Ultrastructure of excretory system in <Emphasis Type="Italic">Bothrioplana semperi</Emphasis> (Platyhelminthes,Turbellaria)

The ultrastructure of flame bulbs and epithelium of excretory canals in Bothrioplana semperi (Turbellaria, Seriata) have been studied. The flame bulbs consist of two cells, the terminal cell and the proximal canal cell. The weir is formed by two rows of longitudinal ribs. The ribs of the internal row originate from the flame cell, external ribs are formed by the proximal canal cell. Each external rib has a remarkable bundle of microfilaments, originating in the cytoplasm of the first canal cell distally to the bases of external ribs. Membrane of internal ribs is marked by small electrondense granules, separate or fused to an electron-dense layer, continuous to dense “membrane,” connecting both external and internal ribs. Sparse internal leptotrichs originate from the bottom of the flame bulb cavity. External leptotrichs are lacking. Septate junction is present only in proximal canal cell at the level of tips of cilia. The apical surface of the canal cell bears rare short microvilli. The basal membrane of canal cells forms long invaginations that may reach nearly the apical membrane. The epithelium of excretory canals lacks the cilia. The ultrastructure of flame bulbs and epithelium of the excretory canals in B. semperi shares representatives of suborder Proseriata (Seriata). The contradiction exists in interpretation of the structure of flame bulbs in Proseriata. Ehlers and Sopott-Ehlers assumed that the external ribs are derivatives of the proximal canal cell and internal ones are outgrowths of the terminal cell, while Rohde has found conversely: the external ribs are outgrowths of the terminal cell, the internal ones are outgrowths of the proximal canal cell. However, the illustrations provided by Rohde do not enable to ascertain what cells the internal and external ribs derive from, while illustrations provided by Ehlers justify his interpretation. The order of weir formation in B. semperi confirms the viewpoint of Ehlers. The implication of ultrastructure of flame bulbs in Proseriata, especially of the order of flame bulb formation, in the Platyhelminthes phylogeny has been discussed.     

Ultrastructure of the protonephridial system of regenerating Stenostomum sp. (Plathelminthes,Catenulida)

Summary The ultrastructure of the flame bulbs, protonephridial capillaries and duct of fully developed and regenerating Stenostomum sp. is described. Flame bulbs are formed by a single cell whose nucleus is located basally or laterally to the weir. The weir is formed by a single row of transverse ribs connected by a thin membrane, apparently of extracellular matrix. Internal leptotriches arise from the proximal cytoplasm and extend in a (usually) single row along the weir and into the lumen of the distal cytoplasmic tube. Many or all leptotriches do not fuse with the distal cytoplasm. Two cilia are each anchored in the proximal cytoplasm by a cross-striated vertical and lateral rootlet, the latter bent forward and extending for some distance into one of the two cytoplasmic cords along the weir. Each cord contains the lateral rootlet in its proximal part, as well as many microtubules. The distal cytoplasmic tube contains two (longitudinal) junctions, i.e. lines of contact between cell processes of the same, terminal cell. Occasionally, more than two junctions were seen, apparently due to branches of the terminal cell in contact with each other. Flame bulbs join capillaries lined by several canal cells type I, containing few or no microvilli but lateral flames. Such capillaries join a duct (or ducts?) lined by canal cells type II with many long microvilli. The large protonephridial duct is lined by numerous cells with lateral flames and many long microvilli. In regenerating tissue (10.5 hours after cutting) some flame bulbs were free, i.e. not connected to capillaries, and some capillaries openly communicated with the surrounding intercellular space. In the presence of a single row of ribs in the weir, of internal leptotriches, and of vertical and lateral ciliary rootlets, the flame bulb of Stenostomum sp. resembles that of other Plathelminthes much more closely than hitherto thought. The species differs from non-catenulid plathelminths mainly in the large number of glandular cells lining the large protonephridial ducts, in the transverse orientation of the ribs in the weir and in the presence of only two cilia in the flame.     

Ultrastructure of the Flame Bulbs and Protonephridial Capillaries of Microstomum sp. (Platyhelminthes,Macrostomida)

The terminal part of the protonephridia of Microstomum is formed by a branching proximal canal cell and (at least?) two terminal cells. Each weir consists of longitudinal (sometimes convoluted) ribs continuous with the cytoplasm of the terminal cell. Internal leptotriches arise from the terminal and proximal canal cells. Near the tip of the flame, the proximal canal cell tube is surrounded by the more external terminal cell and connected to it by a septate junction. Large cristate mitochondria are densely packed in the terminal and canal cells. The flame bulb of Microstomum differs markedly from that of other macrostomids (Macrostomum, Paramalostomum) examined. Phylogenetic implications are discussed.     

Ultrastructure of flame cells and protonephridial capillaries of Craspedella and Didymorchis (Plathelminthes,Rhabdocoela)

Summary The ultrastructure of the flame cells and protonephridial capillaries of the Rhabdocoela Craspedella sp. and Didymorchis sp., ectocommensals on the freshwater crayfish Cherax destructor in eastern Australia is described. The flame cells of both species have variable numbers of cilia without distinct rootlets and with decreasing numbers of axonemal tubules towards the ciliary tips. Bundles of microtubules extend from the cytoplasm adjacent to the ciliary rootlets through the ribs of the weir apparatus into the distal cytoplasmic tube, where the numbers of microtubules gradually decrease. The weir apparatus is formed by a single row of longitudinal ribs connected by a membrane. In Craspedella, but not in Didymorchis, the ribs have external branched leptotriches. Mitochondria are common in the wall of the flame cell of both species. The protonephridial capillary just above the end of the ciliary tuft narrows in both species and bends sharply in Craspedella. The lumen of the flame cell and the capillary is lined by a dark layer of cytoplasm; there is no enlargement of the surface area by microvilli or lamellae. Centrioles were seen in the capillary wall of Craspedella, and in Didymorchis the cytoplasm around the capillaries has a very loose and light appearance. The ultrastructure of the flame cells and capillaries of both species corresponds closely to that of Temnocephala sp.Abbreviations in the figures BB basal body - CE centriole - L leptotrich - M microtubules - ME membrane of weir apparatus - MI mitochondrion - PC protonephridial capillary - R rib (rod) of weir apparatus     

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 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.     

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 ventral epithelium of Trichoplax adhaerens (Placozoa): Cytoskeletal structures,cell contacts and endocytosis

Summary All cilia emerge from ciliary pits supported along their circumference by 22–24 dense rodlets that are connected by filaments to a surrounding sheath of endoplasmic reticulum. The proximal part of the basal body is provided with two short lateral rootlets and one long terminal rootlet, all associated with microtubules. The lateral rootlets are in turn connected by fine fibrous material to the dense supporting rodlets which follow the contour of the ciliary pit and extend along the ciliary membrane beyond the level of the basal plate where the central pair of microtubules originates. The proximal part of the basal body has fine fibrous connections to the endoplasmic reticulum while its distal portion is surrounded by nine curved sheets. The terminal cell contactions are by belt desmosomes that are accompanied by a bundle of microfilaments which encircle the apical region of the cell and insert at the cell membrane. Tight junctions are lacking. Endocytosis was demonstrated by the uptake of cationized ferritin. The structures associated with the ciliary pits are probably associated with the firm anchorage of the ciliary base since Trichoplax adheres to the substrate as it moves propelled by its ventral cilia. The marginal bundle of microfilaments may be involved in folding of the organism during feeding.     

Ultrastructure of the protonephridial system of Dactylogyrus sp. and an unidentified ancyrocephaline (Monogenea: Dactylogyridae)

The ultrastructure of the flame bulbs and capillaries of the protonephridia of Dactylogyrus (probably anchoratus) from Carassius auratus in southeastern Australia, and of an unidentified ancyrocephaline from the marine teleost Priacanthus macracanthus in southern Queensland is described. The cilia of the flame are anchored in the terminal cell by means of basal bodies without distinct rootlets. The nucleus of the terminal cell is basal, and (in Dactylogyrus) partly lateral to the basal bodies. The weir consists of a row of internal and a row of external ribs (rods) connected by a ‘membrane’. The external ribs are continuations of the cytoplasm of a thick-walled ‘cytoplasmic cylinder’ (proximal canal cell) which tightly surrounds most of the flame and contains a septate junction; the internal ribs are continuous with the terminal cell. Internal leptotriches arise from the perikaryon of the terminal cell, and, in the ancyrocephaline, also from the internal ribs. The wall of the protonephridial capillaries contains a septate junction, a reticulum of interconnected spaces and, in the ancyrocephaline, also lamellae. Lateral flames are common in the capillaries of Dactylogyrus.     

False Vertical Rootlets of Epidermal Cilia in the Oncomiracidia of Neoheterocotyle rhinobatidis and Monocotyle spiremae (Platyhelminthes,Monogenea, Monopisthocotylea)

Epidermal cilia of the oncomiracidium of Neoheterocotyle rhinobatidis (Monogenea, Monopisthocotylea, Monocotylidae) have long cross-striated vertical rootlets that are not extensions of the basal bodies as are the vertical rootlets in all catenulid and rhabditophoran turbellarians examined to date. Instead, they originate in the basal part of the horizontal rootlet a short distance from the basal bodies. In Monocotyle spiremae (Monocotylidae), the vertical rootlets are less distinct, with no apparent cross-striation, but they also originate from the basal part of the horizontal rootlets. Epidermal cilia of the oncomiracidium of Zeuxapta seriolae (Monogenea, Polyopisthocotylea, Axinidae) lack vertical rootlets like all other neodermatans examined, but bundles of fibres extend from the basal bodies a short distance into the cytoplasm of the epidermal cells. Monopisthocotylean Monogenea would be intermediate between rhabditophorans and the other neodermatans (in having weakly developed vertical rootles), if these structures were homologous in the two groups. However, in view of the different origin of vertical rootlets in turbellarians and monopisthocolylean oncomiracidia, it is suggested that they are not homologous, and vertical rootlets in the Monopisthocotylea are therefore named “false vertical rootlets”.     

The evolution of protonephridia of the Platyhelminthes

Three types of flame bulbs are distinguished in the Platyhelminthes: type 1 has two cilia arising from a terminal cell and rootlets extending along the weir; type 2 has many cilia arising from a terminal cell and the proximal canal cell closely aligned with it; and type 3 has a non-terminal perikaryon forming many flame bulbs, each with many cilia and a single row of longitudinal ribs. Each type appears in various structural forms. Type 1 is found in the Catenulida; type 2 in the Macrostomida, Polycladida, Prolecithophora, Proseriata, Tricladida, Fecampiidae, and Neodermata; and type 3 in the Rhabdocoela and Lecithoepitheliata. The most likely evolutionary sequence is that type 3 is derived from type 2 and, perhaps, that type 2 is derived from type 1. Characters of the protonephridia show that the Rhabdocoela and the Neodermata form separate phylogenetic lineages; other similarities between these taxa are due to convergent evolution (or horizontal gene transfer?).     

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     

Das Epithel der Tuba uterina des Neugeborenen Elektronenmikroskopische Befunde

Summary An electron microscopical study of the epithelium of the uterine tube was carried out in the newborn. Among the epithelial cells at least two morphologically well defined types can be distinguished: ciliated and non-ciliated cells.The ultrastructure of the cilia and related structures corresponds to what has been described by other authors in ciliated cells of various organs and of different species. Near the basal bodies of the cilia there is a concentration of vesicular mitochondria, which is thought to be evidence of a high metabolic activity in this region of the cell. Large opaque granules in the supranuclear zone of the ciliated cells are, it is suggested, paraplasmatic inclusions, perhaps supporting material for the ciliokinetic processes. There was no evidence of a secretory function of the ciliated cells.Among the non-ciliated cells, which in general show a straight lined luminal border with few microvilli, there are some cells containing dense granules, which are distributed throughout the cytoplasm and concentrated in the luminar side of the cell. The apical parts of these cells are protruding and sometimes digitated or branched; they contain accumulated granular materials and are separated from the rest of the cell after the formation of an intracellular plasmalemma. A similar detachment was found in an other cell type, but here the protruded apical parts of the cells are edematous and do not contain any visible secretory materials. It is uncertain if the detached cytoplasmic substances form a part of a specific secretory product; there are no secretory granules within the cytoplasm. On the contrary, the detachment of cytoplasmic parts may only accompany the excessive proliferation of cells which takes place during this period of growth.     

Ultrastructure of the Protonephridial Terminal Organ in the Terrestrial Nemertean Geonemertes pelaensis (Rhynchocoela,Enopla, Hoplonemertini)

The protonephridial terminal organ of Geonemertes pelaensis consists of two cells that are equal in both size and shape and form mirror images of each other. From the perinucleate lump-shaped region basally in each cell arises a cytoplasmic column which branches at regular intervals to form 3–4 nearly circular bars. The opposed columns and alternating bars of the two cells are arranged in such a way that they form an obviously rigid, cylindrical structure which both supports and gives rise to the thin-walled weir. The fenestration of the weir consists of a single, enormously extended and sinuous cleft which represents part of the boundary between the two terminal cells. The ciliary flame comprises 92–118 cilia that originate basally in the two cells and is surrounded by a palisade of long, straight microvilli, positioned immediately within the weir's wall. The structure is very similar to that of the terminal organ of the land nemertean Pantinonemertes, except that the cytoskeleton which supports columns and bars consists of a fibro-granular substance, not, as in Pantinonemertes, of an abundance of oriented microtubules.     

Ultrastructure of the protonephridia in Pycnophyes kielensis (Kinorhyncha,Homalorhagida)

Summary Pycnophyes kielensis possesses one pair of protonephridia. The single excretory organ of a female consists of 25 cells: 22 terminal cells, 2 canal cells, and 1 nephroporus cell. Generally, all cells exhibit two cilia, the only exception being the nephroporus cell, which contains a diplosome instead. The slashed peripheral cytoplasmic walls of the 22 terminal cells altogether constitute one compound filter and a common filtration area. The protonephridia discharge via cuticularized cavities and six cuticularized tubes. Two accessory cells with modified cilia penetrate the nephroporus cell. These cells are considered to be receptor cells. The protonephridium of the first juvenile stage of P. kielensis is built up of only 5 cells: 3 terminal cells, 1 canal cell, and 1 nephroporus cell. It opens to the outside via 1 cuticularized tube. The protonephridia within both the Kinorhyncha and the Bilateria are discussed. Presumably excretory organs with compound filters developed independently within Bilateria.Abbreviations bb basal body - c canal cell - ca cuticularized cavity - ci cilium - cu cuticle - d dictyosome - de desmosome - di diplosome - dl dorsal longitudinal muscle - dv dorsoventral muscle - ecm extracellular matrix - ep epidermal cell - ex excretory organ - fc filter cleft - fi filter - fm fastening muscle cell - he hemidesmosome - i intestine - if intercellular fluid - m mitochondrium - mv microvilli - n nephroporus cell - nu nucleus - r ciliary rootlet - re accessory cell (presumable receptor cell) - sj septate junction - t terminal cell - tu cuticularized tube - v vesicle - w peripheral cytoplasmic wall     

Ultrastructure of the epidermis of adult and embryonic Paravortex species (Turbellaria,Eulecithophora)

The epidermis and associated structures of adult and embryonic Paravortex cardii and Paravortex karlingi, internal parasites of Cerastoderma edule, have been examined using scanning and transmission electron microscopy. The cellular epidermis of adult Paravortex bears cilia and microvilli which differ in number and distribution between P. karlingi and P. cardii. Cellular organelles include mitochondria, lipid bodies, Golgi bodies, and ultrarhabdites. Epidermal nuclei are located in the proximal portion of the cells. The development of the tegument of embryo Paravortex has been described and a possible origin for the embryo capsule is suggested. These findings are discussed in relation to the phylogenetic status of the Turbellaria in relation to other Platyhelminthes and in the functional adaptation of the epidermis for a parasitic mode of life.Abbreviations bb- basal bodies - bl- basal lamella - c- cilia - cp- capsule - dc- dark cells - e- embryos - ep- epidermis - g- Golgi bodies - int- interdigitation (of cells) - l- lipid - lf- lamellar fold - mc- migrating cell - mf- membranous folds - mt- mitochondria - mv- microvilli - n- nucleus - nb- neoblasts - p- projections of epidermis - par- parenchyma of mother - pr- primary rootlet - rc- rhabditogen cells - sr- secondary rootlet - ur- ultrarhabdites - vt- vitelline material     

Ultrastructure of the protonephridium in Prostoma graecense (Bohmig) (Rhynchocoela, Enopla, Hoplonemertini)

Fine structure studies show that (1) the terminal cell is an elongated thin-walled and fenestrated basket with a multiciliary flame. Many short curved microvilli are confined to the cell lumen, while longer straight microvilli project from the cell's apical end into the proximal part of the protonephri-dial capillary, forming a sheath around the flame. The filtration area consists of slits between narrow cytoplasmic bars and is entirely confined to the terminal cell, which consequently is defined as a flame bulb, not closely similar to those of other phyla. (2) The protonephridial capillary is short and narrow, with few scattered cilia and luminal microvilli. (3) The coiled tubule is thick-walled, with several ciliated cells very rich in glycogen. The luminal border shows specializations probably concerned with modifying the ultrafiltrate.     

The Protonephridia of the Arctic Kinorhynch Echinoderes aquilonius (Cyclorhagida,Echinoderidae)

The cyclorhagid kinorhynch Echinoderes aquilonius Higgins & Kristensen, 1988 possesses a single pair of protonephridia located in segments 10 and 11. The protonephridia consist of: (1) three terminal cells T-1, T-2. T-3, each with two cilia; (2) a single non-ciliated canal cell; (3) a nephridiopore cell with many microvilli and a cuticular sieve plate. The protonephridia of Echinoderes are presumed to develop from the ectoderm near the area of the sieve plate on the eleventh segment, and are suspended in the dorso-lateral pseudocoelomic cavity where they are surrounded by a basal lamina. One of the terminal cells (T-1) secondarily penetrates the basal lamina of the tenth segment and a part of the cell attaches to the cuticle. The kinorhynch protonephridia are compared with the excretory organs of other Bilateria. expecially the ‘aschelminths’, and apomorphic characters of the kinorhynch protonephridia are defined.     

The unique ultrastructure of the uterus of the Gyrocotylidea Poche, 1926 (Cestoda) and its phylogenetic implications

The members of the order Gyrocotylidea are monozoic tapeworms and generally considered to be the most primitive group of the Cestoda in terms of the evolution of this platyhelminth class. As part of a series of ultrastructural studies on Gyrocotyle urna (Wagener, 1852), three regions of the uterus were distinguished. The proximal region of the uterus is characterised by underlying perikarya, the presence of septate junctions within the epithelial wall and two types of specialised outer coverings, lamellae and cilia. The middle, syncytial region of the uterus is covered by short lamellae and marked by a concentration of sunken glandular perikarya (uterine glands). Glandular spheroidal granules (c.0.45 μm in diameter) of moderately dense content and a fine fibrillar texture are liberated by migration through the luminal membrane. The epithelium of the sac-shaped, distal region of the uterine duct is interrupted by cytoplasmic processes of sunken epithelial bodies, covered with lamellae and contains septate junctions. A muscular sphincter surrounds the narrow, terminal region of the distal uterine duct. The ultrastructural pattern of the uterus of the Gyrocotylidea has important discriminating traits (i.e. the presence of sunken perikarya along its entire length, septate junctions within the uterine epithelial cytoplasm of the proximal and distal regions, and cilia on the surface of its proximal region) unique among the Neodermata and which may represent autapomorphic character states for the group.     

Embryogenesis in <Emphasis Type="Italic">Sedum acre</Emphasis> L.: structural and immunocytochemical aspects of suspensor development

The changes in the formation of both the actin and the microtubular cytoskeleton during the differentiation of the embryo-suspensor in Sedum acre were studied in comparison with the development of the embryo-proper. The presence and distribution of the cytoskeletal elements were examined ultrastructurally and with the light microscope using immunolabelling and rhodamine-phalloidin staining. At the globular stage of embryo development extensive array of actin filaments is present in the cytoplasm of basal cell, the microfilament bundles generally run parallel to the long axis of basal cell and pass in close to the nucleus. Microtubules form irregular bundles in the cytoplasm of the basal cell. A strongly fluorescent densely packed microtubules are present in the cytoplasmic layer adjacent to the wall separating the basal cell from the first layer of the chalazal suspensor cells. At the heart-stage of embryo development, in the basal cell, extremely dense arrays of actin materials are located near the micropylar and chalazal end of the cell. At this stage of basal cell formation, numerous actin filaments congregate around the nucleus. In the fully differentiated basal cell and micropylar haustorium, the tubulin cytoskeleton forms a dense prominent network composed of numerous cross-linked filaments. In the distal region of the basal cell, a distinct microtubular cytoskeleton with numerous microtubules is observed in the cytoplasmic layer adjacent to the wall, separating the basal cell from the first layer of the chalazal suspensor cells. The role of cytoskeleton during the development of the suspensor in S. acre is discussed.