Fine structure of the midgut epithelium in the millipede <Emphasis Type="Italic">Telodeinopus aoutii</Emphasis> (Myriapoda,Diplopoda) with special emphasis on epithelial regeneration

The midgut of millipedes is composed of a simple epithelium that rests on a basal lamina, which is surrounded by visceral muscles and hepatic cells. As the material for our studies, we chose Telodeinopus aoutii (Demange, 1971) (Kenyan millipede) (Diplopoda, Spirostreptida), which lives in the rain forests of Central Africa. This commonly reared species is easy to obtain from local breeders and easy to culture in the laboratory. During our studies, we used transmission and scanning electron microscopes and light and fluorescent microscopes. The midgut epithelium of the species examined here shares similarities to the structure of the millipedes analyzed to date. The midgut epithelium is composed of three types of cells—digestive, secretory, and regenerative cells. Evidence of three types of secretion have been observed in the midgut epithelium: merocrine, apocrine, and microapocrine secretion. The regenerative cells of the midgut epithelium in millipedes fulfill the role of midgut stem cells because of their main functions: self-renewal (the ability to divide mitotically and to maintain in an undifferentiated state) and potency (ability to differentiate into digestive cells). We also confirmed that spot desmosomes are common intercellular junctions between the regenerative and digestive cells in millipedes.     

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.     

Fine structure of the midgut of Sinopanorpa tincta (Navás) (Mecoptera: Panorpidae)

Fine structure of the midgut and degeneration of the midgut epithelium of the scorpionfly Sinopanorpa tincta (Navás) adults were investigated using light microscopy and scanning and transmission electron microscopy. The results show that the tubular midgut lacks gastric caeca and is composed of an outer longitudinal and an inner circular muscle layer, a basal lamina, an epithelium and a lumen from the outside to inside. A peritrophic membrane was not found in the lumen. A mass of nodules was observed on the surface of the basal lamina. Three types of cells were recognized in the epithelium: digestive, secretory, and regenerative cells. The digestive cells contain irregular-shaped infoldings in the basal membrane and two types of microvilli in the apical membrane. The secretory cells are characterized by irregular shape and large quantities of secretory granules in the basal cytoplasm. The regenerative cells are triangular in shape and distributed only in the nodules. The epithelial cells are degenerated through programmed cell-death mechanisms (apoptosis and necrosis). The type, function, and degeneration of the epithelial cells of the midgut are briefly discussed.     

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.     

Anatomy and fine structure of the alimentary canal of the spittlebug Lepyronia coleopterata (L.) (Hemiptera: Cercopoidea)

The alimentary canal of the spittlebug Lepyronia coleopterata (L.) differentiates into esophagus, filter chamber, midgut (conical segment, tubular midgut), and hindgut (ileum, rectum). The filter chamber is composed of the anterior extremity of the midgut, posterior extremity of the midgut, proximal Malpighian tubules, and proximal ileum; it is externally enveloped by a thin cellular sheath and thick muscle layers. The sac-like anterior extremity of the midgut is coiled around by the posterior extremity of the midgut and proximal Malpighian tubules. The tubular midgut is subdivided into an anterior tubular midgut, mid-midgut, posterior tubular midgut, and distal tubular midgut. Four Malpighian tubules run alongside the ileum, and each terminates in a rod closely attached to the rectum. Ultrastructurally, the esophagus is lined with a cuticle and enveloped by circular muscles; its cytoplasm contains virus-like fine granules of high electron-density. The anterior extremity of the midgut consists of two cellular types: (1) thin epithelia with well-developed and regularly arranged microvilli, and (2) large cuboidal cells with short and sparse microvilli. Cells of the posterior extremity of the midgut have regularly arranged microvilli and shallow basal infoldings devoid of mitochondria. Cells of the proximal Malpighian tubule possess concentric granules of different electron-density. The internal proximal ileum lined with a cuticle facing the lumen and contains secretory vesicles in its cytoplasm. Dense and long microvilli at the apical border of the conical segment cells are coated with abundant electron-dense fine granules. Cells of the anterior tubular midgut contain spherical secretory granules, oval secretory vesicles of different size, and autophagic vacuoles. Ferritin-like granules exist in the mid-midgut cells. The posterior tubular midgut consists of two cellular types: 1) cells with shallow and bulb-shaped basal infoldings containing numerous mitochondria, homocentric secretory granules, and fine electron-dense granules, and 2) cells with well-developed basal infoldings and regularly-arranged apical microvilli containing vesicles filled with fine granular materials. Cells of the distal tubular midgut are similar to those of the conical segment, but lack electron-dense fine granules coating the microvilli apex. Filamentous materials coat the microvilli of the conical segment, anterior and posterior extremities of the midgut, which are possibly the perimicrovillar membrane closely related to the nutrient absorption. The lumen of the hindgut is lined with a cuticle, beneath which are cells with poorly-developed infoldings possessing numerous mitochondria. Single-membraned or double-membraned microorganisms exist in the anterior and posterior extremities of the midgut, proximal Malpighian tubule and ileum; these are probably symbiotic.     

The fine structure of the midgut epithelium in a centipede,Scolopendra cingulata (Chilopoda,Scolopendridae), with the special emphasis on epithelial regeneration

Scolopendra cingulata has a tube-shaped digestive system that is divided into three distinct regions: fore-, mid- and hindgut. The midgut is lined with a pseudostratified columnar epithelium which is composed of digestive, secretory and regenerative cells. Hemocytes also appear between the digestive cells of the midgut epithelium. The ultrastructure of three types of epithelial cells and hemocytes of the midgut has been described with the special emphasis on the role of regenerative cells in the protection of midgut epithelium. The process of midgut epithelium regeneration proceeds due to the ability of regenerative cells to proliferate and differentiate according to a circadian rhythm. The regenerative cells serve as unipotent stem cells that divide in an asymmetric manner.Additionally, two types of hemocytes have been distinguished among midgut epithelial cells. They enter the midgut epithelium from the body cavity. Because of the fact that numerous microorganisms occur in the cytoplasm of midgut epithelial cells, we discuss the role of hemocytes in elimination of pathogens from the midgut epithelium. The studies were conducted with the use of transmission electron microscope and immunofluorescent methods.     

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.     

Aedes aegypti midgut remodeling during metamorphosis

The Aedes aegypti midgut is restructured during metamorphosis; its epithelium is renewed by replacing the digestive and endocrine cells through stem or regenerative cell differentiation. Shortly after pupation (white pupae) begins, the larval digestive cells are histolized and show signs of degeneration, such as autophagic vacuoles and disintegrating microvilli. Simultaneously, differentiating cells derived from larval stem cells form an electron-dense layer that is visible 24 h after pupation begins. Forty-eight hours after pupation onset, the differentiating cells yield an electron-lucent cytoplasm rich in microvilli and organelles. Dividing stem cells were observed in the fourth instar larvae and during the first 24 h of pupation, which suggests that stem cells proliferate at the end of the larval period and during pupation. This study discusses various aspects of the changes during midgut remodeling for pupating A. aegypti.     

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.     

Ultrastructure and regeneration of midgut epithelial cells in Lithobius forficatus (Chilopoda,Lithobiidae)

Lithobius forficatus (Myriapoda, Chilopoda, Lithobiidae) is a widespread species of centipede that is common across Europe. Its midgut epithelial cells are an important line of defense against toxic substances that originate in food, such as pathogens and metals. Despite this important role, the biology of the midgut epithelium is not well known. Here we describe the ultrastructure of the midgut epithelium, as well as the replacement of degenerated midgut epithelial cells. The midgut epithelium of L. forficatus is composed of digestive, secretory, and regenerative cells. The cytoplasm of digestive cells shows regionalization in organelle distribution, which is consistent with the role of these cells in secretion of enzymes, absorption of nutrients, and accumulation of lipids and glycogen. Secretory cells, which do not reach the luminal surface of the midgut epithelium, possess numerous electron‐dense and electron‐lucent granules and may have an endocrine function. Hemidesmosomes anchor secretory cells to the basal lamina. Regenerative cells play the role of midgut stem cells, as they are able to proliferate and differentiate. Their proliferation occurs in a continuous manner, and their progeny differentiate only into digestive cells. The regeneration of secretory cells was not observed. Mitotic divisions of regenerative cells were confirmed using immunolabeling against BrdU and phosphohistone H3. Hemocytes associate with the midgut epithelium, accumulating between the visceral muscles and beneath the basal lamina of the midgut epithelium. Hemocytes also occur among the digestive cells of the midgut epithelium in animals infected with Rickettsia‐like microorganisms. These hemocytes presumably have an immunoprotective function in the midgut.     

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.     

Ultrastructure and immunolocalization of digestive enzymes in the midgut of Podisus nigrispinus (Heteroptera: Pentatomidae)

The predatory stinkbug Podisus nigrispinus has been utilized in biological control programs. Its midgut is anatomically divided into anterior, middle and posterior regions, which play different roles in the digestive process. We describe the midgut ultrastructure and the secretion of digestive enzymes in the midgut of P. nigrispinus. Midguts were analyzed with transmission electron microscopy and the digestive enzymes amylase, cathepsin L, aminopeptidase and α-glucosidase were immunolocalized. The ultrastructural features of the digestive cells in the anterior, middle and posterior midgut regions suggest that they play a role in digestive enzyme synthesis, ion and nutrient absorption, storage and excretion. The digestive enzymes have different distribution along the midgut regions of the predator P. nigrispinus. Amylase, aminopeptidase and α-glucosidase occur in three midgut regions, whereas cathepsin L occurs in the middle and posterior midgut regions. The anterior midgut region of P. nigrispinus seems to play a role in water absorption, the middle midgut may be involved in nutrient absorption and the posterior midgut region is responsible for water transport to the midgut lumen.     

Changes in the midgut diverticula in the harvestmen Amilenus aurantiacus (Phalangiidae,Opiliones) during winter diapause

The harvestmen Amilenus aurantiacus overwinter in diapause in hypogean habitats. The midgut diverticula have been studied microscopically (light microscopy, TEM) and biochemically (energy-storing compounds: lipids and glycogen) to analyze changes during this programmed starvation period. Throughout the investigated period, the epithelium of the midgut diverticula is composed of secretory cells, digestive cells and adipocytes. Additionally, after the middle of overwintering, the excretory cells appear, and two assemblages of secretory cells are present: the SC1 secretory cells are characterized by electron-dense cytoplasm with numerous protein granules, and the SC2 cells by an electron-lucent cytoplasm with fewer protein granules. The autophagic activity is observed from the middle of overwintering, indicating its vital role in providing nutrients during this non-feeding period. Lipids and glycogen are present in the midgut diverticula cells, except in the excretory cells. Measurements of the lipid droplet diameters and the lipid quantities yielded quite comparable information on their consumption. Lipids are gradually spent in both sexes, more rapidly in females, owing to ripening of the ovaries. Glycogen rates decrease towards the middle, and increase just before the end of overwintering, indicating that individuals are preparing for the epigean active ecophase.     

Regeneration of radiation-damaged digestive tissues in juvenile Pacific oysters (Crassostrea gigas)

Juvenile Pacific oysters, Crassostrea gigas, were irradiated with 16 and 40 krad and their tissues examined histologically. Degenerative syndromes and tissue regeneration processes were determined for the stomach, gut, collecting ducts, and digestive tubules. Following degeneration, tissue regeneration was observed in the digestive tissues of most oysters exposed to 16 krad and in a limited number exposed to 40 krad. Regeneration was first observed in the digestive tubules and subsequently in the stomach, gut, and collecting ducts. Cellular repopulation of the digestive tubules involved epithelialization with large, undifferentiated crypt cells which then differentiated into functional secretory and absorptive cells. Regeneration in the stomach, gut, and collecting ducts was initiated by proliferative islands of small basophilic cells. Mitotic division of those cells and their subsequent differentiation into functional epithelial cells resulted in the rapid restoration and apparent recovery of the affected tissues. The results of these studies indicate that radioresistance of juvenile C. gigas may in part be due to the remarkably efficient regenerative mechanisms involved in replacing injured or lost digestive tissues.     

Ultrastructure of midgut endocrine cells in the adult mosquito, Aedes aegypti

The ultrastructure of endocrine cells in the midgut of the adult mosquito, Aedes aegypti, resembled that of endocrine cells in the vertebrate gastro-intestinal tract. Midgut endocrine cells, positioned basally in the epithelium as single cells, were cone-shaped and smaller than the columnar digestive cells. The most distinctive characteristic of endocrine cells was numerous round secretory granules along the lateral and basal plasma membranes where contents of the granules were released by exocytosis. Secretory granules in each individual cell were exclusively of one type, either solid or 'haloed', and for all cells observed, the range in granule diameter was 60-120 nm. The cytoplasm varied in density from clear to dark. Lamellar bodies were prominent in the apical and lateral cellular regions and did not exhibit acid phosphatase activity. The basal plasma membrane was smooth adjacent to the basal lamina, whereas in digestive cells the membrane formed a labyrinth. Some endocrine cells reached the midgut lumen and were capped by microvilli; a system of vesicles and tubules extended from beneath the microvilli to the cell body. An estimated 500 endocrine cells were distributed in both the thoracic and abdominal regions of the adult midgut. In one midgut, we classified a sample of endocrine cells according to cytoplasmic density and granule type and size; endocrine cells with certain types of granules had specific distributions within the midgut.     

庭疾灶螽中肠及马氏管结构

【目的】本研究旨在以庭疾灶螽Tachycines asynamorus为例探索驼螽消化系统和排泄系统在结构上与其生活环境的适应关系。【方法】运用解剖学方法、石蜡切片技术、冰冻切片技术及超薄切片技术对庭疾灶螽中肠及马氏管的结构进行研究。【结果】庭疾灶螽中肠向前延伸出3个胃盲囊包围着前胃。中肠上皮由再生细胞、柱状上皮细胞和内分泌细胞构成,具有典型的再生细胞龛;闭合型内分泌细胞紧贴在再生细胞龛的外围,基底区聚集大量的分泌颗粒。柱状上皮细胞内聚集有2类大的分泌颗粒：线团状颗粒和电子密度很高的球状颗粒;中肠管腔内有明显的围食膜结构,中肠基底部由基膜和肌肉层组成。马氏管着生在中后肠的交界处,从横切面看马氏管管壁具有3~5个细胞,细胞近管腔端部具有大量长微绒毛,细胞质内分布着电子致密的同心圆球晶体,基底膜内折形成膜迷路。【结论】庭疾灶螽中肠柱状上皮细胞的线团状颗粒由微丝包裹;内分泌细胞由再生细胞龛中的细胞分化而来,产生内分泌颗粒并将其排到血腔;中肠基膜发达,包含微丝与复合糖成分,基膜通过对中肠上皮细胞的支撑作用为肠道蠕动提供保障。庭疾灶螽马氏管细胞中可见大量颗粒和大量同心圆球晶体,推测可能是一种储存排泄。     

Cytological and enzyme-histochemical investigations on the digestive organs of Nautilus pompilius (Cephalopoda, Tetrabranchiata)

The foregut, stomach, caecum, midgut, and rectum of the digestive tract of Nautilus pompilius L.were investigated with ultrastructural and enzyme-cytological methods. Three different cell types were identified within the lamina epithelialis mucosae: main cells, goblet cells, and cells with secretory granules. The main cell type is the epithelial cell with microvilli, a basal nucleus surrounded by dictyosomes, rough endoplasmic reticulum, mitochondria, and electron-dense granules identified as lysosomes in the apical part of the cell. In the caecum this cell type contains endosymbiotic bacteria. The presence of endocytotic vesicles and the storage of lipids in the caecum indicate that this organ is involved in the process of absorption. In the caecum and the longitudinal groove of the rectum the main cells are, in addition, ciliated, facilitating the transport of food particles and faeces. Two types of goblet cells are found in all organs except in the stomach, forming a gliding path for food particles and protecting the epithelium. In the foregut and rectum, cells with electron-dense granules were recognized as the third type. The conspicuous secretory cells of the rectum represent a delimited rectal gland; its possible biological function is discussed. The tunica muscularis in all organs of the digestive tract consists of obliquely striated muscle cells innervated by axons containing transparent, osmiophilic and dense-cored vesicles. Positive reactions for acid and alkaline phosphatase, monoamine oxidase, β-glucuronidase, and trypsin- and chymotrypsin-like enzymes are localized in the lamina epithelialis mucosae.     

Ultrastructural study of cholecystokinin-like immunoreactivity in endocrine cells of the insect midgut

Summary Electron-microscopic immunocytochemistry for the demonstration of CCK-like material in basal endocrine cells of the midgut of Aeschna cyanea, Locusta migratoria, Carausius morosus and Periplaneta americana was performed by use of the peroxidase-antiperoxidase procedure and the colloidal gold method. Immunoreactive cells appeared scattered among digestive and regenerative cells of the epithelium. Immunoreactivity was specifically detected over round to oval electron-dense granules whose size appeared rather different from species to species. Thus, the average size of 30% (d30) of the largest granules ranged from 312 nm in Periplaneta americana to 159 nm in Carausius morosus with intermediate values in Aeschna cyanea (d30=195 nm) and Locusta migratoria (d30=225 nm).     

Degeneration and cell regeneration in the midgut of Podisus nigrispinus (Heteroptera: Pentatomidae) during post-embryonic development

Cell death, proliferation, and differentiation in some developmental stages of insects have been studied in the midgut of ametabolous, which undergo only continuous growth, and holometabolous, which undergo complete metamorphosis. However, in hemimetabolous insects, evolutionarily intermediate between ametabolous and holometabolous, midgut reorganization during the post-embryonic development has been poorly studied. The present study evaluates the post-embryonic development of the midgut of a hemimetabolous insect, Podisus nigrispinus, to test the hypothesis that these insects have programmed cell death and proliferation followed by differentiation of regenerative cells during midgut growth from nymphs to adult. The morphometrical data showed a 6-fold increase in midgut length from the first instar nymph to the adult, which did not result from an increase in the size of the midgut cells, suggesting that the growth of the midgut occurs by an increase in cell number. Cell death was rarely found in the midgut, whereas proliferation of regenerative cells occurred quite frequently. The growth of the midgut of P. nigrispinus appears to result from the proliferation of regenerative cells present in the epithelium; unlike ametabolous and holometabolous insects, the midgut of P. nigrispinus does not undergo extensive remodeling, as shown by the low frequency of digestive cell death.     

Ultrastructure and Immunofluorescence of the midgut of Bombus morio (Hymenoptera: Apidae: Bombini)

Bumblebees are widely distributed across the world and have great economic and ecological importance as pollinators in the forest as well as in agriculture. The insect midgut consists of three cell types, which play various important roles in digestion, absorption, and hormone production. The present study characterized the anterior and posterior midgut regions of the bumblebee, Bombus morio. The digestive, regenerative and endocrine cells in the midgut showed regional differences in their number, nuclear size, as well as the size of the striated border. Ultrastructurally, the digestive cells contained many mitochondria and long microvilli; however, in the anterior midgut region, these cells showed dilated basal labyrinths with a few openings for the hemocoel, whereas the labyrinths of the basal posterior region remained inverse characteristics. Thus, the characterization of the midgut of B. morio supported an ecto-endoperitrophic circulation, contributing to a better understanding of the digestive process in this bee.