Fasciola gigantica: Histology of the digestive tract and the expression of cathepsin L

The digestive tract of Fasciola gigantica is composed of the oral sucker, buccal tube, pharynx, esophagus, and caecum. The tegumental-type epithelium lines the first four parts of the digestive tract while the caecal-type epithelium lines the remaining parts from the caecal bifurcation. The caecal-epithelial cells are classified into 3 types according to their staining properties and ultrastructural characteristics, as related to the amount of food contents in the caecal lumen. All caecal-type epithelial cells synthesize and secrete cathepsin L, a major group of enzymes in the digestive tract, as detected by in situ hybridization and immunolocalization. Moreover, the secreted cathepsin L is also adsorbed on the outer surface of the tegument and the glycocalyx coating of the surface of the tegument, whereas the tegumental cells and tegumental syncytium covering the parasite’s body and lining the proximal part of the digestive tract exhibit no in situ hybridization signal and immunostaining for cathepsin L.     

An ultrastructural approach to feeding behaviour in Philodina roseola and Brachionus calyciflorus (rotifers) II. The oesophagus

The cuticular oesophagus is a simple expansion of the dorsal pharyngeal wall of the mastax. The ciliary oesophagus is the cellular anterior wall of the stomach lumen, but seems to have the same embryological origin as the pharynx.In Brachionus calyciflorus, its cilia are surrounded by cuticular velums which have the same myelin-like structure and the same function as the buccal velum of Philodina roseola. In all cases, the oesophagus prevents the return of food particles from the stomach to the mastax lumen.     

Anatomy and histology of the digestive system of the paddlefish (Polyodon spathula)

The paddlefish (Polyodon spathula) is one of the most primitive and unique freshwater fishes of North America. It is adapted as a plankton filter-feeder. The wide mouth and greatly expansible pharyngeal cavity allows it to process a large quantity of water. Numerous setiform gillrakers, which operate against the flattened surfaces of the gill arches and opercula, are capable of filtering out small food particles. The abundance of taste buds and mucus secreting cells in the buccal cavity and pharynx probably aids in selecting and entangling the food. The first segment of the stomach is provided with peculiar, large longitudinal rods of fat which may help in selecting food from water by causing occlusion of the small lumen when the striated fibers of the muscularis contract. Teeth, unnecessary for microphagous fishes, are absent in adults. The digestive tube is one of the most complex present in fishes. It has two distinct divisions of the stomach and four distinct divisions of the intestine besides possessing a large caecum. Ciliated epithelial cells occur in many parts of the gut and may be an aid in moving or processing the small items of food. Besides the presence of cilia, other primitive features of the digestive tract include the lack of distinction between the esophagus and stomach, the presence of a spiral valve, and the presence of some of the diffuse pancreatic tissue within the wall of the gut. The numerous Peyer's patches in the wall of much of the intestine may be an immunological response to the heavy parasitic infestation.     

Histological and ultrastructural study of the digestive tract of rice field eel, Monopterus albus

The histological characteristics of the digestive tract and the ultrastructure of mucosal cells of the stomach and intestine of rice field eel, Monopterus albus, are described to provide a basis for future studies on its digestive physiology. The digestive tract of the rice field eel is a long and coiled tube composed of four layers: mucosa, lamina propria‐submucosa, muscularis and serosa. The pharynx and oesophagus mucosa is lined with a stratified epithelium. The stomach includes the cardiac and pyloric portions and the fundus. Many gastric pits are formed by invaginations of the mucosal layer and tubular gastric glands formed by the columnar cells in the fundus. The intestine is separated from the stomach by a loop valve and divided into a proximal portion and a distal portion. The proximal intestinal epithelium consists of columnar cells with microvilli towards the lumen and goblet cells. The enterocytes are joined at the apical surface by the junctional complex, including the evident desmosomas. Numerous lysosomes and some vesicles are evident in the upper cytoplasm of the cells, and a moderate amount of endoplasmic reticulum and lysosomes are scattered in the supranuclear cytoplasm. The epithelium becomes progressively thicker and the folds containing large numbers of goblet cells are fewer and shorter in the distal portion of the intestine. At the ultrastuctural level, the columnar cells of the tubular gastric glands have numerous clear vacuoles and channels. A moderate amount of pepsinogen granules are present in the stomach. The enterocytes of the intestinal mucosa display a moderate amount of endoplasmic reticulum and lysosomes, and long and regular microvilli.     

Ultrastructure of introvert and pharynx of Halicryptus spinulosus (priapulida)

The introvert of Halicryptus spinulosus bears three kinds of sensilla: buccal papillae, ordinary scalids, and dentoscalids. They are all characterized by bipolar monociliary receptor cells. The former two have apical openings at which the sensory cilia are in close contact with the ambient sea water. The pharyngeal teeth are composed of slender epithelial cells the tips of which are devoid of organelles and a thick cuticle. The anatomy of the muscle arrangement of the pharynx is described. Glands occur at the junction of the pharynx and midgut.     

An ultrastructural study of alimentary tract development in the cercariae of Prosorhynchoides borealis (Digenea,Bucephalidae)

Podvyaznaya I. 2011. An ultrastructural study of alimentary tract development in the cercariae of Prosorhynchoides borealis (Digenea, Bucephalidae). —Acta Zoologica (Stockholm) 92 : 170–178. The development of digestive system in Prosorhynchoides borealis cercariae was studied using transmission electron microscopy. The foregut and caecum primordia arise in early cercarial embryos as two adjoining cellular cords. The primordial pharynx appears as a cluster of myoblasts in the mid‐part of the foregut primordium whose proximal end abuts onto the ventral embryonic tegument. Later, a lumen develops within the gut primordia and their component cells form the embryonic cellular epithelium with an essentially similar structure in the foregut and caecal regions. Subsequently, the foregut epithelial cells merge to form a syncytium. This process proceeds asynchronously and the most proximal foregut area remains cellular for the longest time. The syncytial lining of the foregut establishes syncytial connections with secretory cytons differentiating in the surrounding parenchyma. These cytons produce secretory granules, which are transported through cytoplasmic connections to the foregut syncytium. Before cercariae reach maturity, their foregut epithelium becomes anucleate and continuous with the external tegument. By the end of cercarial development, numerous short lamellae appear on the luminal surface of the caecal epithelium. The caecal cells become involved in secretory activity as indicated by the presence of Golgi‐derived secretory bodies in their cytoplasm.     

The flow and fate of digestive enzymes in the field cricket,Gryllus bimaculatus

The flow of enzymes, the ratio of bound to unbound enzymes, and their inactivation in the cricket Gryllus bimaculatus was studied. The digestive enzymes are forced forward into the crop by caecal contraction and then they are mixed with freshly chewed food and saliva, forming a crop‐chyme. This chyme is blended by crop peristalsis, and periodic opening of the preproventricular valve (PPV) allows posterior movement into the proventriculus and further into the midgut. The contraction of the crop is modulated by Grybi‐AST and Grybi‐SK peptides, which are partially secreted by the caecal endocrine cells. Most of the aminopeptidase and the four disaccharidases examined are membrane bound (62–80%); the remaining (20–38%) as well all trypsin, chymotrypsin, lipase, and amylase are secreted free into the caecal lumen. Cricket trypsin loses only 30% of its activity in 4 h and very little thereafter. The presence of digestive products in the lumen appears to retard further trypsin autolysis. Cricket trypsin digests 42% of the chymotrypsin, 37% of the lipase, and 45% of the amylase in the caecal fluids over 24 h in vitro no significant difference. Without Ca ion amylase was almost completely digested. About 50% of the membrane bound and free aminopeptidase was digested in the caecal lumen, and about 30–38% of the bound and free maltase. This loss of digestive enzyme activity is possible, because enzyme secretion rates are high, the unbound enzymes are effectively recycled, and the time of nutrient passage is short.     

The fine structure and histochemistry of the caecal epithelium of Calicotyle kröyeri (Monogenea: Monopisthocotylea)

The caecal epithelium of Calicotyle kröyeri consists of a single cell type which functions in the uptake and intracellular digestion of host epidermis and associated mucus. Each cell is columnar with a small basal nucleus and prominent nucleolus. Perinuclear cytoplasm contains narrow profiles of GER and mitochondria with numerous cristae. Golgi complexes are small and indistinct. Most of the cell is filled with vacuoles of heterogeneous content, the largest occupying the cell apex. There is in each cell an apical endocytotic complex comprising cell surface lamellae, apical vesicles and numerous tubular invaginations of the plasmalemma. The limiting membrane of all these components is structurally modified and bears a highly organized array of peg-like structures on its luminal surface. The complex is capable of ingesting particulate food material from the gut lumen for transfer, via vesicles, to the vacuoles for digestion. Most of the vacuoles represent the digestive elements of the cell and, histochemically, are reactive for protein, mucus and carboxylic esterases. Indigestible residues and lipid droplets accumulate in the large apical vacuole and are periodically released to the lumen by exocytosis. Small, undifferentiated caecal cells were occasionally observed in the epithelium, but their development has not been recorded.     

Surface anatomy of branchial food traps of tadpoles: A comparative study

Branchial food traps are regions of specialized secretory tissue in the tadpole pharynx, where suspended food particles are trapped in mucus. Light and scanning electron microscopy were used to study branchial food traps from larvae of ten anuran families (36 species). Most anuran larvae from “advanced” (suborder Neobatrachia) families (e.g., Hylidae, Ranidae, Bufonidae) have distinct secretory pits at the posterior margins of the branchial food traps and secretory ridges elsewhere on these surfaces. The apices of columnar PAS-positive, secretory cells are exposed on the floors of the secretory pits or in rows at the tops of the secretory ridges (secretory zone). Tadpoles from most “archaic” (suborder Archaeobatrachia) families (Ascaphidae, Discoglossidae and Pelobatidae) either lack secretory pits, or have them poorly defined. They also lack secretory ridges but have columnar, mucus-secreting cells whose apices are exposed in a seemingly random fashion in the branchial food traps. Rhinophrynus (Archaeobatrachia: Rhinophrynidae) has secretory ridges, but the apices of secretory cells are not arranged in rows at the tops of the ridges; instead they erupt singly or in small clusters on the epithelial surface, in a pattern similar to that in Ascaphus, the discoglossids and the pelobatids. It is proposed that the generalized condition for the branchial food trap mucosa is one where the apices of secretory cells are exposed haphazardly on a flat epithelium and the derived condition is one where the surface is organized into ridges. The morphology of the branchial food traps in Rhinophrynus suggests that, phylogenetically, ridges preceded the coalescing of secretory cell apices into distinct rows. Pipidae and Microhylidae have unique patterns in the gross and microanatomy of their branchial food traps specific to their families. Branchial food trap morphology relates to diets of tadpoles as well as to taxonomy. Obligate macrophagous (e.g., carnivorous) tadpoles, irrespective of family, tend to have reduced branchial food traps, regularly lack secretory ridges and, in extreme cases, lack columnar mucus-secreting cells. Obligate microphagous forms (midwater suspension feeding of Xenopus, microhylids and Agalychnis), have straight parallel secretory ridges with narrow secretory zones and shallow troughs between the ridges. Secretory ridges may help to form mucus strands in which food particles are trapped, but they are not essential for planktonic entrapment. The hydrodynamic implications of the various topographic patterns remain unclear.     

Ultrastructure and histochemistry,of the stomach of Asplanchna sieboldi

Stomach cells of female Asplanchna sieboldi are specialized for absorption and intracellular digestion of nutrients. Evidence is presented to show that electron-opaque colloidal substances, present in the medium and within digestive vacuoles of the prey (Paramecium), are taken up by the stomach cells at the apical cell membrane and sequestered within food vacuoles which contain hydrolases working in both the acid and alkaline pH range. The stomach cells are also implicated in the absorption of molecules below the resolving power of the electron microscope. In rotifers possessing a complete digestive tract, this task is presumed to be handled by the intestine.     

ORIGINS AND ACTIVATION OF PROPHENOLOXIDASES IN THE DIGESTIVE TRACT OF THE CRICKET,Gryllus bimaculatus

The function of Phenoloxidases (POs) in sclerotization and defense in insects is well understood, but little is known concerning their occurrence, origins, and function in the digestive tract. In Gyrllus bimaculatus gut all of the PO activity is found in the lumen of the digestive tract, and no detectible activity is found in homogenates of the gut epithelium or secretions from incubated epithelial tissues. Prophenoloxidases (PPOs) are synthesized in the hemocytes of Bombyx mori and are transported into the cuticle. It is suggested that the PPOs in the caecal lumen of G. bimaculatus likewise are synthesized in hemocytes and are transported by unknown means into the caecal lumen, where they are activated to POs by trypsin. Peristalsis transports the POs both forward into the crop and posterior within the peritrophic membrane into the hind gut. The PPOs in the hemolymph consist of a trimer (270–280 kDa) and a tetramer (340–370 kDa). The active POs in the gut lumen consist of a monomer (85–95 kDa) in addition to an activated trimer and tetramer.     

On the Fine Structure of the Alimentary Tract of Cephalodiscus gracilis (Pterobranchia,Hemichordata)

The U-shaped alimentary tract of Cephalodiscus is of exclusively epithelial structure; on the basis of fine structural criteria the entire tract can be divided into two large subdivisions: an anterior one with mouth, mouth cavity, pharynx and oesophagus, and a posterior one with stomach and intestine. The anterior subdivision is built up of a relatively uniform, innervated, pseudostratified, ciliated epithelium with mucus cells which are concentrated in the initial parts of the mouth cavity. Cilia and mucus presumably constitute a mechanism transporting food particles into the stomach. In the area of the gill slits specific vacuolated cells occur which may lend rigidity to the walls of the slits. The gastric epithelium consists of prismatic cells characterized by, among others, large inclusion bodies, which may represent digestive vacuoles, small dense rod-shaped granules and an elaborate system of microridges, at the base of which abundant endocytotic vesicles occur. The dorsal gastric pouch contains cells rich in rough ER and secretory granules, probably containing digestive enzymes. Thus morphological evidence points both to intra- and extracellular digestion. The intestinal epithelium resembles that of the stomach, however, it is lower, its organelles are fewer and it bears, beside cilia, mainly microridges, which towards its distal end become sparse. Both in the gastric and intestinal epithelium small granulated cells have been found, which presumably represent endocrine cells.     

Histophysiology of digestion and observations on the structure of the alimentary canal in the ectosymbiont Chaetogaster limnaei limnaei Baer, 1827 (Annelida: Oligochaeta)

The naidid oligochaete Chaetogaster limnaei limnaei has an alimentary canal consisting of a mouth, pharynx with a dorsal pharyngeal pad, esophagus, stomach, anterior and posterior intestine, and anus. The diet is omnivorous but limited by particle size. Unattached food organisms are sucked into the pharynx while sessile organisms are plucked from the substratum. Granules of acid mucosubstances that stain purple with neutral red are secreted into the stomach lumen after food enters, rapidly increasing the acidity from pH 3 to 1.5. Acid induced lysis of the organisms initiates autolysis before the food is passed into the alkaline, pH 7 to 8, anterior intestine. Ciliated intestinal cells showed arylamidase, acid phosphatase and C-esterase active granules indicating primary lysosomes with secondary lysosomes being recognized in electron micrographs suggesting intracellular digestion. Arylamidase and alkaline phosphatase activity appears in the intestinal margins during the alkaline phase of digestion. Scattered, pyramidal cells found only in the anterior intestine contain yellow refractile spheres. The spheres stain alcian blue pH 2.5 and bromophenol blue positive and exhibit a strong acid phosphatase activity all the time with A-esterase active granules surrounding them. Glycogen and lipids are stored mainly in the chlorogague cells. Many of the yellow refractile granules in the stomach and intestinal cells are bacteria.     

Studies on the tardigrades. II. Fine structure of the pharynx of milnesium tardigradum doyère

The fine structure of the pharynx and associated valve of the tardigrade Milnesium tardigradum is presented. The pharynx consists of a triradiate, cuticle lined lumen surrounded by radially arranged muscle cells and special apical cells which cap each of the ventricles of the lumen. The valve is an unusual structure marking the anterior limit of the pharynx. It is a specialization of the cuticular lining of the foregut and the apical cells of the pharynx. The significance and phyletic affinities of these structures are discussed.     

Structure of the digestive tract of the pluteus larva of Dendraster excentricus (Echinodermata: Echinoida)

Summary Histological and ultrastructural observations of the digestive tract of eight-armed plutei of Dendraster excentricus are reported. The esophagus is divided into two regions. The uppermost is a narrow tube comprised of ciliated cells that assist in transporting food to the more bulbous lower esophagus where food particles are formed into a bolus prior to entering the stomach. The esophagus is surrounded by a network of smooth muscle fibers that are predominantly oriented circumferentially in the upper esophagus, and longitudinally in the lower esophagus. The musculature of the upper esophagus produces peristaltic contractions, whereas contractions of the muscle of the lower esophagus open the cardiac sphincter and force food from the lower esophagus into the stomach. Axons are associated with the ciliated cells and the muscles of the upper esophagus. The cardiac sphincter consists of a ring of myoepithelium, with cross-striated myofibrils oriented around the bases of the cells. The gastric epithelium is comprised of two cell types. Type I cells, which predominate, absorb and store nutrients, and may be the source of secreted digestive enzymes. Type II cells apparently phagocytize and intracellularly digest whole algal cells. The intestine is comprised of relatively unspecialized cells and probably functions primarily as a conductive tube for the elimination of undigested materials.     

Electron microscopic study of the anterior midgut in Cenocorixa bifida Hung. (Hemiptera: Corixidae) with reference to its secretory function

The epithelium of anterior midgut of adult Cenocorixa bifida was examined with light and electron microscopy. The folded epithelium is composed of tall columnar cells extending to the lumen, differentiating dark and light cells with interdigitating apices and regenerative basal cells in the nidi surrounded by villiform ridges that penetrate deeply into the epithelium. The columnar cells display microvilli at their luminal surface. Microvilli lined intercellular spaces and basal plasma membrane infoldings are associated with mitochondria. These ultrastructural features suggest their role in absorption of electrolytes and nutrients from the midgut lumen. The columnar cells contain large oval nuclei with prominent nucleoli. Their cytoplasm is rich in rough endoplasmic reticulum, Golgi complexes and electron-dense secretory granules indicating that they are also engaged in synthesis of digestive enzymes. The presence of secretory granules in close proximity of the apical plasma membrane suggests the release of secretion is by exocytosis. The presence of degenerating cells containing secretory granules at the luminal surface and the occurance of empty vesicles and cell fragments in the lumen are consistent with the holocrine secretion of digestive enzymes. Apical extrusions of columnar cells filled with fine granular material are most likely formed in response to the lack of food in the midgut. The presence of laminated concretions in the cytoplasm is indicative of storageexcretion of surplus minerals. The peritrophic membrane is absent from the midgut of C. bifida.     

可口革囊星虫消化道的形态及组织学结构

对可口革囊星虫(Phascolosoma esculenta)消化道各部分结构进行了形态学和组织学观察,表明可口革囊星虫消化道由翻吻、咽、食道、肠、直肠和肛门组成。翻吻收缩性很强;肠道极长,沿纺锤肌螺旋缠绕成肠索。消化道管壁由内向外分为粘膜层、粘膜下层、肌层和外膜。粘膜上皮主要由柱状细胞组成。除肠外,消化道上皮细胞均有发达的纤毛,肠上皮细胞主要具发达的微绒毛;粘膜下层及肌层的发达程度因消化道的部位不同而异。     

Ultrastructure of Enteronephridia and General Description of the Alimentary Canal in Trochonerilla mobilis and Nerillidium troglochaetoides (Polychaeta,Nerillidae)

The alimentary canals of Trochonerilla mobilis and Nerillidium troglochaetoides consist of a ventral pharyngeal organ, oesophagus, stomach, intestine, and rectum. Prominent salivary glands lying lateral to the oesophagus discharge their secretions into the buccal cavity. Ciliated canals, the enteronephridia, embedded in the intestinal epithelium, open into the stomach near its border to the intestine. The ventral pharynx comprises a muscle bulb connected to a tonguelike organ by an investing muscle. The whole alimentary canal is ciliated except for the intestine of T. mobilis. The stomach is built up of absorptive cells and posteriorly also of secretory cells, whereas the intestine consists of only one cell type which is considered to be mainly absorptive. A typical microvillar brush border is present only in the intestine of T. mobilis; elsewhere the density of microvilli is low or the cells have irregular apical processes. In N. troglochaetoides the intestine has a ventral ciliary gutter laterally bordered by cells with highly specialized microvilli. The enteronephridia — 3 in N. troglochaetoides and 13 in T. mobilis — are unicellular tubes up to 130 μm long with a microvillar brush border and other cytological features typical for nephridial ducts. These structures are not known in any other polychaete taxon.     

Fine structure and absorption of ferritin in the digestive organs of Loligo vulgaris and L. Forbesi (Cephalopoda,Teuthoidea)

The digestive organs possibly involved in food absorption in Loligo vulgaris and L. forbesi are the caecum, the intestine, the digestive gland, and the digestive duct appendages. The histology and the fine structure showed that the ciliated organ, the caecal sac, and the intestine are lined with a ciliated epithelium. The ciliary rootlets are particularly well developed in the ciliated organ, apparently in relation to its function of particle collection. Mucous cells are present in the ciliated organ and the intestine. Histologically, the digestive gland appears rather different from that of other cephalopods. However, the fine structure of individual types of squid digestive cell is actually similar to that of comparable organs in other species, and the squid cells undergo the same stages of activity. Digestive cells have a brush border of microvilli, and numerous vacuoles, which sometimes contain “brown bodies.” However, no “boules” (conspicuous protein inclusions of digestive cells in other species) could be identified in their cytoplasm; instead only secretory granules are present. In the digestive duct appendages, numerous membrane infoldings associated with mitochondria are characteristic features of the epithelial cells in all cephalopods. Two unusual features were observed in Loligo: first, the large size of the lipid inclusions in the digestive gland, in the caecal sac, and in the digestive duct appendages; and second, the large number of conspicuous mitochondria with well-developed tubular cristae. When injected into the caecal sac, ferritin molecules can reach the digestive gland and the digestive duct appendages via the digestive ducts, and they are taken up by endocytosis in the digestive cells. Thus, it appears that the digestive gland of Loligo can act as an absorptive organ as it does in other cephalopods.