The larval alimentary canal of the Antarctic insect, Belgica antarctica

On the Antarctica continent the wingless midge, Belgica antarctica (Diptera, Chironomidae) occurs further south than any other insect. The digestive tract of the larval stage of Belgica that inhabits this extreme environment and feeds in detritus of penguin rookeries has been described for the first time. Ingested food passes through a foregut lumen and into a stomodeal valve representing an intussusception of the foregut into the midgut. A sharp discontinuity in microvillar length occurs at an interface separating relatively long microvilli of the stomodeal midgut region, the site where peritrophic membrane originates, from the midgut epithelium lying posterior to this stomodeal region. Although shapes of cells along the length of this non-stomodeal midgut epithelium are similar, the lengths of their microvilli increase over two orders of magnitude from anterior midgut to posterior midgut. Infoldings of the basal membranes also account for a greatly expanded interface between midgut cells and the hemocoel. The epithelial cells of the hindgut seem to be specialized for exchange of water with their environment, with the anterior two-thirds of the hindgut showing highly convoluted luminal membranes and the posterior third having a highly convoluted basal surface. The lumen of the middle third of the hindgut has a dense population of resident bacteria. Regenerative cells are scattered throughout the larval midgut epithelium. These presumably represent stem cells for the adult midgut, while a ring of cells, marked by a discontinuity in nuclear size at the midgut-hindgut interface, presumably represents stem cells for the adult hindgut.     

Phylogenetic diversity of cellular organization in the cardia of muscoid flies (Diptera: Schizophora)

In each of 30 dipteran species, representing 13 acalyptrate and 7 calyptrate families, the cardia is formed from specialized cells at the junction between foregut and midgut. Foregut epithelium forms the stomodeal valve; midgut epithelium envelops the valve to form the cardia's outer wall. Cytological characteristics within these epithelia differ from region to region and from species to species. Since the cardia secretes the peritrophic membrane, cardias with diverse patterns of cellular differentiation may be expected to produce peritrophic membranes with similarly diverse properties. Close relatives often share more details of cardia structure than do distantly related taxa. Within the monophyletic Calyptratae, a common pattern of cellular differentiation includes three distinct zones of columnar midgut cells enclosing a flanged stomodeal valve. Among species in the paraphyletic Acalyptratae, midgut typically includes a single zone of tall columnar cells, while the valve may be spheroidal, cylindrical, conical, or flanged. The correlation of phylogenetic distance with divergence in cardia organization implies a strong influence of ancestry upon current structure, regardless of current diet. However, at least some of the observed diversity in cardia structure is associated with dietary divergence. Calyptrate flies with derived blood-feeding behavior display cellular differentiation that is simplified from that seen in calyptrate relatives with less specialized feeding habits. This evolutionary modification suggests that cardia organization and hence peritrophic membrane structure can adapt to dietary changes, with possible significance for the spatial organization of digestive processes and interactions with ingested microorganisms.     

Interfaces between microbes and membranes of host epithelial cells in hemipteran midguts

Certain families of plant-feeding insects in the order Hemiptera (infraorder Pentatomomorpha) have established symbiotic relationships with microbes that inhabit specific pouches (caeca) of their midgut epithelium. The placement of these caeca in a well-delineated region at the most posterior end of the midgut bordering the hindgut is conserved in these families; in situ the convoluted midgut is predictably folded so that this caecal region lies adjacent to the anterior-most region of the midgut. Depending on the hemipteran family, caeca vary in their number and configuration at a given anterior–posterior location. At the host-microbe interface, epithelial plasma membranes of midgut epithelial cells interact with nonself antigens of microbial surfaces. In the different hemipteran species examined, a continuum of interactions is observed between microbes and host membranes. Bacteria can exist as free living cells within the midgut lumen without contacting host membranes while other host cells physically interact extensively with microbial surfaces by extending numerous processes that interdigitate with microbes; and, in many instances, processes completely envelope the microbes. The host cells can embrace the foreign microbes, completely enveloping each with a single host membrane or sometimes enveloping each with the two additional host membranes of a phagosome.     

Organization of protein digestion in adult Calosoma calidum (Coleoptera: Carabidae)

Organization of protein digestion was examined in adult male Calosoma calidum (Carabidae) fed either ground-beef or waxmoth larvae (Galleria mellonella). Although trypsin activities in the foregut are consistently higher than those in the midgut, the luminal contents of each region are in equilibrium. Movement of fluids between the midgut and foregut is brought about by muscular contractions of the proventriculus. A 42% fall in trypsin activity of the foregut after feeding on ground-beef is due largely to disgorged enzymes being left on the food. A far higher proportion of disgorged trypsin is recovered when C. calidum feed on waxmoth larvae; beetles ingest about 74% prey protein and yet avoid ingesting digestively refractory solids. The retention of undigested macromolecules in the midgut and foregut lumen was determined using [¹⁴C]inulin labelled waxmoth larvae. Nine per cent and 25% of the radiolabel was passed in the faeces in 2 and 4 days respectively, whilst 59 and 91% of the weight gained at feeding was lost in the same intervals. The role that the peritrophic membrane and pyloric valve play in this process, as well as its implications for enzyme conservation, is discussed.     

Energetics of potassium ion transport in Aplysia gut

Basolateral membranes of Aplysia californica foregut epithelia contain an ATP-dependent Na(+)/K(+) transporter (Na(+)/K(+) pump or Na(+)/K (+) -ATPase). This Na(+)/K(+) pump accounts for both the intracellular Na(+) electrochemical potential (micro) being less than the extracelluar Na(+) micro and the intracellular K(+) micro being more than the extracellular K(+ ) micro. Also, K(+) channel activity resides in both luminal and basolateral membranes of the Aplysia foregut epithelial cells. Increased activity of the Na(+)/K(+) pump, coupled to luminal and basolateral membrane depolarization altered the K(+) transport energetics across the basolateral membrane to a greater extent than the alteration in K(+) transport energetics across the luminal membrane. These results suggest that K(+) transport, either into or out of the Aplysia foregut epithelial cells, is rate-limiting at the basolateral membrane.     

The digestive tract of Helicoradomenia (Solenogastres, Mollusca), aplacophoran molluscs from the hydrothermal vents of the East Pacific Rise

Abstract. Species of Helicoradomenia are constantly found at hydrothermal vent sites of the eastern and western Pacific Ocean. The digestive tract of 2 species of the genus was investigated with special focus on the ultrastructure and histochemistry of epithelia and glandular organs. The preoral cavity and foregut epithelia are composed of microvillous main cells, secretory cells producing protein-rich substances, and sensory cells with specialized cilia. The foregut bears a pair of glands with 3 types of extremely long-necked glandular cells surrounded by musculature. Each glandular cell opens directly into the radula pocket without a gland duct. The large radula apparatus consists of pairs of denticulated bars resting on a flexible radular membrane without elaboration of a subradular membrane. The midgut has a narrow, mid-dorsal tract of ciliary cells, but most of the epithelium is composed of digestive cells with a highly developed lysosomal system. The hindgut is lined by ciliated cells and free of glands. The foregut and radula seem to be highly efficient in the capture of relatively large, motile prey. Food contents within the midgut lumen and within some of the large secondary lysosomes indicate a triploblastic metazoan prey of non-cnidarian origin. The digestive tract is not adapted to microvory and there is no indication of a symbiosis with chemoautotrophic bacteria.     

Morphological changes in the oral (buccopharyngeal) membrane in urodelan embryos: development of the mouth opening

The ultrastructure of the oral (buccopharyngeal) membrane in the embryo of the urodelan, Hynobius tokyoensis, was examined by transmission (TEM) and scanning electron microscopy (SEM). The oral membrane consists of the stomodeal ectoderm and foregut endoderm, and is three to five cell layers thick at stage 24. The oral membrane gradually thickens as development proceeds. The stomodeal collar, derived from the ectoderm, is folded inward along the foregut endoderm. Tooth germs are formed partly by cells of the stomodeal collar and partly by mesenchymal cells and calcification takes place before hatching. Secretory granules, which are markers of epithelial differentiation, appear in some cells of the foregut endoderm. Within the oral membrane, the cells of the stomodeal collar become the basal cells, and the endodermal cells of the foregut become the apical cells of the future oral epithelium. Gaps are formed by the epithelial differentiation of the endodermal cells of the foregut in the oral membrane. The gaps connect with each other, with the stomodeum, and with the foregut. As a result of these events, the mouth opens at stage 43, just after hatching.     

Evidence for a sugar receptor (lectin) in the peritrophic membrane of the blowfly larva, Calliphora erythrocephala Mg. (Diptera)

For the first time a sugar receptor (lectin) has been localized by electron microscopy in an invertebrate. The peritrophic membrane of the blowfly larva, Calliphora erythrocephala, is shown here to express lectins with high specificity for mannose. The lectin is restricted to the lumen side of the peritrophic membrane. The surface of the midgut epithelium is devoid of mannose-specific lectins. It is suggested that the midgut epithelium has lost these lectins during the course of evolution in favour of the peritrophic membrane which is secreted by specialized cells only at the beginning of the midgut.Peritrophic membranes and the midgut epithelium lack lectins specific for galactose. The lumen side of the peritrophic membrane of the larvae has mannose and/or glucose residues, and it is densely packed with two species of bacteria, Proteus vulgaris and P. morganii. These also have mannose-specific lectins as well as mannose residues on their pili. The existence of mannose-specific receptors and mannose residues on both, peritrophic membranes and bacteria, leads to the assumption of mutual adherence between the two surfaces.     

Gut morphology of Mastotermes darwiniensis Froggatt (Isoptera : Mastotermitidae)

The digestive system of Mastotermes darwiniensis (Isoptera : Mastotermitidae) is similar to that of other lower termites. The cuticle in the proventriculus of the foregut is sculptured into 6 large dentate plates. Large setae are dispersed over the 1st and 2nd order folds and small setae are on the 3rd order folds. Epithelial cells in the foregut are of one type. The midgut cells are of one functional type, with 3 stages of the developmental cycle clearly distinguishable at any one time. There is no close association of mitochondria with the apical plasmalemma or microvilli. The epithelium rests on a basal lamina and has regularly distributed regenerative crypts. The midgut cells contain a large number of mitochondria close to the basal plasmalemma. Invaginations of the basal plasmalemma are very extensive and small tracheoles are plentiful close to the basal lamina. The presence of rough endoplasmic reticulum, most of which exists in the supranuclear cytoplasm, indicates that protein synthesis occurs in these cells. An abundance of basal plasmalemma invaginations is indicative of some fluid transport occurring across the membranes. The malpighian tubules have accumulations of mineral concretion forms as well as glycogen. Tracheoles are fairly abundant and distributed around the tubules. Epithelial cells in the region where the malpighian tubules join the midgut contain granules which are presumed to be secretory. Dentate plates around the anterior colon entrance are oriented so as not to hinder the flow of the lumen contents towards the posterior colon. Invaginations of the apical plasmalemma of the normal paunch epithelial cell suggest that they are involved in fluid or solute uptake from the lumen. The cuticle of the hindgut consists of areas with adhering bacteria and areas showing an even distribution of pores free of bacteria. The epithelial cells of the anterior colon resemble those in the paunch. The epithelial cells of the posterior colon show 3 variations: a squamous-cuboidal cell with unconstricted lumen; a cuboidal-columnar cell type found at the constriction site close to the rectum; a second squamous-cuboidal cell type constituting the junction of the colon and the rectum. This last cell type shows an abundance of cytoplasmic microtubules and a thick subcuticular zone.     

Cellular organization and peritrophic membrane formation in the cardia (Proventriculus) of Drosophila melanogaster

The peritrophic membrane of Drosophila melanogaster consists of four layers, each associated with a specific region of the folded epithelial lining of the cardia. The epithelium is adapted to produce this multilaminar peritrophic membrane by bringing together several regions of foregut and midgut, each characterized by a distinctively differentiated cell type. The very thin, electron-dense inner layer of the peritrophic membrane originates adjacent to the cuticular surface of the stomadeal valve and so appears to require some contribution by the underlying foregut cells. These foregut cells are characterized by dense concentrations of glycogen, extensive arrays of smooth endoplasmic reticulum, and pleated apical plasma membranes. The second and thickest layer of the peritrophic membrane coalesces from amorphous, periodic acid-Schiff-positive material between the microvilli of midgut cells in the neck of the valve. The third layer of the peritrophic membrane is composed of fine electron-dense granules associated with the tall midgut cells of the outer cardia wall. These columnar cells are characterized by cytoplasm filled with extensive rough endoplasmic reticulum and numerous Golgi bodies and by an apical projection filled with secretory vesicles and covered by microvilli. The fourth, outer layer of the peritrophic membrane originates over the brush border of the cuboidal midgut cells, which connect the cardia with the ventriculus.     

Structural observations on the alimentary canal of Paranthessius anemoniae, a copepod associate of the snakelocks anemone Ammonia sulcata

Light and electron microscopy has shown the alimentary canal of Paranthessius to be composed of clearly defined foregut, midgut and hindgut regions. The spacious foregut is cuticle-lined and separated from the midgut by a valve. The midgut epithelium is composed of columnar cells with an apparent secretary/absorptive rôle, and amoeboid cells thought to engulf material from the lumen. The amoeboid cells have large electron-dense central vacuoles containing carbohydrate-and protein-staining material. These cells appear to be sloughed off into the lumen to form part of a faecal pellet. Apart from their digestive rôle the midgut cells store lipid and it is considered possible that they have an osmoregulatory function. The hindgut epithelium cell type, lacks a cuticular layer and is thought to be mainly concerned with absorption. The alimentary canal is surrounded by strands of longitudinal and circular muscle.     

The functional morphology of the alimentary canal of larval stages of the parasitic copepod Lepeophtheirus salmonis

The functional morphology of the alimentary canal of copepodite and chalimus stages of Lepeophtheirus salmonis (Krøyer, 1837) is described and compared with that found in other copepods studied to date.
The buccal cavity passes into a gut comprising three major regions: foregut (oesophagus), midgut and hindgut. The foregut and hindgut both posscss a cuticular lining whereas the midgut is lined with specialized epithelial cells. The midgut is divided into three recognizable zones, namely anterior midgut caecum, anterior midgut and posterior midgut. Three main types of epithelial cell are recognizable in the midgut: vesicular cells, microvillous cells and basal cells which correspond to the cell types normally described in other parasitic and free-living copepod species.
Digestion is thought to occur in the midgut and be mediated by the epithelial cells that line it. Although several glands appear to discharge into the area of the buccal cavity, none was seen to interface to any other area of the gut. There was no evidence for the involvement of commensal gut bacteria in food digestion.     

Isolation of separate apical,lateral and basal plasma membrane from cells of an insect epithelium. A procedure based on tissue organization and ultrastructure

The tissue used in this study was the midgut of the tobacco hornworm larva, Manduca sexta. The midgut epithelium is a single layer of cells resting on a thin basal lamina and underlying discontinuous muscle layer. The epithelial cells are of two main types, goblet and columnar cells, joined together by the septate junctions characteristic of insect epithelia. From this tissue we were able to isolate four distinct plasma membrane fractions; the lateral membranes, the columnar cell apical membrane, the goblet cell apical membrane and a preparation of basal membranes from both cell types. The lateral membranes were isolated by density gradient centrifugation following gentle homogenization of the midgut hypotonic medium, which caused the cells to rupture at their apical and basal surfaces, releasing long segments of lateral membranes still joined by their septate junctions. For isolation of apical and basal membranes the tissue was disrupted by ultrasound, based on the light microscopic observation that carefully controlled ultrasound can be used to disrupt each cell in layers starting at the apical surface. The top layer contained the columnar cell apical membrane, which consists of microvilli forming a brush border covering the lumenal surface of the epithelium. The second layer contained the goblet cell apical membrane, which is invaginated to form a cavity occupying the apical half of the cell, and the third layer contained the basal membranes. As each layer was stripped off the epithelium it was collected and the plasma membrane purified by differential or density gradient centrifugation. For all four membrane fractions, the isolation procedure was designed to preserve the original structure of the membrane as far as possible. This allowed electron microscopy to be used to follow each step in the isolation procedure, and to identify the constituents of each subcellular preparation. Although developed specifically for M. sexta midgut, these techniques could readily be modified for use on other epithelia.     

The histology and ultrastructure of a nuclear polyhedrosis virus of the webbing clothes moth, Tineola bisselliella

A histological and ultrastructural study of a nuclear polyhedrosis virus in the webbing clothes moth, Tineola bisselliella, was conducted. Polyhedral development was observed in nuclei of cells of the foregut, cardiac valve, midgut, pyloric valve, hindgut, Malpighian tubules, ganglia of the ventral nerve cord, muscle, tracheae, fat, and hypodermis. Observations made with the electron microscope suggest that virions from the gut lumen are transported in vesicles through the cytoplasm into the nuclei of the columnar cells. Here they are released, replicate, take on membrane, and ultimately become multiply occluded in polyhedral protein. Polyhedra observed in nuclei of other tissues appeared identical to those in the gut.     

The ultrastructural investigation of the midgut in the quill mite Syringophilopsis fringilla (Acari,Trombidiformes: Syringophilidae)

The midgut of the females of Syringophilopsis fringilla (Fritsch) composed of anterior midgut and excretory organ (=posterior midgut) was investigated by means of light and transmission electron microscopy. The anterior midgut includes the ventriculus and two pairs of midgut caeca. These organs are lined by a similar epithelium except for the region adjacent to the coxal glands. Four cell subtypes were distinguished in the epithelium of the anterior midgut. All of them evidently represent physiological states of a single cell type. The digestive cells are most abundant. These cells are rich in rough endoplasmic reticulum and participate both in secretion and intracellular digestion. They form macropinocytotic vesicles in the apical region and a lot of secondary lysosomes in the central cytoplasm. After accumulating various residual bodies and spherites, the digestive cells transform into the excretory cells. The latter can be either extruded into the gut lumen or bud off their apical region and enter a new digestive cycle. The secretory cells were not found in all specimens examined. They are characterized by the presence of dense membrane-bounded granules, 2–4 μm in diameter, as well as by an extensive rough endoplasmic reticulum and Golgi bodies. The ventricular wall adjacent to the coxal glands demonstrates features of transporting epithelia. The cells are characterized by irregularly branched apical processes and a high concentration of mitochondria. The main function of the excretory organ (posterior midgut) is the elimination of nitrogenous waste. Formation of guanine-containing granules in the cytoplasm of the epithelial cells was shown to be associated with Golgi activity. The excretory granules are released into the gut lumen by means of eccrine or apocrine secretion. Evacuation of the fecal masses occurs periodically. Mitotic figures have been observed occasionally in the epithelial cells of the anterior midgut.     

Luminal matrices: An inside view on organ morphogenesis

Tubular epithelia come in various shapes and sizes to accommodate the specific needs for transport, excretion and absorption in multicellular organisms. The intestinal tract, glandular organs and conduits for liquids and gases are all lined by a continuous layer of epithelial cells, which form the boundary of the luminal space. Defects in epithelial architecture and lumen dimensions will impair transport and can lead to serious organ malfunctions. Not surprisingly, multiple cellular and molecular mechanisms contribute to the shape of tubular epithelial structures. One intriguing aspect of epithelial organ formation is the highly coordinate behavior of individual cells as they mold the mature lumen. Here, we focus on recent findings, primarily from Drosophila, demonstrating that informative cues can emanate from the developing organ lumen in the form of solid luminal material. The luminal material is produced by the surrounding epithelium and helps to coordinate changes in shape and arrangement of the very same cells, resulting in correct lumen dimensions.     

Ultrastructure of the labrum and foregut of Derocheilocaris remanei (Crustacea,Mystacocarida)

The cuticle-lined foregut of Derocheilocaris remanei consists of the mouth with its associated labrum, and an undifferentiated esophagus. It is separated from the midgut by an esophageal valve. The labrum is a conspicuous structure moved by five pairs of muscles (four dorsoventral and one longitudinal). Four pairs of subcuticular glands open to its inner face forming two longitudinal, lateral rows of cuticular pores. Each secretory unit is composed of a glandular component (with one or two secretory cells), a neck cell, and a duct cell. In addition, a single gland cell opens mesially into the buccal cavity. The ventrally located mouth is a complex structure characterized by a filter-like system, a sensory organ, and epithelial cells with highly developed microvilli. The esophagus is a simple tube with a characteristic curvature following the mouth. It has a rounded cross section and a triradiate lumen. A layer of circular musculature surrounds this region. The end of the esophagus protrudes into the midgut lumen forming the so-called esophageal valve. The ultrastructural features of the foregut, with the presence of a mucus-trapping mechanism, a relatively well-developed filter system and associated structures and an esophagus lacking glands confirm the microphagic feeding habits of mystacocarids. © 1996 Wiley-Liss, Inc.     

Cell renewal in adjoining intestinal and tracheal epithelia of Manduca

Cell renewal continuously replaces dead or dying cells in organs such as human and insect intestinal (midgut) epithelia; in insects, control of self-renewal determines insects’ responses to any of the myriad pathogens and parasites of medical and agricultural importance that enter and cross their midgut epithelia. Regenerative cells occur in the midgut epithelia of many, if not all, insects and are probably derived from a distinctive population of stem cells. The control of proliferation and differentiation of these midgut regenerative cells is assumed to be regulated by an environment of adjacent cells that is referred to as a regenerative cell niche. An antibody to fasciclin II marks cell surfaces of tracheal regenerative cells associated with rapidly growing midgut epithelia. Tracheal regenerative cells and their neighboring midgut regenerative cells proliferate and differentiate in concert during the coordinated growth of the midgut and its associated muscles, nerves and tracheal cells.     

Aplysia gut epithelial cells: luminal membrane chloride channel activity

The Cl(-) energy gradient across the luminal membrane of Aplysia foregut epithelial cells is directed downhill from the lumen to the cellular cytosol. No primary or secondary active transporters had been shown to be involved in Cl(-) translocation across the luminal membrane. Cl(-) channel blockers impeded the movement of Cl(-) from the lumen into the foregut cellular cytosol. It was concluded that the primary means of Cl(-) transport across the luminal membrane was via Cl(-) conductance.     

Placentation in garter snakes: scanning EM of the placental membranes of Thamnophis ordinoides and T. sirtalis

Surface topography and cross-sections of the placental membranes were examined by scanning electron microscopy in two species of Thamnophis. The chorionic epithelium of the chorioallantoic placenta consists of broad, squamous cells that lack surface specializations. The apposed uterine epithelium contains ciliated cells and larger, nonciliated cells. Neither the epithelium of the chorion nor that of the uterus is eroded; thus, underlying capillaries are not exposed to the luminal surface. In both the omphaloplacenta and the omphalallantoic placenta, epithelium of the omphalopleure consists of brush-border cells bearing prominent microvilli, interspersed with cells bearing minuscule microvilli. These surface epithelial cells are joined at their apices and their lateral surfaces are extensively sculpted by intercellular channels, presenting the appearance of an epithelium specialized for absorption. Deep to the epithelium lie the yolk spheres of the isolated yolk mass, interspersed with endodermal cells. Surface topography of the uterine epithelia of the omphaloplacenta and omphalallantoic placenta is relatively unspecialized. The acellular shell membrane separates maternal and fetal tissues in each of the three placental types. Marked differences in surface features of the chorioallantois and omphalopleure probably reflect different roles of these membranes in gas exchange and transfer of water and nutrients.