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.     

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.     

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.     

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     

Ciliated sensory receptors of the unactivated metacestode of Hymenolepis microstoma

The fine structure of the ciliated sensory endings of the unactivated metacestode of Hymenolepis microstoma is described. The type I sensilla, localized in areas of the scolex and sucker tegument, are characterized by a long cilium (1.20 × 0.20 μ) embedded in a bulb containing two electron-dense collars and three to five mitochondria. The type II sensilla, located in tegumentary pits of the rostellum, are characterized by a short cilium (0.62 × 0.28 μ) possessing ciliary rootlets, embedded in a bulb containing a single electron-dense collar and one to three mitochondria. Both sensory cilia are characterized by a 9 + 6 + 1 microtubular substructure. The sensory bulbs contain large numbers of membrane bound electron-lucent vesicles; neurotubules are absent at this stage of development.     

Chaetotaxy and ultrastructure of sensory receptors in the cercaria of a species of Allassogonoporus Olivier, 1938 (Digenea:Lecithodendriidae)

A standard procedure that combines chaetotaxic, ultrastructural and neuromorphological observations has recently provided a new perspective to the study of cercarial sensory systems. In the present work, we aimed to extend the use of this combination of techniques to investigate the chaetotaxy of Allassogonoporus sp. in conjunction with the ultrastructure of sensory receptors and neuromorphology. Five nerve regions were distinguished. A conspicuous bilobed cerebral ganglion was observed at the level of the pharynx. The chaetotaxic pattern was generally consistent with that of other lecithodendriids. Four types of receptors were distinguished with scanning electron microscopy. These types differed in cilium length (short, moderately long or long) and tegumentary collar length (moderately low or high). Internal ultrastructure of receptor type IIAL revealed an unsheathed cilium, a closed basal body, septate extracellular junctional complexes and thickened nerve collars. Some receptor types were site-specific. Long uniciliated receptors were found mainly on the dorsal surface, whereas short uniciliated receptors were widespread across the tegument. Ultrastructure and site-specificity observations suggest that most sensory receptors are mechanoreceptors, probably reflecting the important role mechanoreception plays in host finding.     

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

Summary The ultrastructure of tegumental and subtegumental receptors in the larva of Austramphilina elongata is described. The receptors are terminal swellings of dendrites and contain numerous small vesicles and neurofilaments which are predominantly peripheral. Tegumental receptors, together with a sheath consisting of basal lamina and tegument, project into the epidermis, and cross-striated rootlets were sometimes found in them. Subtegumental receptors lie below the tegument and ciliary rootlets were never observed in them. Anterior dendrites contain single centrioles and clusters of centrioles. The possible function of receptors and centrioles is discussed.Abbreviations in figures bl basal lamina - c centriole - d dendrite - ep epidermis - m microvillus - nt neurotubules - r rootlet of cilium - re receptor - st subtegumental receptor - t tegument     

The ciliary rootlet maintains long-term stability of sensory cilia

The striated ciliary rootlet is a prominent cytoskeleton originating from basal bodies of ciliated cells. Although a familiar structure in cell biology, its function has remained unresolved. In this study, we carried out targeted disruption in mice of the gene for rootletin, a component of the rootlet. In the mutant, ciliated cells are devoid of rootlets. Phototransduction and ciliary beating in sensory and motile cilia initially exhibit no apparent functional deficits. However, photoreceptors degenerate over time, and mutant lungs appear prone to pathological changes consistent with insufficient mucociliary clearance. Further analyses revealed a striking fragility at the ciliary base in photoreceptors lacking rootlets. In vitro assays suggest that the rootlet is among the least dynamic of all cytoskeletons and interacts with actin filaments. Thus, a primary function of the rootlet is to provide structural support for the cilium. Inasmuch as photoreceptors elaborate an exceptionally enlarged sensory cilium, they are especially dependent on the rootlet for structural integrity and long-term survival.     

Topography and ultrastructure of the tegument of Lecithochirium musculus (digenea: Hemiuridae), a parasite of the European eel Anguilla anguilla (osteichthyes: Anguillidae)

The tegumental ultrastructure of the stomach fluke Lecithochirium musculus was studied using scanning and transmission electron microscopy. The surface of the tegument was smooth and covered by transverse cytoplasmic ridges. Cobblestone‐like units of the tegument were observed on the ventral surface. Invagination and evagination of the ecsoma induced variations in the tegumental surface. The ultrastructural study revealed that the tegument of L. musculus had a typical syncytial organization with a distal cytoplasm lying over a basal matrix and cytons. Two types of intra‐tegumental sensory structures were observed. Type 1 sensory receptor was a domed‐like fusiform structure consisting of a smooth elevation of the tegument. Four receptors of this type were observed on the anterior dorsal surface of the fluke. Three nerve bulbs filled with electron‐lucent material and mitochondria composed this receptor. Hemispherical electron‐dense collars were observed at the top of the nerve bulbs. Striated rootlets laid just beneath the hemispherical electron‐dense collars. Type 2 sensory receptor presented two morphological variations, i.e., a bulb‐like monolobed structure, and a bulb‐like bilobed structure observed at two different degrees of evagination. For both variations, the nerve bulb enclosed mitochondria, electron‐lucent material, and a conical electron‐dense collar from which extended a striated rootlet. Numerous sensory receptors of this type were observed around the ventral sucker. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

Fine structural observations on the ciliary receptors in the epidermis of three otoplanid species (Turbellaria proseriata).

In Notocaryoturbella bigermaria, Otoplana truncaspina and Paroto-planella heterorhabditica three types of epidermal receptors are recognized. Type I: with a single cilium running in a duct, piercing the distal dendrite process of the receptor. The internal wall of the dendrite process has eight ridges with longitudinal filaments lying inside them. The ciliary basal body lacks a longitudinal rootlet but is encircled by a thin annular formation. Type II: with a single (A) or several (B) cilia which protrude from the outer epithelial surface and are provided with a large and striped rootlet. Both types are considered as mechanoreceptors. Type III: with two or more short and stumpy cilia devoid of rootlets and displaying the usual 9 + 2 pattern in the proximal part only. They are considered as chemoreceptors.     

Adaptation of a ciliary basal apparatus to cell shape changes in a contractile epithelium

Cilia projecting from the surfaces of highly contractile myoepithelia in the sea anemone Metridium senile maintain their basal orientation, and their ability to propel water, at different states of mesentery contraction, despite substantial changes of myoepithelial cell diameter and length. The ciliary basal apparatus in each monociliated myoepithelial cell is structurally well adapted to provide a stable anchorage for the cilium whilst compensating for these shape changes. It is composed of a distal centriole (basal body), a proximal centriole, a striated rootlet 2-3 micron long which is composed of a bundle of 4-6 nm filaments, and an arched rootlet, also striated, which is composed of a relatively loose bundle of 9-11 nm filaments. A single basal foot projects from the side of the distal centriole in the same direction as the path of the cilium during an effective-stroke; its tip is a focus for many microtubules that radiate outward in all directions toward the cell membrane. The arched rootlet forms a single arch in the cell apex, also in the same plane as the path of the cilium during an effective-stroke. The central axis of the basal apparatus, that is through the distal centriole and the striated rootlet, passes through the apex of the arch. The arched rootlet is apparently flexible so that it can increase or decrease its span as the cell increases or decreases in diameter. In pharnyx and siphonoglyph cells from M. senile, which do not undergo great changes in diameter or length, there is no arched rootlet, and the striated rootlet is much longer. The broad structural diversity of the metazoan ciliary basal apparatus must to a large extent be related to the diversity of the structural and mechanical properties of the cells in which it occurs.     

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.     

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     

Ultrastructure of gill cilia and ciliary rootlets of Chaetoderma nitidulum Lovén 1844 (Mollusca, Chaetodermomorpha)

Cilia and associated structures on the gill lamellae on the ctenidum of Chaetoderma nitidulum were studied. The gill cilia are very long and have a whip-like narrow portion distally, where only three microtubule doublets continue to the distal tip. In the transition zone between the cilium and the centriolar triplet section of the basal body there is a dense plate, an aggregation of granules and a ciliary necklace with four strands. Further down there is a short cross-striated basal foot and two conical cross-striated ciliary rootlets. The first rootlet is flattened and directed forward. It connects distally with the basal feet of other adjacent cilia. The second rootlet is rounded in cross-section and vertically directed. The epithelial structures of Chaetoderma show similarities with other Mollusca. We found no structural characters that could support the current hypothesis of a close relationship of Xenoturbella to the Mollusca.     

Epidermal ciliary ultrastructure of adult and larval sipunculids (Sipunculida)

The ultrastructure of the ciliary apparatus of multiciliated epidermal cells in larval and adult sipunculids is described and the phylogenetic implications discussed. The pelagosphera of Apionsoma misakianum has a dense cover of epidermal cilia on the head region. The cilia have a long, narrow distal part and two long ciliary rootlets, one rostrally and one vertically orientated. The adult Phascolion strombus has cilia on the nuchal organ and on the oral side of the tentacles. These cilia have a narrow distal part as in the A. misakianum larva, but the ciliary rootlets have a different structure. The first rootlet on the anterior face of the basal body is very short and small. The second, vertically orientated rootlet is long and relatively thick. The two ciliary rootlets present in the larval A. misakianum are similar to the basal metazoan type of ciliary apparatus of epidermal multiciliated cells and thus likely represent the plesiomorphic state. The minute first rootlet in the adult P. strombus is viewed as a consequence of a secondary reduction. No possible synapomorphic character with the phylogenetically troublesome Xenoturbella was found.     

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.     

Sense receptors in Lobatostoma manteri (Trematoda, Aspidogastrea)

Sense receptors of the juvenile aspidogastrid Lobatostoma manteri infecting the digestive gland of the prosobranch snail Cerithium moniliferum were examined by transmission electron microscopy. At least eight types of presumptive sense receptors, some uniciliate and some non-ciliate, are described. Some of the receptors occur in thousands. This is the greatest variety of receptors ever described from a parasitic stage of a platyhelminth by electron microscopy.     

Ultrastructure and chaetotaxy of sensory receptors in the cercariae of a species of Crepidostomum Braun, 1900 and Bunodera Railliet, 1896 (Digenea: Allocreadiidae)

Previous investigations of cercarial sensory systems have focused on chaetotaxy and ultrastructure of sensory receptors and have revealed chaetotaxic patterns within families, genera, and species as well as different types of sensory receptors. However, chaetotaxic and ultrastructural observations have rarely been combined. We investigated the ultrastructure of cercarial sensory receptors in conjunction with the chaetotaxy and neuromorphology in 2 allocreadiid species belonging to the genera Crepidostomum and Bunodera. Cercariae were treated with acetylthiocholine iodide and silver nitrate, and for scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Similar cholinergic nerve networks were revealed. Chaetotaxy was consistent with that of other allocreadiids. Seven and 6 types of receptors were distinguished with SEM in Crepidostomum sp. and Bunodera sp., respectively. Types differed in number of cilia (1 or 2), cilium length (short, moderately long, or long), presence or absence of a tegumentary collar and a domelike base, and tegumentary collar length (low, moderately low, or high), TEM of some types revealed unsheathed cilia, basal body, and thickened nerve collars. Some receptor types were site specific. Thus, long uniciliated receptors were concentrated on the dorsal surface. Other types, such as short uniciliated receptors, were widespread throughout most regions. Ultrastructure and site-specificity observations suggest that most receptors are mechanoreceptors.     

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.     

Ultrastructure of the water-movement-sensitive sensilla in the medicinal leech

Behavioral and physiological experiments have shown that medicinal leeches are able to detect low amplitude surface waves, and further, that the transduction of this stimulus modality occurs primarily, if not exclusively, at the annular sensilla (Young, Dedwylder, and Friesen, 1981; Friesen, 1981). Here we examine the morphology of these specialized sensory structures using light, scanning electron, and transmission electron microscopes. We found that three types of ciliated sensory cells occur at the sensilla: (1) a uniciliate cell, with an axial cilium that projects at least 12 μm beyond the cuticle; (2) a multiciliate cell with from two to four grouped cilia that extend 1–3 μm beyond the cuticle; and (3) a second multiciliate cell, whose cilia project parallel to the body surface but remain within the cuticle. The cilia of all three cell types arise from the cuplike depressions which form the apices of slender, elongated cells (approximately 2 μm diameter × 50 μm length). A complexly interconnected ring of microvilli surrounds the cilium of the uniciliate cells. The morphology of the uniciliate cells closely resembles the structure of vibration-sensitive sensory neurons found in other species. We propose, based on previous results and our new findings, that the uniciliate receptor cells are the sensillar movement receptors which mediate leech sensitivity to water movements.