On the structure of the pulmonate osphradium

Summary The cellular organisation of the osphradium of Planorbarius as revealed by the previously described light microscope studies is confirmed in the present study. A third epithelial cell type, the basal cell, is described. Perception is carried out by dendritic processes from osphradial sensory neurones, forming free nerve-endings in the sensory region of the osphradial epithelium. Ciliated cells and secretory cells of the osphradial epithelium are concerned with the transport of material in the osphradial canal. Many of the ultrastructural features of molluscan central ganglia are present in the osphradial ganglion, including inter-axonal chemical synapses. Neuromuscular junctions are present in the sheath surrounding the organ and these may be involved in a behavioural response of the organ. The need for further electrophysiological studies is emphasised.     

Immunolabeled neuroactive substances in the osphradium of the pond snail Lymnaea stagnalis

The osphradium of molluscs is assumed to be a sensory organ. The present investigation in Lymnaea stagnalis has established two ultrastructurally different types of dendrites in the sensory epithelium. Cells immunoreactive to leucine-enkephalin and FMRFamide send processes to the sensory epithelium. These neurons of the osphradial ganglion are thus considered to be part of the sensory system, as are methionine-enkephalin-immunoreactive cells in the mantle wall in the vicinity of the osphradium. The complexity of the osphradial ganglion is further demonstrated by serotonin-immunoreactive neurons innervating the muscular coat around the osphradial canal and methionine-enkephalin-immunoreactive cells sending projections to the central nervous system.     

Ultrastructure of the osphradium of Aplysia californica

Summary The osphradium of Aplysia californica, a sensory organ, is a small yellow-brown epithelial patch located in the mantle cavity immediately anterior to the rostral attachment of the gill. Scanning electron microscopy reveals a round ellipsoid structure of 0.6–1 mm in diameter with a central, occasionally folded, sensory epithelium. The central area is covered with microvilli and surrounded by a densely ciliated epithelium. Transmission electron micrographs show that the columnar supporting cells in the sensory epithelium contain an abundance of apical pigment granules and microvilli. Between the epithelial-supporting cells, the putative sensory elements consist of thin neurites (0.4–1.5 m in diameter) that reach the sea-water side of the osphradium. The neurites contain many neurotubules, mitochondria, vesicles and cilia in their apices. The nerve endings originate from cell bodies up to 40 m below the epithelium or in the osphradial ganglion itself, as revealed by electron microscopy and retrograde labeling with Lucifer yellow. There appear to be two populations of putative sensory cells, a large population of heavily stained cell bodies 4–10 m in diameter and a few scattered cells of large diameter (25–60 m). Following lanthanum impregnation, septate junctions can be seen between all types of cells in the epithelium, 3–5 m below the sea-water surface. This study provides new information for further investigation of osmo- and mechanosensation in Aplysia californica.     

Influence of HgCl2 on the osphradial multisensory system of Lymnaea stagnalis L

The osphradial multisensory system of Lymnaea stagnalis L. (Pulmonata, Basommatophora) was used to demonstrate the modulation of chemosensory information both at periphery and central nervous system (CNS) following heavy metal treatments. A semi-intact preparation including osphradium, CNS and the right inner parietal nerve (r.i.p.n.) connecting them was used to record both extracellular activity of nerve and intracellular activity of central neurons receiving information from osphradium. The ion currents of osphradium were recorded using patch-clamp method. The changes in nerve and neuronal activity were expressed by averaging of firing frequency and interspike intervals. The chemosensory function of osphradium was shown by application of L-aspartate, urea, saccharose and stagnant water to its surface. The central neurons reacting to the stimulation ofosphradium were located to visceral, right parietal, pedal and cerebral ganglia of Lymnaea. Both the acute and chronic treatments with HgCl2 damaged the sensory function of osphradium traced on the flow of information from periphery to central neurons. At the same time, mercury chloride modified the synaptic connections of respiratory pattern generators as well as the Ca- and K-dependent ion currents of osphradial neurons. The results proved the multisensory role of osphradium sensing the alterations in the environment and its usefulness in monitoring the effects of pollutants at various level of regulation from chemosensory epithelium to CNS.     

Ultrastructural investigation of the nuchal organs ofPygospio elegans (Polychaeta). I. Larval nuchal organs

The structural differentiation of the nuchal organs during the post-embryonic development ofPygospio elegans is described. The sensory organs are composed of two cell types: ciliated cells and bipolar primary sensory cells, constituting the nuchal ganglion, which is associated with both the sensory epithelium and the brain. Since the sensory neurons are largely integrated into posterolateral parts of the cerebral ganglion, the nuchal organs are primary presegmental structures. The microvilli of the ciliated cells form a cover over the cuticle with a presumed protective function. An extracellular space extends between cuticle and sensory epithelium. The distal dendrites of the sensory cells terminate in sensory bulbs, bearing one modified sensory cilium each that projects into the olfactory chamber, embedded within the secretion of the ciliated cells. During development, the nuchal organs increase in size. This is accompanied by a shift in position, an expansion of the sensory area, and secretory activity of the ciliated cells. The nuchal ganglion differentiates into three nuchal centres forming three distinct sensory areas around the ciliated region. Each nuchal complex reveals two short nuchal nerves comprising the sensory axons, which enter the posterior circumesophageal connective. The sensory cells lying in the brain exhibit neurosecretory activity; the sensory cilia enlarge their surface area by dilating and branching. Nuchal organs accomplish the basic structural adaptions of chemoreceptors and show structural analogies to arthropod olfactory sensilla; thus, there is every reason to suppose chemoreceptor function.     

Ultrastructure of the osphradium of Siphonaria grisea (Mollusca, Pulmonata)

By means of scanning and transmissive electron microscopic methods osphradium of Siphonaria grisea has been studied. The osphradium of the animal is presented as a small torulus formed by supporting ciliated cells. Among them bodies of receptory cells are situated; they are of smaller size and decorated with a bundle of microvilli. Central processes of the receptory cells, penetrating through a thick layer of the connective tissue, reach the subepithelial neural trunk. Concentration of the neural cells in the periphery of the trunk is small, and in the central area no chemical synapses are revealed. The data presented demonstrate a primitive structure of the osphradial chemoreceptory organ in Siphonaria.     

The fine structure of the newly discovered propodial ganglia of the veliger larva of the nudibranch Onchidoris bilamellata

Summary Two pairs of ganglia are found in the propodial region of the veliger of Onchidoris bilamellata: the anterolateral pair is located at the foremost corners of the propodium, and the frontal pair is located beside the propodial midline. Both sets of ganglia are positioned below the epidermis, and they are joined to the cerebral ganglia by large, common connectives. Each ganglion possesses sensory cells, nerve cells and sheath cells, and the frontal pair contains a complement of secretory cells. Externally, the propodial ganglia are manifested as sensory fields. The fields of the anterolateral pair are elliptical in shape, and each appears as a band of cilia bordering an unciliated zone. The region devoid of cilia is composed of ordinary epidermal cells, whereas the ciliated portion is comprised of dendritic endings originating from cells in the ganglion. Dendrites arise from one type of sensory cell and pass through the epidermis in bundles. Each dendrite terminates as a single cilium at the epidermal surface. Sensory fields of the frontal ganglia are key-shaped and oppose one another on the anterior end of the foot. Each field appears as a flat, circular, unciliated region which extends into a ciliated groove that runs dorsally toward the mouth. The groove contains the terminals of secretory cells, ciliated sensory cells, and the cell bodies of nonciliated sensory cells. The nonciliated sensory cells, characterized by a microvillous apex, are the dominant cells in the flattened circular zone. The space between the frontal ganglia and the epidermis is bridged by bundles of processes which are similar to those of the anterolateral ganglia. However, these tracts contain collections of the apical processes of secretory cells, the dendrites of ciliated sensory cells, and the axons of nonciliated sensory cells. Morphological and behavioral evidence indicates that the propodial ganglia serve a chemosensory function during settlement and metamorphosis.     

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.     

Functional morphology of the stone canal in the sea urchin Eucidaris (Echinodermata: Echinoidea)

The lack of mesocoelomic pores and the existence of a stone canal connecting proto-and mesocoel are characteristic peculiarities of echinoderms in contrast with the situation in other trimeric archicoelomates. The ultrastructure of the stone canal has been studied in order to understand its function in the strategically important position between two coelomic spaces with different functions. The epithelium of the Eucidaris (Echinoidea) stone canal is composed of three cell types: (1) ciliated cells, (2) cells with long basal processes containing myofilaments, and (3) granulated cells, which may represent secretory neurons. Nerve fibres of two types are common in the epithelium. We consider the stone canal to be a structure controlling fluid transport; its wall may exert peristaltic movements or tonic contractions and dilations which are under control of the nervous system. The ciliated cells additionally may have phagocytotic capacities. Similarities with the fine structure of the wall of the mesocoelomic pores in the pterobranch Cephalodiscus are discussed.     

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.     

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.     

Electron microscopic study of intestinal epithelium of <Emphasis Type="Italic">Saccoglossus mereschkowskii</Emphasis> (Enteropneusta,Hemichordata)

The epithelium of the hepatic region of the intestine in Saccoglossus mereschkowskii, a representative of enteropneusts (Enteropneusta, Hemichordata), a group located at the base of Chordata, has been studied by using electron microscopy. The ultrastructure of ciliated and granular epithelial cells, elements of the intraepithelial nerve layer, and intercellular junctions are characterized. The data on the details of the structure of the ciliary apparatus and the system of ciliary rootlets are presented. Justification is provided for the presence of a complicated construction in the ciliated cells, a supportive carcass of cilia that performs a mechanical stabilizing function, and possibly the synchronization of the ciliary movement. The existence of cilia with two centrioles is considered as adaptation to the high functional load on the ciliary apparatus. Well-developed bundles of myofilaments have been revealed in the cytoplasm of the basal parts of ciliated cells, which characterizes these cells as epitheliomuscular. Peculiarities indicating the role of ciliated cells in absorption are described, as well as the capability of these cells for balloon-like secretion. Data are presented on the accumulation of reserved nutritional substances in the cell cytoplasm in the form of lipids and glycogen. With respect to their function, ciliated cells are determined as the ciliated secretory-absorptive epitheliomuscular cells. The location of secretory granules in both apical and basal parts of granular cells indicates the exocrine-endocrine function of these cells. There are no typical endocrine cells in the intestinal epithelium of S. mereschkowskii. Several types of granules are described in the cytoplasm of nerve fibers. Junctions between nerve fibers and basal parts of ciliated and granular epithelial cells have been revealed; the neural regulation of the contractile and secretory functions of epithelial cells is assumed. The intestinal epithelium of enteropneusts is presumed to contain a regulatory neuroendocrine system composed of receptor cells of the open type, secretory endocrine-like cells, and of nerve elements of the nervous layer.     

Histochemical and Ultrastructural Analyses of the Epithelial Cells of the Body Surface Skin from the Terrestrial Slug, Incilaria fruhstorferi

Dorsal and ventral epithelium of the terrestrial slug, Incilaria fruhstorferi, is simple and consists of five cell types: microvillous, ciliated, round mucous, tubular mucous and channel. Microvillous cells were similar to human intestinal epithelial cells morphologically and functionally. At the base of microvilli, pinocytic vesicles which ultimately fused to form larger vacuoles, or multivesicular bodies were present. At the edge of tail or mouth, ciliated epithelial cells possessed the typical axonemes (9 plus 2 arrangement of microtubles). Mucous secretory cells were either tubular or round and their granules were membrane-bound and secreted by exocytosis. Granules of round mucous cells were proteinaceous but those of tubular cells were acidic mucopolysaccharides. Channel cells were elongate U-shaped and the central lumen was filled with a large amount of fluid (hemolymph). The function of channel cells is thought to remove hemolymph accumulated during hyperhydration. Our experiments of some markers-injection revealed that the fluid containing large molecules passed transcellularly from the hemolymph, across the basal or side region of the cell and into the central lumen. These results suggest that channel cell of the slug skin and vertebrate nephron showed some parallels in structure and function.     

The pelvic kidney of male Ambystoma maculatum (Amphibia,urodela, ambystomatidae) with special reference to the sexual collecting ducts

This study details the gross and microscopic anatomy of the pelvic kidney in male Ambystoma maculatum. The nephron of male Ambystoma maculatum is divided into six distinct regions leading sequentially away from a renal corpuscle: (1) neck segment, which communicates with the coelomic cavity via a ventrally positioned pleuroperitoneal funnel, (2) proximal tubule, (3) intermediate segment, (4) distal tubule, (5) collecting tubule, and (6) collecting duct. The proximal tubule is divided into a vacuolated proximal region and a distal lysosomic region. The basal plasma membrane is modified into intertwining microvillus lamellae. The epithelium of the distal tubule varies little along its length and is demarcated by columns of mitochondria with their long axes oriented perpendicular to the basal lamina. The distal tubule possesses highly interdigitating microvillus lamellae from the lateral membranes and pronounced foot processes of the basal membrane that are not intertwined, but perpendicular to the basal lamina. The collecting tubule is lined by an epithelium with dark and light cells. Light cells are similar to those observed in the distal tuble except with less mitochondria and microvillus lamellae of the lateral and basal plasma membrane. Dark cells possess dark euchromatic nuclei and are filled with numerous small mitochondria. The epithelium of the neck segment, pleuroperitoneal funnel, and intermediate segment is composed entirely of ciliated cells with cilia protruding from only the central portion of the apical plasma membrane. The collecting duct is lined by a highly secretory epithelium that produces numerous membrane bound granules that stain positively for neutral carbohydrates and proteins. Apically positioned ciliated cells are intercalated between secretory cells. The collecting ducts anastomose caudally and unite with the Wolffian duct via a common collecting duct. The Wolffian duct is secretory, but not to the extent of the collecting duct, synthesizes neutral carbohydrates and proteins, and is also lined by apical ciliated cells intercalated between secretory cells. Although functional aspects associated with the morphological variation along the length of the proximal portions of the nephron have been investigated, the role of a highly secretory collecting duct has not. Historical data that implicated secretory activity concordant with mating activity, and similarity of structure and chemistry to sexual segments of the kidneys in other vertebrates, lead us to believe that the collecting duct functions as a secondary sexual organ in Ambystoma maculatum. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Cell division in the olfactory epithelium of the lamprey,Lampetra fluviatilis

Summary The turnover of cells within the olfactory epithelium of the lamprey Lampetra fluviatilis was investigated using tritiated thymidine followed by autoradiography. It was found that cell division occurred in three distinct regions of the olfactory lamellae. Two of these regions — a distal lamellar region and a proximal lamellar region occurred outside the sensory area proper, but appeared to contribute cells to the sensory area as well as giving rise to secretory or ciliated cells outside the sensory area. A third region of division occurrred at the base of the sensory area. Division of specialised basal or blastema cells in this region gives rise to cells that are confined to the sensory region of the lamellae. These findings are discussed in the light of previous studies on cell replacement within the olfactory epithelium.     

Serotonin-like immunoreactivity in the central nervous system and gonad of the scallop,Patinopecten yessoensis

Summary The localization of neurons containing serotonin in the central nervous system and the gonad of the scallop, Patinopecten yessoensis, was examined immunohistochemically. In the central nervous system a large number of immunoreactive perikarya were observed in the following regions: a part of the anterior lobe of the cerebral ganglion; the posterior lobe of the cerebral ganglion; the pedal ganglion; and the accessory ganglion. No immunoreactive perikarya were found in the visceral ganglion. Numerous immunoreactive fibers were revealed in the neuropil of all central ganglia. In the gonadal region immunoreactive fibers were distributed around the gonoduct and along the germinal epithelium.This work was supported by a grant from the Ministry of Education, Science and Culture, Japan     

The Wheel Organ and Hatschek's Groove in the Lancelet,Branchiostoma lanceolatum (Cephalochordata)

The wheel organ is a specialized epithelium in the roof and sides of the adult lancelet oral cavity. It borders the oral epithelium proper, separated by a thin strip of margin cells which are not ciliated but contain a few large dense-cored vesicles apically. The wheel organ cells are tall and strongly ciliated and have dark, heterochromatin-rich nuclei. Dorsally, and slightly paramedially, the organ is further specialized, forming the so-called Hatschek's groove (pit), which consists of two ciliated cell types. The first type synthesizes a dense granular material, the granules being approximately 95 nm in diameter. This is stored basally and apparently it is also released through the basal cell membrane into the blood cavities. The cells at the bottom of Hatschek's groove have peculiar rod-shaped apical cellular regions. Each cell bears one tall cilium surrounded by microvilli and it is apparently involved in the production of secretory material into the groove. It is evident that the histology, and probably also the function, of the wheel organ and its groove is much more complex than hitherto believed.     

Ultrastructure of the ampullae of Lorenzini of <Emphasis Type="Italic">Aptychotrema rostrata</Emphasis> (Rhinobatidae)

Small epidermal pores of the electrosensory ampullae of Lorenzini located both ventrally and dorsally on the disk of Aptychotrema rostrata (Shaw and Nodder, 1794) open to jelly-filled canals, the distal end of which widens forming an ampulla that contains 6 ± 0.7 alveolar bulbs (n = 13). The sensory epithelium is restricted to the alveolar bulbs and consists of receptor cells and supportive cells. The receptor cells are ellipsoid and their apical surfaces are exposed to the alveolar lumen with each bearing a single central kinocilium. Presynaptic bodies occur in the basal region of the receptor cell immediately proximal to the synaptic terminals. The supportive cells that surround receptor cells vary in shape. Microvilli originate from their apical surface and extend into the alveolar lumen. Tight junctions and desmosomes connect the supportive cells with adjacent supportive and receptor cells in the apical region. The canal wall consists of two cell layers, of which the luminal cells are squamous and interconnect via desmosomes and tight junctions, whereas the cells of the deeper layer are heavily interdigitated, presumably mechanically strengthening the canal wall. Columnar epithelial cells form folds that separate adjacent alveoli. The same cells separate the ampulla and canal wall. An afferent sensory nerve composed of up to nine myelinated nerve axons is surrounded by several layers of collagen fibers and extends from the ampulla. Each single afferent neuron can make contacts with multiple receptor cells. The ultrastructural characteristics of the ampullae of Lorenzini in Aptychotrema rostrata are very similar to those of other elasmobranch species that use electroreception for foraging.     

Ultrastructure of the pineal organ of the killifish,Fundulus heteroclitus,with special reference to the secretory function

Summary The pineal organ of the killifish, Fundulus heteroclitus, was investigated by electron microscopy under experimental conditions; its general and characteristic features are discussed with respect to the photosensory and secretory function. The strongly convoluted pineal epithelium is usually composed of photoreceptor, ganglion and supporting cells. In addition to the well-differentiated photosensory apparatus, the photoreceptor cell contains presumably immature dense-cored vesicles (140–220 nm in diameter) associated with a well-developed granular endoplasmic reticulum in the perinuclear region and the basal process. These dense-cored vesicles appear rather prominent in fish subjected to darkness. The ganglion cell shows the typical features of a nerve cell; granular endoplasmic reticulum, polysomes, mitochondria and Golgi apparatus are scattered in the electron-lucent cytoplasm around the spherical or oval nucleus. The dendrites of these cells divide into smaller branches and form many sensory synapses with the photoreceptor basal processes. Lipid droplets appear exclusively in the supporting cell, which also contains well-developed granular endoplasmic reticulum and Golgi apparatus. Cytoplasmic protrusions filled with compact dense-cored vesicles (90–220 nm in diameter) are found in dark-adapted fish. The origin of these cytoplasmic protrusions, however, remains unresolved. Thus, the pineal organ of the killifish contains two types of dense-cored vesicles which appear predominantly in darkness. The ultrastructural results suggest that the pineal organ of fish functions not only as a photoreceptor but also as a secretory organ.We thank Dr. Grace Pickford for the fishes.     

The ultrastructural organization of olfactory epithelium of two species of gadoid fish

The untrastructural organization of the olfactory epithelium of the cod Gadus morhua (L.) and the haddock Melanogrammus aeglefinus (L.) was studied using both transmission and scanning electron microscopy. The olfactory rosette was found to exhibit regional differences; the faces of the olfactory lamella were composed of sensory epithelium, the edges were non-sensory. The cellular organization of the olfactory epithelium was determined and consisted of bi-polar sensory neurones, supporting cells, mucous cells and basal cells. The ultrastructure of the sensory cells was consistent, having an elongate cell, the free surface of which terminated in an olfactory vesicle from which arose either four olfactory cilia or numerous microvilli. Ciliary aggregations have been found in the two species of gadoid fish studied; it is suggested that these structures aid in the separation and in the circulation of fluid between the lamellae. The surface structure of the supporting cells was found to be of two types: either ciliated or ridged; the former presenting distinct ciliated tufts, the latter showing definite, but unorganized, ridges over the epithelium surface.