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.     

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.     

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.     

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。     

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.     

Functional ultrastructure of the midgut of the fire ant Solenopsis saevissima Forel 1904 (Formicidae: Myrmicinae)

Solenopsis saevissima has a midgut composed of columnar, regenerative, and goblet cells. The midgut epithelium was covered by a basal lamina. Outside the basal lamina, layers of inner oblique, circular, and outer longitudinal muscles were present. Columnar cells showed a basal plasma membrane containing numerous folds, mitochondria, and the nucleus. Rough endoplasmic reticulum, Golgi bodies, membrane bounded vacuoles, and spherocrystals were found in this region. The apical plasma membrane was constituted by microvilli, which were above a region rich in mitochondria. Regenerative cells were found in groups lying by the basal lamina. Goblet cells were associated with an ion-transporting mechanism between the haemolymph and the midgut epithelium. These cells were lying by the midgut lumen and large microvilli were evident, but the cytoplasmic features were similar to the columnar cells.     

Morphology and ultrastructure of the midgut in Piscicola geometra (Annelida, Hirudinea)

This paper presents information on the organization of the midgut and its epithelium ultrastructure in juvenile and adult specimens of Piscicola geometra (Annelida, Hirudinea), a species which is a widespread ectoparasite found on the body and gills and in the mouth of many types of fish. The analysis of juvenile nonfeeding specimens helped in the explanation of all alterations in the midgut epithelium which are connected with digestion. The endodermal portion (midgut) of the digestive system is composed of four regions: the esophagus, the crop, the posterior crop caecum, and the intestine. Their epithelia are formed by flat, cuboidal, or columnar digestive cells; however, single small cells which do not contact the midgut lumen were also observed. The ultrastructure of all of the regions of the midgut are described and discussed with a special emphasis on their functions in the digestion of blood. In P. geometra, the part of the midgut that is devoid of microvilli is responsible for the accumulation of blood, while the epithelium of the remaining part of the midgut, which has a distinct regionalization in the distribution of organelles, plays a role in its absorption and secretion. Glycogen granules in the intestinal epithelium indicate its role in the accumulation of sugar. The comparison of the ultrastructure of midgut epithelium in juvenile and adult specimens suggests that electron-dense granules observed in the apical cytoplasm of digestive cells take part in enzyme accumulation. Numerous microorganisms were observed in the mycetome, which is composed of two large oval diverticles that connect with the esophagus via thin ducts. Similar microorganisms also occurred in the cytoplasm of the epithelium in the esophagus, the crop, the intestine, and in their lumen. Microorganisms were observed both in fed adult and unfed juvenile specimens of P. geometra, which strongly suggests that vertical transmission occurs from parent to offspring.     

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.     

Structural and ultrastructural features of boar seminal vesicles

The morphological features of boar seminal vesicles were examined by light and transmission microscopy. Boar seminal vesicles consist of glandular tissue arranged in multiple lobules containing a system of ramified secretory tubules. The secretory tubules are composed of a mucosa formed by an epithelium and an underlying lamina propria and, are surrounded by a muscular layer. The epithelium is made up of columnar cells and occasional basal cells. Mast cells are frequently found among epithelial cells. Three types of columnar cells, considered different stages of the secretory cell cycle, are present: principal cells, clear cells and dense cells. Principal cells are functionally differentiated cells characterised by abundant mitochondria, great development of the rough endoplasmic reticulum and presence of secretory granules in their cytoplasm. The apical surface of many principal cells shows apical blebs filled with PAS-positive material. No acid mucosubstances are detected. Microvilli cover the apical surface except in the apical blebs. Dense cells, arranged between principal cells, are also functional differentiated cells but with signs of cellular degeneration. Clear cells are an initial differentiated stage of columnar cells and are characterised by the presence of a poorly developed rough endoplasmic reticulum and by the absence of secretory granules. Proliferating cells are present among columnar cells. Basal cells contain scarce cytoplasm, few organelles and no secretory granules. The lack of mitotic activity in these cells suggests that they do not act as precursors of columnar cells.     

ACTIVE TRANSPORT BY THE CECROPIA MIDGUT : II. Fine Structure of the Midgut Epithelium

A morphological basis for transcellular potassium transport in the midgut of the mature fifth instar larvae of Hyalophora cecropia has been established through studies with the light and electron microscopes. The single-layered epithelium consists of two distinct cell types, the columnar cell and the goblet cell. No regenerative cells are present. Both columnar and goblet cells rest on a well developed basement lamina. The basal portion of the columnar cell is incompletely divided into compartments by deep infoldings of the plasma membrane, whereas the apical end consists of numerous cytoplasmic projections, each of which is covered with a fine fuzzy or filamentous material. The cytoplasm of this cell contains large amounts of rough endoplasmic reticulum, microtubules, and mitochondria. In the basal region of the cell the mitochondria are oriented parallel to the long axes of the folded plasma-lemma, but in the intermediate and apical portions they are randomly scattered within the cytoplasmic matrix. Compared to the columnar cell, the goblet cell has relatively little endoplasmic reticulum. On the other hand, the plications of the plasma membrane of the goblet cell greatly exceed those of the columnar cell. One can distinguish at least four characteristic types of folding: (a) basal podocytelike extensions, (b) lateral evaginations, (c) apical microvilli, and (d) specialized cytoplasmic projections which line the goblet chamber. Apically, the projections are large and branch to form villus-like units, whereas in the major portion of the cavity each projection appears to contain an elongate mitochondrion. Junctional complexes of similar kind and position appear between neighboring columnar cells and between adjacent columnar and goblet cells as follows: a zonula adherens is found near the luminal surface and is followed by one or more zonulae occludentes. The morphological data obtained in this study and the physiological information on ion transport through the midgut epithelium have encouraged us to suggest that the goblet cell may be the principal unit of active potassium transport from the hemolymph to the lumen of the midgut. We have postulated that ion accumulation by mitochondria in close association with plicated plasma membranes may play a role in the active movement of potassium across the midgut.     

Light and electron microscopy studies of the oesophagus and crop epithelium in Aplysia depilans (Mollusca, Opisthobranchia)

The oesophagus and crop epithelium of Aplysia depilans consist in a single layer of columnar cells with apical microvilli, and some of them also possess cilia. Cell membrane invaginations, small vesicles, multivesicular bodies and many dense lysosomes were observed in the apical region of the cytoplasm. In most cells, a very large lipid droplet was observed above the nucleus and a smaller one was frequently found below the nucleus; glycogen granules are also present. Considering these ultrastructural features, it seems that these cells collect nutritive substances from the lumen by endocytosis, digest them in the apical lysosomes and store the resulting products. The cell bodies of mucus secreting flask-shaped cells are subepithelial in the oesophagus and intraepithelial in the crop. Histochemistry methods showed that the secretion stored in these cells contains acidic polysaccharides. Secretory vesicles with thin electron-dense filaments scattered in an electron-lucent background fill most of these cells, and the basal nucleus is surrounded by dilated rough endoplasmic reticulum cisternae containing small tubular structures. Considering the relatively low number of secretory cells, mucus production cannot be high. Moreover, since protein secreting cells were not observed in either oesophagus or crop, extracellular digestion in the lumen of these anterior segments of the digestive tract most probably depend on the enzymes secreted by the salivary and digestive glands.     

Structural and ultrastructural features of boar bulbourethral glands

The morphological features of boar bulbourethral glands were examined by light and transmission microscopy. Bulbourethral glands are compound tubuloalveolar glands surrounded by a capsule of dense connective tissue and arranged in multiple lobules formed by endpieces and excretory ducts. Endpieces and excretory ducts are both lined by a single epithelium of mucous cells with a basal nucleus. Epithelial cells accumulate secretory granules containing neutral and carboxylated acid mucosubstances and a small amount of sulphated acid mucosubstances. The ultrastructure of epithelial cells varies according to the secretory cycle. In initial stages, the cells show a columnar shape and secretory granules unevenly distributed in the cytoplasm. As the synthesis of mucosubstances progresses, the amount of the secretory granules increases and the cellular shape becomes pyramidal. Secretory granules can contain inclusions and present differences among them according to their different phases of formation. In pyramidal cells, secretory products are released into the lumen by a merocrine mechanism.     

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.     

Cytochemical and ultrastructural characteristics of the lining epithelium of the distal part of the epididymis in hamsters (Mesocricetus auratus)

In the terminal segment of the hamster epididymidis there was some evidence of micro-merocrine protein secretion a the level of the principal cells and clear evidence of granular secretion in the light cells, presumable of glycoproteins. The PAS and protein cytochemistry reactivities observed in both these cells, of the ductus epithelial lining, but especially in the light cells, are suggestive of mucopolysaccharides and protein complexes synthesis and secretion. This secretion is carried out to the epididymal epithelium from the lumen and luminal content. A complex of small vacuoles and vesicles appeared to form from the Golgi complex is showed in the principal cells. It was suggested that this complex may represented merocrine secretory vacuoles and vesicles in these cells. Dense granules, at the TEM level, are observed in all the cytoplasm of the light cells, with correspondence to similar PAS-positive granules observed in these cells, at the light microscope level. These granules, at the TEM level, are actually secreted to the epididymal duct lumen, by the apical cytoplasms of the light cells. Signs of absorption were suggested to the principal and light columnar cells through the ultrastructural observations of micropinocytosis, apical multivesicular bodies or great membrane-bounded vacuoles in the adluminal cytoplasms.     

Ultrastructural, histochemical and cytochemical characterization of intestinal epithelial cells in Aplysia depilans (Gastropoda, Opisthobranchia)

To improve the current knowledge about the digestive system in opisthobranchs, light and electron microscopy methods were used to characterize the epithelial cells in the mid‐intestine of Aplysia depilans. This epithelium is mainly formed by columnar cells intermingled with two types of secretory cells, named mucous cells and granular cells. Columnar cells bear microvilli on their apical surface and most of them are ciliated. Mitochondria, multivesicular bodies, lysosomes and lipid droplets are the main components of the cytoplasm in the region above the nucleus of these cells. Peroxisomes are mainly found in middle and basal regions, usually close to mitochondria. Mucous cells are filled with large secretory vesicles containing thin electron‐dense filaments surrounded by electron‐lucent material in which acidic mucopolysaccharides were detected. The basal region includes the nucleus, several Golgi stacks and many dilated rough endoplasmic reticulum cisternae containing tubular structures. The granular cells are characterized by very high amounts of flat rough endoplasmic reticulum cisternae and electron‐dense spherical secretory granules containing glycoproteins. Enteroendocrine cells containing small electron‐dense granules are occasionally present in the basal region of the epithelium. Intraepithelial nerve fibres are abundant and seem to establish contacts with secretory and enteroendocrine cells.     

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.     

Ultrastructural changes of the midgut epithelium in Isohypsibius granulifer granulifer Thulin, 1928 (Tardigrada: Eutardigrada) during oogenesis

The midgut epithelium of Isohypsibius granulifer granulifer (Eutardigrada) is composed of columnar digestive cells. At its anterior end, a group of cells with cytoplasm which differs from the cytoplasm of digestive cells is present. Probably, those cells respond to crescent-like cells (midgut regenerative cells) described for some tardigrade species. Their mitotic divisions have not been observed. We analyzed the ultrastructure of midgut digestive cells in relation to five different stages of oogenesis (previtellogenesis, beginning of the vitellogenesis, vitellogenesis—early choriogenesis, vitellogenesis—middle choriogenesis, late choriogenesis). In the midgut epithelium cells, the gradual accumulation of glycogen granules, lipid droplets and structures of varying electron density occurs. During vitellogenesis and choriogenesis, in the cytoplasm of midgut cells we observed the increasing number of organelles which are responsible for the intensive synthesis of lipids, proteins and saccharides such as cisterns of endoplasmic reticulum and Golgi complexes. At the end of oogenesis, autophagy also intensifies in midgut epithelial cells, which is probably caused by the great amount of reserve material. Midgut epithelium of analyzed species takes part in the yolk precursor synthesis.     

Formation and structure of the surface coat in the midgut of a waterstrider,Gerris najas deg. (Heteroptera : Gerridae)

The midgut epithelium of Gerris najas (Heteroptera : Gerridae) consists of a single cell type in different differentiating and functioning states. Cells elongate from nests of regenerative cells, forming together with their differentiating neighbours an interepithelial cavity, which contains coat material of membrane-like structure, and aggregations of cubic crystals visible only at this stage. Having reached a fold, the contents of the cavity are discharged into the lumen, and the young cells join the columnar cells. Exocytosis of coat material continues, while the cells gradually change to typical digestive columnar cells. The membrane-like material spreads from the folds into the lumen in whorls without forming a continuous cover of the luminal surface. Therefore, a peritrophic membrane does not exist in Gerris.     

Fine structure of the larval midgut of the fly Rhynchosciara and its physiological implications

The midgut of Rhynchosciara americana larvae consists of a cylindrical ventriculus from which protrudes two gastric caeca formed by polyhedral cells with microvilli covering their apical faces. The basal plasma membrane of these cells is infolded and displays associated mitochondria which are, nevertheless, more conspicuous in the apical cytoplasm. The anterior ventricular cells possess elaborate mitochondria-associated basal plasma membrane infoldings extending almost to the tips of the cells, and small microvilli disposed in the cell apexes. Distal posterior ventricular cells with long apical microvilli are grouped into major epithelial foldings forming multicellular crypts. In these cells the majority of the mitochondria are dispersed in the apical cytoplasm, minor amounts being associated with moderately-developed basal plasma membrane infoldings. The proximal posterior ventriculus represents a transition region between the anterior ventriculus and the distal posterior ventriculus. The resemblance between the gastric caeca and distal posterior ventricular cells is stressed by the finding that their microvilli preparations display similar alkaline phosphatase-specific activities. The results lend support to the proposal, based mainly on previous data on enzyme excretion rates, that the endo-ectoperitrophic circulation of digestive enzymes is a consequence of fluid fluxes caused by the transport of water into the first two thirds of midgut lumen, and its transference back to the haemolymph in the gastric caeca and in the distal posterior ventriculus.