Penetration of the mosquito midgut wall by the ookinetes of Plasmodium yoelii nigeriensis

Peritrophic matrix formation and Brugia malayi microfilaria invasion of the midgut of a susceptible vector, Ochlerotatus togoi (Diptera: Culicidae)

The mosquito midgut is the first site that vector-borne pathogens contact during their multiplication, differentiation, or migration from blood meal to other tissues before transmission. After blood feeding, the mosquitoes synthesize a chitinous structure called peritrophic matrix (PM) that envelops the blood meal and separates the food bolus from the midgut epithelium. In this study, a systematic investigation of the PM formation and the interaction of Brugia malayi within the midgut of a susceptible vector, Ochlerotatus togoi, were performed using scanning electron microscopy (SEM). SEM analysis of the midguts dissected at different time points post feeding on a B. malayi-infected blood meal (PIBM) revealed that the PM was formed from 45 min PIBM and gradually thickened and matured during 8-18 h PIBM. The PM degraded from 24 to 72 h PIBM, when digestion was completed. The invasion process of the microfilariae was observed between 3 and 4 h PIBM. In the beginning of the process, only sheathed microfilariae interacted with the internal face of the PM by its anterior part, and then the midgut epithelium before entering the hemocoel, after that they exsheathed. Microfilarial sheaths lying within the hemocoel were observed suggesting that they may serve as a decoy to induce the immune systems of the mosquitoes to respond to the antigens on the sheaths, thereby protecting the exsheathed microfilariae. These initial findings would lead to further study on the proteins, chemicals, and factors in the midgut that are involved in the susceptibility of O. togoi as a vector of filariasis.

Filarial worms reduce Plasmodium infectivity in mosquitoes

BACKGROUND

Co-occurrence of malaria and filarial worm parasites has been reported, but little is known about the interaction between filarial worm and malaria parasites with the same Anopheles vector. Herein, we present data evaluating the interaction between Wuchereria bancrofti and Anopheles punctulatus in Papua New Guinea (PNG). Our field studies in PNG demonstrated that An. punctulatus utilizes the melanization immune response as a natural mechanism of filarial worm resistance against invading W. bancrofti microfilariae. We then conducted laboratory studies utilizing the mosquitoes Armigeres subalbatus and Aedes aegypti and the parasites Brugia malayi, Brugia pahangi, Dirofilaria immitis, and Plasmodium gallinaceum to evaluate the hypothesis that immune activation and/or development by filarial worms negatively impact Plasmodium development in co-infected mosquitoes. Ar. subalbatus used in this study are natural vectors of P. gallinaceum and B. pahangi and they are naturally refractory to B. malayi (melanization-based refractoriness).

METHODOLOGY/PRINCIPAL FINDINGS

Mosquitoes were dissected and Plasmodium development was analyzed six days after blood feeding on either P. gallinaceum alone or after taking a bloodmeal containing both P. gallinaceum and B. malayi or a bloodmeal containing both P. gallinaceum and B. pahangi. There was a significant reduction in the prevalence and mean intensity of Plasmodium infections in two species of mosquito that had dual infections as compared to those mosquitoes that were infected with Plasmodium alone, and was independent of whether the mosquito had a melanization immune response to the filarial worm or not. However, there was no reduction in Plasmodium development when filarial worms were present in the bloodmeal (D. immitis) but midgut penetration was absent, suggesting that factors associated with penetration of the midgut by filarial worms likely are responsible for the observed reduction in malaria parasite infections.

CONCLUSIONS/SIGNIFICANCE

These results could have an impact on vector infection and transmission dynamics in areas where Anopheles transmit both parasites, i.e., the elimination of filarial worms in a co-endemic locale could enhance malaria transmission.

Do malaria ookinete surface proteins P25 and P28 mediate parasite entry into mosquito midgut epithelial cells?

Background

P25 and P28 are related ookinete surface proteins highly conserved throughout the Plasmodium genus that are under consideration as candidates for inclusion in transmission-blocking vaccines. Previous research using transgenic rodent malaria parasites lacking P25 and P28 has demonstrated that these proteins have multiple partially redundant functions during parasite infection of the mosquito vector, including an undefined role in ookinete traversal of the mosquito midgut epithelium, and it has been suggested that, unlike wild-type parasites, Dko P25/P28 parasites migrate across the midgut epithelium via an intercellular, rather than intracellular, route.

Presentation of the hypothesis

This paper presents an alternative interpretation for the previous observations of Dko P25/P28 parasites, based upon a recently published model of the route of ookinete invasion across the midgut epithelium. This model claims ookinete invasion is intracellular, with entry occurring through the lateral apical plasma membrane of midgut epithelial cells, and is associated with significant invagination of the midgut epithelium localised at the site of parasite penetration. Following this model, it is hypothesized that: (1) a sub-population of Dko P25/P28 ookinetes invaginate, but do not penetrate, the apical surface of the midgut epithelium and thus remain within the midgut lumen; and (2) another sub-population of Dko P25/P28 parasites successfully enters and migrates across the midgut epithelium via an intracellular route similar to wild-type parasites and subsequently develops into oocysts.

Testing the hypothesis

These hypotheses are tested by showing how they can account for previously published observations and incorporate them into a coherent and consistent explanatory framework. Based upon these hypotheses, several quantitative predictions are made, which can be experimentally tested, about the relationship between the densities of invading Dko P25/P28 ookinetes in different regions of the midgut epithelium and the number of oocyst stage parasites to which these mutant ookinetes give rise.

Implications of the hypothesis

The recently published model of ookinete invasion implies that Dko P25/P28 parasites are greatly, although not completely, impaired in their ability to enter the midgut epithelium. Therefore, P25 and/or P28 have a novel, previously unrecognized, function in mediating ookinete entry into midgut epithelial cells, suggesting that one mode of action of transmission-blocking antibodies to these ookinete surface proteins is to inhibit this function.

Genetics of anti-parasite resistance in invertebrates

This review summarizes and compares available data on genetic and molecular aspects of resistance in four well-described invertebrate host-parasite systems: snail-schistosome, mosquito-malaria, mosquito-filarial worm, and Drosophila-wasp associations. It underlies that the major components of the immune reaction, such as hemocyte proliferation and/or activation, and production of cytotoxic radicals are common to invertebrate hosts. Identifying genes responsible for naturally occurring resistance will then be helpful to understand the mechanisms of invertebrate immune defenses and to determine how virulence factors are used by parasites to overcome host resistance. Based on these four well-studied models, invertebrate resistance appears as generally determined by one major locus or a few loci, displaying at least partial dominance. Interestingly, specificity of resistance is highly variable and would involve processes other than simple recognition mechanisms. Finally, resistance was shown to be generally costly but is nevertheless observed at high frequencies in many natural populations, suggesting a high potential for host parasite coevolution.

Plasmodium falciparumookinete invasion of the midgut epithelium ofAnopheles stephensiis consistent with the Time Bomb model

Plasmodium falciparumgametocytes grownin vitrowere fed through membrane feeders to laboratory-rearedAnopheles stephensimosquitoes. Intact midguts, including entire bloodmeal contents, were removed between 24 and 48 h post-bloodfeeding. Giemsa-stained histological sections were prepared from the midguts and examined by light microscopy. Contrary to previous reports, ookinetes were clearly visible within midgut epithelial cells, demonstrating intracellular migration across the midgut wall. Ookinetes entered epithelial cells through the lateral apical membrane at sites where 3 adjacent cells converged. There was no evidence for the existence of a morphologically distinct group of epithelial cells preferentially invaded by ookinetes. However, ookinete penetration was associated with significant morphological changes to invaded cells, including differential staining, condensation and fragmentation of the nucleus, vacuolization, loss of microvilli and various degrees of extrusion into the midgut lumen. Epithelial cells completely separated from the midgut wall were found within the midgut lumen. These cells were associated with invading parasites suggesting that ookinete penetration resulted in complete ejection of invaded cells from the midgut wall. Small clusters of morphologically altered midgut cells and invading parasites spanning the membranes of adjacent abnormal epithelial cells were observed, consistent with intracellular movement of ookinetes between neighbouring midgut cells. Extruded epithelial cells were also observed rarely in uninfected midguts. Epithelial cell extrusion, therefore, may be a general mechanism of tissue repair through which damaged cells are removed from the midgut wall rather than a parasite-specific response. These observations demonstrate that human malaria parasite infection of mosquitoes is consistent with, and provides further support for, the Time Bomb model of ookinete invasion of the mosquito midgut epithelium previously proposed for rodent malaria parasites.

A snake venom phospholipase A2 blocks malaria parasite development in the mosquito midgut by inhibiting ookinete association with the midgut surface

Oocyst formation is a critical stage in the development of the malaria parasite in the mosquito. We have discovered that the phospholipase A2 (PLA2) from the venom of the eastern diamondback rattlesnake (Crotalus adamanteus) inhibits oocyst formation when added to infected chicken blood and fed to mosquitoes. A similar transmission-blocking activity was demonstrated for PLA2s from the venom of other snakes and from the honeybee. This effect is seen both with the avian malaria parasite Plasmodium gallinaceum and with the human parasite Plasmodium falciparum developing in their respective mosquito hosts. The inhibition occurs even in the presence of an irreversible inhibitor of the active site of PLA2, indicating that the hydrolytic activity of the enzyme is not required for the antiparasitic effect. Inhibition is also seen when the enzyme is fed to mosquitoes together with ookinetes, suggesting that the inhibition occurs after ookinete maturation. PLA2 has no direct effect on the parasite. However, pretreatment of midguts with PLA2 (catalytically active or inactive) dramatically lowers the level of ookinete/midgut association in vitro. It appears, therefore, that PLA2 is acting by associating with the midgut surface and preventing ookinete attachment to this surface. Thus, PLA2 is an excellent candidate for expression in transgenic mosquitoes as a means of inhibiting the transmission of malaria.

Effects of malaria infection on vitellogenesis in Anopheles gambiae during two gonotrophic cycles

We report changes in the abundance of vitellogenin (Vg) mRNA, and concentration of haemolymph Vg and ovarian vitellin (Vn) in Anopheles gambiae following infection with Plasmodium yoelii nigeriensis. A parasite-induced reduction in Vg mRNA abundance was first detected 24 h after feeding on an infective blood meal, when ookinetes were invading the midgut. During a second gonotrophic cycle post-infection, developing oocysts reduced Vg mRNA abundance by up to 33% and the effect was detected from 2 h post blood meal. Concentrations of Vg were initially reduced by infection during the second cycle, as predicted from Vg mRNA measurements. However, after 24 h, excess Vg had accumulated in the haemolymph. This accumulation may be due to impaired uptake, since ovarian vitellin accumulation was significantly decreased by infection during both gonotrophic cycles.

Cryofracture electron microscopy of the ookinete pellicle of Plasmodium gallinaceum reveals the existence of novel pores in the alveolar membranes

The malaria parasite invades the midgut tissue of its mosquito host as a motile form called the ookinete. We have examined the pellicle of the ookinete of Plasmodium gallinaceum by freeze-fracture and quick-freeze, deep-etch electron microscopy. The general organization is analogous to that of invasive stages of other members of Apicomplexa. The pellicle is composed of three membranes: the plasma membrane, and the two linked intermediate and inner membranes, which in the ookinete form one flattened vacuole that is located beneath the plasma membrane. The edges of this vacuole form a longitudinal suture. Beneath the vacuole is found an array of microtubules that are connected to the inner membrane by intramembranous particles. During freeze-fracture, the membranes can split along their hydrophobic planes, thus yielding six fracture faces, each of which displays a characteristic pattern of intramembranous particles. Additionally, we find that the ookinete pellicle differs from all other apicomplexan motile stages by the presence of large pores. These pores are of unknown function, but clearly might constitute a novel pathway for the transport of molecules to and from the cortex, which is independent of the well-described route through the apical micronemal/rhoptry complex. The pores may be the route by which motor proteins or other non micronemal surface proteins are trafficked, such as P25/P28 and SOAP, some of which are implicated in transmission blocking immunity.

Plasmodium vivax: ookinete destruction and oocyst development arrest are responsible for Anopheles albimanus resistance to circumsporozoite phenotype VK247 parasites

Anopheles albimanus and An. pseudopunctipennis differ in their susceptibilities to Plasmodium vivax circumsporozoite phenotypes. An. pseudopunctipennis is susceptible to phenotype VK247 but almost refractory to VK210. In contrast, An. albimanus is almost refractory to VK247 but susceptible to VK210. To investigate the site in the mosquito and the parasite stage at which resistance mechanisms affect VK247 development in An. albimanus, parasite development was followed in a series of experiments in which both mosquitoes species were simultaneously infected with blood from patients. Parasite phenotype was determined in mature oocysts and salivary gland sporozoites by use of immunofluorescence and Western blot assays and/or gene identification. Ookinete maturation and their densities within the bloodmeal bolus were similar in both mosquito species. Ookinete densities on the internal midgut surface of An. albimanus were 4.7 times higher than those in An. pseudopunctipennis; however, the densities of developing oocysts on the external midgut surface were 6.12 times higher in the latter species. Electron microscopy observation of ookinetes in An. albimanus midgut epithelium indicated severe parasite damage. These results indicate that P. vivax VK247 parasites are destroyed at different parasite stages during migration in An. albimanus midguts. A portion, accumulated on the internal midgut surface, is probably destroyed by the mosquito's digestive enzymes and another portion is most likely destroyed by mosquito defense molecules within the midgut epithelium. A third group, reaching the external midgut surface, initiates oocyst development, but over 90% of them interrupt their development and die. The identification of mechanisms that participate in parasite destruction could provide new elements to construct transgenic mosquitoes resistant to malaria parasites.

Spatial distribution of factors that determine sporogonic development of malaria parasites in mosquitoes

Mosquitoes transmit malaria, but only a few species permit the complete development and transmission of the parasite. Also, only a fraction of the ingested parasites develop in the vector. The attrition occurs in different compartments during the parasite's complex developmental scheme in the insect. A number of factors, both physical and biochemical, that affect the development have been proposed or demonstrated. Each of these factors is located within a specific space in the insect. We have divided this space into six compartments, which are distinct in their biochemical and biophysical nature: Endoperitrophic space, Peritrophic matrix, Ectopretrophic space, Midgut epithelium, Haemocoel and Salivary gland. Because factors that influence a particular stage of parasite development share the same microenvironment within these compartments, they must be considered collectively to exploit them for designing effective transmission blocking strategies. In this article we discuss these factors according to their spatial location in the mosquito.

Invasion in vitro of mosquito midgut cells by the malaria parasite proceeds by a conserved mechanism and results in death of the invaded midgut cells

Using an in vitro culture system, we observed the migration of malaria ookinetes on the surface of the mosquito midgut and invasion of the midgut epithelium. Ookinetes display constrictions during migration to the midgut surface and a gliding motion once on the luminal midgut surface. Invasion of a midgut cell always occurs at its lateral apical surface. Invasion is rapid and is often followed by invasion of a neighboring midgut cell by the ookinete. The morphology of the invaded cells changes dramatically after invasion, and invaded cells die rapidly. Midgut cell death is accompanied by activation of a caspase-3-like protease, suggesting cell death is apoptotic. The events occurring during invasion were identical for two different species of Plasmodium and two different genera of mosquitoes; they probably represent a universal mechanism of mosquito midgut penetration by the malaria parasite.

Plasmodium gallinaceum ookinetes adhere specifically to the midgut epithelium of Aedes aegypti by interaction with a carbohydrate ligand

During the course of its development in the mosquito and transmission to a new vertebrate host, the malaria parasite must interact with the mosquito midgut and invade the gut epithelium. To investigate how the parasite recognizes the midgut before invasion, we have developed an in vitro adhesion assay based on combining fluorescently labelled ookinetes with isolated midgut epithelia from blood-fed mosquitoes. Using this assay, we found that Plasmodium gallinaceum ookinetes readily adhered to midguts of Aedes aegypti, mimicking the natural recognition of the epithelium by the parasite. This interaction is specific: the ookinetes preferentially adhered to the lumen (microvillar) side of the gut epithelium and did not bind to other mosquito tissues. Conversely, the binding was not due to a non-specific adhesive property of the midguts, because a variety of other cell types, including untransformed P. gallinaceum zygotes or macrogametes, did not show similar binding to the midguts. High concentrations of glycosylated (fetuin, orosomucoid, ovalbumin) or non-glycosylated (bovine serum albumin) proteins, added as non-specific competitors, failed to compete with the ookinetes in binding assays. We also found that the adhesion of ookinetes to the midgut surface is necessary for sporogonic development of the parasite in the mosquito. Antibodies and other reagents that blocked adhesion in vitro also reduced oocyst formation when these reagents were combined with mature ookinetes and fed to mosquitoes. Chemical modification of the midguts with sodium periodate at pH 5.5 destroyed adhesion, indicating that the ookinete binds to a carbohydrate ligand on the surface of the midgut. The ligand is sensitive to periodate concentrations of less than 1 mmol l-1, suggesting that it may contain sialic-acid-like sugars. Furthermore, free N-acetylneuraminic acid competed with the ookinetes in binding aasays, while other monosaccharides had no effect. However, in agreement with the current belief that adult insects do not contain sialic acids, we were unable to detect any sialic acids in mosquito midguts using the most sensitive HPLC-based fluorometric assay currently available. We postulate that a specific carbohydrate group is used by the ookinete to recognize the midgut epithelium and to attach to its surface. This is the first receptor-ligand interaction demonstrated for the ookinete stage of a malaria parasite. Further characterization of the midgut ligand and its parasite counterpart may lead to novel strategies of blocking oocyst development in the mosquito.

Adhesion of Plasmodium gallinaceum ookinetes to the Aedes aegypti midgut: sites of parasite attachment and morphological changes in the ookinete

Plasmodium gallinaceum ookinetes adhered to Aedes aegypti midgut epithelia when purified ookinetes and isolated midguts were combined in vitro. Ookinetes preferentially bound to the microvillated luminal surface of the midgut, and they seemed to interact with three types of structures on the midgut surface. First, they adhered to and migrated through a network-like matrix, which we have termed microvilli-associated network, that covers the surface of the microvilli. This network forms on the luminal midgut surface in response to blood or protein meals. Second, the ookinetes bound directly to the microvilli on the surface of the midgut and were occasionally found immersed in the thick microvillar layer. Third, the ookinetes associated with accumulations of vesicular structures found interspersed between the microvillated cells of the midgut. The origin of these vesicular structures is unknown, but they correlated with the surface of midgut cells invaded by ookinetes as observed by TEM. After binding to the midgut, ookinetes underwent extensive morphological changes: they frequently developed one or more annular constrictions, and their surface roughened considerably, suggesting that midgut components remain bound to the parasite surface. Our observations suggest that, in a natural infection, the ookinete interacts in a sequential manner with specific components of the midgut surface. Initial binding to the midgut surface may activate the ookinete and cause morphological changes in preparation for invasion of the midgut cells.

Plasmodium ookinete development in the mosquito midgut: a case of reciprocal manipulation

The ookinete is one of the most important stages of Plasmodium development in the mosquito. It is morphologically and biochemically distinct from the earlier sexual stages--gametocytes and zygote, and from the later stages--oocyst and sporozoites. Development to ookinete allows the parasite to escape from the tightly packed blood bolus, to cross the sturdy peritrophic matrix (PM), to be protected from the digestive environment of the midgut lumen, and to invade the gut epithelium. The success of each of these activities may depend on the degree of the biochemical and physical barriers in the mosquito (such as density of blood bolus, thickness of peritrophic matrix, proteolytic activities in the gut lumen etc.) and the ability of the ookinete to overcome these barriers. Ookinete motility, secretion of chitinase, resistance to the digestive enzymes, and recognition/invasion of the midgut epithelium all may play crucial roles in the transformation to oocyst. The overall sporogonic development of Plasmodium, therefore, depends on the results of the two-way manipulations between the parasite and the vector mosquito. Study of ookinete development and of the cellular and biochemical complexities of the mosquito gut may therefore lead to the design of novel strategies to block the transmission of malaria. This article reviews the intricate interactions between the parasite and the mosquito midgut in the context of development and transmission of Plasmodium parasites.

Plasmodium gallinaceum preferentially invades vesicular ATPase-expressing cells in Aedes aegypti midgut

Penetration of the mosquito midgut epithelium is obligatory for the further development of Plasmodium parasites. Therefore, blocking the parasite from invading the midgut wall disrupts the transmission of malaria. Despite such a pivotal role in malaria transmission, the cellular and molecular interactions that occur during the invasion are not understood. Here, we demonstrate that the ookinetes of Plasmodium gallinaceum, which is related closely to the human malaria parasite Plasmodium falciparum, selectively invade a cell type in the Aedes aegypti midgut. These cells, unlike the majority of the cells in the midgut, do not stain with a basophilic dye (toluidine blue) and are less osmiophilic. In addition, they contain minimal endoplasmic reticulum, lack secretory granules, and have few microvilli. Instead, these cells are highly vacuolated and express large amounts of vesicular ATPase. The enzyme is associated with the apical plasma membrane, cytoplasmic vesicles, and tubular extensions of the basal membrane of the invaded cells. The high cost of insecticide use in endemic areas and the emergence of drug resistant malaria parasites call for alternative approaches such as modifying the mosquito to block the transmission of malaria. One of the targets for such modification is the parasite receptor on midgut cells. A step toward the identification of this receptor is the realization that malaria parasites invade a special cell type in the mosquito midgut.

The biology of malarial parasite in the mosquito--a review

The purpose of this review is to summarize the biology of Plasmodium in the mosquito including recent data to contribute to better understanding of the developmental interaction between mosquito and malarial parasite. The entire sporogonic cycle is discussed taking into consideration different parasite/vector interactions and factors affecting parasite development to the mosquito.

Sporogonic development of Leucocytozoon smithi

The sporogonic development of Leucocytozoon smithi in its black fly vector was studied by light and electron microscopy and was compared with that of other haemosporidians. Within 18 to 24 h after ingestion of gametocytes by black flies, ookinetes passing through the midgut epithelium were observed. Intracellular migration of ookinetes resulted in the apparent disruption and degeneration of host cells. Intercellular migration also occurred as was evidenced by the presence of ookinetes between midgut cells. Transformation of ookinete to spherical oocyst occurred extracellularly in three different sites. Although most oocysts were found between the host cell basal membrane and the basal lamina, large numbers also were found attached to the external surface of the basal lamina, projecting into the hemocoel. Ectopic development of oocysts in the midgut epithelium between cells was observed much less frequently than development on the basal side of the midgut. The oocyst wall of dense granules, believed to be of parasite origin, was distinguishable from the basal lamina of the host's midgut epithelium. As in other Leucocytozoidae, the cytoplasm of the oocyst differentiated into a single sporoblastoid from which 30-50 sporozoites were formed. Beginning on the third day post infection, elongation of segregated dense sporoblastoid material associated with pellicle thickening led to the formation of the finger-like sporozoite buds which projected into the oocyst cavity. Sporozoites within mature oocysts and salivary glands were structurally similar to sporozoites as described for other haemosporidians.

Penetration of the mosquito (Aedes aegypti) midgut wall by the ookinetes of Plasmodium gallinaceum

We observed Plasmodium gallinaceum ookinetes in both intracellular and intercellular positions in the midgut epithelium of the mosquito Aedes aegypti. After epithelial cell invasion intracellular ookinetes lacked a parasitophorous vacuolar membrane and were surrounded solely by their own pellicle. Thus, the ookinete in the midgut epithelium of the mosquito differs from erythrocytic and hepatic stages in that the parasite in the vertebrate host is surrounded by a vacuole. The midgut epithelial cytoplasm around the apical end of invading ookinetes was replaced by fine granular material deprived of normal organelles. Membranous structure was observed within the fine granular area. Most ookinetes were seen intracellularly on the luminal side and intercellularly on the haemocoel side of the midgut epithelial cells. These observations suggest that the ookinete first enters into the midgut epithelial cell, then exists to the space between the epithelial cells and moves to the basal lamina where the ookinete develops to the oocyst.

Development of Plasmodium berghei ookinetes to young oocysts in vitro

The mosquito stage of Plasmodium berghei was cultivated in vitro, with special attention to ookinete transformation into early oocyst. The ookinetes were obtained by in vitro culture of gametocytes taken from infected mice, purified by density gradient of metrizoic acid or a lymphocyte separation medium, and incubated either in acellular culture or in co-cultivations with mosquito cells. In acellular culture, the ookinetes were found to aggregate with each other and transformed from banana to round shapes. Their inner pellicular membranes and subpellicular microtubules partially disappeared, indicating that development to early oocyst had occurred. Co-cultivation wtih Aedes albopictus cells (C6/36 clone) revealed that ookinetes transformed into early oocyst in the medium, or invaded the cells and then transformed to early oocysts within the cell cytoplasm as well. However all of these transformed cells failed to develop further, i.e., neither deposition of the oocyst capsule nor nuclear division was observed. Many ookinetes which failed to penetrate the Aedes cells were phagocytized within three days of culture. A significant difference between invaded and transformed oocysts and phagocytized ookinetes was seen in that the former lacked vacuole membrane. Co-cultivation with Toxorhynchites amboinensis cells (TRA-284-SFG clone) permitted transformation of ookinetes into early oocysts in the medium as in the acellular culture, but no ookinete invasion nor phagocytosis by the cell was observed.

A scanning electron microscopic study of the sporogonic development of Plasmodium falciparum in Anopheles stephensi

The full development of Plasmodium falciparum in Anopheles stephensi mosquitoes was studied by scanning electron microscopy. Ookinetic development was described from in vitro cultures. Growing oocysts beneath the basal lamina of the midgut wall mechanically stretch this lamina until it is torn and displaced by day 7. In young oocysts the wall appears smooth. In older oocysts wrinkles in the wall are visible after routine fixation. Osmium tetroxide postfixation greatly reduced the occurrence of these wrinkles. Intracapsular development of sporozoites was visualized after mechanical manipulation of the oocysts during sample preparation. In contrast to P. berghei, no ectopic development was seen in P. falciparum in the mosquito midgut. The mechanism of sporozoite escape from the oocyst appears to be similar to that described for rodent malaria. Fracturing of salivary glands provided the first view by scanning electron microscopy of sporozoites located in proximal and distal gland cells and in the draining duct.