Double membrane-bounded intestinal microvilli in Oncopeltus fasciatus

Perimicrovillar membrane assembly: the fate of phospholipids synthesised by the midgut of Rhodnius prolixus

In this study, we describe the fate of fatty acids that are incorporated from the lumen by the posterior midgut epithelium of Rhodnius prolixus and the biosynthesis of lipids. We also demonstrate that neutral lipids (NL) are transferred to the haemolymphatic lipophorin (Lp) and that phospholipids remain in the tissue in which they are organised into perimicrovillar membranes (PMMs). 3H-palmitic acid added at the luminal side of isolated midguts of R. prolixus females was readily absorbed and was used to synthesise phospholipids (80%) and NL (20%). The highest incorporation of 3H-palmitic acid was on the first day after a blood meal. The amounts of diacylglycerol (DG) and triacylglycerol synthesised by the tissue decreased in the presence of Lp in the incubation medium. The metabolic fates of 3H-lipids synthesised by the posterior midgut were followed and it was observed that DG was the major lipid released to Lp particles. However, the majority of phospholipids were not transferred to Lp, but remained in the tissue. The phospholipids that were synthesised and accumulated in the posterior midgut were found to be associated with Rhodnius luminal contents as structural components of PMMs.

Dynamic role of microvilli in peritrophic membrane formation

Although the peritrophic membrane (PM) is a common extracellular construction in many invertebrate groups, evidence of the location of its secretion has never been reported. In this study a specific marker for chitin has been developed, enabling a separate examination of secretion of the chitinous and proteinaceous components of the PM in the millipede, Glomeris marginata. Chitin appears first at the base of the microvilli (MV), synchronized in adjacent cells along the entire length of the midgut. Evidence showing that it originates at the plasma membrane is discussed. Proteinaceous components appear to be added from the MV to the chitinous sheet as it moves along the MV toward the lumen. Precedence for such a dynamic role for MV in formation of extracellular structures is reviewed. The completed PM extends around individual items in the gut contents as well as forming a multilayered envelope; this may enhance both its digestive and protective functions.

Host—parasite relationships ofBlastocrithidia familiarisinLygaeus pandurusScop. (Hemiptera: Lygaeidae)

Blastocrithidia familiariswere found to be parasitic in the midgut, ileum and rectum ofLygaeus pandurus. The host—parasite relationship is described. Attachment of parasites in the midgut and ileum occurs by interdigitation of expanded flagella over and between the microvilli. No attachment to microvilli was observed where extracellular membranes form well-organized layers which lie parallel to the gut wall and completely separate the microvilli border from the lumen. The extracellular membranes originate from delamination of the outer unit membrane of microvilli which consists of a double plasma membrane. The function of the extracellular membrane layers and their possible role as a peritrophic membrane in preventing parasite attachment is discussed in relation to previous studies on midgut cells in Hemiptera with a similar apical coat on midgut microvilli. In the rectum, parasites attach to the cuticle of the gland cells and not to the rest of the rectal wall. Attachment to the cuticle occurs by the formation of hemidesmosomes. A comparison of the relationship ofB. familiarisand its host to previous ultrastructural studies of associations between kinetoplastid flagellates and their respective hosts is discussed.

Biphasic perimicrovillar membrane production following feeding by previously starved Dysdercus peruvianus (Hemiptera: Pyrrhocoridae)

The development of perimicrovillar membranes (PMM) from midgut cells of starved and fed Dysdercus peruvianus was studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and assays for specific enzymatic markers of the perimicrovillar membranes (alpha-glucosidase), perimicrovillar space (aminopeptidase) and microvillar membranes (beta-glucosidase). High activities of these enzymes were observed 6h post-feeding and significant production of membranes was observed at 30 h post-feeding. In the gut cells of starved insects, the rough endoplasmic reticulum was organized in concentric bundles, with a greater number of mitochondria in the cellular apex. The presence of electron dense double-membrane vesicles and the production of PMM were not observed in this condition. Thirty hours post-feeding, a disorganization of the rough endoplasmic reticulum was observed, and it was possible to see double-membrane vesicles close to the cell apex. The membrane system formation was evident with a significant development of PMM in the midgut lumen. The luminal surface of the midgut during starvation and up to 48 h post-feeding was monitored using SEM. It was demonstrated that in the starved condition, the PMM was virtually absent from gut cells, except at the base of the microvilli. At 6h post-feeding, the microvilli were already completely covered with PMM, but with a maximum of PMM formation seen at 30 h post-feeding. Signals of PMM degradation were observed 48 h after pulse feeding.

Interaction of Leptomonas wallacei with the intestinal tract of its natural host Oncopeltus fasciatus (Hemiptera: Lygaeidae)

While investigating the distribution of Leptomonas wallacei in the intestine of the insect host Oncopeltus fasciatus, promastigotes and cyst-like forms of L. wallacei were observed only in the midgut ventricles V(3) and V(4) and the hindgut. In video-microscopy, once contact had occurred, the parasites remained attached to the midgut epithelium. Scanning electron microscopy revealed the adhesion of flagellates and cyst-like forms to the midgut wall and to the rectal pads of the hindgut. Using transmission electron microscopy, we observed that adhesion occurred mainly between the flagellum and the perimicrovillar membranes secreted by the midgut epithelium. No modifications were observed either in the parasite or in the epithelial cells. In the hindgut, adhesion to the superficial wax layer of the epithelial cells of the rectal pads was via flagellum. Host cell morphology appeared unaffected by L. wallacei.

Ultrastructure of the alimentary canal of the sheep scab mite, Psoroptes ovis (Acari: Psoroptidae)

The mite Psoroptes ovis causes sheep scab disease in flocks throughout much of the world. A serious impediment to the development of novel control measures for this mite is our inability to produce in vitro colonies of the mite. Here, we describe the alimentary canal of the mite with the particular aim of determining what it feeds on, as part of a longer term goal, to develop in vitro culture techniques for P. ovis. The alimentary canal of P. ovis consists of a cuticle-lined foregut and hindgut separated by a microvilli-lined midgut. The foregut is divided into a pre-oral cavity and a muscular pharynx and oesophagus. The midgut consists of three ultrastructurally discrete areas: a stomach with bi-lobed ventriculi, a colon and a post-colon. The stomach is composed of two cell types. The most common cells (Type 1) are either cuboidal or squamous depending on the degree of gut distension and possess short microvilli, a single basally located nucleus, extensive rough endoplasmic reticulum and other components suggesting active secretion. The less common cells (Type 2), possess an extensive apical network of tubules and basally food vacuoles suggesting intracellular digestion. These cells extend into the gut lumen, become free-floating and degenerate. The colon and post-colon are composed of cells similar to Type 1 stomach cells but the post-colon epithelium possesses significantly longer microvilli. Cells from these areas have not been observed to leave the surrounding epithelium and enter the gut lumen, but it is suggested that significant absorption occurs in these two areas. Faecal pellets, often containing a significant number of bacteria, leave the digestive system through the cuticle-lined anal atrium.

Trypanosoma (Megatrypanum) incertum from Pipistrellus pipistrellus: development and transmission by cimicid bugs

Trypanosoma (Megatrypanum) incertum Pittaluga 1905 was found in 33 out of 206 Pipistrellus pipistrellus caught at various sites in Britain. The trypanosome is described from blood smears. Development took place in laboratory-reared Cimex pipistrelli and Cimex lectularius. Epimastigote forms initially multiplied rapidly in the ventriculus and midgut of Cimex. Metacyclic trypanosomes were found in the rectum of both species of Cimex after 8 days when bugs were maintained at 20 degrees C and as early as 3 days at 30 degrees C. Electron microscopy of infected bugs revealed that there was no attachment to epithelial cells of the ventriculus or midgut, but within the rectum epimastigotes were attached by their flagella to the cuticle of the rectum by hemidesmosomes. Transmission was achieved by feeding experimentally infected bugs to bats kept in the laboratory. These bats were negative as judged by xenodiagnosis using laboratory-reared Cimex. Bats which had been caught in the wild demonstrated low-grade or sub-patent parasitaemias (positive in xenodiagnosis) for up to 400 days after the day of capture. Despite an extensive search of impression smears of tissues immediately after trypanosomes first appeared in the blood of experimentally infected bats no multiplicative stages were found.