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.     

Ultrastructural study of the ciliated cyst in the human uterine tube epithelium

Ciliated cysts in the human uterine tube epithelium were investigated with the transmission electron microscope. The cysts were about 3-9 microns in diameter and were provided with many ciliary apparatuses and microvilli. Degenerative changes of these cilia, such as electron-dense round or irregular bodies and amorphous substance, were observed in many cysts, but complete disappearance of ciliary structures was not detected in any ciliated cysts. The ciliated cysts were mostly observed in basal cells and were occasionally found in ciliated cells bordering the tubal lumen. In the basal cells, these cysts distended with the increase in degenerated cilia. Distended ciliated-cyst-containing cells became exposed directly to the tubal lumen. U- or reverse omega-shaped deep indentations of the apical surface of ciliated cells confirmed the opening of ciliated cysts into the lumen. It was suggested that the ciliated cysts result from the premature differentiation of basal cells or disturbed migration of centrioles in ciliogenic cells.     

Identification and activity-dependent labeling of peripheral sensory structures on a spionid polychaete

In marine sedimentary habitats, chemoreception is thought to coordinate feeding in many deposit-feeding invertebrates such as polychaetes, snails, and clams. Relatively little is known, however, about the chemosensory structures and mechanism of signal transduction in deposit feeders. Using electron microscopy, confocal laser scanning microscopy (CLSM), and immunohistochemistry, we investigated the structure and function of putative chemosensory cells on the feeding appendages of a deposit-feeding polychaete species, Dipolydora quadrilobata. Tufts of putative sensory cilia were distributed over the prostomium and feeding palps and typically occurred next to pores. Examination of these regions with transmission electron microscopy revealed multiciliated cells with adjacent glandular cells beneath the pores. The sensory cells of prostomium and palps were similar, displaying an abundance of apical mitochondria and relatively short ciliary rootlets. Staining with antiserum against acetylated alpha-tubulin was examined by CLSM, and revealed axonal processes from putative sensory tufts on the palp surface to palp nerves, as well as many free nerve endings. Activity-dependent cell labeling experiments were used to test the sensitivity of putative sensory cells on the palps to an amino acid mixture that elicited feeding in previous behavioral experiments. In static exposures, the number of lateral and abfrontal cells labeled in response to the amino acid mixture was significantly greater than in the controls. Ultrastructural, positional, and now physiological evidence strongly suggests that spionid feeding palps are equipped with sensory cells, at least some of which function as chemoreceptors.     

Light and electron microscopic observations on ciliated vacuoles and cysts in the oviductal and endocervical epithelia of the rabbit

Ciliated vacuoles and intraepithelial cysts have been observed in oviductal and endocervical epithelia of rabbits. In this study, rabbits under various hormonal conditions were studied by light and transmission electron microscopy and tissue culture in an attempt to determine their distribution and origin. Ciliated vacuoles most frequently lay in the basal cytoplasm, below or beside the nucleus, and very close to the basal lamina. A few were apically located. Their average diameter was 8.8 by 5.1 microns. Cilia and microvilli projected into the vacuolar lumen. These vacuoles were located intracellularly as evidenced first by the degeneration of both their cilia and microvilli and the moderately dense matrix that often filled the vacuolar lumen, as observed by electron microscopy. Secondly, phase microscopy of the living endocervical epithelium allowed us to observe the beating of the cilia within the vacuoles, not on the surface of such cells. Thirdly, ruthenium red stained the surface glycocalyx of ciliated and secretory cells, but not that of the cilia and microvilli within the vacuoles. The intraepithelial cysts were not observed in all tissue blocks. The largest numbers were found in ovariectomized animals treated for 3 and 5 days with estradiol. More were seen in the isthmus and cervix than in the fimbria and ampulla. The cysts were located most often within the epithelium along the sides of, and at the bases of, the mucosal folds. They were lined by flattened epithelium of various combinations of secretory and ciliated cells. An unusual cell type was associated with some of the cysts and ciliated vacuoles. Its cytoplasm contained aggregates of mitochondria and vesicles whose contents varied in density. Although the genesis of the ciliated vacuoles is not certain, our results indicate that they may arise from aberrant positioning of proliferating procentrioles or from a defect in targeting or transporting the centrioles to the apical plasma membrane to serve as basal bodies. Fusion of adjacent ciliated vacuoles with lumina lined by secretory cells having deep apical invaginations appeared to contribute to the formation of cysts.     

Putative sensory structures in marine bryozoans

Abstract. SEM studies of 21 species of marine bryozoans demonstrated that the abfrontal side of the tentacles bears a row of mono- or multiciliated cells, which are presumably sensory. In stenolaemates, the abfrontal cells, as well as the cells at the tentacle tips and the laterofrontal cells, are monociliated. In the 17 gymnolaemate species studied, each tentacle tip bears at least 3 multiciliated cells, each with a tuft of 5–7 stiff cilia of various lengths. On the abfrontal tentacle surface, mono- and multiciliated cells alternate, but all species studied have multiciliated cells at the base and the tip of each tentacle. In live animals, single cilia perform occasional flicks, whereas the tufts of 7–15 cilia on the multiciliated cells are immotile. Length and number of abfrontal cilia vary between species. Two types of multiciliated, putative sensory organs were found on the introvert of some gymnolaemates. One has an apical knob surrounded by a ring of cilia; the other has an apical tuft of cilia. The ultrastructure of the sensory cells of tentacles and introvert was studied in Rhamphostomella ovata . Our observations on both fixed and living material all suggest that these cells are primitive mechanoreceptors. The few species lacking ciliary structures on the introvert have long proximal ciliary tufts on the abfrontal tentacle surface.     

Stereoscan studies of eggs, free-swimming and penetrating miracidia and early sporocysts of Fasciola hepatica.

The egg of Fasciola hepatica has a smooth surface with a slightly elevated circle marking the fracture of the operculum. The operculum and the aperture have crenated edges. The epithelial cells of the miracidium are covered with long cilia. When miracidia are vibrated in an ultrasonic cleaner the cilia of the epithelial cells of the four posteroir tiers are broken off only leaving longitudinal rows of cilium stubs, whereas the cilia of the first tier are still retained. The apical papilla is provided with a dorso-ventral furrow, multiciliated pits and isolated sensory cilia. The narrow intercellular ridge is smooth, whereas the epithelial cells have small cytoplasmic knobs between the cilia. The penetration into the snail (Lymnaea truncatula) and the transforamtion into sporocyst may be separated into three phases. (1) Less than 1 min after attachment to the snail the ciliated cells of the anterior tier are shed and swim away. (2) The cilia of the remaining cells beat violently and after about 5 min most cilia are broken off near the cell surface. The miracidium remains for about 15 min embedded as far as the intracellular ridge receptors (lateral papillae and sheathed ciliated nerve endings). During this period extensive contraction and relaxation of the body are performed. (3) The final penetration of the snail epithelium takes about 15 min. Simultaneously with the penetration into the snail tissue the "bald" cells (epithelial cells with cilium stubs only) of the four posterior tiers loosen, florm globules and fall off. The surface below the cells is smooth and in cytoplasmic continuity with the intercellular ridge and the apical papilla, and this syncytium forms the later tegument of the sporocyst. After a few days the tegument of the sporocyst is provided with microvillus-like projections and the apical papilla and sensory structures are lost.     

Fine Structure of the Female Intoshia variabili(Alexandrov & Sljusarev) (Mesozoa: Orthonectida)

The morphology and fine structure of female Intoshia variabili, new combination for Rhopalura variabiliAlexandrov & Sljusarev, 1992, were studied with transmission electron microscopy. The body surface is covered with a 3-layered cuticula, under which is a layer of ciliated + non-ciliated cells arranged in alternating rings around the body. Ciliated cells have lateral extensions that intercalate with the non-ciliated cells. The kinetosome of each cilium has two longitudinally oriented cross-striated rootlets. The outer surface of the ciliated cells is covered with small tubercles, and the cytoplasm of these cells contains granules, vacuoles, mitochondria, fibrillar structures and lamellary bodies. A band of dense fibrils passes through the upper part of each ring of cells, going from one cell junction to another, encircling the entire body. Between the layer of ciliated + non–ciliated cells and the oocytes, elongated contractile cells from 4–5 longitudinal columns and 1 ring, the latter at the level of ciliated rings 7–9. The contractile cells contain thick and thin longitudinally oriented fibrils. The oocytes contain a large nucleus, numerous mitochondria, electron–dense granules and 1–2 spherical structures. An anteriorly situated, ciliated goblet–like receptor, not described for any other orthonectids, consists of three closely apposed cells, the upper part of which contains densely packed cilia. The genital pore opens through a non–ciliated cell and is surrounded by several cells with granules.     

Ultrastructural Analysis of the Sensilla of Austroperipatus aequabilis Reid, 1996 (Onychophora,Peripatopsidae)

The head of Austroperipatus aequabilis bears five types of sensilla. which were examined by electron microscopy. They differ from each other in position, shape of outer sensory elements and cuticular socket structures. Thus, we distinguish sensilla with sensory hairs, sensilla with sensory bulbs, cone-shaped sensilla. sensilla with sensory bristles, and sensilla of the lips. They are composed of up to 15 cells, which can he separated into four cell types. The most frequent cell type is the bipolar receptor cell that occurs in all sensilla. The apical surface of this primary receptor cell is characterized by one or two partly branched cilia with a basal 9 × 2 + 0 pattern of microtubules. A modified bipolar receptor cell was found in all sensilla bearing a sensory peg except for the sensilla equipped with sensory bristles. The apical dendrite extends to a long pale process which exclusively contains mitochondria and single microtubules. In all sensilla examined in this study at least one supporting cell occurs which is characterized by parallel microvilli. An additional function of this cell type as a part of the stimulus-conducting system is possible. In the sensillum with a sensory bulb two kinds of supporting cells occur. A unique cell type with an upside down position has regularly been found in all sensilla bearing a sensory peg. Apart from the sensilla they also occur within the labial epidermis. Since most sensilla contain several different receptor cells, they can be considered as complex sense organs. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved     

The External Structure and Distribution of Sensilla in the Medicinal Leech

Abstract The body of Hirudo medicinalis consists of 32 segments. The quinquannulate midbody segments 3–18 bear 14 sensilla on the central (neural) annulus. Elsewhere segments are represented by fewer than five annuli but sensilla are retained on the neural annulus. From neural sensilla protrude S hairs (cilia from uniciliate cells) which are thought to detect water currents. Two categories of multiciliate cell, of unknown function, are also present within neural sensilla; (i) grouped cilia extending beyond the cuticle (G hairs), and (ii) cilia which radiate out beneath the cuticle. Studies of the entire external surface of leech body wall with a scanning electron microscope revealed the presence of large numbers of sensilla on every annulus including the neural annuli. These sensilla lack S hairs. Our results show: (i) annular sensilla have a significantly smaller surface area than neural sensilla (p < 0.001 Mann-Whitney test, 2 tailed), (ii) The position and number of the small sensilla varies from annulus to annulus, segment to segment and individual to individual, (iii) Significantly higher numbers of small sensilla were found where non-neural annuli remain single and have not undergone a further division during development as happens in the quinquannulate segments. The data suggest that small sensilla continue to be added during the adult life of the leech.     

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.     

Ultrastructural Observations on the Larva of Loxosoma pectinaricola Franzén (Entoprocta,Loxosomatidae)

The larva of Loxosoma pectinaricola Franzén has been studied using scanning and transmission electron microscopy. The embryo develops surrounded by an egg envelope attached to the brood chamber. The newly released larva measures about 100 μm in length and is characterized by a prominent apical organ, stalked vesicles, paired lateral sense organs and a prototroch. The apical organ consists of at least four cell types: (1, 2) two types of ciliated cells, (3) vacuolated cells and (4) myoepithelial cells. The apical organ and frontal ganglion are tightly juxtaposed in the upper tier of the episphere. The stalked vesicles each consisting of two cells are unique evaginations of the epidermis. There are about twenty stalked vesicles with a maximum diameter of about 20.0 μm. The ciliated, knob-shaped, paired lateral sense organs are situated fronto-laterally on the episphere. The prototroch is comprised of a row of contiguous prototroch cells each containing about eighteen long cilia. The apical organ, frontal ganglion and paired lateral sense organs are suggested to be sensory structures that play an important role in active locomotion, settlement site selection and metamorphosis.     

Cell shape, spatial patterns of cilia, and mucus-net construction in the ascidian endostyle

The endostyle of the ascidian Ciona intestinalis secretes a mucus-net composed of regularly spaced longitudinal and transverse mucoprotein filaments. It is a gutter-shaped organ composed of eight different longitudinally aligned epithelial zones numbered 1-8. Each zone, with the exception of zone 7 which is unciliatcd. has a characteristic cell shape and spatial pattern of cilia and microvilli. Zones 1.5 and 8 are composed of multiciliated cells, and zones 2,3,4 and 6 of monociliated cells. Cell apices in zones 2, 3 and 4 are rectangular in cross-section, and bear highly ordered rows of cilia. Spacings between cilia both within and between rows are different in each of these zones, but they are similar to interfilament spacings in the mucus-net. The basic structure of the mucus-net is probably secreted and constructed by secretory cells and cilia in zones 1-4. Further secretion may be added in zones 6 and 7 whilst cilia in zones 5, 6 and 8 propel the net out on to the inner surface of the pharynx where it acts as a food filter. The highly organized and structurally complex pattern of ciliated cpithelia in the aseidian endostyle is surprising when compared with the endostyle of the cephalochordate Branchiostoma lanceolatum, which is composed entirely of monociliated cells that differ very little in structure between epithelial zones.     

Frequency of occurrence and position of cilia in fibroblasts of the periodontal ligament of the mouse incisor

Solitary cilia occur in motile as well as in non-motile fibroblasts of the peridontal ligament. The cells which moved with the erupting incisor are bipolar and oriented with their long axes parallel to the tooth surface. In cross section these cells have a flattened appearance. Cilia are localized in close vicinity to the nuclear area and show a definite orientation with respect to the transverse cell axis. The frequency of occurrence of this organelle was estimated from the percentage of diplosomes containing a basal body. Analysis of the composition of the paired structures indicated that at least 70 per cent of the fibroblasts are ciliated. The frequency of cilia in motile fibroblasts does not differ from that in non-motile cells suggesting that the presence of this organelle is not directly associated with cell locomotion.     

Bitter substances suppress afferent responses to an appetitive mixture: Evidence for peripheral integration of chemosensory stimuli

The processes that lead from detection of chemicals, transduction, and coding with the appropriate message to initiate ingestion of a palatable meal or to reject a potentially noxious substance are poorly understood in vertebrates owing to the complex organization of the taste system. As a first step in elucidating the cellular basis of the behavioral differences elicited by appetitive stimuli and bitter compounds, we recorded from the afferent nerves conveying peripheral chemosensory information to the CNS in the head of the leech, Hirudo medicinalis. Superfusion of the chemosensory region of the lip of Hirudo with a mixture of NaCl (150 mM) and arginine (1 mM), an appetitive solution that elicits ingestion, increased the neuronal activity in the afferent cephalic nerves, for example (Zhang X, Wilson RJ, Li Y, Kleinhaus AL. 2000. Chemical and thermal stimuli have short‐lived effects on the Retzius cell in the medicinal leech. J Neurobiol 43:304–311.). In the present paper we show that superfusing the lip with quinine or denatonium reduced the basal neural activity in the afferents. Furthermore, these bitter substances in the appetitive solution counteracted the increased activity the appetitive solution evoked in the cephalic nerves. Thus, the neural activity evoked by the application of appetitive and aversive stimuli to the chemosensory area of the lip paralleled the opposite behavioral responses to the same chemicals. The results suggest that individual leech taste cells possess receptors for both types of stimuli. Therefore, the leech may be a good model system in which to study peripheral taste events in cells that may possess multiple receptors and transduction mechanisms that interact to integrate information. © 2001 John Wiley & Sons, Inc. J Neurobiol 49: 255–263, 2001     

Mutant sensory cilia in the nematode Caenorhabditis elegans

Eight classes of chemosensory neurons in C. elegans fill with fluorescein when living animals are placed in a dye solution. Fluorescein enters the neurons through their exposed sensory cilia. Mutations in 14 genes prevent dye uptake and disrupt chemosensory behaviors. Each of these genes affects the ultrastructure of the chemosensory cilia or their accessory cells. In each case, the cilia are shorter or less exposed than normal, suggesting that dye contact is the principal factor under selection. Ten genes affect many or all of the sensory cilia in the head. The daf-19 (m86) mutation eliminates all cilia, leaving only occasional centrioles in the dendrites. The cilia in che-13 (e1805), osm-1 (p808), osm-5 (p813), and osm-6 (p811) mutants have normal transition zones and severely shortened axonemes. Doublet-microtubules, attached to the membrane by Y links, assemble ectopically proximal to the cilia in these mutants. The amphid cilia in che-11 (e1810) are irregular in diameter and contain dark ground material in the middle of the axonemes. Certain mechanocilia are also affected. The amphid cilia in che-10 (e1809) apparently degenerate, leaving dendrites with bulb-shaped endings filled with dark ground material. The mechanocilia lack striated rootlets. Cilia defects have also been found in che-2, che-3, and daf-10 mutants. The osm-3 (p802) mutation specifically eliminates the distal segment of the amphid cilia. Mutations in three genes affect sensillar support cells. The che-12 (e1812) mutation eliminates matrix material normally secreted by the amphid sheath cell. The che-14 (e1960) mutation disrupts the joining of the amphid sheath and socket cells to form the receptor channel. A similar defect has been observed in daf-6 mutants. Four additional genes affect specific classes of ciliated sensory neurons. The mec-1 and mec-8 (e398) mutations disrupt the fasciculation of the amphid cilia. The cat-6 (e1861) mutation disrupts the tubular bodies of the CEP mechanocilia. A cryophilic thermotaxis mutant, ttx-1 (p767), lacks fingers on the AFD dendrite, suggesting this neuron is thermosensory.     

Chemical and thermal stimuli have short-lived effects on the retzius cell in the medicinal leech

During the appetitive phase of feeding, hungry leeches detect a prey by the integration of signals perceived by different sensory systems. Earlier reports suggested that chemical or thermal sensory stimulation of the lip was associated with increased afferent activity in cephalic nerves connecting the lip to the central nervous system. These authors further suggested that this activity was relayed to Retzius cells in segmental ganglia, which then released serotonin to initiate and control all aspects of feeding behavior. In this study, we show that chemosensory or thermal activation of the lip lasting for at least 5 min produces a distinct signal in the cephalic nerves consisting of action potentials of low amplitude. These small amplitude signals are clearly distinguishable from the large action potentials evoked by mechanosensory stimuli applied to the same area of the lip. Both types of sensory stimuli also evoke an increase in the firing frequency of the Retzius cells in segmental ganglia. However, the response recorded in the nerves and the Retzius cells during a maintained stimulus is not constant but decreases with an exponential time course. These results agree with our earlier observations on a semi-intact feeding preparation in which we showed that the firing frequency of the Retzius cell decreased as soon as the leech began to ingest its meal. Therefore, our data provide further evidence suggesting that it is unlikely that heat or chemical cues maintain the Retzius cell in an active state throughout the consummatory phase of feeding.     

Motile ciliogenesis and the mitotic prism

Motile cilia of epithelial multiciliated cells transport vital fluids along organ lumens to promote essential respiratory, reproductive and brain functions. Progenitors of multiciliated cells undergo massive and coordinated organelle remodelling during their differentiation for subsequent motile ciliogenesis. Defects in multiciliated cell differentiation lead to severe cilia‐related diseases by perturbing cilia‐based flows. Recent work designated the machinery of mitosis as the orchestrator of the orderly progression of differentiation associated with multiple motile cilia formation. By examining the events leading to motile ciliogenesis with a methodological prism of mitosis, we contextualise and discuss the recent findings to broaden the spectrum of questions related to the differentiation of mammalian multiciliated cells.     

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.     

Bitter substances suppress afferent responses to an appetitive mixture: evidence for peripheral integration of chemosensory stimuli.

The processes that lead from detection of chemicals, transduction, and coding with the appropriate message to initiate ingestion of a palatable meal or to reject a potentially noxious substance are poorly understood in vertebrates owing to the complex organization of the taste system. As a first step in elucidating the cellular basis of the behavioral differences elicited by appetitive stimuli and bitter compounds, we recorded from the afferent nerves conveying peripheral chemosensory information to the CNS in the head of the leech, Hirudo medicinalis. Superfusion of the chemosensory region of the lip of Hirudo with a mixture of NaCl (150 mM) and arginine (1 mM), an appetitive solution that elicits ingestion, increased the neuronal activity in the afferent cephalic nerves, for example (Zhang X, Wilson RJ, Li Y, Kleinhaus AL. 2000. Chemical and thermal stimuli have short-lived effects on the Retzius cell in the medicinal leech. J Neurobiol 43:304-311.). In the present paper we show that superfusing the lip with quinine or denatonium reduced the basal neural activity in the afferents. Furthermore, these bitter substances in the appetitive solution counteracted the increased activity the appetitive solution evoked in the cephalic nerves. Thus, the neural activity evoked by the application of appetitive and aversive stimuli to the chemosensory area of the lip paralleled the opposite behavioral responses to the same chemicals. The results suggest that individual leech taste cells possess receptors for both types of stimuli. Therefore, the leech may be a good model system in which to study peripheral taste events in cells that may possess multiple receptors and transduction mechanisms that interact to integrate information.