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

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

Ultrastructural investigation of the nuchal organs of Pygospio elegans (Polychaeta)

Summary The differentiation of the dorsal organs as well as the structure of the nuchal organs and their relation to the central nervous system in adult Pygospio elegans were studied by electron microscopy and compared to the nuchal organs of the larvae. The nuchal organs are represented by paired ciliary bands on the dorsal side of the first setiger, delimiting a median caruncle that is completely filled with epidermal and nervous tissue. They are composed of ciliated supporting cells and bipolar primary sensory cells constituting the nuchal ganglia, which are integrated into the brain. Microvillus-like processes of the ciliated cells give rise to a secondary covering layer over the sensory epithelium. The size of the nuchal organs is a sexually dimorphic feature.Dorsal organ formation is concomitant with the onset of sexual maturation in the male sex only. They appear as metameric ciliary bands on the dorsal side of the anterior body region and consist of ciliated cells accompanied by lateral accumulations of tubular gland cells. In the gametogenic segments they are structurally associated with the male genital pores and may be involved in reproduction. The results refute previous theories that dorsal organs are sensory and have a common origin to nuchal organs.Abbreviations ac anterior commissure of the brain - ace anterior circumesophageal connective - bb basal body - bl basal lamina - c cuticle - ca caruncle - cc ciliated cell - ci sensory cilium - co microvillar cover - d septate desmosome - db dorsal blood vessel - dn dorsal nerve cord - ea efferent axons - ec epidermal cell - eg elementary granules - g Golgi complex - i filamentous inclusion - lm longitudinal muscles - ly lysosome - mc motile cilia - mv microvillus - n neuron - ng nuchal ganglion - nn nuchal nerve - nu nucleus - oc olfactory chamber - pa palp - pc posterior commissure of the brain - pce posterior circumesophageal connective - rer rough endoplasmic reticulum - sI setiger I - sb sensory bulb - sc sensory cell - sd sensory dendrite - ser smooth endoplasmic reticulum - tf tonofilament bundle - v clear vesicles - za zonula adherens     

Ultrastructure of the nuchal organ in the interstitial polychaete Stygocapitella subterranea (Parergodrilidae)

The nuchal organs of Stygocapitella subterranea are paired narrow pits. They are lined by unciliated cells at the opening and by ciliated cells at the basal parts. The primary sensory cells (6–8) are arranged in a single patch at the bottom of the nuchal pit. The nuclei of the sensory cells are located in the posterior portion of the brain. Their dendrites form the nuchal nerve which is sheathed by the ciliated cells. Each sensory cell bears up to 4 modified sensory cilia and several microvilli extending into the olfactory chamber. The sensory cilia show various patterns of axonemal organization and have no rootlets. The olfactory chamber is covered by a cuticular matrix. Another primary sensory cell lies at the opening of the nuchal pit. It bears cilia which penetrate the cuticle but are enveloped by the epicuticle. Retractor muscles insert caudally on the organ. The nuchal organ of S. subterranea shows similarities to those of opheliids but exhibits several features not to be found in other nuchal organs.     

Ultrastructure of nuchal organs in some marine polychaetes

Polychaetes normally possess one pair of nuchal organs at the posterior edge of the prostomium or peristomium. They have been regarded as chemosensory organs. The nuchal organs of four marine polychaete species with different habits were investigated by electron microscopy. Although the shapes of nuchal organs can vary greatly from simple ciliary bands (Scolelepis squamata, Spionidae) to retractile tongue-like, piston- or finger-shaped forms (Eteone longa, Anaitides mucosa, Phyllodocidae; Heteromastus filiformis, Capitellidae), the structural components, including the ciliated supporting cells, sensory cells, and nuchal epidermal cells, are essentially similar. The differences basically concern 1) the position of the sensory cells with relation to the ciliated supporting cells, 2) the location and structure of the nuchal nerve, and 3) the structure of the nuchal cuticle. The diverging nature of this modified cuticle is described and discussed in detail. Comparisons are made with the fine structure of nuchal organs of other polychaete species. Similarities of cellular components of nuchal organs are found not only in the four species studied here but also in all nuchal organs investigated so far. This is hypothesized to be due to the fact that the polychaete stem species already possessed nuchal organs with the respective cell types. Differences in the number and distribution of cellular components and in the overall shape of nuchal organs are thought to have evolved in correlation with the equipment of other cephalic appendages and with different habits and modes of nutrition.     

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

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

Ultrastructure of the extensively developed nuchal organs of Laonice bahusiensis (Annelida: Canalipalpata: Spionidae)

The nuchal organs of annelid Laonice bahusiensis (Spionidae) from northern Europe have been studied using scanning and transmission electron microscopy. L. bahusiensis is the first spionid species in which extensively developed, continuous nuchal organs are described. The nuchal organs of this genus are the longest known among polychaete annelids. They consist of paired double bands extending from the prostomium on a mid‐dorsal caruncle for about 24–30 setigers. Their microanatomy corresponds to the general structural plan of nuchal organs: there are ciliated supporting cells and bipolar sensory cells with sensory cilia traversing an olfactory chamber. The organs are overlaid by a secondary paving‐stone‐like cover and innervated by one pair of longitudinally elongated nuchal nerves. These findings clearly favor the hypothesis that the paired, extensively developed ciliated structures found in some Spionidae are homologous with the prostomial nuchal organs characteristic of polychaete annelids. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

Ultrastructural investigations on the cephalic and metameric nuchal organs of Spio cf. filicornis (Polychaeta, Spionidae)

The nuchal organs of Spio cf. filicornis from northern Europe have been studied by scanning and transmission electron microscopy. Spio cf. filicornis is the first species in which metameric nuchal organs are described. The nuchal organs consist of a distinct cephalic nuchal complex followed by metameric structures for a variable number of chaetigers. Their microanatomy corresponds to the general structural plan of nuchal organs: these are ciliated supporting cells and bipolar sensory cells with sensory cilia traversing an olfactory chamber. The organs are overlaid by a secondary paving-stone-like cover and innervated by longitudinally elongated paired nuchal nerves. The findings clearly favour the hypothesis that the paired metameric ciliated structures found in some Spionidae are in fact homologous with the prostomial nuchal organs characteristic of Polychaeta.     

Ultrastructural investigations on the nuchal organ of the protandric polychaete,Ophryotrocha puerilis (Polychaeta,Dorvilleidae)

Summary The nuchal organs of the protandric hermaphrodite Ophryotrocha puerilis were studied by electron microscopy. Ophryotrocha puerilis is the first species hitherto described which possesses four instead of two nuchal organs. These sensory structures are located as ciliary pits at the posterior margin of the prostomium. Histologically, the nuchal organs are composed of supporting cells with long motile cilia and bipolar sensory cells, the perikarya of which form four distinct nuchal ganglia adjoining the brain. These structural components are concentrically arranged around the central sensory area. This area is covered by a modified cuticle, whereas the cuticle above the peripheral region of the sense organ exhibits the appearance typical for polychaetes. Two types of vesicular material are produced in the basal supporting cells, a dense-cored one within the central supporting cells only and a clear irregular-shaped one in all of these cells. The first type is considered to be responsible for the formation of the modified cuticle. The significance of these most probably long-distance chemoreceptory organs and their possible role in reproductive behaviour is discussed.     

Ultrastructure of the nuchal organ and cerebral organ in Onchnesoma squamatum (Sipuncula, Phascolionidae)

 The ultrastructure of the nuchal organ and cerebral organ is described for the first time in a species of the Sipuncula, Onchnesoma squamatum. The nuchal organ is an unpaired structure lying outside and dorsal to the tentacular crown; furrows give the organ a paired appearance. The cerebral organ is an unciliated pad anterior to the nuchal organ. The nuchal organ consists of ciliated supporting cells, non-ciliated supporting cells and bipolar primary sensory cells. The cerebral organ is composed of unciliated supporting cells and numerous bipolar sensory cells. This clearly favours the hypothesis that this structure has a sensory function in adults rather than being a vestige of a larval organ. The sensory cells are similar in both organs and exhibit features indicative of chemoreception. Since the density of the sensory cells is low in the nuchal organ, an exclusively sensory function is questioned. There is some evidence that the two organs represent a functional unit. The present findings do not support the view that the nuchal organs of Sipuncula and ”Polychaeta” are homologous, but instead suggest that they are convergent structures. Accepted: 18 September 1996     

Ultrastructure of Nuchal Organs in Polychaetes (Annelida) — New Results and Review

Nuchal organs are epidermal sensory structures present in most polychaetes. They are situated at the posterior edge of the prostomium and may extend posteriorly onto the peristomium. Although there is considerable external variation, they all consist of ciliated supporting cells, bipolar primary sensory cells and retractor muscles. They are innervated directly from the brain by paired nerves. The sensory cells are usually monociliated; their sensory processes lie in subcuticular spaces, the olfactory chambers. Structural variability is to be observed in the location of the sensory cells, the course of the nuchal nerve, position of nuchal ganglia as well as in cytological features of sensory and supporting cells. These differences provide useful characters for phylogenetic considerations to establish supraspecific taxa within the phylogenetic system of the Annelida. Special emphasis is laid on the problem of whether the nuchal organs represent an autapomorphy of the Polychaeta or the Annelida and thus whether the lack of nuchal organs in Clitellata is primary or secondary. As is discussed, the probability of a loss of the nuchal organs in Clitellata is higher, which favours the second hypothesis: that nuchal organs are part of the ground pattern of the Annelida and very likely are an autapomorphy of this group.     

Histology and ultrastructure of the olfactory organ of the freshwater pulmonate Helisoma trivolvis

Summary The olfactory organ of Helisoma trivolvis is located on the surface of the body at the base of the cephalic tentacles. An evagination of skin, the olfactory plica, at the base of the tentacle extends over the olfactory organ dorsally. The epithelium of the olfactory organs contains unspecialized epithelial cells, ciliated epithelial cells, basal cells, mucous secretory cells, and sensory dendrites. The surface of the epithelium has a complex brush border of thick plasmatic processes, which branch to form several terminal microvillar twigs. Long slender cytoplasmic processes form a dense spongy layer among the plasmatic processes beneath the level of the terminal twigs. Bipolar primary sensory neurons clustered beneath the epithelium of the olfactory organ send dendrites through the epithelium to the free surface. Some sensory endings have a few short cilia, but most bear only microvilli. Cilia of sensory endings and epithelial cells extend beyond the brush border of the epithelium. Small axons arise from the perikarya of the sensory neurons and enter a branch of the olfactory nerve. HRP tracing indicates that the axons pass to the cerebral ganglion without interruption. Histochemical tests indicate that the sensory neurons are neither aminergic nor cholinergic.     

On the structure of the pulmonate osphradium

Summary The osphradium of Planorbarius consists of a blindly-ending ciliated canal, formed by an infolding of the mantle epithelium, and a basal ganglion of nerve cells which is comparable in complexity with ganglia of the central nervous system. The distribution of cell types in the osphradial epithelium is specialised so that three regions can be recognised; the ciliated, the secretory and the sensory regions. The basal sensory region of the canal epithelium consists of ciliated cells and is innervated by sensory neurones of the osphradial ganglion. The middle secretory region contains mainly of mucus-secreting cells and the epithelium adjacent to the osphradial aperture of ciliated cells and secretory cells of a second type. The sensory neurones of the osphradial ganglion are bipolar or of a modified monopolar type. Other monopolar neurones, similar to those common in the central nervous system are of non-sensory function. The osphradium of Paludina, although of typical prosobranch form, possesses ciliated pits similar to the single canal of Planorbarius, which may indicate a shared modality of receptor function. A definite function cannot be ascribed to the pulmonate osphradium based on morphological evidence alone.     

Sense organs in polychaetes (Annelida)

Polychaetes possess a wide range of sensory structures. These form sense organs of several kinds, including the appendages of the head region (palps, antennae, tentacular cirri), the appendages of the trunk region and pygidium (parapodial and pygidial cirri), the nuchal organs, the dorsal organs, the lateral organs, the eyes, the photoreceptor-like sense organs, the statocysts, various kinds of pharyngeal papillae as well as structurally peculiar sensory organs of still unknown function and the apical organs of trochophore larvae. Moreover, isolated or clustered sensory cells not obviously associated with other cell types are distributed all over the body. Whereas nuchal organs are typical for polychaetes and are lacking only in a few species, all other kinds of sensory organs are restricted to certain groups of taxa or species. Some have only been described in single species till now. Sensory cells are generally bipolar sensory cells and their cell bodies are either located peripherally within the epidermis or within the central nervous system. These sensory cells are usually ciliated and different types can be disinguished. Structure, function and phylogenetic importance of the sensory structures observed in polychaetes so far are reviewed. For evaluation of the relationships of the higher taxa in Annelida palps, nuchal organs and pigmented ocelli appear to be of special importance.     

Sensory innervation in the rim of the octopus sucker

Anatomical components of afferent innervation in the rim of the octopus sucker are described. In the sensory epithelium under the smooth cuticle two associated ciliated receptor cell-types (presumably chemosensitive) occur in clusters. A third ciliated receptor cell-type under the toothed cuticle may be a mechanoreceptor. A non-ciliated receptor cell-type of unknown function, under the toothed cuticle, is characterized by a microvillus-lined apical canal containing dense granular material. The axons of the latter two receptors go directly into large nerve tracts which nm through the infundibular muscle and on to the ganglion of the sucker. The axons of the first cell-types terminate on interneurons either in the base of the epithelium or below the epithelium. All the interneurons of the basal region of the epithelium migrate centripetally and develop into encapsulated interneurons. Within the epithelium, fine fibers provide collateral contact among cluster receptors. Collateral interaction among basal and encapsulated interneurons occur in the infundibular plexus. The microanatomy of the rim of the sucker suggests that chemosensory cues are funneled into the interneurons where they are concentrated into integrated signals, while other sensory input is probably sent directly to the ganglia of the sucker and/or arm.     

Olfactory metamorphosis in the coastal tailed frog Ascaphus truei (Amphibia,Anura, Leiopelmatidae)

The structure of the olfactory organ in larvae and adults of the basal anuran Ascaphus truei was examined using light micrography, electron micrography, and resin casts of the nasal cavity. The larval olfactory organ consists of nonsensory anterior and posterior nasal tubes connected to a large, main olfactory cavity containing olfactory epithelium; the vomeronasal organ is a ventrolateral diverticulum of this cavity. A small patch of olfactory epithelium (the “epithelial band”) also is present in the preoral buccal cavity, anterolateral to the choana. The main olfactory epithelium and epithelial band have both microvillar and ciliated receptor cells, and both microvillar and ciliated supporting cells. The epithelial band also contains secretory ciliated supporting cells. The vomeronasal epithelium contains only microvillar receptor cells. After metamorphosis, the adult olfactory organ is divided into the three typical anuran olfactory chambers: the principal, middle, and inferior cavities. The anterior part of the principal cavity contains a “larval type” epithelium that has both microvillar and ciliated receptor cells and both microvillar and ciliated supporting cells, whereas the posterior part is lined with an “adult‐type” epithelium that has only ciliated receptor cells and microvillar supporting cells. The middle cavity is nonsensory. The vomeronasal epithelium of the inferior cavity resembles that of larvae but is distinguished by a novel type of microvillar cell. The presence of two distinct types of olfactory epithelium in the principal cavity of adult A. truei is unique among previously described anuran olfactory organs. A comparative review suggests that the anterior olfactory epithelium is homologous with the “recessus olfactorius” of other anurans and with the accessory nasal cavity of pipids and functions to detect water‐borne odorants. J. Morphol. 2011. © 2011 Wiley Periodicals, Inc.     

Observations of serotonin and FMRFamide-like immunoreactivity in palp sensory structures and the anterior nervous system of spionid polychaetes

Evidence suggests that ciliated sensory structures on the feeding palps of spionid polychaetes may function as chemoreceptors to modulate deposit-feeding activity. To investigate the probable sensory nature of these ciliated cells, we used immunohistochemistry, epi-fluorescence, and confocal laser scanning microscopy to label and image sensory cells, nerves, and their organization relative to the anterior central nervous system in several spionid polychaete species. Antibodies directed against acetylated alphatubulin were used to label the nervous system and detail the innervation of palp sensory cells in all species. In addition, the distribution of serotonin (5-HT) and FMRFamide-like immunoreactivity was compared in the spionid polychaetes Dipolydora quadrilobata and Pygospio elegans. The distribution of serotonin immunoreactivity was also examined in the palps of Polydora cornuta and Streblospio benedicti. Serotonin immunoreactivity was concentrated in cells underlying the food groove of the palps, in the palp nerves, and in the cerebral ganglion. FMRFamide-like immunoreactivity was associated with the cerebral ganglia, nuchal organs and palp nerves, and also with the perikarya of ciliated sensory cells on the palps.     

Surface ultrastructure of the olfactory rosette of an air-breathing catfish,Heteropneustes fossilis (Bloch)

The surface architecture of the olfactory rosette ofHeteropneustes fossilis (Bloch) has been studied by scanning electron microscopy. The olfactory rosette is an oval structure composed of a number of lamellae arranged pinnately on a median raphe. The raphe is invested with epithelial cells and pits which represent goblet cell openings. On the basis of cellular characteristics and their distribution the lateral surface of each olfactory lamella is identified as sensory, ciliated non-sensory and non-ciliated non-sensory epithelium. The sensory epithelium is provided with receptor and supporting cells. The ciliated non-sensory epithelium is covered with dense cilia obscuring the presence of other cell types. The non-ciliated non-sensory epithelium is with many polygonal areas containing cells.     

Integument and epidermal sensory structures of Myzostoma cirriferum (Myzostomida)

Summary The fine structure of the integument of Myzostoma cirriferum is described with special attention to the integument sensory areas. Hypotheses about the function and a functional model of these are proposed. The integument consists of an external pseudostratified epithelium with cuticle (the epidermis) covering a parenchymo-muscular layer (the dermis). The dermis includes two types of cells: muscular fibers of the double obliquely striated type and parenchymal cells. Differences occur in the epidermis, which consists either of a large non-innervated myoepithelial area (viz. the regular epidermis). or of several rather localized sensory-secretory areas associated with discrete nerve proceses (viz. the sensory epidermis). The regular epidermis is made up of three types of cell: covering cells, ciliated cells and myoepithelial cells. The sensory epidermis shows small or marked structural variations from the regular epidermis. Small variations occur in the cirri, the buccal papilla, the body margin, the parapodia and the parapodial folds where nerve processes insinuate between epidermal cells. They are thought to be mechanoreceptor sites that could give information on the structural variations of the host's integument and participate in the recognition of individuals of the same species. The sensory epidermis differs markedly from the regular eidermis in the four pairs of lateral organs. Each lateral organ consists of a villous and ciliated dome-like central part, surrounded by a peripheral fold. The epidermis of the fold's inner part (viz. the part facing the central dome) is made up of secretory cells, while that of the fold's outer part is similar to the regular epidermis. The epidermis of the dome includes vacuolar cells, sensory cells and a different type of secretory cell. Lateral organs are presumed to be both chemoreceptors and mechanoreceptors. They could allow the myzostomids to recognize the host's integument and prevent them from shifting on the surrounding inhospitable substrate.     

鲻鱼嗅囊组织形态结构观察及功能探讨

实验用鱼为全长35.5~40.0 cm的野生鲻(Mugil cephalus),采用石蜡切片以及透射电镜技术对鲻的嗅囊以及嗅板细胞进行观察。结果表明:鲻的嗅觉器官由左右两个呈扁平椭球形嗅囊构成,分别由前后两个鼻孔与外界相通。嗅囊长径与眼径之比为0.80,长径与短径之比为2.09。嗅囊的嗅轴左右两边分别有垂直于嗅轴并向上倾斜排列整齐的18~25个披针形嗅板,只有初级嗅板未见次级嗅板。嗅板由中央髓和两侧的嗅上皮两部分构成,中央髓由疏松的结缔组织和毛细血管组成。嗅上皮又分为感觉区和非感觉区,感觉区位于嗅板的内侧,具有发达纤毛,呈连续分布状态,非感觉区位于嗅板边缘,细胞纤毛较少。通过光镜和电镜的综合研究结果显示嗅上皮细胞大致可分为5类:基细胞、支持细胞、纤毛非感觉细胞、纤毛感觉细胞和柱状细胞。文章讨论了鲻的感官活动类型。     

PACAP in developing sensory and peripheral organs of the zebrafish, Danio rerio

The anatomical distribution of PACAP-like immunoreactivity was investigated in sensory and peripheral organs of the zebrafish, Danio rerio, during the pharyngula, hatching and larval periods, by using indirect immunofluorescence methods. First PACAP-like immunoreactive (ir) elements appeared during the pharyngula period, at 24 hours post fertilization (hpf), within the most superficial layer of the retina and the dorsal aorta. At 48 hpf, additional ir cells were found in the olfactory placode and esophagus. At 72 hpf (hatching period), PACAP-like immunoreactivity was first detected in the ganglion cell layer of the retina, the otic sensory epithelium, pharyngeal arches, swim bladder and pancreatic progenitor cells. During day 5 of larval development, new groups of ir cells appeared in the liver, whereas no ir elements were observed in the olfactory placode. Subsequently, at day 13 of larval development, additional ir elements were found for the first time in some gut epithelial cells while those previously observed in the retina and otic sensory epithelium were absent. The transient expression of PACAP-like ir material in sensory organs suggests that the peptide could be implicated in neurotrophic activities and neurosensorial connections in the migration and/or differentiation processes. The appearance of PACAP-like ir elements in peripheral organs at different developmental stages, indicates that this peptide could be involved in the control of more specific functions as soon as these peripheral structures begin to operate.