Fine structure of the eyes of Pseudoceros canadensis (Turbellaria,Polycladida)

Summary The cerebral and epidermal ocelli of the Müller's larva and the cerebral and tentacular eyes of the adult turbellarian Pseudoceros canadensis were studied by electron microscopy. The right cerebral ocellus of the larva consists of one cup-shaped pigmented cell and three sensory cells that bear microvilli. The left cerebral eye of the larva has the above named cells plus a sensory cell with many cilia. Evolutionary significance is attributed to the presence of both ciliary and microvillar photoreceptors in an eye of a flatworm. The one epidermal ocellus of the larva is composed of two cells: a cup-shaped pigmented one bearing flattened cilia, the presumed photoreceptors, and a cell above the cup that adds a few nonciliary lamellae to the stack of ciliary ones from the pigmented cell. The adult eyes contain only microvillar receptors; cilia were not observed.     

Evolution of cerebral vesicles and their sensory organs in an ascidian larva

Sorrentino M., Manni L., Lane N. J. and Burighel P. 2000. Evolution of cerebral vesicles and their sensory organs in an ascidian larva. —Acta Zoologica (Stockholm) 81 : 243–258 The ascidian larval nervous system consists of the brain (comprising the visceral ganglion and the sensory vesicle), and, continuous with it, a caudal nerve cord. In most species two organs, a statocyst and an ocellus with ciliary photoreceptors, are contained in the sensory vesicle. A third presumptive sensory organ was sometimes found in an ‘auxiliary’ ganglionic vesicle. The development and morphology of the sensory and auxiliary ganglionic vesicles in Botryllus schlosseri and their associated organs was studied. The sensory vesicle contains a unique organ, the photolith, responding to both gravity and light. It consists of a unicellular statocyst, in the form of an expanded pigment cup receiving six photoreceptor cell extensions. Presumptive mechano‐receptor cells (S1 cells), send ciliary and microvillar protrusions to contact the pigment cup. A second group of distinctive cells (S2), slightly dorsal to the S1 cells, have characteristic microvillar extensions, resembling photoreceptor. We concur with the idea that the photolith is new and derived from a primitive statocyst and the S2 cells are the remnant of a primitive ocellus. In the ganglionic vesicle some cells contain modified cilia and microvillar extensions, which resemble the photoreceptor endings of the photolith. Our results are discussed in the light of two possible scenarios regarding the evolution of the nervous system of protochordates.     

Fine structure of the cephalic sensory organ in veliger larvae of Littorina littorea, (L.) (Mesogastropoda,Littorinidae)

The cephalic sensory organ (CSO) in planktonic veliger larvae of Littorina littorea is situated dorsally between the velar lobes at the level of the shell aperture. It consists of ciliated primary sensory cells, adjacent accessory cells and supporting epithelial cells. Cell bodies of the ciliated cells originate in the cerebral commissure and their dendrites pass to the epidermis. The flask-shaped sensory cells are characterized by a deep invaginated lumen with modified cilia arising from the cell surface in the lumen. These cilia are presumed to be non-motile because they lack striated rootlets and show a modified microtubular pattern (6 + 2, 7 + 2 and 8 + 2). The adjacent accessory cells never possess an invaginated lumen; occasionally cilia and branched microvilli arise from the apical surface. These cells may be sensory, but there is no obvious direct connection with the nervous system. The supporting epithelial cells are part of the epidermis and flank the apical necks of the sensory and accessory cells. Morphological evidence suggests that the CSO may function in chemoreception related to substrate selection at settlement, feeding or other behaviour.     

Sensory endings of the ascidian static organ (Chordata,Ascidiacea)

Summary The ultrastructure of the static organ is examined in larvae of Diplosoma macdonaldi, a colonial ascidian, and Styela plicata, a solitary ascidian; the results are similar. As previous workers found, the cell body of a unicellular statocyte lies in the lumen of the sensory vesicle and contains the statolith. A narrow neck connects the cell body to an anchoring foot in the floor of the sensory vesicle. Two previously undescribed sensory endings project into the lumen just to the left of the statocyte, one anterior and one posterior to the neck. A network of fine processes from each ending contacts the statocyte body. It is proposed that movements of the statocyte cell body are detected by these endings. They arise from neurons in the ventral wall of the sensory vesicle that project axons to the visceral ganglion. The placement of the sensory endings may allow discrimination of the directon of statocyte deflection.Abbreviations ax axons - bb ciliary basal body - bl basal lamina - c cilium - cr striated ciliary rootlet - ec ependymal cells - en endoderm - h hemocoel - ly lysosome - mv microvilli - n neuron - nf neurofilaments - ns neck of the statocyte - sb statocyte cell body - sd sensory dendrite - sn sensory neuron - sp sensory processes - stf statocyte foot - svl sensory vesicle lumen - zo zonula occludens     

Primitive Coronet Cells in the Brain of Oikopleura (Appendicularia,Tunicata)

Abstract A peculiar cell type is described from the sensory vesicle of the brain of the appendicularian tunicate Oikopleura dioica. The cells carry globular, modified cilia and resemble in several other respects the coronet cells of Saccus vasculosus in many fishes. This finding, together with earlier reports of similar cells in the sensory vesicle of ascidian tadpoles, makes it probable that a primitive form of coronet cells is a regular constituent of the tunicate brain cavity. The existence of coronet cells in the tunicate line of evolution is of interest from a phylogenetical as well as functional point of view. The author proposes that the tunicate sensory vesicle corresponds to part of the craniate third ventricle and that the tunicate coronet cells are involved in the regulation of the ventricle fluid composition.     

Sensory Structures in Tadpole Larvae of the Ascidians Microcosmus exasperatus Heller and Herdmania momus (Savigny)

In this paper we describe the larval morphology of two species from the ascidian family Pyuridae, Microcosmus exasperatus and Herdmania momus, with special emphasis on components of the cerebral vesicle. Larvae have not previously been described for any species in the large genus Microcosmus. Besides a difference in size (larvae of H. momus are about 40% larger than those of M. exasperatus), larvae of the two species differ primarily in the number and arrangement of sensory structures. Both species possess a well-developed statocyte but only H. momus has an ocellus. The absence of an ocellus in M. exasperatus is unique among pyurid ascidians. An auxiliary vesicle was found situated on the left side of the cerebral vesicle in both species. However, unlike the larvae of H. momus and other pyurid species, there is no apparent communication between the auxiliary and cerebral vesicles of M. exasperatus. Epithelial cells in the auxiliary vesicles of both species carry modified cilia about 2 μm in diameter; auxiliary vesicles of H. momus also have simple cilia with axonemes in a 9 + 0 microtubule configuration. In H. momus the membranes of the epithelial cells are highly convoluted and extend into the lumen of the auxiliary vesicle. Morphological arrangements of auxiliary vesicles and globular cilia reported so far in ascidian tadpoles are contrasted and discussed.     

Cytoskeletal elements in arthropod sensilla and mammalian photoreceptors.

Ciliary receptor cells, typified by cilia or modified cilia, are very common in the animal kingdom. In addition to the cytoskeleton of their ciliary processes these receptors possess other specific prominent cytoskeletal elements. Two representative systems are presented: i) scolopidia, mechanosensitive sensilla of various arthropod species; and ii) photoreceptor cells of the retina of the bovine eye. Two cytoskeletal structures are characteristic for arthropod scolopidia: a scolopale typifies the innermost auxiliary cell, and long ciliary rootlets are extending well into the sensory cells. The latter element is also characteristic for the inner segment of the photoreceptor cells in bovine. The scolopale of scolopidia is mainly composed of actin filaments. In the absence of myosin, the uniform polarity of the actin filaments and their association with tropomyosin all indicate a stabilizing role of the filament bundles within the scolopale. This function and a certain elasticity of actin filament bundles may be important during stimulation of the sensilla. The ciliary rootlets of both systems originate at the basal bodies at the ciliary base of the sensory cells and project proximally. These rootlets are composed of longitudinally oriented, fine filaments forming a characteristic regular cross-striation. An alpha-actinin immunoreactivity was detected within the ciliary rootlets of scolopidia. In addition, antibodies to centrin react with the rootlets of both types of receptors. Since centrin is largely responsible for the contraction of the flagellar rootlets in green algae, contraction may also occur in the ciliary rootlets of insect sensilla and vertebrate photoreceptors. In both systems, contraction or relaxation of the ciliary rootlets could serve in sensory transduction or adaptation.     

Ultrastructure of epidermal eyespots of Microstomum lineare (Turbellaria,macrostomida)

Summary The eyespots of Microstomum lineare were studied by electron microscopy, light microscopy, and fluorescence microscopy. Each eyespot consists of two ciliary photoreceptor cells shielded by pigment cells and additional sensory cells. The photoreceptor cells are characterized by a distal intracellular cavity lined with 50–100 interwoven cilia. The other sensory cells are of two ultrastructurally different types, one with long cilia predominating and the other with balloonlike cilia. The pigment cells, which envelop processes of the sensory cells, contain pigment vacuoles varying in size and content and give a bright red fluorescence by the Falck-Hillarp method. The eyespots are suggested to perform a dual function as photoreceptors and chemoreceptors. The evolutionary significance of ciliary photoreceptors in Turbellaria is discussed.     

The role of hair cells, cilia and ciliary motility in otolith formation in the zebrafish otic vesicle

Otoliths are biomineralised structures required for the sensation of gravity, linear acceleration and sound in the zebrafish ear. Otolith precursor particles, initially distributed throughout the otic vesicle lumen, become tethered to the tips of hair cell kinocilia (tether cilia) at the otic vesicle poles, forming two otoliths. We have used high-speed video microscopy to investigate the role of cilia and ciliary motility in otolith formation. In wild-type ears, groups of motile cilia are present at the otic vesicle poles, surrounding the immotile tether cilia. A few motile cilia are also found on the medial wall, but most cilia (92-98%) in the otic vesicle are immotile. In mutants with defective cilia (iguana) or ciliary motility (lrrc50), otoliths are frequently ectopic, untethered or fused. Nevertheless, neither cilia nor ciliary motility are absolutely required for otolith tethering: a mutant that lacks cilia completely (MZovl) is still capable of tethering otoliths at the otic vesicle poles. In embryos with attenuated Notch signalling [mindbomb mutant or Su(H) morphant], supernumerary hair cells develop and otolith precursor particles bind to the tips of all kinocilia, or bind directly to the hair cells' apical surface if cilia are absent [MZovl injected with a Su(H)1+2 morpholino]. However, if the first hair cells are missing (atoh1b morphant), otolith formation is severely disrupted and delayed. Our data support a model in which hair cells produce an otolith precursor-binding factor, normally localised to tether cell kinocilia. We also show that embryonic movement plays a minor role in the formation of normal otoliths.     

The infundibular balance organ in amphioxus larvae and related aspects of cerebral vesicle organization

Serial EM reconstructions were used to examine the organization and constituent cell types of the infundibular region of the cerebral vesicle (c.v.) in a 12.5-day larva of Branchiostoma floridae . The balance organ lies just in front of the infundibular cells and consists of 10 electron-dense cells with long, bulbous cilia, each surrounded by a ring of accessory cells. The ciliary bulb cells have axons that terminate in vesicle-filled swellings that lack identifiable synapses. The accessory cells have short basal processes that are minor contributors to the adjacent neuropile. Based on morphology, we suggest a mechanosensory function for the ciliary bulb cells, possibly related to balance or motion detection. Scattered cells of similar type are found elsewhere in the cerebral vesicle, along with a variety of other neurones with caudally projecting axons and varicosities, but few synapses. Instead, nonsynaptic, paracrine secretion appears to be the predominant mode of transmitter release in the neuropile and ventral tracts of the cerebral vesicle. The closest vertebrate homologue of this part of the amphioxus brain is arguably the limbic core of the caudal diencephalon and mesencephalon, including the homeostatic control centres of the hypothalamus. We postulate that this limbic core is an ancient structure traceable at least as far back in evolution as the common ancestor of amphioxus and vertebrates.     

Intraflagellar transport proteins in ciliogenesis of photoreceptor cells

Background information. The assembly and maintenance of cilia depend on IFT (intraflagellar transport) mediated by molecular motors and their interplay with IFT proteins. Here, we have analysed the involvement of IFT proteins in the ciliogenesis of mammalian photoreceptor cilia. Results. Electron microscopy revealed that ciliogenesis in mouse photoreceptor cells follows an intracellular ciliogenesis pathway, divided into six distinct stages. The first stages are characterized by electron‐dense centriolar satellites and a ciliary vesicle, whereas the formations of the ciliary shaft and the light‐sensitive outer segment discs are features of the later stages. IFT proteins were associated with ciliary apparatus during all stages of photoreceptor cell development. Conclusions. Our data conclusively provide evidence for the participation of IFT proteins in photoreceptor cell ciliogenesis, including the formation of the ciliary vesicle and the elongation of the primary cilium. In advanced stages of ciliogenesis the ciliary localization of IFT proteins indicates a role in IFT as is seen in mature cilia. A prominent accumulation of IFT proteins in the periciliary cytoplasm at the base of the cilia in these stages most probably resembles a reserve pool of IFT molecules for further delivery into the growing ciliary shaft and their subsequent function in IFT. Nevertheless, the cytoplasmic localization of IFT proteins in the absence of a ciliary shaft in early stages of ciliogenesis may indicate roles of IFT proteins beyond their well‐established function for IFT in mature cilia and flagella.     

Ultrastructural investigations of presumed photoreceptive organs in two Saccocirrus species (polychaeta,saccocirridae)

In addition to the pigmented ocelli, four different types of photoreceptor-like organs without shading pigment have been found in Saccocirrus papillocercus and S. krusadensis. The sensory cells of these presumed ocelli are either ciliary or rhabdomeric with ciliary rudiments. With the exception of the multicellular type-2 ocelli they are bicellular consisting of a sensory cell and a supportive cell. In each ocellus the supportive cell forms a thin cup-shaped envelope around the sensory elements. In the type-2 ocellus, 7 supportive cells form an ovoid cavity leaving openings through which dendritic processes of an equal number of sensory cells enter the cavity. The pigmented ocelli possess an ocellar cavity communicating with the exterior through a pore in the eyecup, ciliary rudiments in both sensory and supportive cell, and additional non-photoreceptive sensory cells in the opening of the eyecup. The sensory organs show characteristic differences between the two species, such as presence or absence of a particular type of ocellus (type 2 is absent in S. krusadensis, type 3 in S. papillocercus), number of cilia in type-4 ocelli, density of microvilli, number of non-photoreceptive sensory cells in the pore of the pigmented ocellus, etc. These differences provide important characters which can be used for discrimination either of species or of subgeneric taxa in Saccocirrus. The phylogenetic significance of the different photoreceptive organs is discussed.     

Ultrastructural investigations of presumed photoreceptors as a means of discrimination and identification of closely related species of the genus Microphthalmus (Polychaeta,Hesionidae)

Summary Differences in the ultrastructure of presumed photoreceptors of three morphologically similar Microphthalmus populations on the opposite sides of the Atlantic (German North Sea coast and coasts of North Carolina and Massachusetts) suggest the existence of three different species. Only the European M. listensis possesses three pairs of prostomial eyes, of which one pair has rhabdomeric receptors and pigment cells. The two other pairs are unpigmented and can be found in all three species. The frontal one has ciliary receptors, the posterior one rhabdomeric sensory cells. An additional unpaired potential photoreceptor organ in the segment with the first pair of tentacular cirri is present in all individuals of this species complex. It has a relatively high number of cilia with numerous microvillar projections. — For each type of ocellus there are slight but distinct and constant differences among the species such as relative position of sensory cells, presence of dilations of the ciliary shafts, number of cilia, and shape of the sensory cells. Presence of both ciliary and rhabdomeric light-sensitive cells is discussed with reference to various theories of the evolution of photoreceptors.Abbreviations ax axonema - bb basal body - cc cup cell - ci cilium - cu cuticle - epc epidermal cell - g Golgi apparatus - gp glycogen particles - mi mitochondrion - mv microvilli - mvb multivesicular body - nu nucleus - pc pigment cell - pg pigment granule - rer rough ER - smc submicrovillar cysternae - sr striated rootlet     

The Fine Structure of a Sensory Organ of a Cladocop Ostracode (Crustacea) Belonging to the Organ of Bellonci (Sensory Pore) Complex

Andersson, A. 1980. The fine structure of a sensory organ of a cladocop ostracode (Crustacea) belonging to the organ of Bellonci (sensory pore) complex. (Department of Zoology, University of Lund, Sweden.) — Acta zool. (Stockh.) 61(1): 51–58. The organ of Bellonci, a complex of cephalic receptors, has previously been reported from two ostracode groups. On morphologic grounds, a cephalic receptor of a third ostracode group (Cladocopa) is believed to be an organ of Bellonci. The organ is situated on the forehead above the first pair of antennae and consists of two feathered hairs. Two nerves, each formed by one dendrite, run from the protocerebrum into the hairs where they terminate with ramose cilia. The dendrites, as well as the cilia and ciliary branches, are enveloped by glial cells. Distally, these cells form cavities around the ciliary branches. The ciliated neuronal connection and the glial cavities, together with other morphologic characteristics of the organ, support a homologization with the organ of Bellonci of other myodocopid ostracodes.     

Ultrastructure of copulatory organs in Turbellaria

Summary The copulatory organs in Macrostomum sp. and Microstomum sp. contain simple tubular stylets which are intracellular specializations. The stylet in Macrostomum sp. is produced in a syncytium covering part of the prostatic vesicle. The proximal region of the stylet surrounds the vesicle which contains six prostatic gland ducts and six accessory (sensory) cells containing ciliary rootlets. The stylet in Microstomum sp. is produced in an extension of a syncytium which lines the combined seminal-prostatic vesicle. The stylet is connected to the combined vesicle by a narrow bridge of matrix syncytium through which sperm, prostatic gland products and sensory cilia pass from the vesicle to the stylet lumen. In both species the matrix syncytium can be interpreted as a specialized terminal end of the male canal epithelium. Stylets of Turbellaria and other lower Metazoa are discussed in regards to structure (one or several pieces) and location (in separate cells, in a syncytium, or extracellular).Abbreviations used in figures ac accessory cell - b basal body - c cilium - cv combined vesicle - d prostatic gland duct - dc degenerative cell - di dictyosome - e epidermis - ed ejaculatory duct - g prostatic gland cell - h hemidesmosome - i intercellular matrix - im internal muscle - in intestine; - l lumen of male canal - lm longitudinal muscle - m matrix syncytium - mc male canal epithelial cell - mi microfilaments - mt microtubules - mu muscle cell - mv microvilli - n nucleus - np nerve process - ns neurosecretory (?) granule - p prostatic vesicle - pv prostatic part of combined vesicle - r rootlet - s stylet - sm stylet material - sp sperm - sv seminal part of combined vesicle     

The proteome of rat olfactory sensory cilia

Olfactory sensory neurons expose to the inhaled air chemosensory cilia which bind odorants and operate as transduction organelles. Odorant receptors in the ciliary membrane activate a transduction cascade which uses cAMP and Ca²⁺ for sensory signaling in the ciliary lumen. Although the canonical transduction pathway is well established, molecular components for more complex aspects of sensory transduction, like adaptation, regulation, and termination of the receptor response have not been systematically identified. Moreover, open questions in olfactory physiology include how the cilia exchange solutes with the surrounding mucus, assemble their highly polarized set of proteins, and cope with noxious substances in the ambient air. A specific ciliary proteome would promote research efforts in all of these fields. We have improved a method to detach cilia from rat olfactory sensory neurons and have isolated a preparation specifically enriched in ciliary membrane proteins. Using LC‐ESI‐MS/MS analysis, we identified 377 proteins which constitute the olfactory cilia proteome. These proteins represent a comprehensive data set for olfactory research since more than 80% can be attributed to the characteristic functions of olfactory sensory neurons and their cilia: signal processing, protein targeting, neurogenesis, solute transport, and cytoprotection. Organellar proteomics thus yielded decisive information about the diverse physiological functions of a sensory organelle.     

Fine structure of the cephalic sensory organ in the larva of the nudibranch Rostanga pulchra (Mollusca,Opisthobranchia, Nudibranchia)

Summary The cephalic sensory organ in the veliger larva of Rostanga pulchra is situated dorsally between the rhinophores, emerging as a tuft of cilia. This organ is made up of three types of sensory cells, and based on their morphology have been termed ampullary, parampullary and ciliary tuft cells. The cell bodies of the organ originate in the cerebral commissure, and their dendrites pass to the epidermis as three tracts. Dendrites terminate in the epidermis to form a sectorial field. Axons of these cells run into the mass of neurites in the cerebral commissure but no synapses were observed in this area. Morphological evidence suggests that the cephalic sensory organ may function in chemoreception and mechanoreception related to substrate selection at settlement, feeding, or other behaviors.     

The fine structure of thermo-/hygrosensitive sensilla in the silkmoth Bombyx mori: Receptor membrane substructure and sensory cell contacts

Summary The thermo-/hygroreceptive sensilla styloconica of the silkmoth Bombyx mori are located on the tips of the antennal branches. A small poreless cuticular peg is innervated by three sensory cells. The outer dendritic segments of two type-1 receptor cells, the presumed hygroreceptors, almost completely fill the peg lumen and are in close contact with each other. The outer dendritic segment of the third (type-2) receptor cell, the presumed thermoreceptor, forms lamellae below the peg base. The membranes of these lamellae are studded with knobs in orthogonal array, protruding into the extracellular space with the same orientation on facing lamellae. This Bossy Orthogonal Surface Substructure (BOSS) is assumed to play a role in thermoreceptor function. Contacts are observed between the outer dendritic segments, between the inner dendritic segments immediately below the ciliary segments, and between the sensory cell somata. These contacts, which are not found in the olfactory sensilla (s. trichodea and basiconica) of this species, indicate electrical interactions between the three sensory cells of the styloconic sensillum and possibly are involved in the antagonistic and/or bimodal response characteristics of thermo-/hygroreceptor cells.     

Arf4 Is Required for Mammalian Development but Dispensable for Ciliary Assembly

The primary cilium is a sensory organelle, defects in which cause a wide range of human diseases including retinal degeneration, polycystic kidney disease and birth defects. The sensory functions of cilia require specific receptors to be targeted to the ciliary subdomain of the plasma membrane. Arf4 has been proposed to sort cargo destined for the cilium at the Golgi complex and deemed a key regulator of ciliary protein trafficking. In this work, we show that Arf4 binds to the ciliary targeting sequence (CTS) of fibrocystin. Knockdown of Arf4 indicates that it is not absolutely required for trafficking of the fibrocystin CTS to cilia as steady-state CTS levels are unaffected. However, we did observe a delay in delivery of newly synthesized CTS from the Golgi complex to the cilium when Arf4 was reduced. Arf4 mutant mice are embryonic lethal and die at mid-gestation shortly after node formation. Nodal cilia appeared normal and functioned properly to break left-right symmetry in Arf4 mutant embryos. At this stage of development Arf4 expression is highest in the visceral endoderm but we did not detect cilia on these cells. In the visceral endoderm, the lack of Arf4 caused defects in cell structure and apical protein localization. This work suggests that while Arf4 is not required for ciliary assembly, it is important for the efficient transport of fibrocystin to cilia, and also plays critical roles in non-ciliary processes.