Ultrastructure of the nerve cells and sensilla of Geocentrophora baltica (Platyhelminthes,Lecithoepitheliata) and the surface sensilla in the Geocentrophora group

Two types of nerve cell could be distinguished ultrastructurally in the central nervous system of Geocentrophora baltica (Prorhynchida, Lecithoepitheliata). Both show invaginations in the plasma membrane, but they differ in the character of the cytoplasm (light or densely stained) and the distribution of the neuronal vesicles (evenly or in groups). Different kinds of vesicles and neuronal release sites are observed. Special features of the synapses are pronounced local thickenings of the presynaptic membrane connected to paramembranous densities. In G. baltica and five endemic Geocentrophora spp. from Lake Baikal six types of surface sensillum were observed at the epidermal surface: 1. those with a long thin rootlet; 2. a short, balloon-shaped cilium with an aberrant axoneme and a reduced rootlet; 3. a rootlet branching into many striated bundles; 4. a thick rootlet; 5. a reduced rootlet and numerous neurotubules;and 6. collared sensilla each with one cilium in a deep pit surrounded by a collar of 11 to 12 microvilli. The variable number of microvilli in the collared sensillum is considered plesiomorphic relative to the stable number of eight microvilli known in sensilla of the Prolecithophora, Proseriata, and Rhabdocoela. The ultrastructure of the collar sensillum indicates that the Lecithoepitheliata is only distantly related to the Prolecithophora and higher turbellarians.     

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     

Ultrastructure of three types of sense receptors of larval Austramphilina elongata (Amphilinidea)

Ultrastructure of three types of sense receptors of larval Austramphilina elongata (Amphilinidea). International Journal for Parasitology16: 245–251. The ultrastructure of three sense receptors is described. The first is the ending of an anterior dendrite which penetrates deeply into the epidermis and is surrounded by an invagination into the epidermis of the tegument, basal lamina and a thick layer of underlying fibrous matrix, and contains electrondense collars, a basal body and a short ciliary rootlet; it does not reach the surface and is without a cilium. The second is a dendrite which penetrates through the tegument and epidermis and does not contain electron-dense collars, a basal body or a cilium; it forms a free ending on the surface. The third is a nerve ending in the epidermis without electron-dense collars or a basal body, with microvilli-like structures (rhabdomere) and flanked by a densely granular (pigment?) body on one side; it is interpreted as a simple photoreceptor. Diagrams of six sense receptors previously described are given, and the distribution of the receptors in the larva is discussed.     

Sensory and secretory cells ofOphryotrocha puerilis (Polychaeta)

Summary As revealed by glyoxylic acid induced fluorescence, the protandric polychaeteOphryotrocha puerilis possesses different types of catecholaminergic primary bipolar sensory cells, the perikarya of which are located beneath the epidermis. About 20 of such receptors are situated in each segment but they are mostly found on antennae, palps, urites and parapodial cirri. The dendrites of these sensory neurones run to the cuticle and dilate to form receptive endings. Three different types of dendritic endings could be distinguished: (1) multiciliary receptors with 4–8 cilia and ciliary rootlets, (2) monociliary receptors with microvilli arranged like a funnel and electron-dense cuffs and (3) monociliary receptors of the collar-type with, constantly, ten microvilli surrounding one single central cilium. The latter type is also characterized by rootlet fragments. Dendrites and dilated receptive endings of all three types contain clear (putative secretory) vesicles, multivesicular bodies and mitochondria. Pharmacological treatment (dopamine, reserpine) does not affect the number of secretory vesicles of the receptor neurones. Extra vesicular storage of catecholamines is discussed. Secretory cells of unknown function containing large numbers of electron-dense vesicles are usually found in close association with sensory cells.Abbreviations CA catecholamines - DA dopamine - RE reserpine     

Monociliary receptors in interstitial Proseriata and Neorhabdocoela (Turbellaria Neoophora)

Summary The ultrastructure of monociliary receptors in 10 species of the Proseriata and Neorhabdocoela is described, with particular reference to the epidermal dendritic part.Sensory cells with a single kinocilium situated at the level of the distal epidermis membrane are considered as mechano- or chemoreceptors.There exist sensory cells with a dendrite penetrating one epidermis cell and bearing an embedded kinocilium and a collar of 8 stereocilia or ridges with a fribrillose substructure. These collared receptors probably function as mechanoreceptors.In comparison with collared sensory cells in species of other turbellarian orders, the embedded receptors in the Proseriata and Neorhabdocoela are more advanced and possess synapomorphous characteristics. With the embedded receptors a new evidence is given for the close phylogenetic relationship between the Proseriata and Neorhabdocoela.The distribution of collared cells in the animal system and their phylogenetic implication for a choanoflagellate origin of the Metazoa are briefly discussed.List of abbreviations ar annular rootlet - bm basement membrane - cb crystalline body - cc collar cell - cw cell web - cwt cell web-thickening - d dendrite - kc kinocilium - lm longitudinal musculature - mv microvilli - n nerve - nt neurotubuli - pb parenchymal branches - r rootlet - rd ridges - rh rhabdite - rm ring musculature - sc stereocilia - sd septate desmosomes - tm transversal musculature - u ultrarhabdites - za zonula adhaerens     

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.     

Ultrastructure of multiciliated collar cells in the pilidium larva of Lineus bilineatus (Nemertini)

Summary The ultrastructure of the apical plate of the free-swimming pilidium larva of Lineus bilineatus (Renier 1804) is described with particular reference to the multiciliated collar cells. In the multiciliary collar cells there are several, up to 12, cilia surrounded by a collar of about 20 microvilli extending from the cells' apical surface. The cilia have the typical 9+2 axoneme arrangement and are equipped with striated caudal rootlets extending from the basal bodies. No accessary centriole or rostral rootlet were observed. Microvilli surrounding the cilia are joined in a cylindrical manner by a mucus-like substance to form a collar. In comparison with many sensory receptor cells built on a collar cell plan the multiciliary collar cells of the pilidium larva apical plate are rather simple and unspecialized. In other pilidium larvae monociliated collar cells are found in the apical plate. The possible function and phylogenetic implications of multiciliated collar cells in Nemertini are briefly discussed.List of Abbreviations a axoneme - b basal body - c cilia or flagella - d desmosome - G Golgi apparatus - m mitochondria - mf microfilaments - mu mucus - mv microvilli - n nucleus - nt neurotubules - pm plasma membrane - r rootlet - ri ribosomes - v secretory vesicles     

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.     

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.     

Sensory receptors of the rosette organ of Gyrocotyle rugosa

The fine structure of three sensory receptors of the rosette organ of Gyrocotyle rugosa, is described. The Type I sensory receptors, localised towards the edge of both upper and lower surfaces, are characterized by a long cilium embedded in a bulb containing two electron-dense collars and several mitochondria. The Type II sensory receptors, larger than Type I, are located on the upper surface of the rosette and have a long cilium and a ciliary rootlet. They also have two electron-dense collars and one or two mitochondria. The sensory cilia of both types are characterized by 9 + 2 axonemes. The Type III sensory receptors, localised on the under surface, lack a sensory cilium but have a ciliary rootlet and are enclosed in the tegument and musculature; there is a complicated three-dimensional spherical lattice of microfibrils associated with the rootlet. The sensory bulbs contain large numbers of membrane bound vesicles and neurotubules. A function is postulated for each of the three types of sensory receptors.     

Description and surface topography of the cercaria of Austrobilharzia sp. (Digenea: Schistosomatidae)

The cercaria of Austrobilharzia sp. from the marine prosobranch gastropod Planaxis sulcatus in Kuwait Bay is described. The surface microtopography and pattern of the tegumentary sensory receptors are examined using scanning electron microscopy. The general microtopography of the surface of the cercaria is similar to that previously observed in cercariae of mammalian schistosomes, although differences are recorded in the types, numbers and distribution of the sensory receptors. The study identified more than 13 types of receptors comprising aciliated, uniciliated and for the first time a multiciliated receptor in a strigeid cercaria. The ciliated receptor types differ in the cilium length and structure of the surrounding collar and tegumentary base. The receptor types are site specific: (1) the aciliated and pitlike on the anterior organ-neck region and ventral sucker; (2) the uniciliated with a long flexible cilium with or without collar or a tegumentary base on the body and tail; and (3) the uniciliated with a short rigid cilium and a robust collar and tegumentary base, and the multiciliated with 6 flexible cilia and a high cylindrical collar on the anterior organ tip. The reported SEM information on the sensory receptors may contribute to elucidating their functional role and to establishing morphological characters for the phylogeny of the family Schistosomatidae.     

Ciliary receptors associated with the mouth and pharynx of Acoela (Acoelomorpha): a comparative ultrastructural study

The ultrastructure and distribution of receptor cells near the mouth and (where present) the pharynx of Hofstenia miamia, Proporus bermudensis, Conaperta thela, and Convoluta convoluta (Acoela) were investigated by transmission electron microscopy and confocal laser scanning microscopy of specimens stained with a fluorescence marker for actin. Five types of monociliary receptors were identified: (1) non‐collared receptors with a single long and narrow ciliary rootlet; (2) non‐collared receptors with a wide main ciliary rootlet and a smaller posterior rootlet; (3) non‐collared receptors with a single wide and hollow ciliary rootlet with a granulated core; (4) Collar (?) receptors with obliquely radial filament bundles in the cell apex and with a single hollow ciliary rootlet composed of numerous strand‐like elements; and (5) Collar receptors lacking a striated rootlet but with a granular body (swallow's nest rootlet). While H. miamia bears the first two receptor types, P. bermudensis has receptors of type 1, 3 and 5, and Cona. thela and Conv. convoluta have receptors of type 3, 4 and 5. The density of receptors is generally highest at the anterior body tip, regardless of where the mouth is located. Most receptor types occur scattered over the whole body but type 2 receptors of H. miamia are restricted to the pharynx and mouth region. The lack of a common receptor type specific for the mouth and pharynx of the investigated species points to an independent origin of the pharynges in Hofsteniidae and in Proporidae and of the mouth tube in Convolutidae. Moreover, the homology of the so‐called collar receptors in Acoela with typical collar receptors in other invertebrates is questioned.     

Ultrastructure of the sense receptors of adult Multicotyle purvisi (Trematoda, Aspidogastrea)

The following presumptive sense receptors of adult Multicotyle purvisi from the intestine of freshwater turtles in Malaya are described by transmission electron microscopy: disc-like receptor with many electron-dense collars and modified ciliary rootlet forming a 'disc'; non-ciliate receptor with long rootlet; non-ciliate receptor with branching rootlet and dense mass of irregularly arranged microtubules; non-ciliate receptor with rootlet fanning out from basal body, cross-striated in its upper and with electron-dense structures in its lower part; uniciliate receptor with thick layer of cytoplasm around axoneme; receptor with short cilium, at base of deep invagination of tegument; receptor with short cilium terminating in an electron-dense apical cap; and uniciliate receptor with long cilium. In addition, there may be a small non-ciliate receptor with a long ciliary rootlet at the base of the thick dorsal tegument, and uniciliate receptors differing from the uniciliate receptor with long cilium in the number of electron-dense collars and the length of the cilium and ciliary rootlet. Implications of the findings for the phylogeny of the parasitic Platyhelminthes and for evolutionary trends within that group arc discussed. The considerable degree of divergence of receptor types between the species of one family is attributed to the archaic nature of the group.     

Epidermis and sensory receptors of Temnocephala minor (Plathelminthes, Rhabdocoela, Temnocephalida): an electron microscopic study

 The epidermis, rhabditic glands and receptors of the commensal flatworm Temnocephala minor are described using electron microscopic techniques. The epidermis is syncytial and non-ciliated at the anterior body end; it bears folds, microvilli and other structures which differ according to the body side. The nuclei are located intraepithelially and distally from the basal membrane. Long cilia occur at the posterior end anteriorly from the sucker. All receptor structures described belong to a single morphological type and stand in groups arising from epidermal pits. On the tentacles these groups are regularly distributed. Each receptor has a single cilium and a long rootlet. More than 15 000 receptors of this type have been estimated to occur on the surface of a single medium-sized specimen of T. minor. Although the total number of receptor structures appears very high, the number of different receptor types is extremely low in comparison to other taxa of flatworms. Accepted: 8 July 1997     

Ultrastructure of the tube-foot of an ophiuroid echinoderm, Hemipholis elongata

The morphology of the tube-foot of the brittlestar, Hemipholis elongata was examined by transmission and scanning electron microscopy. The entire surface of the tubefoot is covered by microvilli and a thick cuticle layer. At the tip of the tube-foot the cuticle layer is thinner and sensory-secretory complexes are seen. These are composed of two secretary cells and a central ciliated sensory cell. Scanning electron microscopy shows that the cilium which extends from the receptor cell ends flush with the surrounding microvilli. These studies suggest that the tube-feet in ophiuroids are important structures in sensory-reception, gasexchange as well as locomotion. The hemoglobin containing cells within the lumen of the tubefeet may serve in oxygen transport and storage.     

A reexamination of the epithelial sensory cells of amphioxus (Branchiostoma)

The rostral epithelium of a newly metamorphosed juvenile of Branchiostoma floridae was examined at the EM level to confirm previous reports on its sensory cells. The majority of the sensory cells are of three types: two type I variants, with simple collars of unbranched microvilli surrounding their cilia, and one kind of type II cell, with an extended collar of repeatedly branched microvilli. The two type I variants differ in the structure and arrangement of the microvilli, basal body and rootlet, and the length of the cilium. Both variants are probably primary sensory cells (i.e. each has its own axon), but the data supporting this conclusion are much better for one variant than for the other. Type II cells are secondary sensory cells, with synaptic terminals borne on short extensions of the cell body. The presence of degenerating type II cells suggests that they may be subject to a regular process of loss and renewal. The results do not resolve the evolutionary issue of how amphioxus sensory cells relate to the epithelial sensory and receptor cells of vertebrates. Being primary, the type I cells resemble the supposed ancestral type more closely than do type II cells. Type II cells may be chemosensory, however, and should not be ruled out a priori as possible homologues of either primary or secondary chemosensory cells in vertebrates.     

Morphology of the proboscis of Hubrechtella Juliae (nemertea,pilidiophora): Implications for pilidiophoran monophyly

The proboscis of Hubrechtella juliae was examined using transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy to reveal more features of basal pilidiophoran nemerteans for morphological and phylogenetic analysis. The proboscis glandular epithelium consists of sensory cells and four types of gland cells (granular, bacillary, mucoid, and pseudocnidae‐containing cells) that are not associated with any glandular systems; rod‐shaped pseudocnidae are 15–25 μm in length; the central cilium of the sensory cells is enclosed by two rings of microvilli. The nervous plexus lies in the basal part of glandular epithelium and includes 26–33 (11–12 in juvenile) irregularly anastomosing nerve trunks. The proboscis musculature includes four layers: endothelial circular, inner diagonal, longitudinal, and outer diagonal; inner and outer diagonal muscles consist of noncrossing fibers; in juvenile specimen, the proboscis longitudinal musculature is divided into 7–8 bands. The endothelium consists of apically situated support cells with rudimentary cilia and subapical myocytes. Unique features of Hubrechtella's proboscis include: acentric filaments of the pseudocnidae; absence of tonofilament‐containing support cells; two rings of microvilli around the central cilium of sensory cells; the occurrence of subendothelial diagonal muscles and the lack of an outer diagonal musculature (both states were known only in Baseodiscus species). The significance of these characters for nemertean taxonomy and phylogeny is discussed. The proboscis musculature in H. juliae and most heteronemerteans is bilaterally arranged, which can be considered a possible synapomorphy of Hubrechtellidae + Heteronemertea (= Pilidiophora). J. Morphol. 274:1397–1414, 2013. © 2013 Wiley Periodicals, Inc.     

Occurrence and Ultrastructure of Collar Cells in the Stomach Gastrodermis of Polypodium hydriforme Ussov (Cnidaria)

The existence of collar cells lining the stomach gastrodermis in free-living Polypodium hydriforme and their ultrastructure are described. The collar cells are provided with a collar consisting of 9–10 microvilli which encircles a central flagellum and forms a flagellar pit. At the bottom of the pit around the basal part of the flagellum there is fine crystalline material which extends also in the spaces between the microvilli and keeps them straight. The flagellum has a typical axoneme (9+2), its basal body is located below the apical surface of the collar cell and continues into a striated rootlet. An accessory centriole is situated close to the upper part of the rootlet. The cell nucleus is located in the basal part of the cell. Prominent mitochondria with tubular cristae, Golgi cisternae and fragments of rough endoplasmic reticulum are situated mostly in the basal part of the cytoplasm. Discoidal vesicles are abundant in the apical cytoplasm. The collar cells are connected to each other by septate junctions and interdigitations. The ultrastructure of collar cells described here is discussed in comparison to that of other Cnidarians and in connection with the problem of Polypodium's systematic position.     

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.     

Ultrastructural studies of epidermis, sense receptors and sperm of Craspedella sp. and Didymorchis sp. (Platyhelminthes, Rhabdocoela)

Craspedella has a non-ciliated epidermis with nuclei located in the epidermis and with short microvilli. There is a thin basal lamina and thick underlying fibrous matrix. Rhabdites are secreted through ducts lined by microtubules. Multiciliate sense receptors consist of bundles of dendrites in a depression of the epidermis. Each dendrite has a cilium with a cross-striated rootlet; there are no electron-dense collars. Spermatozoa have peripheral microtubules which in cross-section are arranged in a ring-like or spiral fashion, numerous electron-dense granules, mitochondria and a nucleus; axonemes of the 9 +'1'type are free for most of their length. Centrioles occur in some nerve fibres. In Didymorchis parts of the epidermis are ciliated and epidermal perikarya are 'insunk', connected to the surface part of the epidermis by a single cytoplasmic process. Epidermal cilia have cross-striated vertical and horizontal rootlets. In the ciliary tips a short electron-dense rod along the central pair of tubules extends to the tip, where it widens to become a terminal plate; peripheral doublets gradually disappear by losing their microtubules. Receptors observed are uniciliate. Spermatozoa are as in Craspedella . Ultrastructural evidence indicates that Craspedella and Didymorchis arc closely related and belong to the Rhabdocoela.