Ultrastructure of the renopericardial complex in Hypselodoris tricolor (Gastropoda, Nudibranchia)

The histology and ultrastructure of the renopericardial complex of Hypselodoris tricolor (Gastropoda, Nudibranchia, Doridoidea) have been investigated by means of semithin serial sections and transmission electron microscopy (TEM). The examinations revealed a functional metanephridial system comprising a monotocardian heart with ventricle and auricle in a spacious pericardium that is linked with the single, large kidney by a renopericardial duct with prominent ciliation towards its opening. Podocytes as the site of ultrafiltration were not only detected in the auricular epicardium, but also line the entire outer pericardial epithelium. The cuboidal, highly vacuolated excretory cells of the kidney epithelium with extensive basal infoldings and an apical microvillous border indicate secretory and reabsorptive activity. Solitary rhogocytes (pore cells) of the connective tissue and haemocoel represent additional loci of ultrafiltration with a fine structure identical to that of the podocytes (slits between cytoplasmic processes, bridged by fine diaphragms and covered by extracellular matrix). The presence of podocytes situated in the epicardial wall of the auricle is regarded as plesiomorphic for the Mollusca and is confirmed for the Nudibranchia. An additional, extensive and separate ultrafiltration site in the outer pericardial wall is not known from any other taxon of the Mollusca and strongly suggests a significantly increased ultrafiltration activity in H. tricolor.     

Ultrastructure of possible sites of ultrafiltration in some gastropoda,with particular reference to the auricle of the freshwater prosobranch Viviparus viviparus L.

Summary In the Gastropoda pressure-ultrafiltration of the blood is assumed to be the first step in urine formation. The most probable site of ultrafiltration is the wall of the heart. Since in other animal groups ultrafilters are characterized by a special cell type, the podocyte, the hearts of two pulmonates (Lymnaea stagnalis, Biomphalaria glabrata) and of four prosobranchs (Viviparus viviparus, Bithynia tentaculata, Ampullaria gigas, Littorina littorea) were ultrastructurally investigated, in order to establish whether or not podocytes occur in these structures. It appeared that only in the wall of the auricle of Viviparus podocytes are present. They form a layer underneath the epicardium, the epithelium covering the auricle. It is assumed that in Viviparus ultrafiltration proceeds in the auricle. The possible route of the pro-urine is discussed. The location of the ultrafilters in the other species studied remains still unknown.     

Fine Structure of Heart,Pericardium and Glomerular Vessel in Cephalodiscus gracilis M'Intosh, 1882 (Pterobranchia,Hemichordata)

Heart, pericardium and glomerular vessel of Cephalodiscus gracilis have been studied with the electron microscope. The lumen of the heart is lined by a basal lamina and an associated epithelium, composed of myoepithelial cells with well developed thin and thick myofilaments. The heart is located in the pericardial cavity, which is deliminated by the pericardium. The latter is composed of two flat layers of myoepithelia with fused basal laminae. The outer layer of the pericardium is the protocoelomic lining, and the inner layer is the ‘parietal’ pericardial epithelium. The myoepithelium forming the heart wall can be considered to represent the ‘visceral’ pericardial epithelium. The spacious glomerular vessel is lined by a basal lamina, on which typical podocytes rest. These cells indicate that ultrafiltration takes place through the wall of the glomerular vessel. The lumen of the vessel contains fine granular material (presumably precipitated blood proteins), fibrils with a faint cross striation, suggesting that they represent collagen, and stellate cells, which in part line the vessel. Since ultrafiltration requires hydrostatic pressure, it is inferred that the blood flow is from the dorsal region then through the heart and into the glomerular vessel.     

Morphology,morphometry and ultrastructure of the maxillary gland of the terrestrial isopod Porcellio scaber (Crustacea,Oniscidea)

Summary The ultrastructure of the maxillary gland of the terrestrial isopod Porcellio scaber is described. The gland is composed of an end sac, an excretory duct and a terminal duct which opens by a valve at the base of maxilla 2. An epithelium of podocytes in the end sac enables passive ultrafiltration by haemolymph pressure. The excretory duct shows ultrastructural adaptations to secretion and resorption. SEM micrographs reveal the location and the morphology of the valve at the excretory pore. A model reconstructed from serial sections allowed the calculation of morphometric data of the gland. The ultrafiltration area of the end sac and the area of resorption and secretion of the excretory duct amount to 0.091 and 0.157 cm² per 1 g of fresh body weight, respectively. The total volume of the gland is calculated to be 0.04 mm³ in a specimen of 13.7 mm length. In comparison with the marine species Mesidotea entomon, the relative areas for ultrafiltration and resorption of the gland of P. scaber are more than twice as large as the corresponding areas of the marine species. This relative enlargement of the gland in P. scaber and the form of the valve at the excretory pore are seen as adaptations to terrestrial life.     

The ultrastructure of the heart and kidney of the pilid gastropod mollusc Marisa cornuarietis, with special reference to filtration throughout the Architaenioglossa

Transmission and scanning electron micrographs of the heart and kidney of Marisa cornuarietis (L.) reveal that the auricle is largely responsible for filtration supplemented to a small extent by the ventricle. The epicardial cells of the auricle are specialized to allow the escape of a filtrate of the blood, but do not bear any resemblance to podocytes. The posterior part of the kidney is lined by a characteristic renal epithelium excreting uric acid and resorbing glucose whilst the anterior part has an epithelium specialized for active transport of ions. A possible mechanism for filtration is suggested, and the functional significance of the structural variations in the heart and kidney of pilids, viviparids and cyclophorids is discussed.     

Comparative Functional Morphology of the Bivalve Excretory System

Combining injection techniques with ultrastructural observations,and relating these findings to the more traditional physiologicaland morphological studies have shed new light onthe excretorymechanisms underlying the processes of ultrafiltration, secretionand reabsorption in some bivalve molluscs. These basic processesare further elucidated by comparing normal excretory tissueswith those in bivalves that have been subjected to stress bypollutants in either the natural environment or under laboratoryexperimentation. The process of ultrafiltration is size andcharge dependent and occurs at the filtration barrier at thebase of the podocytesin the pericardial gland. Primary urinemay be modified by secretion (primarily from the kidney cellsbut also from the podocytes), reabsorbtion in the kidney, andby the addition of hemocytes passing from blood spaces throughthe epithelium into the lumen of the kidney. Numerous concrements(granules, concretions and membranes) that result from lysosomalactivities inthe podocytes, kidney cells and hemocytes alongwith the fluid are excreted into the mantle cavity     

Microscopic Anatomy and Ultrastructure of Nephridium in the Sipunculan Thysanocardia nigra Ikeda, 1904 from the Sea of Japan

The microscopic anatomy and ultrastructure of nephridium have been studied in the sipunculan Thysanocardia nigra Ikeda, 1904 (Sipuncula, Sipunculidea) from the Sea of Japan using histological and electron microscopic techniques (SEM and TEM). This paper describes ultrastructural features of nephridial epithelium, muscle grid, and coelomic epithelium on the surface of the nephridium, the area of the ciliary funnel, and the tongue. Several types of cells were distinguished in the excretory tube of the nephridium: (1) a columnar epithelium of the excretory bunches; (2) a cubical or flattened epithelium of flask-shaped infoldings; and (3) granulocytes that migrate from the coelom to the extracellular matrix of the nephridial wall. The system of podocytes and multiciliary cells were described in the nephridial coelothelium. Two types of secretion of nephridial epithelium have been discovered: a merocrine secretion of columnar cells and an apocrine secretion of cells of the flask-shaped infoldings. Using ultrastructural data, two zones of filtration through the wall of excretory tube have been found, namely (1) the tips of flask-shaped infoldings (via the extracellular matrix and microvillary canals between the epithelial cells) and (2) areas between the flask-shaped infoldings (via the contacts of podocytes, extracellular matrix, and the basal labyrinth of the columnar cells). Unlike previously studied representatives of the genus Phascolosoma, no coelomic epithelium is present on the tips of the flask-shaped infoldings in Th. nigra. This data on the anatomy and histology allow us to conclude that the funnel only works like a gonoduct.     

MORPHOLOGICAL BASIS OF EXCRETORY FUNCTION IN NAUSITORA FUSTICULA (JEFFREYS, 1860) (BIVALVIA: TEREDINIDAE)

The function of the excretory system of the teredinid bivalveNausitora fusticula is discussed on the basis of new informationon histology and ultrastructure. The wall of the auricles islined with podocytes that allow haemolymph ultrafiltration tothe pericardial cavity. These podocytes also show apical microvilliwith absorptive activity. The primary urine is drained fromthe pericardial cavity to the afferent ducts by a citiated bulb-likestructure. Theafferent and efferent ducts together form thekidney body. The afferent duct shows structures related to absorption,excretion and conduction of the urine. The efferent ducts, however,have structures concerned only with urine absorption and conduction. (Received 13 January 1997; accepted 15 July 1997)     

Development of the excretory system in a polyplacophoran mollusc: stages in metanephridial system development

ABSTRACT: BACKGROUND: Two types of excretory systems, protonephridia and metanephridial systems are common among bilaterians. The homology of protonephridia of lophotrochozoan taxa has been widely accepted. In contrast, the homology of metanephridial systems -- including coelomic cavities as functional units -- among taxa as well as the homology between the two excretory systems is a matter of ongoing discussion. This particularly concerns the molluscan kidneys, which are mostly regarded as being derived convergently to the metanephridia of e.g. annelids because of different ontogenetic origin. A reinvestigation of nephrogenesis in polyplacophorans, which carry many primitive traits within molluscs, could shed light on these questions. RESULTS: The metanephridial system of Lepidochitona corrugata develops rapidly in the early juvenile phase. It is formed from a coelomic anlage that soon achieves endothelial organization. The pericardium and heart are formed from the central portion of the anlage. The nephridial components are formed by outgrowth from lateral differentiations of the anlage. Simultaneously with formation of the heart, podocytes appear in the atrial wall of the pericardium. In addition, renopericardial ducts, kidneys and efferent nephroducts, all showing downstream ciliation towards the internal lumen, become differentiated (specimen length: 0.62 mm). Further development consists of elongation of the kidney and reinforcement of filtration and reabsorptive structures. CONCLUSIONS: During development and in fully formed condition the metanephridial system of Lepidochitona corrugata shares many detailed traits (cellular and overall organization) with the protonephridia of the same species. Accordingly, we suggest a serial homology of various cell types and between the two excretory systems and the organs as a whole. The formation of the metanephridial system varies significantly within Mollusca, thus the mode of formation cannot be used as a homology criterion. Because of similarities in overall organization, we conclude that the molluscan metanephridial system is homologous with that of the annelids not only at the cellular but also at the organ level.     

The microscopic anatomy and ultrastructure of the nephridium of the sipunculan Themiste hexadactyla Satô 1930) (Sipuncula: Sipunculidea)

The microscopic anatomy and ultrastructure of the nephridia of the sipunculan Themiste hexadactyla (Satô, 1930) from the Sea of Japan were studied by the histological and electron microscopic methods. The fine structures of the ciliary funnel, muscular “tongue,” excretory sac, and excretory tube of the nephridium were described. The ultrastructural features of the excretory epithelium, cupola-shaped epithelial infoldings, excretory canals, and muscular layer in the extracellular matrix of the nephridial wall were examined and described in detail. The ultrastructure of the nephridial coelomic epithelium composed of podocytes with long processes and multiciliary cells was also examined and illustrated. Characteristic cell contacts between the processes of podocytes, viz., paired “double diaphragms,” were described and illustrated for the first time.     

Morphological analysis of the hagfish heart. II. The venous pole and the pericardium

The morphological characteristics of the venous pole and pericardium of the heart were examined in three hagfish species, Myxine glutinosa, Eptatretus stoutii, and Eptatretus cirrhatus. In these species, the atrioventricular (AV) canal is long, funnel‐shaped and contains small amounts of myocardium. The AV valve is formed by two pocket‐like leaflets that lack a papillary system. The atrial wall is formed by interconnected muscle trabeculae and a well‐defined collagenous system. The sinus venosus (SV) shows a collagenous wall and is connected to the left side of the atrium. An abrupt collagen‐muscle boundary marks the SV‐atrium transition. It is hypothesized that the SV is not homologous to that of other vertebrates which could have important implications for understanding heart evolution. In M. glutinosa and E. stoutii, the pericardium is a closed bag that hangs from the tissues dorsal to the heart and encloses both the heart and the ventral aorta. In contrast, the pericardium is continuous with the loose periaortic tissue in E. cirrhatus. In all three species, the pericardium ends at the level of the SV excluding most of the atrium from the pericardial cavity. In M. glutinosa and E. stoutii, connective bridges extend between the base of the aorta and the ventricular wall. In E. cirrhatus, the connections between the periaortic tissue and the ventricle may carry blood vessels that reach the ventricular base. A further difference specific to E. cirrhatus is that the adipose tissue associated with the pericardium contains thyroid follicles. J. Morphol. 277:853–865, 2016. © 2016 Wiley Periodicals, Inc.     

Structure of the Excretory System of Hawaiian Nerites (Gastropoda: Neritoidea)

Neritoidean gastropods are present in freshwater, brackish andmarine environments, which vary in salinity and exposure todehydration. In this study we examine the structure of the excretory systemof Hawaiian nerites, which indicates possible processes thatenable these gastropods to survive in a wide range of environmentsin the Hawaiian Islands. As is true of other nerites studied to date, the main excretorymechanism of Hawaiian nerites is through filtration of the bloodbetween podocytes in the auricle epicardium, resulting in productionof an ultra-filtrate, which collects in the pericardial cavity.No podocytes are present on the surface of the ventricle ofHawaiian nerites. The reno-pericardial canal conveys the urine tothe kidney, the epithelium of which is composed mainly of acidophiliccells in marine nerites. In brackish and fresh water species,baso-philic cells are present in addition to the acidophiliccells present in marine nerites. It is proposed that the basophilickidney cells allows non-marine nerites to osmoregulate and producea hyperosomotic urine at low salinities. A bladder is present,and empties into the mantle cavity near the gill by way of a ureter. (Received 26 November 1997; accepted 15 May 1998)     

Involvement of actin and Na+-K+ ATPase in urine formation of the freshwater pulmonate Helisoma

The site and process of urine formation in the renopericardial system of Helisoma have been investigated. Osmotic pressure and protein content of hemolymph from the heart, pericardial fluid from the pericardial cavity, prourine from the kidney sac, and urine from the ureter have been determined. Osmotic pressure is equal in hemolymph, pericardial fluid, and prourine, but less in urine. Protein content is similar in hemolymph and pericardial fluid, but much less in prourine and urine. Hemoglobin molecules are present in hemolymph and pericardial fluid but not in prourine. It is suggested that in Helisoma the kidney sac is the site of prourine formation, and prourine is an ultrafiltrate of hemolymph. The kidney epithelial cells contain 6- to 7-nm microfilaments which react with heavy meromyosin producing unidirectional arrowheads. Numerous actin filaments are present in the vicinity of the lateral cell membranes and basal processes. It is possible that the actin filaments regulate the extracellular spaces for prourine passage. It is postulated that the actin-rich kidney epithelium may generate hydrostatic pressure for ultrafiltration. Na⁺-K⁺ ATPase is located on the luminal side of the kidney epithelium, which may regulate intracellular fluid level of the kidney epithelial cells, and thereby regulate their cell volume. Thus Na⁺-K⁺ ATPase may be involved in the regulation of extracellular spaces in kidney epithelial cells. The enzyme may participate in the production of hyposmotic urine.     

Development of the excretory system in the polyplacophoran mollusc,Lepidochitona corrugata: The protonephridium

A single pair of protonephridia is the typical larval excretory organ of molluscs. Their presence in postlarval developmental stages was discovered only recently. We found that the protonephridia of the polyplacophoran mollusc, Lepidochitona corrugata, achieve their most elaborate differentiation and become largest during the postlarval period. This study describes the protonephridia of L. corrugata using light and electron microscopy and interactive three‐dimensional visualization. We focus on the postlarval developmental period, in which the protonephridia consist of three parts: the terminal part with the ultrafiltration sites at the distal end, the voluminous protonephridial kidney, and the efferent nephroduct leading to the nephropore. The ultrafiltration sites show filtration slits between regularly arranged thin pedicles. The ciliary flame originates from both the terminal cell and the duct cells of the terminal portion. The efferent duct also shows ciliation. The most conspicuous structures, the protonephridial kidneys, are voluminous swellings composed of reabsorptive cells (“nephrocytes”). These cells exhibit strong vacuolization and an infolding system increasing the basal surface. The protonephridial kidneys, previously not reported at such a level of organization in molluscs, strikingly resemble (metanephridial) kidneys of adult molluscan excretory systems. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Ultrastructure and function of the labial nephridia and the rectum of Orchesella cincta (L.) (Collembola)

Summary The collembolan Orchesella cincta possesses a well-developed coelomoduct kidney. The presence of podocytes in the wall of the sacculus and the fact that the epithelium of the nephridial tubule has the ultrastructural characteristics of resorbing cells, indicate that this is an ultrafiltration-reabsorption kidney.Apparently also the rectum is lined by a reabsorptive epithelium; the cells possess an extensive system of apical and basal infoldings. This view is sustained by the fact that the stereology of the apical channel system varies in animals kept under different moisture conditions. During the intermoult period, both organs are subject to strong morphological changes, which are obviously related to the feeding rhythm.The authors wish to thank Dr. T. Sminia for his stimulating interest during the investigations, Dr. J.C. Jager for statistical advice and Mr. G.W.H. van den Berg for drawing the figures     

Comparative anatomy of the heart–glomerulus complex of Cephalodiscus gracilis (Pterobranchia): structure,function, and phylogenetic implications

Cephalodiscus gracilis Harmer, 1905 is a semi-sessile deuterostome that shares with fish-like chordates pharyngeal gill slits and a dorsally situated brain. In order to reveal structures potentially homologous among deuterostomes and to infer their functional roles, we investigated the axial complex, associated blood vessels and structures of C. gracilis using transmission electron microscopy, light microscopy, and digital 3D reconstructions. We describe the smooth, bipartite cephalic shield retractor muscles that originate as solid compact muscles and fan out to traverse the protocoel as individual muscle cells. The axial complex consists of a cap-shaped coelomic sac, the pericardium that surrounds the central heart. The pericardium is constituted of myoepithelial cells, with the cells facing the heart being thicker and richer in myofilaments. A prominent dorsal median blood vessel opens into the heart, which gives rise to a short median ventral vessel that opens into the paired glomeruli connected to the ventral side of the stomochord. The tip of the curved stomochord rests precisely above the connection of the dorsal median vessel with the heart, a position that would allow the stomochord to function as a valve facilitating unidirectional blood flow. Glomeruli are lined by podocytes of the spacious protocoel and are considered to be the site of ultrafiltration. Two pairs of blood vessels enter the median dorsal blood vessel from the tentacles. The median dorsal blood vessel is separated from the brain by a thin basement membrane. This arrangement is consistent with the hypothesis that blood vessels in the tentacles increase oxygen supply for the brain. Based on detailed similarities, the heart–glomerulus complex of C. gracilis is considered homologous with the heart–glomerulus complex in Rhabdopleura spp., and Enteropneusta, and the axial complex in Echinodermata. In addition, we hypothesize homology to the excretory complex including Hatschek’s nephridium in Cephalochordata. Thus, the heart–glomerulus complex does not support a sister-group relationship between Echinodermata and Hemichordata, whereas the organization of the cephalic shield retractor muscles is consistent with the evolution of pterobranchs within enteropneusts.     

Trb2, a mouse homolog of tribbles, is dispensable for kidney and mouse development

Glomeruli comprise an important filtering apparatus in the kidney and are derived from the metanephric mesenchyme. A nuclear protein, Sall1, is expressed in this mesenchyme, and we previously reported that Trb2, a mouse homolog of Drosophila tribbles, is expressed in the mesenchyme-derived tissues of the kidney by microarray analyses using Sall1-GFP knock-in mice. In the present report, we detected Trb2 expression in a variety of organs during gestation, including the kidneys, mesonephros, testes, heart, eyes, thymus, blood vessels, muscle, bones, tongue, spinal cord, and ganglions. In the developing kidney, Trb2 signals were detected in podocytes and the prospective mesangium of the glomeruli, as well as in ureteric bud tips. However, Trb2 mutant mice did not display any apparent phenotypes and no proteinuria was observed, indicating normal glomerular functions. These results suggest that Trb2 plays minimal roles during kidney and mouse development.     

Ultrastructure of the coxal gland of the horseshoe crab Limulus polyphemus: Evidence for ultrafiltration and osmoregulation

Electron microscopic examination of the paired coxal glands of the horseshoe crab Limulus polyphemus, focusing on urinary and vascular channels, shows six morphologically distinct regions. Each of four nephridial lobes consists of two cortical layers surrounding a medulla. The outer and inner cortexes contain blood vessels separated by a basement membrane from the urinary space lined by podocytes. Podocyte foot processes are applied to the basement membrane, interdigitate with those from other podocytes, and have a filtration slit diaphragm between them. Cortical morphology demonstrates ultrafiltration of blood, a previously undescribed function of the gland, as well as possible endocytic reabsorption of materials by the podocytes. The medulla drains into the stolon connecting the four lobes. These two areas have urinary tubules of cuboidal epithelium featuring microvillous-like apical projections, cytoplasmic vesicles and vacuoles, elaborate lateral interdigitations with septate junctions, and basal invaginations containing numerous mitochondria. These tubules are closely surrounded by blood channels, lined by a basement membrane containing embedded support cells. The medulla and stolon morphology are suggestive of both ion transport and water movement, in keeping with the gland's role in osmoregulation. The stolon empties into the end sac in the base of the most posterior lobe. It is lined by tall epithelium exhibiting apical overlap, blunt projections into the lumen, apparent endocytic vesicles, and basal plasma membrane infoldings with mitochondria. The end sac drains into the conducting nephric duct, the proximal end of which is lined by a cuticle. J. Morphol. 234:233–252, 1997. © 1997 Wiley-Liss, Inc.