Comparative ultrastructure of juvenile and adult nuchal organs of an annelid (Polychaeta: Opheliidae)

Opheliid nuchal organs are composed of ciliated cells, retractor muscles, and sensory cells. The perikarya of sensory cells are located in the posterior portion of the brain, and their distal processes extend along the body wall, as the nuchal nerve, and terminate just anterior to the ciliated region. The nuchal nerve of the juvenile is composed of 30–35 dendrites; the adult nuchal nerve has 35–40 dendrites. The ends of the sensory dendrites form sensory bulbs which are clustered around the olfactory chamber, and each bulb bears a modified cilium. Sensory cilia lose their axonemes and extend as microvillous-like structures into the olfactory chamber. Supportive cells delineate approximately the posterior and dorsal portions of the chamber with sensory bulbs forming the remaining ventral and anterior portions. On the lateral aspect of the chamber, cuticular matrix extends into it, and in this area supportive cells bear microvilli which extend into the matrix. The adult nuchal organ is larger than that of the juvenile, and the sensory portion of the olfactory chamber wall is expanded. Expansion of the sensory area is apparently the result of size increase in sensory bulbs and by intrusion of supportive cells between sensory bulbs.     

Ultrastructural study of sensory cells of the proboscidial glandular epithelium of Riseriellus occultus (Nemertea,Heteronemertea)

Only one sensory cell type has been observed within the glandular epithelium of the proboscis in the heteronemertine Riseriellus occultus. These bipolar cells are abundant and scattered singly throughout the proboscis length. The apical surface of each dendrite bears a single cilium enclosed by a ring of six to eight prominent microvilli. The cilium has the typical 9×2 + 2 axoneme arrangement and is equipped with a cross-striated vertical rootlet extending from the basal body. No accessory centriole or horizontal rootlet was observed. Large, modified microvilli (stereovilli) surrounding the cilium are joined together by a system of fine filaments derived from the glycocalyx. Each microvillus contains a bundle of actin-like filaments which anchor on the indented inner surface of a dense, apical ring situated beneath the level of the ciliary basal body. The tip of the cilium is expanded and modified to form a bulb-like structure which lies above the level where the surrounding microvilli terminate. In the region where the cilium emerges from the microvillar cone, the membrane of the microvillar apices makes contact with a corresponding portion of the ciliary membrane. At this level microvilli and cilium are apparently firmly linked by junctional systems resembling adherens junctions. The results suggest that these sensory cells may be mechanoreceptors. © 1996 Wiley-Liss, Inc.     

Two ciliate sense receptors in the larva of Austramphilina elongata Johnston, 1931 (Amphilinidea)

Summary The ultrastructure of a uniciliate and a quadruciliate receptor in the anterior end of the larva of Austramphilina elongata is described on the basis of serial sections. The uniciliate receptor has numerous branched and interconnected microvilli at its surface, several rings forming the electron dense collar, and cross-striated rootlets diverging from the basal body of the cilium. The quadruciliate receptor has four short club-shaped sensory cilia and a single electron-dense collar.Abbreviations used in figures ec electron-dense collar - ep epidermis - m microvilli - nt neurotubules - pe process of electron-dense collar - r rootlet of cilium - sc sensory cilium - sd septate desmosome     

The paracrystalline structure of an insect rhabdomere (Calliphora erythrocephala)

Summary By use of a modified fixation technique, the receptor cells of the compound eye of the blowfly Calliphora erythrocephala were found to contain a regular, paracrystalline array of alternating rows of hexagonally shaped microvilli. The receptor cells R1 to R6 have a cell-specific number of microvilli per row in a cross section. Every microvillus has a filament cluster connecting the axial skeleton with the microvillar membrane. This cluster is preferentially right-left oriented relative to the longitudinal axis of the microvillar array. Three adjacent microvilli are interconnected by an electron-dense substance. A mirroring technique indicated that this intermicrovillar structure consists of three subunits, although these subunits could not be conclusively demonstrated by classical densitometry or image subtraction techniques. The electron-dense substance can be seen in all cross sections of the proximal and distal parts of the microvilli. They are cylindrical structures separating the microvilli along their entire length. It is suggested that these cylindrical aggregates contain an enzymatic complex separating the rhodopsin-containing microvillar membrane into six compartments.     

Structure of the digestive tract of tornaria larva inEnteropneusta (Hemichordata)

The ultrastructure of the digestive tract of tornaria larva of enteropneusts was investigated. It showed that the digestive tract consists of three parts: esophagus, stomach, and intestine. The esophagus epithelium consists of two types of multiciliated epithelial cells and solitary muscle cells. Axonal tracts and neurons were found in the ventral wall of the esophagus. The cardiac sphincter contains an anterior band of strongly ciliated cells and a posterior band of cells with long vacuolized processes which partition the sphincter lumen. The stomach consists of three cell types: (1) cells with electron-opaque cytoplasm, bearing a fringed border on their apical sides; (2, 3) sparse cells with electron-light cytoplasm and different patterns of apical microvilli. Cells of the pyloric sphincter bear numerous cilia and almost no microvilli. The intestine consists of three parts. The anterior part is formed of multiciliated cells which bear the fringed border. The middle part consists of flattened cells bearing rare cilia and vast numbers of mace-like microvilli. The posterior part of the intestine is formed of cells bearing numerous cilia and few microvilli. Muscle cells were not found in either stomach or intestine epithelium. One noticed that the structure of the digestive tract of enteropneust tornaria larva differs from that of echinoid pluteus larva.     

Cytoskeletal modifications of the sensorimotor interneurons ofHydra vulgaris (Cnidaria,Hydrozoa), indicating a sensory function similar to chordotonal receptors of insects

Summary The battery mother cell complexes in the tentacles ofHydra vulgaris contain a neuronal cell known as sensorimotor interneuron that is characterized by a modified cilium lying parallel to the mesoglea. The cilium is surrounded by up to three rings of microvilli. Microvilli and cilium arise in an unusual antiparallel orientation from the opposite poles of a central cellular cavity. The lumen of this cavity communicates with the extracellular environment by way of a straight channel-like opening that is encircled by the microvillar rings. The modified cilium extends into the channel and terminates outside in the intercellular space. The wall of the cavity and the channel are stabilized by bundles of microtubules. A prominent glycocalyx interconnects all microvilli and links the innermost microvillar ring to the cilium. Within this contact region approximately 0.7 m in length the ciliary axoneme is specifically modified: all nine microtubule doublets and up to six additional microtubules are embedded in electron-dense material. The microtubule doublets are connected to the ciliary membrane by ledges of Y-shaped cross-bridging elements. These axonemal modifications resemble those known from the hydrozoan cnidocil complex or from the outer segments of insect mechanoreceptor cells. Distribution and orientation of the sensorimotor interneuron within the tentacles indicate a mechanosensory function of the cell similar to that of chordotonal receptors of insects.     

Femoral chordotonal organ in the legs of an insect,Chrysoperla carnea(Neuroptera)

The femoral chordotonal organ (FCO) inChrysoperla carneais situated in the distal part of the femur and consists of two scoloparia, which are fused at their distal end. The distal scoloparium contains 17-20 scolopidia, and the proximal one six scolopidia. Each scolopidium consists of two sensory cells and three types of enveloping cells (glial, scolopale and attachment cell). The sensory cells of different scolopidia do not lie at the same level in the FCO. Therefore the attachment cells of different scolopidia have different lengths. In the FCO, three types of ciliary roots are found in different sensory cells. The dendrite of the sensory cell terminates in a distal process, which has the structure of a modified cilium (9x2+0). The very distal part of the cilium is surrounded by an extracellular electron dense material, the cap, and ends in a terminal dilation. The scolopale cell contains the electron dense scolopale rods, consisting of plentiful microtubules. In their middle third the scolopale rods are fused and form the scolopale. In the FCO septate junctions, desmosomes and hemidesmosomes are found.     

Immunohistochemical and ultrastructural studies on ciliary sense organs of arrow worms (Chaetognatha)

We examined epibenthic and pelagic species of Chaetognatha (Spadellidae and Sagittidae) using immunohistofluorescence and confocal laser scanning microscopy to detect tubulin and cell nuclei in whole-mount preparations and scanning and transmission electron microscopy to visualize the ultrastructural organisation of their ciliary sense organs. All chaetognaths bear three types of ciliary sense organs distributed throughout the body: (1) transversally oriented ciliary fence organs, (2) longitudinally (parallel to the anterior-posterior axis) oriented ciliary tuft organs, and (3) a ciliary loop, the corona ciliata. This study targets the ciliary fence as well as the ciliary tuft organs. Two types of primary receptor cells constitute the ciliary fence and ciliary tuft organs. The first type of receptor cells forms a single cell line along the midline axis of the organs, whereas the second type of receptor cells forms multiple lines of cells at either side of type 1 cells. Each receptor cell extends a single, non-locomotory cilium from its narrow apex collared by slender, non-reinforced microvilli; however, both types of sensory cells considerably differ on ultrastructural level. Type 1 sensory cells have thicker cilia than those protruded by the type 2 sensory cells which are characterized by rootlets consisting of an elongated, amorphous distal as well as a cross-striated proximal portion. These results likely reveal that both types of sensory cells have distinct functions.     

SEM and TEM study of the armed male terminal genitalia of the tapeworm Paraechinophallus japonicus (Cestoda: Bothriocephalidea)

For the first time, the ultrastructure of the armed cirrus of an echinophallid cestode, Paraechinophallus japonicus (Yamaguti, 1934), has been studied with the use of scanning and transmission electron microscopy. Two sets of eversible copulatory organs (approximately 300 microm in length and approximately 130 microm in width) are present on the dorsal side of each segment near the lateral margin of the strobila. Except for the terminal portion, the cirrus is covered with large spines (up to 40 mircom long, measured from SEM photomicrographs) composed of 2 parts. The basal portion contains a lobed electron-dense outer region that gives way to a reticular meshwork of electron-dense material. The apical region of the spines, composed of a homogeneous, moderately electron-dense matrix, is slightly curved distally. Spines are covered with a cortical zone. Between the spines, the distal cytoplasm is covered with microvilli of about 1.2 microm in length. The wall of the cirrus sac, which is approximately 500 microm long and approximately 250 microm wide, is composed of 2 layers of muscles, i.e., an internal layer of circular muscles and external longitudinal muscles. Microvilli on the cirrus of P. japonicus are reported for the first time in the Cestoda, whereas the spines on the cirrus may represent a synapomorphy of bothriocephalidean cestodes of the Echinophallidae.     

THE FINE STRUCTURE OF THE ABDOMINAL SENSE ORGANS OF PTERIOMORPHA (MOLLUSCA, BIVALVIA)

An ultrastructural study of the abdominal sense organs (ASO)of nine species, representing all three groups Of Pteriomorpha(Mytilina, Arcina, Pteriina), is offered. In all species investigated the sensory epithelium is high (40–130µm) and possesses two predominant cell types: (a) electron-densesupporting cells with microvilli only, pigment granules, andoval distal nuclei; (b) sensory cells with round proximal nucleiand electron-lucent plasma. Their narrow processes, always bunched,reach the surface bearing long (100–300 µm) stiffcilia. Each cilium has a polarized, orientated basal foot andseveral strong roots and is surrounded by nine specialized "stereo-microvilli"forming a basal plate in connexion with the basal body. Basalcontact of the sensory cells with nervous tissue proves theirreceptive nature. Structural uniformity as well as identical innervation provideevidence for the homology of the ASO in all Pteriomorpha. Thereis no homology between sensory cells and the similar, so-called"choanocyte-like cells".     

Ultrastructure of the daughter sporocyst and developing cercaria of Schistosoma japonicum in experimentally infected snails, Oncomelania hupensis hupensis

Ultrastructure of daughter sporocysts and cercariae of Schistosoma japonicum were studied 2 and 4 months after infection of Oncomelania hupensis hupensis. The body walls of daughter sporocysts are similar at all infectious stages. They consist of an external syncytial tegument on a basement membrane, and an internal cellular subtegument surrounding a body cavity containing developing cercariae. The cercariae embryos develop 2 months after infection from germinal balls in the brood chamber of the daughter sporocyst. They are at first enveloped by a primitive epithelium rising from the daughter sporocyst. Four months after infection, the cercariae were almost fully developed and the primitive epithelium had degenerated. The body wall of the cercaria consists of a thin tegument covered by a surface coat of fibrous material and connected to the subtegumental cells by cytoplasmic processes. The matrix of the tegument contains numerous dense bodies, vacuoles, and spines. Two types of sensory structures - uniciliated and multiciliated - are found at the anterior tip of the cercaria. There are five pairs of penetration gland cells of two distinct types differentiated by the morphology of secretory granules. Flame cells are found in both daughter sporocysts and in cercariae. The cilia of the flame cells are characterized by the typical 9 and 2 cilium pattern.     

Ultrastructural investigation of the nuchal organs ofPygospio elegans (Polychaeta). I. Larval nuchal organs

The structural differentiation of the nuchal organs during the post-embryonic development ofPygospio elegans is described. The sensory organs are composed of two cell types: ciliated cells and bipolar primary sensory cells, constituting the nuchal ganglion, which is associated with both the sensory epithelium and the brain. Since the sensory neurons are largely integrated into posterolateral parts of the cerebral ganglion, the nuchal organs are primary presegmental structures. The microvilli of the ciliated cells form a cover over the cuticle with a presumed protective function. An extracellular space extends between cuticle and sensory epithelium. The distal dendrites of the sensory cells terminate in sensory bulbs, bearing one modified sensory cilium each that projects into the olfactory chamber, embedded within the secretion of the ciliated cells. During development, the nuchal organs increase in size. This is accompanied by a shift in position, an expansion of the sensory area, and secretory activity of the ciliated cells. The nuchal ganglion differentiates into three nuchal centres forming three distinct sensory areas around the ciliated region. Each nuchal complex reveals two short nuchal nerves comprising the sensory axons, which enter the posterior circumesophageal connective. The sensory cells lying in the brain exhibit neurosecretory activity; the sensory cilia enlarge their surface area by dilating and branching. Nuchal organs accomplish the basic structural adaptions of chemoreceptors and show structural analogies to arthropod olfactory sensilla; thus, there is every reason to suppose chemoreceptor function.     

Ultrastructure of the endolymphatic sac in the mouse.

The ultrastructure of the endolymphatic sac (ES) in the mouse was examined by light and electron microscopy. This organ was divided into three parts: proximal, intermediate and distal. In the proximal portion of the ES, the epithelium consisted of thin squamous cells. The epithelial cells had acquired basolateral processes, numerous small vesicles and well-developed Golgi apparatus. In the intermediate portion, the epithelium consisted of columnar or cuboidal cells. The epithelial cells could be classified into two types: type I and type II. The type I cells had abundant microvilli, pinocytotic vesicles, vacuoles, multivesicular bodies, lysosomes and mitochondria. The type II cells had fewer numbers of these organelles. A few free-floating cells could be observed in the lumen of this intermediate portion, most of which were macrophages. In the distal portion, the epithelium consisted of squamous or cuboidal cells. The epithelial cells had a few cytoplasmic organelles. In the ultrastructural study, each portion of the mouse ES was found to have a very distinct morphological feature. It was suggested that each of these three portions has a different function.     

Ultrastructure of the Epithelium and the Sensory Receptors in the Body Wall, the Proboscis and the Pharynx of Gyratrix hermaphroditus (Turbellaria, Rhabdocoela)

The epidermis of Gyratrix hermaphroditus can be described as semi-syn-cytial. Its ultrastructure is characterized by microvilli and cilia with two strong rootlets perpendicular to each other. The apical part of the epithelium contains mitochondria and vacuoles. The basal synthesizing layer is provided with cell boundaries, at least between the type II penetrating receptors in the anterior and posterior end of the worm. Four different types of sensory receptors are described. The type I receptor has a protruding cilium-bearing process and is found all over the body. The type II receptor is found in the anterior and posterior end and has a retracted process with a kinocilium surrounded by eight stereocilia. The type III receptor bears a balloon-shaped modified cilium and is located at the anterior end. The type IV receptor has a short cilium with an unstable ciliary membrane and occurs in the proboscis epithelium as well as in the pharynx epithelium. Phylogenetical aspects of the semi-syncytial epithelium and functional aspects of the sensory receptors are discussed.     

Lateral organs in sedentary polychaetes (Annelida) – Ultrastructure and phylogenetic significance of an insufficiently known sense organ

Lateral organs are sense organs visible as densely ciliated pits or papillae between the noto‐ and the neuropodia in certain taxa of sedentary polychaetes. Ultrastructural studies in about 10 species of the following taxa Maldanidae, Opheliidae, Orbiniidae, Paraonidae, Magelonidae, Spionidae, Poecilochaetidae and Terebellidae have been designed to evaluate whether these organs are homologous among polychaetes. In spite of great external diversity, the investigations revealed an overall ultrastructural similarity. Differences between species investigated mainly concern the size of the organs as well as the number and arrangement of cells. The organs comprise supportive cells and uniciliated penetrative sensory cells. Their dendrites are closely arranged and thus their cilia may resemble multiciliated cells. There are two types of sensory cells: one type possesses no or mainly thin microvilli of which usually only a few reach the cuticular surface, and in the other type the cilium is consistently surrounded by 10 strong microvilli, which form a pore‐like opening in the cuticle. Further differences occur in the structure of the rootlet system. Basally, a retractor muscle attaches to the organ. The systematic significance of these organs within Annelida is discussed with respect to the conflicting phylogenetic hypotheses explaining the relationships of annelid taxa.     

Ultrastructure of the nuchal organs in the polychaete Platynereis dumerilii (Annelida,Nereididae)

Abstract. We examined the nuchal organs of adults of the nereidid polychaete Platynereis dumerilii by means of scanning and transmission electron microscopy. The most prominent features of the nuchal organs are paired ciliary bands located dorsolaterally at the posterior margin of the prostomium. They are composed of primary sensory cells and multiciliated supporting cells, both covered by a thin cuticle. The supporting cells have motile cilia that penetrate the cuticle and are responsible for the movement of water. Subapically, they have a narrowed neck region; the spaces between the neck regions of these supporting cells comprise the olfactory chamber. The dendrites of the sensory cells give rise to a single modified cilium that crosses the olfactory chamber; numerous thin microvillus-like processes, presumably extending from the sensory cells, also traverse the olfactory chamber. At the periphery of the ciliated epithelium runs a large nervous process between the ciliated supporting cells. It consists of smaller bundles of sensory dendrites that unite to form the nuchal nerve, which leaves the ciliated epithelium basally and runs toward the posterior part of the brain, where the perikarya of the sensory cells are located in clusters. The ciliated epithelium of the nuchal organs is surrounded by non-ciliated, peripheral epidermal cells. Those immediately adjacent to the ciliated supporting cells have a granular cuticle; those further away have a smooth cuticle. The nuchal organs of epitokous individuals of P. dumerilii are similar to those described previously in other species of polychaetes and are a useful model for understanding the development of nuchal organs in polychaetes.     

Structure of the auditory system of the weta Hemideina crassidens (Blanchard, 1851) (Orthoptera,Ensifera, Gryllacridoidea,Stenopelmatidae)

Summary This study of the ultrastructure of the auditory sensilla of the New Zealand weta, Hemideina crassidens, is the first such study on a member of the orthopteran Superfamily Gryllacridoidea. Ultrastructure of the auditory sensilla is similar in all of the tibial mechanosensory organs, here called subgenual organ, intermediate organ and crista acoustica by analogy with comparable structures in Tettigoniidae.Distal to each sensory soma is a dendrite containing multiple ciliary rootlets that fuse into a single ciliary root. This splits into nine root processes that pass around the outside of the proximal basal body and then rejoin at the level of the distal basal body, distal to which the dendrite has a modified ciliary structure with a circlet of nine peripheral paired tubes and rods as it passes through the proximal extracellular space. It is then enclosed by a zone of scolopale cell cytoplasm before expanding into a dilatation within the distal extracellular space. In some sensilla this space is partially occluded by electron dense material which is part of the scolopale cell. Distal to the dilatation the cilium shrinks and ends surrounded by the scolopale cap.Accessory cells consist of glia enwrapping the sensory neuron in the region of its soma, the scolopale cell surrounding the ciliary portion of the dendrite, and the attachment cell surrounding the scolopale cell and scolopale cap and connected to them by desmosomes. The attachment cells are filled with microtubules in differing densities and orientations. Lamellae are present in the acellular matrix surrounding the attachment cells. Banded fibres, presumably of collagen, are also present in the matrix.     

Fine Structure of the Ampullary Organs of the Bichir Polypterus senegalus Cuvier, 1829 (Pisces: Brachiopterygii) With Some Notes on the Phylogenetic Development of Electroreceptors

The ampullary organs of the bichir were examined by light and electron microscopy. Unlike most other ampullary organs, they are exclusively found in the epidermis and are never sunk into the subepidermal connective tissue. The sensory epithelium consists of sensory cells and supporting cells surrounded by mantle cells. The luminal surface of the sensory cell is provided with a cilium surrounded by several microvilli. In the apical cytoplasm are found numerous mitochondria and microtubules. In the basal part of the cell synaptic sheets or synaptic bodies opposite to afferent nerve endings are frequent.     

Structure and development of the nephridia of Tomopteris helgolandica (Annelida)

 Nephridial diversity is high in Phyllodocida (Annelida) and ranges from protonephridia to metanephridia. The nephridia of Tomopteris helgolandica (Tomopteridae) can be characterized as metanephridia which bear a multiciliated solenocyte. This cell is medially apposed to the proximal part of the nephridial duct and bears several cilia, each of which is surrounded by a ring of 13 microvilli. An extracellular matrix connects the microvilli and thus leads to the impression of a tube surrounding the central cilium. Each tube separately enters a subjacent duct cell and the cilia extend into a cup-shaped compartment within the duct cell. This compartment is not connected to the duct. The funnel consists of eight multiciliated cells and is connected to the nephridial duct, which initially runs intercellularly and later percellularly. The last duct cell bears a neck-like process which pierces the subepidermal basal membrane and is connected to epidermal cells forming a small invagination, the nephropore. The nephridia of T. helgolandica develop from a band of cells and all structural components are differentiated at an early developmental stage. Further development is characterized by enlargment of the funnel, ciliogenesis in the solenocyte, merging of different sections of the duct and, finally, the formation of the nephropore. An evaluation of the nephridia of T. helgolandica leads to the hypothesis that the nephridial diversity in Phyllodocida can be explained by the retainment of different stages in the transition of protonephridia into metanephridia; this is caused by the formation of a ciliated funnel at different ontogenetic stages. Although the protonephridia in Phyllodocida are regarded as primary nephridial organs, protonephridia are also presumed to have evolved secondarily in progenetic interstitial species of the Annelida by an incomplete differentiation of the nephridial anlage. Accepted: 18 December 1996     

Morphological aspects of Culex quinquefasciatus salivary glands

The salivary glands of Culex quinquefasciatus female mosquitoes are paired organs composed of two lateral lobes with proximal and distal secretory portions, and a medial lobe. All portions comprise a simple epithelium that surrounds a salivary duct. In the apical portion of the medial lobe, non-secretory cells strongly resemble cells involved in ion and water transport. The general architecture of the secretory portions is similar between lobes. The appearance of the secretory material and the morphological aspect of the apical cell membrane are the most distinctive features among the three secretory portions. Cells in the lateral proximal lobe display thin membrane projections extending into a translucent and finely filamentous secretory product. At the lateral distal portion, the apical cell membrane forms an intricate meshwork that encloses a dark secretory product. Medial lobe secretory cells also contain secretory cavities surrounded by intracytoplasmic vesicles, all containing a very dark and uniform product. Scattered cells holding numerous vacuoles, some of them containing a small and electron-dense granule eccentrically located and resembling those of the diffuse endocrine system, are frequently observed in the periphery of all secretory portions. Immunofluorescence assays revealed that the distal portion of the lateral lobes contains apyrase, an enzyme putatively responsible for platelet aggregation inhibition, diffusely distributed in the cell cytoplasm.