Specialization of the endoplasmic reticulum in the apex of the midgut cells of Aeshna cyanea (Insecta,Odonata)

Summary Lamellar stacks of cisternae regularly spaced and bound to each other by fine filaments are observed in the apex of midgut cells of Aeshna cyanea. These cisternae are connected with the rough endoplasmic reticulum but are devoid of ribosomes except on their terminal sacs. Their location and their structure, quite different in fed and starved animals, suggest that they are involved in intracellular transport.     

Cytology of the midgut epithelium of Aeshna larvae (Insecta,Odonata)

Summary The midgut epithelium of the larval dragonfly Aeshna cyanea consists of four types of differentiated cells, which all display secretory activity. Pure secretory cells are the mucocytes and two morphologically distinguishable types of endocrine cells, while the enterocytes exert the dual function of secretion and absorption. Both functions can be performed more or less synchronously; however, appropriate feeding and starvation experiments can stress one function over the other. The heaviest accumulation of secretion granules was observed after a few days of starvation following a long period of regular feeding. Then the enterocytes resembled typical protein-secreting exocrine gland cells. It is still unknown whether the two main secretions, namely digestive enzymes and peritrophic membrane material, are jointly or separately distributed in the secretion granules. The morphological findings suggest that the secretory products follow the same cisternal pathway via endoplasmic reticulum, Golgi apparatus, membrane-bound storage granules and exocytotic extrusion that is well established for the exocrine pancreas of vertebrates. The routes of protein secretion and lipid absorption through the enterocytes of Aeshna are discussed in comparison with the enterocytes and mammocytes of vertebrates.Dedicated to Prof. Dr. K.E. Wohlfarth-Bottermann on the occasion of his 65^th birthday     

Constancy of ascending projections in the metamorphosing brain of the meal-beetle Tenebrio molitor L. (Insecta: Coleoptera)

Summary The fate of ascending projections of thoracic interneurons in the metamorphosing brain of Tenebrio molitor is described. Persistent brain neurons were identified and their fate is described during metamorphosis. The projection sites of ascending elements are invariable throughout metamorphosis both in quantitative and in qualitative terms. Some of these ascending neurons are serotonin-immunoreactive and this set of neurons maintains a constant projection site within the metamorphosing brain. The alterations in the projection sites of these and other ascending neurons in the ventral nerve cord were analysed experimentally. The central projection sites of these persistent ascending neurons are not important for the maintenance of their nerve cord projections throughout metamorphosis. Experimental deletion of ascending neurons which project into the suboesophageal ganglion varies the shape of persistent central neurons.     

Multiple peptide immunoreactivities in the nervous system of Aeschna cyanea (Insecta,Odonata)

Summary By use of the indirect immunoperoxidase method, the brain, the suboesophageal ganglion and the corpora cardiaca of the dragonfly Aeschna cyanea have been shown to be immunoreactive to proctolin antiserum and to several mammalian peptide antisera including unsulfated cholecystokinin octapeptide (CCK-8 NS) (Andriès et al. 1989), vasoactive intestinal peptide (VIP), human somatoliberin (hGRF) (Andriès et al. 1984) and motilin antisera. Immunohistochemical studies have been performed on material fixed in a solution of picricacid paraformaldehyde or in Bouin Hollande's sublimate solution. Antisera were applied on alternate sections or, according to the elution-restaining method of Tramu et al. (1978), one after another on the same section. Multiple peptide immunoreactivities appear expressed in the brain and the suboesophageal ganglion. Cells reactive to both hGRF and VIP antisera show also gastrin/CCK-like immunoreactivity and some of them are also detected by motilin antiserum. Besides, some cells immunopositive to CCK-8 NS and motilin antisera do not show hGRF or VIP immunoreactivity. At last, two pairs of protocerebral cells appear immunoreactive to both CCK-8 NS and proctolin antisera. Therefore, the present observations support our previously developed idea (Andriès et al. 1989) that the population of CCK-like cells is heterogenous.     

Functional morphology of the midgut of Aedes aegypti L. (Insecta,Diptera) during blood digestion

Morphometric analysis of the epithelial lining of the stomach of A. aegypti suggests that digestion of the first blood meal in the stomach of this species can be viewed as a series of phases that can be correlated with physiological data from the literature. In phase Ia (0-10 h after blood meal [abm]) the whorls of the rough endoplasmic reticulum unfold, the Golgi zones increase, and the basal labyrinth is enlarged. This coincides with processes of synthesis and secretion (e.g., peritrophic membrane, esterases and lipases) and transport by the stomach epithelium. In phase Ib (10-20 habm) the cellular parameters measured further increase, indicating high synthetic and secretory activities (e.g., digestive enzymes). In phase Ic (20-30 habm) cell structures involved in synthesis and secretion still exhibit high values coinciding with maximal activity of proteases in the gut. Enhanced surface area of microvilli, prominent lipid inclusions, and appearance of glycogen deposits in the gut epithelium suggest increased absorption, storage, and transport functions of the stomach cells. In phase II (30-36 habm) structural alteration points to a gradual shift from synthesis and secretion to absorption, partial storage, and transport of nutrients. In phase III (36-72 habm) the cellular apparatus is reduced concomitant with the ending of the digestive cycle. Lipid inclusions and glycogen deposits disappear from the stomach epithelium.     

A morphometric study of the midgut in resistant and non-resistant Anticarsia gemmatalis (Hübner) (Lepidoptera: Noctuidae) larvae to its nucleopolyhedrovirus (AgMNPV)

In this investigation, the anterior and posterior regions of the midgut of resistant (RL) and non-resistant (SL) Anticarsia gemmatalis larvae were analyzed morphometrically to characterize different regions along their length. Also, this investigation compares the results between SL and RL to improve the understanding of the resistance mechanisms to the virus. Histological sections were analyzed in a computerized system and the data were statistically analyzed by the Kruskal-Wallis test and by multivariate analysis. The midguts are morphometrically different in the two larval populations; we observed higher values in RL. The morphometric analysis of the epithelial cells showed that only columnar and goblet cells were distinct along the midgut, in both larvae, with the higher values found in the anterior region. Comparing the results between the two larval populations, all the epithelial cells presented significant differences, with RL showing the higher morphometric values. We concluded that there are regional differences along the length of midgut in SL and RL that confirm the idea of two morpho-functional distinct regions. The consistently morphometric superior values in RL indicate that this variability can be related with the resistance of A. gemmatalis to its AgMNPV.     

Peroxisomes of the midgut epithelium, malpighian tubules and fat body of larvae of the dragonfly, Aeshna cyanea

The enterocytes of the midgut epithelium of Aeshna cyanea larvae are rich in peroxisomes while the nidal regenerative and endocrine cells contain only a few. Most of the enterocytic peroxisomes are microperoxisomes lacking a crystalloid nucleoid, but peroxisomes with well developed nucleoid are also present. The peroxisomes are usually concentrated in the basal region of the cells but may also spread into the apical region and closely intermingle with absorptive lipid droplets. They significantly increase in number, when the larvae are regularly fed lipid-rich natural food or long-chain monounsaturated fatty acids that are unusual dietary components of these animals. This observation seems to indicate that the enterocytic peroxisomes are involved in chain shortening and degradation of fatty acids absorbed from the gut lumen. Numerous microperoxisomes are also present in the lipid-storing cells of the Malpighian tubules and fat body.     

The terminal bar in the larval midgut epithelium ofAeshna cyanea

Summary Thin section and freeze-fracture electron microscopy revealed that the terminal bars of the larval midgut epithelium ofAeshna cyanea consisted of extended smooth septate junctions (SSJ), multiple adhesive junctions and rare gap junctions. Freeze-fractures of native tissue suggested that the septal building units were anchored only in the external membrane leaflet by partially integrated proteins while the interseptal pegs were anchored partly in both leaflets by completely integrated proteins and partly by presumed peripheral proteins.Reversible depletion of the physiological Ca⁺⁺ concentration had no apparent structural effect on the SSJ of the terminal bars, but led to a reversible formation of junctional septa between the foot processes concomitant with a rearrangement of IMPs in the basolateral plasma membranes. The basolateral SSJ assembly and disassembly induced by reversible Ca⁺⁺ deprivation was interpreted as exaggerated response of an intrinsic capability normally related to the apical growth of regenerative cells and to the extrusion of degenerating cells. Lanthanum tracer ingested with hyperosmotic drinking solution was always found excluded from the basolateral intercellular spaces underneath the terminal bar, but there was a dual effect on the SSJ structure. Part of the junctions remained structurally intact, part was dissociated in the apical portion and invaded by tracer.Abbreviations EF exoplasmic fracture face - EGTA ethylenglycol-bis(2-aminoethylether)-N,N-tetraacetic acid - IMP intramembrane particle - PAS periodic acid Schiff reagent - PF protoplasmic fracture face - PSJ pleated septate junction - SDS sodium dodecyl sulphate - SSJ smooth septate junction Dedicated to Prof. Dr. E.Scholtyseck in honour of his 65th birthday.     

Crevice organs: Sensory structures on the wings of dragonflies (Insecta,Odonata)

Summary Crevice organs are small, elongate, innervated indentations in the hard cuticle of one wing vein of aeshnid dragonflies. There are four groups on each wing. The structure and orientation of crevice organs suggest that they detect strains in the cuticle during wing movements.I wish to thank Mr. R. Whitty and staff for technical assistance with the scanning electron microscope and Dr. E.E. Ball for helpful discussion     

Sperm ultrastructure in Chironomoidea (Insecta, Diptera)

The fine structure of spermatozoa from several species of chironomids, of Culicoides sp. (Ceratopogonidae) and of Odagmia pontina (Simulidae) was studied. A synapomorphic feature, consisting of nine kidney-shaped structures forming the centriole adjunct, was found in the chironomid species. All members of Chironomoidea share a mono-layered acrosome and a flagellar axoneme, provided with accessory tubules with 15 protofilaments in their tubular wall. The axoneme has a 9+9+2 pattern, but in an unidentified species of chironomid, a 9+9+0 model was observed where the central complex and the spokes are missing. Sperm motility is, however, maintained in all the examined species. The spermatozoa of this taxon have the tendency to complete maturation during their progression along the deferent ducts. Thus, in the proximal region of these ducts, they often show remnants of the spermatid cytoplasm.     

Sperm ultrastructure and spermiogenesis of Coniopterygidae (Neuroptera, Insecta)

The spermiogenesis and the sperm ultrastructure of several species of Coniopterygidae have been examined. The spermatozoa consist of a three-layered acrosome, an elongated elliptical nucleus, a long flagellum provided with a 9+9+3 axoneme and two mitochondrial derivatives. No accessory bodies were observed. The axoneme exhibits accessory microtubules provided with 13, rather than 16, protofilaments in their tubular wall; the intertubular material is reduced and distributed differently from that observed in other Neuropterida. Sperm axoneme organization supports the isolated position of the family previously proposed on the basis of morphological data.     

Ultrastructural and immunohistochemical study of endocrine cells in the midgut of the cockroach Blaberus craniifer (Insecta,Dictyoptera)

Summary The midgut of Blaberus craniifer is principally made up of columnar epithelial cells which are derived from small regenerative cells found grouped in nidi. Between them, small sparsely granulated cells with clear cytoplasm can be observed lying on the basal lamina. Mainly based on the size, shape and texture of their secretory granules, at least ten types of such endocrine cells have been identified. Five cell types contain a uniform population of dense granules: (1) medium-sized, round to oval granules; (2) small elongated granules; (3) large irregular granules; (4) oval granules with a highly osmiophilic core; (5) oval, haloed granules. Five others are characterized by a heterogeneous population of granules: (6) small, round to oval, variably electron-dense granules; (7) oval medium-sized granules of variable electron density; (8) large irregular granules of variable electron density; (9) small dense granules and large vesicles with filamentous material; (10) small dense granules and very large pale vesicles.In addition, near the regenerative cells, large cells characterized by very large, irregular, dense granules (up to 4 m), lack contact with the lumen, and reach the basal lamina only by slender cytoplasmic processes.Several antisera raised against mammalian peptides and amine were used to reveal axonal fibers and endocrine cells. Serotonin-like immunoreactivity is localized in a profuse innervation of the muscle layers that surround the epithelium, whereas cholecystokinin and methionine-enkephalin antisera stain a more moderate number of axonal fibers. Cholecystokinin-, methionine-enkephalin-, substance P-, vasoactive intestinal peptide-, somatoliberin-, and gonadoliberin-like immunoreactivities were detected in endocrine cells of the epithelium. While most of the cells appear pyramidal, oval, fusiform or bowl-shaped, and seem to lack contact with the lumen, cells reaching it have been detected reacting with antisera to cholecystokinin, substance P, vasoactive intestinal peptide, somatoliberin and gonadoliberin.     

Midgut epithelium formation in Thermobia domestica (Packard) (Insecta, Zygentoma)

The origin of midgut epithelium may begin either from yolk cells (energids), tips of stomo- and proctodaeum (ectoderm), inner layer (endoderm) or from both kinds of the above mentioned cells. The origin of the midgut epithelium in wingless insects (Apterygota) has still not been determined. In Thermobia domestica the formation of midgut is much delayed, and it completes in the post-embryonic stage, while the stomo- and the proctodaeum are well-developed in the embryonic period. The energids, which remain inside the yolk, start to migrate to its periphery, where they arrange singly close to cell membrane. The yolk mass with the energids at the 14th day of embryogenesis are referred to as the primary midgut. During the first instar larval stage more and more energids migrate to the yolk periphery and the cell membrane starts to form numerous foldings surrounding the groups of energids, which in turn lead to formation of isolated regenerative cell groups. Eventually the cell membrane invaginations reach the center of the yolk mass. Large cells of the primary epithelium, surrounding the newly formed midgut lumen are formed. The cells of the primary epithelium are filled with yolk and are equipped with microvilli pointing to the midgut lumen. As the yolk is being digested, the process of the primary epithelium cells degeneration begins. The cells are getting shorter and start to degenerate. The definitive midgut epithelium is formed from proliferating regenerative cells. It consists of regularly spaced regenerative cell groups as well as the epithelial cells. The ultrastructure of both these cell groups has been described.     

Neuropeptide F and the corn earworm, Helicoverpa zea: A midgut peptide revisited

The neuropeptide Y family of peptides is implicated in the regulation of feeding across a broad range of animals, including insects. Among vertebrates, neuropeptide Y exerts its actions mainly centrally, whereas peptide YY and pancreatic polypeptide arise from digestive tissues. Among invertebrates, neuropeptide F (NPF) is the sole counterpart of the NPY family. Shared features of NPF sequences derived for Lepidoptera indicate that the midgut peptide (Hez-MP-I) of the corn earworm, Helicoverpa zea, characterized more than a decade ago, is a carboxyl fragment of a full-length NPF. An antibody to Hez-MP-I was used to characterize the peptide's distribution in tissues of larvae, pupae, and adults. Immunostaining demonstrated NPF-related material both in nervous tissues and in abundant endocrine cells of the midgut. Radioimmunoassay of Hez-MP-I in the head, midgut and hemolymph of fifth instar larvae revealed concentration changes corresponding to development and feeding state. As with the vertebrate homologs, NPF may arise both centrally and peripherally to modulate the physiology of feeding and digestion of Lepidoptera.     

Cytochemistry and X-ray microprobe analysis of the midgut of Tomocerus minor lubbock (Insecta,Collembola) with special reference to the physiological significance of the mineral concretions

Summary Histochemical and cytochemical analyses have been made on the mineral concretions within the midgut cells of Tomocerus minor. The classical histochemical methods are not specific and precise enough and have been supplemented with cytochemical techniques on ultrathin sections. The most interesting of these was the K-pyroantimonate technique combined with glutaraldehyde-osmium fixation. This technique shows the distribution of cations such as Ca⁺⁺, K⁺, Mg⁺⁺ and Na⁺ on the concentric layers of the concretions. Chloride ions can be detected by means of the silver lactate technique. The action of calcium chelators such as E.D.T.A. shows an important distribution of calcium ions in the concretions. The spectra obtained by electron probe microanalysis from areas of fresh, dried and carbon coated midguts as well as from carbon coated semithin or ultrathin sections reveal the presence of Ca, K, Mg, S, Cl and P principally. Other elements such as aluminium, silicon and manganese have also been detected. Iron is not always present. The chemical and X-ray analytical investigations indicate that the midgut concretions are mainly built up of calcium, potassium, magnesium and sodium phosphates, perhaps associated with chlorides and carbonates. An organic matrix formed by polysaccharides seems to join the different mineral layers. These concretions may be formed within the vesicles of rough endoplasmic reticulum. The midgut cells are highly differentiated and very active in transport. Extensive basal infoldings and apical microvilli as well as lateral membranes are a site of small cationic deposits. The possible pathway of ion transport in the cell and the physiological significance of the concretions are discussed. The principal function of these concretions seems to be the maintenance of the mineral balance and to trap foreign and excess ions.

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Résumé L'analyse chimique des sphérocristaux de l'intestin moyen de Tomocerus minor a été réalisée. Les méthodes histochimiques courantes manquant souvent de spécificité et de sensibilité ont été complétées avec des méthodes cytochimiques sur coupes ultrafines. La plus intéressante a été la technique du pyroantimonate de K montrant la distribution des cations Ca⁺⁺, K⁺, Mg⁺⁺, Na⁺ sur les couches concentriques des sphérocristaux. La technique au lactate d'argent permet de déceler les ions Cl^-. L'action d'agents chélateurs du Ca tels l'E.D.TA. montre une importante distribution du calcium dans les sphérocristaux. L'analyse spectrographique d'étalements de mésentérons séches, carbonés et de coupes semi-fines ou ultrafines carbonées montre la présence de Ca, K, Mg, S, Cl, P, Na. D'autres éléments tels l'Al et le Si ont pu être détectés. Le Fe n'est pas toujours présent. Les sphérocristaux semblent formés essentiellement de phosphates de calcium, de potassium, de magnésium, de sodium associés peut-être à des chlorures ou des carbonates. Une matrice organique constituée essentiellement par des polysaccharides semble lier les différentes couches minérales. Ces sphérocristaux prennent naissance à l'intérieur des vésicules de l'ergastoplasme. Les cellules de l'intestin moyen sont très différenciées et sont le siège de nombreux transports actifs. Les replis basaux de la membrane plasmique, les microvillosités apicales, de même que les membranes latérales sont le siège de dépôts de cations. Le transport des ions dans les cellules ainsi que le rôle physiologique des sphérocristaux sont discutés. Le maintien de la balance hydrique ainsi que le piégeage d'ions étrangers ou en surplus semblent être la principale fonction des sphérocristaux.

     

The ultrastructure of the midgut epithelium in millipedes (Myriapoda,Diplopoda)

The midgut epithelia of the millipedes Polyxenus lagurus, Archispirostreptus gigas and Julus scandinavius were analyzed under light and transmission electron microscopies. In order to detect the proliferation of regenerative cells, labeling with BrdU and antibodies against phosphohistone H3 were employed. A tube-shaped midgut of three millipedes examined spreads along the entire length of the middle region of the body. The epithelium is composed of digestive, secretory and regenerative cells. The digestive cells are responsible for the accumulation of metals and the reserve material as well as the synthesis of substances, which are then secreted into the midgut lumen. The secretions are of three types – merocrine, apocrine and microapocrine. The oval or pear-like shaped secretory cells do not come into contact with the midgut lumen and represent the closed type of secretory cells. They possess many electron-dense granules (J. scandinavius) or electron-dense granules and electron-lucent vesicles (A. gigas, P. lagurus), which are accompanied by cisterns of the rough endoplasmic reticulum. The regenerative cells are distributed individually among the basal regions of the digestive cells. The proliferation and differentiation of regenerative cells into the digestive cells occurred in J. scandinavius and A. gigas, while these processes were not observed in P. lagurus. As a result of the mitotic division of regenerative cells, one of the newly formed cells fulfills the role of a regenerative cell, while the second one differentiates into a digestive cell. We concluded that regenerative cells play the role of unipotent midgut stem cells.     

Digestive enzymes in larvae of the leaf cutting ant, Acromyrmex subterraneus (Hymenoptera: Formicidae: Attini)

The digestive physiology and biochemistry of larvae of the leaf-cutting ant Acromyrmex subterraneus were investigated here. The activity of digestive enzymes was evaluated in the labial glands, midgut epithelium (soluble and particulate fractions), and in the lumen contents, separated into endo and ectoperitrophic regions. Enzymes with high levels of activity were partially characterised using chromatography and electrophoresis techniques. Microscope observations were carried out and the anatomy of the larval digestive tract was described here for the first time. Larvae fed with pH indicator solutions showed the anterior portion of the midgut to be acidic and the posterior portion neutral to alkaline, indicating that the pH of the different regions of the midgut could optimise certain enzyme activities, whilst inhibiting others. The flow rate of the intestinal contents was also evaluated in larvae fed with a dye solution. The slow flow rate is probably due to closure of the rear end of the larval midgut. No compartmentalisation of digestive enzymes acting on oligosaccharides and disaccharides in the ectoperitrophic space and on polysaccharides in the endoperitrophic space was observed here, which could also be related to the closure of the midgut. The digestive physiology of these larvae is therefore similar to ancestral Holometabola, a paradox when considering the highly evolved nature of these insects. The larval midgut demonstrated a large diversity of enzyme activities with high levels of alpha-amylase, alpha-mannosidase, chitinase, alpha-glucosidase, beta-glucosidase and proteinase. High levels of chitinase and amylase activities were detected in the labial glands of larvae. The enzyme profile reflected the necessity of the larvae to degrade the fungal substrate, their sole diet, and a probable source of some of the digestive enzymes detected here. When compared to adults, the larvae had a greater diversity and higher levels of enzyme activity, highlighting their importance as the "digestive caste" of the colony.