles and small efferent arteriole may provide sufficient pressure for Techniques for Electron Microscopy, Hayat MA (ed.) Vol 3, filtering but without damming up the glomerulus. Van Nostrand Reinhold Publ Co, New York, 239-296.

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V.H. Gattone: How are the cranially extended portions of the AdditionalReferences kidney different from the main mass? Authors: The paired cranial extensions of the kidney show a Aharinejad S, Franz P, Bock P, Lametschwandtner A, very loose interstitium, surrounding only two or three pairs of Breiteneder W, Firbas W (1990). Sphincterlike structures in nephrons per segment. Also the glomeruli in this region corrosion casts. Scanning 12, 280-289. appear more slack (thinner capillary walls, less mesangium, bigger intercapillary distances) if compared to the main mass Fretschner M, Endlich K, Fester C, Parekh N, Stein of kidney tissue. hausen M (1990). A narrow segment of the efferent arteriole controls efferent resistance in the hydronephrotic rat kidney. V.H. Gattone: How does the glomerular structure (SEM and Kidney Int. TI, 1227-1240. TEM) differ from that of higher teleostean species? Authors: Besides size and possible dynamic processes (comp. Murakami T, Miyoshi M, Fujita T (1971). Glomerular e.g. Bonga, 1973; deRuiter, 1980), the construction of the vessels of the rat kidney with special reference to double teleost glomeruli seems very uniform (see also Hickman and efferent arterioles. A scanning electron microscope study of Trump, 1969). The podocyte processes are rather irregular corrosion casts. Arch. Histol. Jap. TI, 179-198. with regard to their branching and arrangement. The basal me~brane seems to show normally cellular (mesangial) in Murakami T (1976). Double efferent arterioles of the rat clusions. From corrosion casts of various species, we may renal glomerulus as studied by the injection replica scanning generalize that the glomerulus in teleosts is comparatively electron microscope method. Arch. Histol. Jap. 39, 327-332. large with numerous, branching capillaries of considerable diameter (nucleated erythrocytes).

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