The fine structure of the protonephridial system in the land nemertean Pantinonemertes californiensis (Rhynchocoela,Enopla, Hoplonemertini)

Summary The protonephridial terminal organ in the nemertean Pantinonemertes californiensis is composed of two cells that are similar in size and shape and are mirror images of each other. Basally in the organ the two cells combine to form a binucleate cytoplasmic mass. Apically they are intimately joined to form a subcylindrical thin-walled weir apparatus; this part is supported by two opposed cytoplasmic columns running the length of the weir region, one originating from each of the two cells, and by a number of regularly spaced circular bars that arise from the two columns. The ciliary flame consists of 94–114 cilia that originate in the bases of the two cells, and it is surrounded by a palisade of incomplete circlets of long, straight microvilli. The convoluted protonephridial tubule is rich in structures that indicate intensive reabsorption from the primary urine. It is argued that the terminal organs in Pantinonemertes and Geonemertes are fundamentally similar and differ only in the amount of microtubules present in the longitudinal supports.Abbreviations BL basal lamina - CF ciliary flame - CT connective tissue - CV coated vesicle - E endocytotic pit - FM filtration membrane - G Golgi complex - LC longitudinal cytoplasmic column - M mitochondrion - MT microtubules - MV microvilli - N nucleus - NPC nucleus of protonephridial capillary cell - PC protonephridial capillary cell - R rootlets - TB transverse bar - TC terminal cell - WE weir, exterior of fenestrated wall - WI weir, interior of same     

Ultrastructure of the Protonephridium in Acteonemertes bathamae Pantin (Rhynchocoela: Enopla: Hoplonemertini)

The protonephridial system consists of terminal cell, protonephridial capillary, protonephridial tubule and efferent duct. The terminal cell is an elongated, thin-walled, fenestrated basket containing a ciliary flame circumscribed by a palisade of straight microvilli. The filtration area is confined to the terminal cell and consists of slits bridged by a filtration membrane. The cilia, as well as the microvilli, projects into the proximal bell-shaped part of the thin-walled protonephridial capillary. The terminal cells are often found in pairs connected to the same capillary, which has a very narrow lumen. The proximal part of the thick-walled, convoluted protonephridial tubule is ciliated and shows characteristic foldings of the luminal plasma membrane and numerous small vesicles in the cytoplasm. The cells of the following, non-ciliated part of the tubule have interdigitating lateral surfaces and the bases deeply invaginated to form compartments with numerous mitochondria; in the cytoplasm are many large vesicles, possibly containing lipid droplets, and small amounts of glycogen. The distal protonephridial tubule resembles various epithelia with an osmoregulatory function, including the vertebrate nephron.     

No direct contact between the excretory system and the circulatory system in Prostomatella arenicola Friedrich (Hoplonemertini)

Excretory and circulatory systems in Prostomatella arenicola are examined at the ultrastructural level. Interdigitating cells, which rest on a thin fibrillar basal lamina, line the lumina of the lateral vessels. A layer of muscle cells and an underlying sheath of fibrillar extracellular material surround each vessel.The excretory system consists of one pair of laterally situated branched protonephridia. Each protonephridium is composed of several terminal cells, an efferent duct and a nephridiopore. The terminal parts of the protonephridia are not restricted to the vicinity of the circulatory system; they can also be found dorsally or laterally to the nerve cords between muscle cells. The presumed filtration area arises as a hollow cylinder from the terminal cell. This cylinder is perforated by numerous clefts which are never bridged by a filter diaphragm. Instead, each terminal cell cylinder is surrounded by an extracellular matrix. The terminal cells neither extend into the lumen of the lateral vessel nor contact the vessel lining cells.Phylogenetic implications of the results are discussed.     

Ultrastructure of the lycophora larva of Gyrocotyle urna (Cestoda,Gyrocotylidea)

Summary The ultrastructure of the protonephridial system of the lycophore larva of Gyrocotyle urna Grube and Wagener, 1852, is described. It consists of six terminal cells, at least two proximal canal cells, two distal canal cells and two nephridiopore cells. The terminal cells and the proximal canal cell build up the filtration weir with its two circles of weir rods. The proximal canal cell constitutes a solid, hollow cylinder without a cell gap and desmosome. The distal canal cell is characterized by a strong reduction of the canal lumen by irregularly shaped microvilli. The nephridiopore region is formed by a nephridiopore cell; its cell body is located at some distance proximally within the larva. The connection among different canal cells is brought about by septate desmosomes. Morphological, evolutionary and functional aspects of the protonephridial system within Platyhelminthes are discussed. The structure of the proximal canal cells without a desmosome is considered an autapomorphy of Cestoda.Abbreviations ci cilia of the terminal cell - Co distal canal cell - col lumen of the distal canal cell - Ep epidermis - er outer rods of the filtration weir - il inner leptotriches - ir inner rods of the filtration weir - ld lipid droplets - mt microtubule - mv microvilli - Nc nephridiopore cell - Ne neodermis anlage cells - nu nucleus - pC proximal canal cell - ro ciliary rootlets - sd septate desmosome - Tc terminal cell     

Ultrastructure of protonephridia in Xenotrichula carolinensis syltensis and Chaetonotus maximus (Gastrotricha: Chaetonotida): comparative evaluation of the gastrotrich excretory organs

In an attempt to obtain detailed information on the entire protonephridial system in Gastrotricha, we have studied the protonephridial ultrastructure of two paucitubulatan species, Xenotrichula carolinensis syltensis and Chaetonotus maximus by means of complete sets of ultrathin sections. In spite of some differences in detail, the morphology of protonephridia in both examined species shows a common pattern: Both species have one pair of protonephridia that consist of a bicellular terminal organ, a voluminous, aciliar canal cell and an adjacent, aciliar nephridiopore cell. The terminal organ consists of two monociliar terminal cells each with a distal cytoplasmic lobe. These lobes interdigitate and surround cilia and microvilli of the terminal cells. Where both lobes interdigitate, a meandering cleft is formed that is covered by the filtration barrier. We here term the entire structure composite filter. The elongated, in some regions convoluted protonephridial lumen opens distally to the outside via a permanent nephridiopore. A comparison with the protonephridia of other species of the Gastrotricha allows hypothesising the following autapomorphies of the Paucitubulata: The bicellular terminal organ with a composite filter, the convoluted distal canal cell lumen and the absence of cilia, ciliary basal structures and microvilli within the canal cell. Moreover, this comparative survey could confirm important characteristics of the protonephridial system assumed for the ground pattern of Gastrotricha like, for example, the single terminal cell with one cilium surrounded by eight microvilli.     

The excretory organs in Sphaerodorum flavum (Phyllodocida, Sphaerodoridae): a rare case of co-occurrence of protonephridia, coelom and blood vascular system in Annelida

The excretory organs of Sphaerodorum flavum (Sphaerodoridae) were investigated by TEM and reconstructed from serial ultrathin sections. These organs are segmentally arranged paired protonephridia, which are in close association with a well-developed blood vascular system. Each protonephridium consists of a terminal part made up of two monociliary terminal cells (solenocytes), and a nephridioduct, formed by two cells. The two solenocytes lie close together. Each cilium is surrounded by 12 microvillar rods projecting from the perikaryon of each solenocyte. These rods form a weir-like structure in the coelomic space. The distal part of the weir is embedded in the proximal nephridioduct. The largest part of the cell bodies of the solenocytes, containing the nucleus, is lateral or basal to the weir-like structures. The lumen of the nephridioduct is formed by two multiciliated cells, which enclose the extracellular nephridial canal one behind the other. The canal opens through the nephropore beneath the cuticle without penetrating the cuticle. Both nephridioduct cells are surrounded by a blood vessel, which is partially folded into several layers. The significance of a simultaneous occurrence of protonephridial excretory organs and a well-developed blood vascular system as well as coelomic cavities is discussed. The results of this investigation indicate a close relationship of Sphaerodoridae to Phyllodocidae instead of to Syllidae within the Phyllodocida. Accepted: 27 November 2000     

The fine structure of protonephridia in Gnathostomulida and their comparison within Bilateria

Summary The fine structure of the protonephridia of Haplognathia rosea (Filospermoidea) and Gnathostomula paradoxa (Bursovaginoidea) is described. Each protonephridium consists of three different cells: (1) a monociliated terminal cell which constitutes the filtration area, (2) a nonciliated canal cell showing a special protonephridial outlet system, and (3) an intraepidermal cell — the nephroporus cell — constituting the nephroporus. The protonephridia are arranged serially. There is no canal system connecting the protonephridial units.Protonephridial characters in other Bilateria are considered. The pattern of characters in the protonephridia in the last common gnathostomulid stem species and presumed apomorphies in the protonephridia of the Gnathostomulida investigated are discussed.Abbreviations used in figures ac acessory centriole - AC additional epidermal cell - bb basal body - bl basal lamina - bm bundle of microvilli - c cilium - cc cilium duct cell - cd cilium duct - cr ciliary rootlet - crs structures resembling ciliary rootlets - di diplosome - ds desmosome - dy dictyosome - f filtration area - g granules - m mitochondrium - mv microvillus - n nucleus - NC nephroporus cell - np nephroporus - oc outlet canal - TC terminal cell - tl tubules of lacunar system     

Ultrastructure of the tubular nephron of the lizard Podarcis (= Lacerta) Taurica

The proximal, intermediate, and distal convoluted tubules of the neprhon of Podarcis (= Lacerta) taurica were examined by electron microscopy. Proximal tubule cells have large, apical cytoplasmic protrusions and microvilli interpreted to function in urate secretion. Adjacent cells are bound apically by tight junctions and desmosomes but interdigitate in their basal region. This situation is repeated in the other tubules with significant differences in intercellular space width. The basal surfaces bear numerous cytoplasmic processes. The intermediate tubule has proximal and distal segments each with dark, ciliated, and light cells, the cuboidal dark cells with dense cytoplasm constituting the main bulk of the wall. As the cells of the proximal and distal segments resemble those of the proximal and distal convoluted tubules, respectively, the intermediate tubule is considered as a transition region. The ciliated cell body has two broad processes extending from the lumen, one to the basement membrane and one to a foot process of a light cell. The light cell is surrounded by dark and ciliated cells. It does not reach the lumen, but contacts the basement membrane through a process running below a ciliated cell to form a mushroom-shaped structure in tubule cross-section, the light cell process forming the stalk and a ciliated cell the cap. The cilia probably propel the glomerular filtrate towards the distal convoluted tubule. This latter tubule has initial, middle, and terminal zones, all nonciliated but with different lumen widths and cell shapes.     

The Terminal Protonephridial Complex of Haplopharynx rostratus (Platyhelminthes,Haplopharyngida)

The terminal protonephridial complex of Haplopharynx rostratus consists of three terminal cells. There are no weirs consisting of ribs connected by a filtration “membrane”, but some cytoplasmic outgrowths into the lumen of the terminal cells. Excretion is by exocytotic vesicles. The terminal cells also contain Golgi complexes and large membrane-bound vacuoles containing electron-dense material. The ciliary bundles (flames) of terminal cells 2 and 3 protrude into the lumen of the centrally located terminal cell I. The complex is surrounded by a sheath containing numerous filaments. The terminal complex of H. rostratus resembles that of the macrostomid Paromalostomum proceracauda, lending support to the view that the two taxa are closely related. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

Ultrastructure of the protonephridial system in Neodasys chaetonotoideus (Gastrotricha: Chaetonotida) and in the ground pattern of Gastrotricha

The taxon Neodasys has a basal position within Gastrotricha. This makes it very interesting for phylogenetic considerations in this group. To complete the reconstruction of the nephridial system in the stem species of Gastrotricha started earlier, we have studied the whole protonephridial system of Neodasys chaetonotoideus by means of complete sets of ultrathin sections and TEM. In many characters, protonephridia of N. chaetonotoideus resemble those of macrodasyidan gastrotrich species. For example, each of the six protonephridia, arranged in three pairs, consists of three distinct cells that constitute the continuous protonephridial lumen. Especially, the terminal cell of the protonephridia of N. chaetonotoideus shows a striking pattern: The perforation of the filter region is a meandering cleft that is continuous with the seam of the enfolded lumen of that cell. With the results presented here and that of former TEM studies, we give a comprehensive idea of the excretory organs in the ground pattern of Gastrotricha. Moreover, we can elaborate on the hypothesized protonephridial system in the stem species of Bilateria. We suggest that a meandering filtration cleft is a feature of the ground pattern of the Bilateria.     

Investigations on the protonephridial system of post-larval Gyrocotyle urna and Amphilina foliacea (Cestoda).

The gross morphology and ultrastructure of the different parts of the protonephridial system of the monozoic tapeworms Gyrocotyle urna and Amphilina foliacea are described. The terminal cell in both species has numerous cilia which are interconnected and extend into the lumen of the first canal cell. The filtration area is built up from projections of two cells, the terminal cell and the first canal cell. The first canal cell forms a solid hollow cylinder without a cell gap and a desmosome as found in Neodermata other than cestodes and Udonella. In Gyroctyle the nucleus of the first canal cell is located in the wall cytoplasma whereas more distally located ductules of both species have subepithelial cell bodies containing the nuclei. In both taxa the protonephridial canal system is reticulate. In Amphilina the distal canals lack non-terminal ciliary flames, such ciliary tufts can be found in the larger capillaries of Gyrocotyle. The capillary cilia have rootlets and the ultrastructure of the duct wall cytoplasm containing large numbers of vesicles indicates highly active transport processes. The morphology of the protonephridial systems is discussed with regard to the evolution of Neodermata (especially of the Cestoda) and the function of the protonephridial system in cestodes as a probable organ of nutrient distribution.     

Ultrastructure of the Flame Bulbs and Protonephridial Capillaries of Artioposthia sp. (Platyhelminthes,Tricladida, Geoplanidae)

The protonephridial terminal complex of Artioposthia is formed by one or two terminal cells, each with a nucleus located in the lateral wall of the flame bulb, and probably two proximal canal cells forming the wall of the protonephridial capillary. The weir is restricted to the proximal parts of the flame bulbs and consists of convoluted slits separated by thick cytoplasmic columns. Cross-striated ciliary rootlets running parallel with and obliquely or transversely to the longitudinal axis of the flame bulbs strengthen the walls of the flame bulbs and, to a lesser degree, that of the capillary. Numerous cristate mitochondria are present in the terminal and proximal canal cells. Cytoplasmic processes extend from the terminal cells into the adjacent tissue, and narrow internal leptotriches extend from the cytoplasm of the terminal cells into the lumen of the flame bulbs. The wall of the capillary contains many interconnected, liquid filled spaces that communicate with the lumen of the capillary, and two septate junctions. Phylogenetic implications of the findings are discussed.     

The ultrastructure of the protonephridial system of Götte's larva of Stylochus mediterraneus (Polycladida, Platyhelminthes)

The protonephridial system of Götte's larva of Stylochus mediterraneus was studied by electron microscopy. There is one protonephridium on each side of the body, formed by one terminal and one canal cell. The terminal filtration apparatus is formed by a single cell (the terminal cell) with several globular processes, the largest of which includes the nucleus. Fingers of cytoplasm (leptotriches) from each process penetrate the lumen surrounding the bundle of cilia and fingers from adjacent processes interdigitate to form a pattern of convoluted slits which constitute the weir. The single canal cell is inserted internally to the terminal cell at the top of the weir and encloses the lumen without a junction. Septate junctions are present between the terminal and canal cells. The lumen of the canal cell is smooth-walled for most of its length and cilia arise and terminate at all levels of the terminal and canal cells. Posterior to the larval mouth opening, the canal cell crosses the epithelium and the lumen ramifies to form the excretory opening. The terminal apparatus closely resembles that found in the freshwater planarian Bdellocephala brunnea .     

Ultrastructure of the protonephridia of Seison annulatus (Rotifera)

Summary Each of the two protonephridial systems of Seison annulatus consists of three sections which are separated by cell borders with septate junctions: (a) a terminal syncytium with eight terminal organs and a capillary canal, (b) a canal syncytium which is divided into a multiciliary canal region and a main canal region, and (c) a nephroporus cell. The terminal syncytium is branched and linked twice to the canal syncytium. The supporting structure of each filtration barrier is a hollow cylinder which is perforated by pores and lacks microvilli (pillars). A protonephridial spine is situated in the multiciliary canal region and stabilizes the neck region. The ored, hollow cylinder and the protonephridial spine are new characteristics for the Rotifera.     

Light and electron microscopic studies on the excretory system of Macrobiotus richtersi Murray, 1911 (Eutardigrada)

Summary The excretory system of Macrobiotus richtersi consists of one dorsal and two lateral components and shows a high degree of structural complexity. In each of these a tricellular external lobe and a column can be distinguished, the two parts being connected distally. The surface of the lobe cells is increased by deep basal infoldings and fingerlike processes which form a labyrinth next to the basal lamina. Their cytoplasm contains numerous mitochondria, a well developed rough endoplasmic reticulum, dictyosomes, and granules in amounts depending on the physiological state of the animal. Excretory crystals occur in caveolae located in the lobe: between the fingershaped processes of the cell and in the space enclosed by the basal lamina on one side and the column on the other.The column faces an extracellular channel meandering along its whole length which is surrounded on the outside by a basal lamina. Morphologically the column is similar to the protonephridial channel of Rotifera. At the ultrastructural level, the cytoplasm of the column shows numerous mitochondria, rough endoplasmic reticulum, lysosomes, and a well developed Golgi apparatus. The lumen of the channel is coated by glycocalyx. At the base of the column several small cells form the proximal part of a duct that communicates with the gut.The morphology and ultrastructure of the excretory system of M. richtersi have been compared with similar a system in Isohypsibius megalonyx (Greven, 1979), and on these grounds a proposal is put forward to call the excretory organs of Tardigrada nephridia instead of Malpighian tubules.     

Ultrastructure and relationship between protonephridia and metanephridia in Phoronis muelleri (Phoronida)

Summary The actinotrocha of Phoronis muelleri has one pair of ectodermally derived, monociliated protonephridia. The duct runs mainly between the epidermis and the lining of the hyposphere coelom, pierces the septum and extends into the blastocoel. The proximal part is branched and closed up by terminal complexes consisting of two morphologically different cells which both serve filtration. During metamorphosis, the terminal complexes and the branches of the duct are cast off. The cells degenerate, pass into the remaining duct and are endocytosed by the duct cells. After metamorphosis the remaining part of the protonephridial duct is U-shaped, blindly closed and borders on the prospective lophophoral vessel. In a later stage the duct receives a ciliated funnel, which consists of monociliated epithelio-muscle cells and is a derivative of the lining of the metacoel. Thus, a part of the protonephridial duct of the larva and the whole metanephridial duct of the adult are identical. Aspects of a possible homology between phoronid nephridia and such organs in other bilaterians are discussed.     

The Nephridium of the Bonellia viridis Male (Echiura)

The nephridium of the dwarf male of Bonellia viridis was investigated by means of transmission electron microscopy. The nephridium proved to be of a distinct protonephridial type and not a metanephridium as maintained in the older literature. The nephridium is composed of a ciliated duct that projects into the coelom. Five crown cells at the end of the duct function as terminal filtration cells. Each crown cell has a bundle of about 20 cilia, surrounded by a labyrinthic weir of cell processes that are presumably involved in filtration. The ciliary bundles enter the nephridial duct through perforations of the adjacent tubule cells. This finding of a protonephridium in a minute, coelomate animal that lacks a circulatory system corroborates a recently formulated functional theory on the distribution of nephridial types.     

Protonephridial organs in Myzostoma cirriferum (Myzostomida)

Myzostoma cirriferum Leuckart, 1836 possesses five paired, serially arranged, blindending nephridial organs which are described for the first time. Ultrastructural investigations reveal that each nephridium is composed of three terminal cells and one tubular cell that forms the emission tubule. The central lumen of the individual terminal cells contains six to nine flagella, each of which is surrounded regularly by cytoplasmic rods arranged in parallel. Weir-like fenestrations in the peripheral wall of the terminal cells make up the connection between the central lumina and the extracellular space around the nephridial organ. The canal of the emission tubule possesses cilia, microvilli and cytoplasmic structures, suggesting involvement of this cell with active transport and storage. It opens into the cuticle at the ventral surface of the animal.     

The nephridia of the interstitial polychaete Hesionides arenaria and their phylogenetic significance (Polychaeta,Hesionidae)

Summary The small hesionid polychaete Hesionides arenaria possesses paired segmental excretory organs that closely resemble solenocytic protonephridia. The nephridium consists of one terminal cell and four tubule cells which form the emission channel. From the terminal cell, up to six flagella arise each surrounded by a weir of ten regularly arranged cytoplasmic rods. The structure of the cytoplasm of three of the following cells suggests that they function in active transport and storage. Because all of the larger, more primitive species of this family are equipped with metanephridia, the possibility is discussed that these organs have been developed out of metanephridia. The Hesionides arenaria nephridium may be a morphological stage in the evolutionary pathway from metanephridia to solenocytes. This would mean that solenocytes can no longer be considered to be homologous in every case with other protonephridial organs in polychaetes and may well be derived several times independently out of metanephridia or true protonephridia.     

PROTONEPHRIDIA

(1) The flame cell of platyhelminths is a composite organ formed from two cells. One cell contains a large nucleus and bears the flagella which form the flame. The other cell which contributes the barrel is the first tubule cell. There is a region of interdigitation between the two cells at the top of the barrel and the interdigitations are joined along their length by desmosomes. The tubule lumina are extracellular, the smaller tubules, at least, being formed by encircling projections from a single cell, joined at their tips by desmosomes. Cestodes apparently have no desmosomes in their tubule walls so that the tubule lumina may be intracellular. The tubules of most platyhelminths are lined by folds or microvilli, and flagella may be present in the lumen. The protonephridia of nemertines, entoprocts and priapulids appear to be of this type. (2) Direct evidence of the physiological role of flame cell systems is limited. There is, for example, no proven instance of the production of urine hypo-osmotic to body fluids by any fresh-water platyhelminth or nemertine. Endoparasitic platyhelminths are apparently unable to osmoregulate. The relative permeability of the body surface of marine, fresh-water, terrestrial and parasitic platyhelminths and nemertines may be related to protonephridial function. It seems highly likely that the function of the flame cell is to filter interstitial fluid, separating water and crystalloids from macromolecules. The ultrafiltrate produced then flows down the tubules as a result of the hydrostatic pressure generated by the beating of flame flagella, or as a result of peristaltic waves of the whole body generated by the musculature of the worm. The fluid may be modified in the canal lumen by both active and passive resorption of solutes or the secretion of material from the walls into the lumen. Experiments with the larger platyhelminths suggest that the main function of the system is to remove organic metabolites from the interstitial spaces of the deeper tissues of the worm by a mechanism more efficient than simple diffusion. (3) The flame bulbs of rotifers are fan-shaped and the nucleus is in the tubule not the cap. The barrel of the flame bulb is composed of a series of columns in scalloped formation, each arc of the scallop being supported by a cytoplasmic pillar. A membrane interconnects the columns and each column is linked to its neighbouring central pillar by fibrils. The tubules leading from the flame bulbs are a complex system of three to four multinucleate cells. They empty into a contractile bladder. The protonephridia of acanthocephalans and gastrotrichs may be of this type. (4) The mode of action of flame bulbs is probably to filter the pseudocoelomic fluid which is then modified by selective reabsorption in the tubule system. Rotifers are able to osmoregulate and this may be the chief function of their protonephridia. (5) Solenocytes are morphologically diverse, usually with a cytoplasmic cap containing a nucleus, and a long tubule, in the lumen of which lie one or two flagella. The walls of the tubule are pierced by fenestrations, probably the site of fluid ‘filtration’. (6) Kümmel (1962) has suggested that the many types of terminal organ are the result of divergent evolution from a single ancestral type. We suggest that protone-phridial terminal organs can be divided on structural grounds into three or four different groups which are probably not inter-related. This would mean that the apparent structural similarities which do appear would be the result of convergent evohtion imposing a conformity based on functional requirements.