Structure of the tertiary component of the myenteric plexus in the guinea-pig small intestine

The tertiary component of the myenteric plexus consists of interlacing fine nerve fibre bundles that run between its principal ganglia and connecting nerve strands. It was revealed by zinc iodide-osmium impregnation and substance P immunohistochemistry at the light-microscope level. The plexus was situated against the inner face of the longitudinal muscle and was present along the length of the small intestine at a density that did not vary markedly from proximal to distal. Nerve bundles did not appear to be present in the longitudinal muscle as judged by light microscopy, although numberous fibre bundles were encountered within the circular muscle layer. At the ultrastructural level, nerve fibre bundles of the tertiary plexus were found in grooves formed by the innermost layer of longitudinal smooth muscle cells. In the distal parts of the small intestine, some of these nerve fibre bundles occasionally penetrated the longitudinal muscle coat. Vesiculated profiles in nerve fibre bundles of the tertiary plexus contained variable proportions of small clear and large granular vesicles; they often approached to within 50–200 nm of the longitudinal smooth muscle cells. Fibroblast-like cells lay between strands of the tertiary plexus and the circular muscle but were never intercalated between nerve fibre varicosities and the longitudinal muscle. These anatomical relationships are consistent with the tertiary plexus being the major site of neurotransmission to the longitudinal muscle of the guinea-pig small intestine.     

The fine structure of the buccal tentacles of Holothuria forskali (Echinodermata,Holothuroidea)

Summary Ultrastructural study of the buccal tentacles of Holothuria forskali revealed that each tentacle bears numerous apical papillae. Each papilla consists of several differentiated sensory buds.The epidermis of the buds is composed of three cell types, i.e. mucus cells, ciliated cells, and glandular vesicular cells (GV cells). The GV cells have apical microvilli; they contain bundles of cross striated fibrillae associated with microtubules. Ciliated cells have a short non-motile cilium. Bud epidermal cells intimately contact an epineural nervous plate which is located slightly above the basement membrane of the epidermis. The epineural plate of each bud connects with the hyponeural nerve plexus of the tentacle. This nerve plexus consists of an axonic meshwork surrounded in places by sheath cells. The buccal tentacles have well-developed mesothelial muscles. Direct innervation of these muscles by the hyponeural nerve plexus was not seen.It is suggested that the buccal tentacles of H. forskali are sensory organs. They would recognize the organically richest areas of the sediment surface through the chemosensitive abilities of their apical buds. Tentacles presumably trap particles by wedging them between their buds and papillae.     

Microscopic anatomy and ultrastructure of the nervous system of <Emphasis Type="Italic">Phoronopsis harmeri</Emphasis> Pixell, 1912 (Lophophorata: Phoronida)

The microscopic anatomy and ultrastructure of the nervous system of Phoronopsis harmeri was investigated using histological techniques and electron microscopy. The collar nerve ring is basically formed by circular nerve fibers originating from sensitive cells of tentacles. The dorsal nerve plexus principally consists of large motor neurons. It is shown for the first time that the sensitive collar nerve ring immediately passes into the motor dorsal nerve plexus. The basic components of the nervous system have similar cytoarchitectonics and a layered structure. The first layer is formed by numerous nerve fibers surrounded by the processes of glia-like cells. The bodies of glia-like cells constitute the second layer. The third layer consists of neuron bodies overarched by the bodies of epidermal cells. The giant nervous fiber is accompanied by more than one hundred nerve fibers of a common structure and, thus, marks the true longitudinal nerve. The phoronids possess one or two longitudinal nerves. It is supposed that the plexus nature of the nervous system in phoronids may be related to their phylogenesis. A comparison of the nervous system organization and body plans among the Lophophorata suggests that the nervous system of phoronids cannot be considered as a reductive variant of the brachiopod nervous system. At the same time, the structure of the nervous system of bryozoans can be derived from that of phoronids.     

Ultrastructure of neurons and synapses in the tentacle gastrodermis of the sea anemone Calliactis parasitica

Little is known about gastrodermal neurons and synapses in the tentacles of sea anemones. Using transmission electron microscopy of serial thin sections of Calliactis parasitica, we have identified both a sensory cell and a ganglion cell with granular vesicles originating from the Golgi complex and have identified four types of synapses in the tentacular gastrodermal nerve plexus. The sensory cell has a recessed apical cilium with a basal body and a perpendicularly oriented centriole, below which are several strands of striated rootlets surrounded by mitochondria. The ganglion cell lacks a cilium and resembles a bipolar neuron, with oppositely directed processes lying parallel to the basally located circular smooth muscle. Both one-way and two-way interneuronal synapses are present with 60- to 90-nm granular vesicles of various densities aligned at the paired electron-dense membranes and fine cross filaments in the intervening 13-nm cleft. Two types of neuroeffector synapses have been located. Dense granular vesicles are present at neuromuscular synapses, whereas less dense vesicles are present at neuroglandular synapses. Most of the synaptic vesicles range from 60 to 120 nm in diameter. Two types of nerve cells and a variety of synaptic loci provide morphological substrates for the spontaneous SS2 conduction pulses in the tentacular gastrodermis of C. parasitica. J Morphol 231:217–223, 1997. © 1997 Wiley-Liss, Inc.     

Ultrastructure of sea urchin tube feet

Summary An analysis of the ultrastructure of the tube feet of three species of sea urchins (Strongylocentrotus franciscanus, Arbacia lixula and Echinus esculentus) revealed that the smooth muscle, although known to be cholinoceptive, receives no motor innervation.The muscle fibers are attached to a double layer of circular and longitudinal connective tissue which surrounds the muscle layer and contains numerous bundles of collagen fibers. On its outside, the connective tissue cylinder is invested by a basal lamina of the outer epithelium to which numerous nerve terminals are attached. These are part of a nerve plexus which surrounds the connective tissue cylinder. The plexus itself is an extension of a longitudinal nerve that extends the whole length of the tube foot. It is composed of axons, but nerve cell bodies and synapses are conspicuously lacking, suggesting that the axons and terminals derive from cells of the radial nerve. Processes of the epithelial cells penetrate the nerve plexus and attach to the basal lamina. There is no evidence that the epithelial cells function as sensory cells.On the basis of supporting evidence it is suggested that the transmitter released by the nerve terminals diffuses to the muscle cells over a distance of several microns and in doing so affects the mechanical properties of the connective tissue.Supported by the Sonderforschungsbereich 138 of the Deutsche Forschungsgemeinschaft     

Ultrastructural analysis of spinal primary afferent fibers within the circular muscle of the cat lower esophageal sphincter

We have analyzed the ultrastructural characteristics and environment of spinal primary afferent fibers that run within the circular muscle of the cat lower esophageal sphincter. These were selectively labeled by anterogradely transported cholera toxin B subunit conjugated with horseradish peroxidase. Most of the labeled fibers were perpendicular to the muscle cells but some ran sinuously or parallel to the muscle cells. All the labeled fibers were unmyelinated and exhibited relatively rare varicosities. Most of the fibers were in large nerve fiber bundles surrounded by perineurium and probably project to the mucosa. Only some fibers that were in small nerve fiber bundles with no perineurium ran parallel to the musculature and established close relationships with smooth muscle cells. They might be a small subpopulation of the spinal tension receptors, most of the other spinal tension receptors being located in the myenteric plexus area, between the circular and longitudinal muscle. Accepted: 2 December 1999     

Scanning electron microscopy of echinoid podia

Summary Tube feet of the sea urchin Strongylocentrotus franciscanus were studied with the scanning electron microscope (SEM). By use of fractured preparations it was possible to obtain views of all components of the layered tube-foot wall.The outer epithelium was found to bear tufts of cilia possibly belonging to sensory cells. The nerve plexus was clearly revealed as being composed of bundles of varicose axons. The basal lamina, which covers the outer and inner surfaces of the connective tissue layer, was found to be a mechanically resistant and elastic membrane. The connective tissue appears as dense bundles of (collagen) fibers. The luminal epithelium (coelothelium) is a single layer of flagellated collar cells.There is no indication that the muscle fibers, which insert on the inner basal lamina of the connective tissue layer are innervated by axons from the basiepithelial nerve plexus.The results agree with previous conclusions concerning tube-foot structure based on transmission electron microscopy, and provide additional information, particularly with regard to the outer and inner epithelia.This investigation was supported by the Sonderforschungsbereich 138 of the Deutsche Forschungsgemeinschaft. The work was carried out at the Friday Harbor Laboratories of the University of Washington. The authors are indebted to the Director, Professor A.O.D. Willows for use of the facilities, and to Drs. Christopher Reed and Tom Schroeder for invaluable instruction and assistance     

Neurobiology of the gorgonian coelenterates,Murcea californca and Lophogorgia chilensis

Summary Diffuse and synaptic nerve nets are present in the coenenchymal mesoglea and ectoderm of Muricea and Lophogorgia colonies. The nerve nets extend into the polyp column and tentacles maintaining a subectodermalmesogleal position. The density of nerve elements is low in comparison with similar nerve nets found in pennatulids.In the column of the polyp anthocodium, and throughout the oral disk region, neurons cross the mesoglea and enter the polyp endoderm. These neurons presumably connect with the endodermal nerve net which innervates the septal musculature. The trans-mesogleal neurons probably represent the connection between colonial and polyp nervous systems.In the tentacles, longitudinal ectodermal musculature is present with an overlying nerve plexus. These muscles and nerves, as well as tentacular sensory cells, are well represented in the oral side of the tentacles only.Presumed sensory cells form ciliary cone complexes in which one cell possesses an apical cilium. The other cells as well as the centrally located nematocyte contribute microvilli to the cone. The basal portion of the sensory cells is drawn into one or more neurite-like processes which enter the ectodermal nerve plexus. Similar processes form synapses with longitudinal muscle cells and nematocytes. The sensory cells of the ciliary cones presumably include chemoreceptors which can activate or modify nematocyst discharge, local muscle twitches, and tentacle bending.This work was supported by Office of Naval Research Contract N00014-75-C-0242, NSF Grant BMS 74-23242 and General Research Funds of the University of California, Santa Barbara. We wish to thank Dr. Steven K. Fisher for the use of facilities in his lab. This paper is part of a thesis to be submitted by R.A.S. to the Department of Biological Sciences, University of California, Santa Barbara in partial fulfillment of the requirements for the Ph. D.     

Ultrastructure of the adhesive tentacles of the limnomedusa Vallentinia gabriella (Hydrozoa,Olindiadidae)

Summary The specialized adhesive exumbrellar tentacles of the limnomedusa Vallentinia gabriella were examined by light microscopy and scanning and transmission electron microscopy. The adhesive region first differentiates some distance from the tentacle tip. As differentiation proceeds the distal part is reduced and the adhesive region comes to lie at the tentacle tip. The adhesive epithelium consists of flagellated and non-flagellated glandular cells, a few nematocytes, and a nerve plexus. The glandular cells are characterized by electron-dense granules and bundles of microtubules. The microtubules, being anchored to the mesoglea, are oriented parallel to the longitudinal axis of the cell and extend up to the cell apex. It can be assumed that the microtubules are involved in the transport of secretory granules to the cell apex. Bundles of neurites run adjacent to the mesoglea between the basal processes of the glandular cells. The neurites form interneural synapses and synapses with glandular cells. It is suggested that detachment of the specialized adhesive tentacles is under nervous control.     

Histology and Ultrastructure of the Body Wall in the Phoronid Phoronopsis harmeri

The histology and ultrastructure of the body wall in Phoronopsis harmeriwere studied using light microscopy and TEM. The ectoderm epithelium of tentacles, anterior body region, and ampulla consists of monociliary cells. Gram-negative bacteria were found between microvilli, in the protocuticle of the anterior region, and in the ampulla. The epithelium of the posterior body region lacks both monociliary cells and bacteria. The bundles of nerve fibers run between the layer of epithelial cells and basal membrane. The musculature of the body wall comprises circular and longitudinal muscles. The circular muscle fibers are applied to the basal membrane and constitute a solid layer extending almost throughout the length of the body. This pattern is broken in the posterior body region, where there is no solid layer of circular musculature, and the latter is arranged in isolated muscle bands. In the ampullar (terminal) body region, the inversion of circular and longitudinal muscle layers takes place, so that the latter appears to be pressed against the basal membrane. The apical surfaces of longitudinal muscle cells bear cytoplasmic processes; some of the cells have a flagellum. The basal portion of the longitudinal muscle cells forms a cytoplasmic process containing bundles of tonofilaments. The processes of all cells making up the muscle bands are interwoven and anchored to the basal membrane.     

Identification of interstitial cells of Cajal in the digestive tract of turkeys (Meleagris gallopavo)

Interstitial cells of Cajal (ICCs) were identified in the digestive tract of turkeys by electron microscopy. ICCs have been implicated as sources of pacemaking slow wave potentials that initiate peristalsis in the stomach and intestines in mammals. The gastroduodenal contraction cycle in turkeys, however, is uniquely coordinated by a neurogenic pacemaker in the isthmus area between the glandular stomach and the gizzard, and this controls the coordinated phasic contractions of the muscles of the gizzard, duodenum and glandular stomach. Thus, it becomes important to look for the presence and distribution of ICCs in the avian digestive tract, especially in the gizzard and duodenum. This investigation has identified that cells are present which contain the typical characteristics of ICCs including: numerous mitochondria, caveolae, thin processes, basement membrane, filaments, rough ER, Golgi, and occasional gap junctions. They were mostly located in the region of the myenteric plexus between the longitudinal and circular muscle layers and occasionally within the longitudinal muscle layer. They were frequently near nerve axon bundles and were usually surrounded by collagen, elastin fibers, and occasional fibroblasts or blood vessels. ICCs were easily found in the ileum, but were also present in the duodenum, cecum, and rectum. None were found in the serosal region of the thick muscle of the gizzard. The presence of ICCs in the turkey duodenum, which like the gizzard is under neurogenic control, suggests that ICCs may play a role(s) in addition to initiating peristalsis.     

Presence of 3,4-dihydroxyphenylalanine and 3,4,5-trihydroxyphenylalanine in a coelenterate nervous system: Possible tyrosinase-mediated accumulation

3,4-dihydroxyphenylalanine (DOPA), 3,4,5-trihydroxyphenylalanine (5-OH-DOPA), 5-S-cysteinylDOPA (5-SC.D) and 2-S-cysteinylDOPA (2-SC.D) in the tentacles of the sea anemone, Metridium senile, were studied by the combined use of differential centrifugation of tissue homogenates, ultracentrifugation on sucrose density gradients, HPLC and electron microscopy. DOPA, 5-OH-DOPA and o-diphenol:O₂ oxidoreductase (Tyrosinase) were enriched in fractions containing membranes and subcellular particles, and the cysteinylDOPAs in the cytosol fractions. Ultrastructurally studied fractions rich in DOPA and 5-OH-DOPA contained large numbers of highly osmium-reducing vesicles. Identical structures were localized in ectodermal nerves and epidermal sensory cells.The results suggest that previous findings of catecholderivatives in the tentacles of Metridium and other sea anemone species by histochemical methods, are explained by a tyrosinase-based accumulation of DOPA and 5-OH-DOPA in the ectodermal nerve net. These substances are confined in specific compartments (vesicles) in the neurons and sensory cells.     

Localization and distribution of nitric oxide synthase and other neuronal markers in the podia of Holothuria arguinensis

The organization of the nervous system of the holothurian podia—the tentacles, papillae, and tube feet—is still poorly understood, which limits the development of functional studies. Knowledge of nitric oxide (NO) signaling in sea cucumbers is nonexistent, although it is known to play an important role in many essential biological functions, including neurotransmission, throughout the animal kingdom. The objective of this study was to characterize the holothurian podia in Holothuria arguinensis. To this end, we used classical histology, nitric oxide synthase (NOS) distribution, using NADPH‐diaphorase histochemistry and NOS immunostaining, and neuronal immunohistochemistry. Our results revealed an abundant distribution of NO in the nervous components of the holothurian podia, suggesting an important role for NO as a neuronal messenger in these structures. Nitrergic fibers were intensely labeled in the longitudinal nerve and the nerve plexus surrounding the stem, but were more weakly labeled in the mesothelium. NOS was also found in scattered cell bodies and abundant fibers in the podia terminal end (i.e., the discs in tentacles and tube feet, and the pointed conical structures in the papillae), with evident neuronal projections to the bud surface, especially in the tentacles. The podia terminal end was the most specialized area and was characterized by a specific nervous arrangement, consisting of a distinct nerve plate, rich in cells and fibers containing potential sensory cells staining positively for neuronal markers, which makes this the most likely candidate to be a chemosensory region and an important candidate for future exploration.     

Nitroxidergic Innervation of the Digestive Tract of the Shishamo Smelt Hypomesus japonicus (Teleostei: Salmoniformes)

The distribution and morphology of nitroxidergic elements in the esophagus, stomach, and intestine of the shishamo smelt Hypomesus japonicus (Teleostei: Salmoniformes) were studied. Nitroxidergic cells and fibers were found in all examined parts of the digestive tract, occurring most frequently in the stomach and rectal portion of the intestine. The spatial density of the nerve cells and fibers was maximum in the myenteric plexus and circular muscle layer and decreased in the longitudinal muscle layer and in the submucous plexus.     

The tentacles of Ichthyophis (Amphibia: Caecilia) with special reference to the skin

The skin of the paired tentacles of Ichthyophis consists of a cornified epidermis of 5–7 layers of epidermal cells, and a glandular dermis of ducted mucous glands, in association with collagen, blood vessels, fibroblasts, granulocytes, sparse melanophores and characteristic laminophores of unknown function. The epidermis is highly innervated at all levels below the stratum corneum by naked neurites, which originate as branches from large unmyelinated nerve bundles (and associated Schwann cells), located sub-epidermally, and which are part of the trigeminal cranial nerve. Myelinated nerves are also present below the epidermis, spatially associated with capillaries and glands. The study of the ultrastructure of the tentacle supports a concept of a sensory function, possibly tactile, though until further information from experimentation is available, any ideas on the specific nature of these sensory activities must remain speculative.     

The fine structure of the hypostome and mouth of hydra

The hypostome and mouth of fresh-water Hydra were examined by scanning electron microscopy. The external surface of the hypostome possesses cnidocils, possibly sensory hairs, and small spiny protrusions surrounding the mouth; the internal surface has cylindrical microvilli, free flagella and adherent flagella. The adherent flagella are most numerous close to the mouth where they cause the cell surface to appear smooth when viewed at low magnifications. Free flagella and leaf-like microvilli increase in prominence towards the tentacles and enter on proper. The edge of the mouth has an abrupt boundary marking the apposition of epidermal and gastrodermal cells. A transitional groove occurs at the boundary and the cells underlying the groove are smaller than those on other regions of the hypostome. The transition groove may represent a site of cell loss in normal cell turnover. Some of the small underlying cells may represent nervous elements involved in regulating hypostome activity during the feeding reation.     

Comparative ultrastructure of the pharynx simplex in turbellaria

Summary The simple pharynges in thirteen species of Turbellaria in the orders Macrostomida, Haplopharyngida, Catenulida, and Acoela have been studied by electron microscopy. After consideration of the functional aspects of the pharynx simplex, the relationship of the pharynx simplex ultrastructure to the phylogeny of the above mentioned groups is analyzed.The Haplopharyngida and Macrostomida are united as a group by the following characters: a pharynx transition zone of 1–5 circles of insunk cells with modified ciliary rootlets or no cilia, pharynx sensory cells without stereocilia collars and with a variable number of cilia, a prominent nerve ring with more than 30 axons circling the pharynx at the level of the beginning of the pharynx proper distal to the gland ring, 2 or more gland cell types in the pharynx, with at least two layers of muscle present and the longitudinal muscles derived from regular and special body wall circular muscles and a prominent post-oral nerve commissure. This specific arrangement can be distinguished from the other pharynx simplex types and is called the pharynx simplex coronatus.The catenulid pharynx simplex is characterized by the lack of a prominent nerve ring, no prominent post-oral commissure, a transition zone with epidermal type ciliary rootlets, recessed monociliated sensory cells, and one or no type of pharynx gland cell. The Acoela are specialized because of the epidermal type rootlets in the pharynx proper. They also lack a transition zone and a prominent nerve ring and have monociliated sensory cells different from the catenulid type.Ultrastructural characters of the pharynx simplex support the view that the Haplopharyngida-Macrostomida are monophyletic. The more primitive catenulid pharynx probably arose from a common ancestral pool with the Haplopharyngida and Macrostomida, although it does not appear possible presently to establish a clear monophyletic line for these forms. The various pharynx types within the Acoela appear to indicate independent origins with no clear link to the basic pharynx simplex type in the three other orders.Abbreviations Used in Figures a nerve axon - ar accessory rootlet - bb basal body - bn brain-nerve ring commissure - c caudal rootlet - ce centriole - ci cilium - cm circular muscle - cp ciliary pit - cu cuticle - cw cell web - d dictyosome - dp proximal pharynx proper cell - e epidermis - er rough endoplasmic reticulum - f fibrous rod - g gastrodermis - gc gastrodermal gland cell - he heterochromatin - i intercellular matrix - lc lateral nerve cord - lm longitudinal muscle - m mitochondria - mo mouth - mt microtubules - mv microvilli - n nucleus - nr nerve ring - ns neurosecretory granules - p pharynx proper - ph pharynx - po post-oral commissure - r rostral rootlet - rm radial muscle - s sphincter - sc sensory cell - sj septate junction - sr sensory rootlet - t transition zone - u ultrarhabdite - v vertical rootlet - va food vacuole - za zonula adhaerens - 1 type I gland cell - 2 type II gland cell - 3 type III gland cell - 4 type IV gland cell - 5 type V gland cell - 6 type VI gland cell - 7 type VII gland cell     

Studies on Stenostomum grande Child, 1902 (Platyhelminthes,Catenulida): fine structure of the digestive tract and the endocytotic activity of the gastrodermis

Abstract The digestive tract and its endocytotic activity in the catenulid Stenostomum grande were studied by electron microscopy. The pharynx was typical of the simplex type. At the mouth, between the integumental epithelium and the pharyngeal epithelium proper, was a transition zone. Among the epithelial cells of this transition were monociliated sensory cells and the necks of bucco-pharyngeal secretory cells of two types. The pharyngeal epithelium proper was densely ciliated, with long ciliary rootlets and mitochondria. It was surrounded by two layers of muscles. The gastrodermis consisted of phagocytes and typical secretory Minotian cells. It was underlain by a delicate basal lamina and muscle fibers. Distinctive of the phagocytes was the presence of differentiated cilia, cup-shaped mitochondria, and vacuoles with dense inclusions. Morphological differences between pharyngeal and gastrodermal cilia suggest functional differences. Experiments using latex beads as tracers and the identification of acid phosphatase in cytoplasmic vacuoles pointed to a high level of endocytotic and digestive activity in the phagocytes. Our data demonstrate that the basic structure of the digestive tract in S. grande conforms well to that of other free-living platyhelminths, but it does have ultrastructural peculiarities.     

A qualitative and quantitative study on the enkephalinergic innervation of the pig gastrointestinal tract.

Enkephalins are involved in neural control of digestive functions such as motility, secretion, and absorption. To better understand their role in pigs, we analyzed the qualitative and quantitative distribution of enkephalin immunoreactivity (ENK-IR) in components of the intestinal wall from the esophagus to the anal sphincter. Immunohistochemical labelings were analyzed using conventional fluorescence and confocal microscopy. ENK-IR was compared with the synaptophysin immunoreactivity (SYN-IR). The results show that maximal ENK-IR levels in the entire digestive tract are reached in the myenteric plexuses and, to a lesser extent, in the external submucous plexus and the circular muscle layer. In the longitudinal muscle layer, ENK-IR was present in the esophagus, stomach, rectum, and anal sphincter, whereas it was absent from the duodenum to the distal colon. In the ENK-IR plexuses and muscle layers, more than 60% of the nerve fibers identified by SYN-IR expressed ENK-IR. No ENK-IR was observed in the internal submucous plexus and the mucosa; the latter was found to contain ENK-IR endocrine cells. These results strongly suggest that, in pigs, enkephalins play a major role in the regulatory mechanisms that underlie the neural control of digestive motility.