Investigation of the midgut structure and ultrastructure in <Emphasis Type="Italic">Cimex lectularius</Emphasis> and <Emphasis Type="Italic">Cimex pipistrelli</Emphasis> (Hemiptera: Cimicidae)

Cimicidae are temporary ectoparasites, which means that they cannot obtain food continuously. Both Cimex species examined here, Cimex lectularius (Linnaeus 1758) and Cimex pipistrelli (Jenyns 1839), can feed on a non-natal host, C. lectularius from humans on bats, C. pipistrelli on humans, but never naturally. The midgut of C. lectularius and C. pipistrelli is composed of three distinct regions—the anterior midgut (AMG), which has a sack-like shape, the long tube-shaped middle midgut (MMG), and the posterior midgut (PMG). The different ultrastructures of the AMG, MMG, and PMG in both of the species examined suggest that these regions must fulfill different functions in the digestive system. Ultrastructural analysis showed that the AMG fulfills the role of storing food and synthesizing and secreting enzymes, while the MMG is the main organ for the synthesis of enzymes, secretion, and the storage of the reserve material. Additionally, both regions, the AMG and MMG, are involved in water absorption in the digestive system of both Cimex species. The PMG is the part of the midgut in which spherites accumulate. The results of our studies confirm the suggestion of former authors that the structure of the digestive tract of insects is not attributed solely to diet but to the basic adaptation of an ancestor.     

Histochemical changes in the midgut of two ixodid tick speciesBoophilus microplus andRhipicephalus appendiculatus during digestion of the blood meal

The changes in the midgut epithelia of two ixodid tick species,Boophilus microplus andRhipicephalus appendiculatus, have been studied using several histochemical techniques. It was revealed that there is an accumulation of RNA at the time of tick attachment to the host and prior to the arrival of the blood meal, indicating that the midgut digest cell is furnished with the machinery characteristic of a synthetic cell. There appears to be a synchrony in the appearance of granules with peroxidase activity and the uptake of haemoglobin into the midgut digest cells. Alkaline phosphatase activity was observed in the midgut epithelia of all ticks except in a few of the long-starved ticks, and was concentrated in the apical plasma membrane regions of those digest cells involved in absorption and the intracellular digestion of haemoglobin. The presence of these enzymes suggests that the midgut digest cell is a multifunctional cell capable of both secretory and digestive activities. The colloidal material in the midgut lumen was found to result from the accretion of several products both secreted and excreted by the midgut epithelial cells and exhibited different staining reactions depending on which component dominated. The nature of the material suggests that in addition to its digestive function it may serve as a sink to bind all the by-products of digestion and thereby facilitate their excretion.     

Ultrastructure of the midgut in Ornithodoros papillipes (Acarina, Argasidae) nymphs in the blood assimilation period

Three types of cells (reserve, digestive and secretory) differing in their ultrastructural characters were found to exist in the epithelium of the midgut of nymphs of O. papillipes during the blood assimilation. Reserve and secretory cells are dominant at the first stage of digestion while digestive cells are most abundant at the second one. Ultrafine structure of secretory and digestive cells points to the labile relationship between these types of cells. The same digestive cells assimilate blood both by means of phagocytosis of formed elements and by pinocytosis of its liquid components. The identity of ultrastructural peculiarities of endocytosis and intracellular blood digestion in ixodid and argasid ticks is shown.     

Ultrastructural studies on the midgut epithelium and digestion in the female tickArgas (Persicargas) arboreus (Ixodoidea: Argasidae)

The ultrastructure of the midgut epithelium and digestion in the female tickArgas (Persicargas) arboreus are described before and after feeding, up to oviposition. The epithelium consists of secretory cells, digestive cells (DI and DII), and regenerative cells which may differentiate into any of the other cell types. In unfed ticks, the midgut wall consists mainly of type DII digestive cells retained from a previous feeding, and a few regenerative cells. Within 3 days after the tick feeding, haemolysis of the host blood components occurs in the midgut lumen. Secretory cells, the first differentiation of the regenerative cells, are presumed to produce a haemolysin and an anticoagulant which are released by merocrine and holocrine secretions. The DII cells seen in unfed ticks, and secretory cells which have completed their secretory cycle, start to have a specialized surface for endocytosis characteristic of type DI digestive cells. From 5 to 7 days after feeding up to the female oviposition, type DI cells which have completed their endocytosis are transformed into type DII digestive cells specialized for intracellular digestion and the storage of reserve nutrients required by the tick for long starvation. The various phases of the digestive cycle are considered according to ultrastructural changes of the midgut epithelium.     

Cellular localisation of aminopeptidase in the midgut of Rhodnius prolixus Stål (Hemiptera:Reduviidae) during blood digestion

Summary By use of the artificial substrate leucyl--naphthylamide, aminopeptidase was localised in the midgut cells of the haematophagous insect Rhodnius prolixus before and at various times up to 25 days after a meal of rabbit blood. The enzyme was primarily associated with the membranes of the microvilli, with extracellular membrane layers and with the lysosomes of the midgut cells. Aminopeptidase activity was also detected on the rough endoplasmic reticulum and at the periphery of intracellular storage vesicles. The absence of aminopeptidase on the microvilli of the crop supports the conclusion that the crop is not involved in the digestion of blood-meal proteins and that protein digestion is restricted to the intestine. The sites of localisation are in accordance with models for the spatial separation of digestive enzymes in the midgut of several non-haematophagous insects, and this suggests that aminopeptidase plays a major role in the terminal digestion of the blood meal. The changes in enzyme localisation during the digestive period correlate with previously described cycles of digestive-enzyme activity and changes in midgut ultrastructure. A model for blood protein digestion in R. prolixus is described.     

Morphofunctional changes in the midgut of tick females of the genus Ixodes (Acari: Ixodidae) during and after feeding

Cyclic changes of the midgut epithelium were observed in females of 5 ticks species of the genus Ixodes during 7-10 days of feeding. The midgut epithelium of unfed females is represented by the digestive cells of nymphal phase and stem cells. The digestive cells of nymphal phase are functional during 1.5-2 days after attachment of the tick, and then, after the tearing away they go into the gut lumen. The secretory cells substitute the digestive cells of nymphal phase and finish their growth during the 4-4.5 days. Secretion of digestive enzymes is performed by the holocrine type with tearing away a whole cell. Intracellular digestion takes place in the digestive cells of four consequent generations. The secretory and digestive cells form a peritrophic matrix on their surface. The presence of peritrophic matrix gives an evidence the maturity and functional activity of the secretory and digestive cells. We suggest, that the peritrophic matrix takes part in intracellular digestion, namely in the process of micropinocytosis. The phagocytosis was not found in the ticks investigated. Digestion in the midgut lumen is performed by enzymes of the ruptured secretory and digestive cells, that is proved by the haemolysis of erythrocytes in the zone of their contact with these cells. The digestive cells of each generation functioned almost synchronously, with largest difference in starting about 12 hours.     

Signaling by intracellular Ca2+ and H+ in larval mosquito (Aedes aegypti) midgut epithelium in response to serosal serotonin and lumen pH

The midgut of larval mosquitoes (Aedes aegypti) mediates a cycle of alkali secretion in the anterior segment (AMG) followed by partial reacidification in the posterior segment (PMG); both processes are serotonin-dependent. Here we report that intracellular Ca(2+)(Ca(i)(2+)) as indicated by Fura-2 fluorescence, is elevated in both tissues in response to serotonin, but the time courses differ characteristically in the two gut segments, and Ca(2+)-free solution abolishes the serotonin response in AMG, but not in PMG, whereas Thapsigargin, an inhibitor of endoplasmic Ca(2+) transport, abolished responsiveness to 5-HT in PMG. These results suggest the origins for the Ca(2+) signal differ between the two tissues. Quantitative real-time RT-PCR revealed expression of 5 putative 5-HT receptor types in AMG, including 5-HT(2)-like receptors which would be expected to initiate a Ca(2+) signal. None of these receptors were highly expressed in PMG. Cyclic AMP (cAMP) is a secretagogue for both tissues, but H89, an inhibitor of Protein Kinase A (PKA), is also a secretagogue, suggesting that the stimulatory effect of cAMP involves a non-PKA pathway. Cytochalasins B and D block the effect of 5-HT in AMG, suggesting a vesicle-fusion mechanism of activation of the basal V-ATPase in this tissue. Finally, in PMG, elevation of luminal pH increases (Ca(i)(2+)) and decreases intracellular pH as measured by BCECF fluorescence. These responses suggest that the rate of acid secretion by PMG might be responsive to local demand for luminal reacidification as well as to serosal serotonin.     

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.     

Ultrastructural localisation of cathepsin B in the midgut of Rhodnius prolixus Stål ( Hemiptera : Reduviidae) during blood digestion

The artificial substrate N-benzoyl-dl-arginine-β-naphthylamine, was used to localise cathepsin B in midgut cells of the haematophagous insect, Rhodnius prolixus Stål (Hemiptera : Reduviidae), during blood digestion. Cathepsin B was localised primarily in the lysosomes of cells from all 3 midgut regions and in Golgi vesicles of the digestive intestinal regions, but not in association with any other cellular structures. The timing of localisation correlated with previously described cycles of endoproteinase activity and with known ultrastructural modifications to the midgut cells. Secretory vesicles, which originated from the Golgi complexes, were present only in the intestinal regions, and in the anterior intestine, they showed a strong positive correlation (r = 0.939, P = 0.01) with post-feeding cathepsin B activity. Cathepsin B plays a major role in primary extracellular digestion of blood proteins, and is active in the midgut lumen and lysosomes rather than in association with the microvilli.     

Morphometric analysis of Rhodnius prolixus Stal (Hemiptera:Reduviidae) midgut cells during blood digestion

Post-feeding ultrastructural modifications to the midgut cells of Rhodnius prolixus are quantified using morphometry. Changes in relative and absolute volumes and/or surface areas are demonstrated for the whole cells, nuclei, mitochondria, rough endoplasmic reticulum, lysosomes, Golgi apparatus, storage vesicles, glycogen, microvilli, and basal labyrinth, before and during blood digestion. These parameters are separately determined for cells from each of the three midgut regions, and are correlated against previously published cycles of digestive enzyme activities. The results support the proposed division of the midgut of R. prolixus into three functional regions: the anterior midgut or crop is the site of water transport immediately after feeding, and of lipid and glycogen storage. No protein digestion occurs in this region. The anterior intestine is the site of most proteinase synthesis and secretion, although limited absorption and nutrient storage also occurs. The posterior intestine is responsible for some secretory activity, but is also implicated as the most important region for absorption of digested nutrients and for carbohydrate absorption and storage.     

Ultrastructure and functional features of midgut of an adult water mite Teutonia cometes (Koch, 1837) (Hydrachnidia: Teutoniidae)

Shatrov, A. B. 2010. Ultrastructure and functional features of midgut of an adult water mite Teutonia cometes (Koch 1837) (Hydrachnidia: Teutoniidae). —Acta Zoologica (Stockholm) 91 : 222–232 The midgut of the adult water mite Teutonia cometes (Koch 1837) (Hydrachnidia: Teutoniidae) was investigated by means of transmission electron microscopy and on semi‐thin sections. The midgut is represented by a blind sac composed of the narrow ventriculus, two proventricular lateral diverticula and three pairs of postventricular caeca. A single‐layered epithelium consists of one type of endodermal digestive cells of quite different shape and size, which may form protrusions into the midgut lumen. The large nuclei are frequently lobed and contain one to three nucleoli. The apical cell membrane forms short scarce microvilli, between their bases the pinocytotic vesicles of unspecific macropinocytosis as well as the narrow pinocytotic canals are formed and immersed into the cell. The intracellular digestion of the food ingested into the midgut after extraintestinal digestion is predominant. The pinocytotic vesicles fuse with small clear vesicles of proposed Golgi origin to form secondary lysosomes. The digestive cells also contain small amounts of rough endoplasmic reticulum, variously structured heterolysosomes, residual materials in the form of both the small electron‐dense bodies and the large variously granulated substances, reserve nutritive materials such as lipid and glycogen, as well as clear vacuoles. Residual materials are obviously extruded from the cells into the gut lumen.     

银盾革蜱的中肠上皮变化与血餐消化

用光学显微镜及电子显微镜对不同生理状况下银盾革蜱Dermacentor niveus Neumann雌虫中肠上皮及血餐消化进行了研究.饿蜱的中肠只由一种干细胞组成,脂滴作为饿蜱营养的贮藏形式.非滞育蜱消化分三个阶段,即第一连续消化阶段,减慢消化阶段及第二连续消化阶段.吸血后中肠上皮共观察到四种细胞类型,即替代细胞、分泌细胞、消化细胞及卵赞原细胞.滞育蜱第一连续消化阶段延长.饱血后60天到120天,消化作用几近停止,为停滞消化阶段.卵黄原细胞的超微结构有明显改变.滞育解除后,开始进行减慢消化阶段及第二连续消化阶段.     

Increased expression of ATG genes during nonfeeding periods in the tick Haemaphysalis longicornis

Ticks are long-lived hematophagous arthropods and have tolerance to starvation. They can survive without food during the host-seeking period for several months to years. To understand how ticks obtain energy over a long period of non-feeding (starvation), we focused on autophagy, a crucial proteolysis system via the lysosomes for various cellular processes that is induced during starvation in eukaryotes. In the present study, EST databases for several organs of the tick Haemaphysalis longicornis led to the identification of HlATG3, HlATG4 and HlATG8, homologues of 3 autophagy-related (ATG) genes, ATG3, ATG4 and ATG8/LC3/GABARAP, respectively, which are essential for the Atg8 conjugation system in model animals. Real-time PCR results revealed that the expression of HlATG3, HlATG4 and HlATG8 in the tick showed higher levels during the non-feeding period than the feeding period, suggesting that the Atg8 conjugation system is at work in unfed ticks. Notably, their expression levels were higher in the midgut, a digestive organ, of unfed than fed adults. Histological analysis demonstrated that lipids and glycogen accumulated within the epithelial cells of the midgut in unfed ticks, implying that the midgut of unfed ticks serves as storage of those components as nutrients during non-feeding. Furthermore, autophagic organelles were found in the midgut undifferentiated cells of unfed ticks. The starved condition appears to be associated with the increased expression of HlATG genes in the midgut of unfed ticks. Tick autophagy might help compensate for the loss of nutrients derived from host blood components during the non-feeding period.     

The role of autophagy in the midgut epithelium of Eubranchipus grubii (Crustacea,Branchiopoda, Anostraca)

Eubranchipus grubii (Crustacea, Branchiopoda, Anostraca) is an omnivorous filter feeder whose life span lasts no more than 12 weeks. Adult males and females of E. grubii were used for ultrastructural studies of the midgut epithelium and an analysis of autophagy. The midgut epithelium is formed by columnar digestive cells and no regenerative cells were observed. A distinct regionalization in the distribution of organelles appears – basal, perinuclear and apical regions were distinguished. No differences in the ultrastructure of digestive cells were observed between males and females. Autophagic disintegration of organelles occurs throughout the midgut epithelium. Degenerated organelles accumulate in the neighborhood of Golgi complexes, and these complexes presumably take part in phagophore and autophagosome formation. In some cases, the phagophore also surrounds small autophagosomes, which had appeared earlier. Fusion of autophagosomes and lysosomes was not observed, but lysosomes are enclosed during autophagosome formation. Autophagosomes and autolysosomes are discharged into the midgut lumen due to apocrine secretion. Autophagy plays a role in cell survival by protecting the cell from cell death.     

Digestive cells in the midgut of Triatoma vitticeps (Stal, 1859) in different starvation periods

Triatoma vitticeps (Stal, 1859) is a hematophagous Hemiptera that, although being considered wild, can be found in households, being a potential Chagas’ disease vector. This work describes the histology and ultrastructure of the midgut of T. vitticeps under different starvation periods. Fifteen adults of both sexes starved for 3, 7, 20 and 25 days were studied. In general, digestive cells had apical microvilli, basal plasma membrane infoldings and central nucleus. The perimicrovillar membrane was found in all insects examined. Digestive cells of anterior midgut had lipid droplets, glycogen granules, developed basal labyrinth associated with mitochondria suggesting their role in nutrient storage and in fluid and ion transport. The cells of median and posterior regions of the midgut were rich in rough endoplasmic reticulum, lysosomes, vesicles and granules with different electron-densities. Moreover, cells of the posterior portion of the midgut had hemozoyn granules and mitochondria in the apical cytoplasm close to microvilli, suggesting their role in blood digestion and active nutrient absorption. The midgut of T. vitticeps showed differences in digestive cells associated with the time after feeding, and the increase of vesicles amount in long starvation periods, which suggests enzyme storage, which is readily used after a blood meal.     

Morphological Study of Digestive Cycle in Bloodsucking Biting Midges of Genus Cu1icoides

Changes in the midgut of biting midges Culicoides punctatus (Mg.) and C. grisescens (Edw.) in the course of digestion of the single blood portion were studied using methods of light and electron microscopy. Essential differences are shown in structure of intestinal epithelial cells of young non-fed females and adult individuals completing each digestive cycle. Blood digestion in adult females of both species takes about 3.5–4.0 days and is not accompanied by formation of blood thrombus. Formation of the peritrophic membrane occurs at the period of 12 h to 2.5 days after blood-feeding and is associated with secretory activity of cells of the posterior part of midgut. Functions of secretion, absorption, and transport of substances are performed by all cells of intestinal epithelium and to the great extent are overlapped in time. Peculiarities of structural organization of digestion in midges in comparison with other blood-sucking insects are discussed.__________Translated from Zhurnal Evolyutsionnoi Biokhimii i Fiziologii, Vol. 41, No. 2, 2005, pp. 176–185.Original Russian Text Copyright © 2005 by Filimonova.     

Ultrastructural changes in the abdominal midgut of the mosquito, Culiseta melanura, during the gonotrophic cycle

Abdominal midguts of the mosquito, Culiseta melanura, were examined by light and electron microscopy 1 hr-14 days days after blood feeding. Epithelial cells were drastically altered from columnar to squamous in form after engorgement, and returned to columnar by day 4 after feeding. Accumulation of mitochondria along brush borders of digestive cells, followed by the appearance of large secondary lysosomes, accompanied blood digestion. Evidence was obtained that myelin-like material in the lysosomes, probably the result of mitochondrial autolysis, is extruded into the lumen. Digestive cells resumed their pre-blood meal appearance by 10-14 days post-engorgement. Regenerative cells were scattered throughout the basal portion of the epithelium, along with endocrine cells. Other midgut cells containing large, microvilli-lined apical cavities were identified in most specimens. No evidence of division or differentiation was obtained for any cell types.