Electron Microscopic Studies of the Development of Kinetes in Theileria annulata Dschunkowsky & Luhs, 1904 (Sporozoa, Piroplasmea)

SYNOPSIS. Gamogony of Theileria annulata Dschunkowsky & Luhs occurs within the intestine of nymphs of the tick Hyalomma anatolicum excavatum Koch. After the 5th day post repletionem (p.r.) of the ticks spherical and ovoid parasites were found within the intestinal cells. These stages were thought to represent fertilized macrogametes. These underwent a transformation process leading ultimately to the differentiation of a motile stage, the kinete , which leaves the intestinal cells on the 14th-17th day p.r. and penetrates the alveoles of the salivary glands. The transformation of the stationary into a motile stage takes place by formation of a growing protrusion (= anlage) into an inner, enlarging vacuole. During this process the limiting membrane of the vacuole serves as the outer membrane of the developing motile stage, whereas the 2 inner membranes of its pellicle are newly formed. The steps of this differentiation in T. annulata are compared to the process of ookinete formation in haemosporina.     

Membrane recruitment of Aut7p in the autophagy and cytoplasm to vacuole targeting pathways requires Aut1p, Aut2p, and the autophagy conjugation complex

Autophagy is a degradative pathway by which cells sequester nonessential, bulk cytosol into double-membrane vesicles (autophagosomes) and deliver them to the vacuole for recycling. Using this strategy, eukaryotic cells survive periods of nutritional starvation. Under nutrient-rich conditions, autophagy machinery is required for the delivery of a resident vacuolar hydrolase, aminopeptidase I, by the cytoplasm to vacuole targeting (Cvt) pathway. In both pathways, the vesicle formation process requires the function of the starvation-induced Aut7 protein, which is recruited from the cytosol to the forming Cvt vesicles and autophagosomes. The membrane binding of Aut7p represents an early step in vesicle formation. In this study, we identify several requirements for Aut7p membrane association. After synthesis in the cytosol, Aut7p is proteolytically cleaved in an Aut2p-dependent manner. While this novel processing event is essential for Aut7p membrane binding, Aut7p must undergo additional physical interactions with Aut1p and the autophagy (Apg) conjugation complex before recruitment to the membrane. Lack of these interactions results in a cytosolic distribution of Aut7p rather than localization to forming Cvt vesicles and autophagosomes. This study assigns a functional role for the Apg conjugation system as a mediator of Aut7p membrane recruitment. Further, we demonstrate that Aut1p, which physically interacts with components of the Apg conjugation complex and Aut7p, constitutes an additional factor required for Aut7p membrane recruitment. These findings define a series of steps that results in the modification of Aut7p and its subsequent binding to the sequestering transport vesicles of the autophagy and cytoplasm to vacuole targeting pathways.     

Electron-Microscopic Studies on Theileria ovis Rodhain, 1916: Development of Kinetes in the Gut of the Vector Tick, Rhipicephalus evertsi evertsi Neumann, 1897, and Their Transformation Within Cells of the Salivary Glands

SYNOPSIS. Gamogony of Theileria ovis Rodhain occurs within the gut of nymphs of the tick Rhipicephalus evertsi evertsi Neumann. After molting, spherical and ovoid parasites are found within the intestinal cells of the ticks. These stages are thought to be zygotes, because they undergo a transformation process leading ultimately (in 3 days) to the formation of a motile stage, the kinete , the fine structure of which is very similar to that of the ookinetes of the hemosporidia. The kinete leaves the gut cells of the tick and penetrates the salivary gland cells where it produces infective stages (the sporozoites ). These stages may be transmitted to sheep during the next blood meal of the tick. The developmental processes of T. ovis are compared to those of Hemosporina.     

Electron-microscopic studies on Theileria ovis Rodhain, 1916: development of kinetics in the gut of the vector tick, Rhipicephalus evertsi evertsi Neumann, 1897, and their transformation within cells of the salivary glands.

Gamogony of Theileria ovis Rodhain occurs within the gut of nymphs of the tick Rhipicephalus evertsi evertsi Neumann. After molting, spherical and ovoid parasites are found within the intestinal cells of the ticks. These stages are thought to be zygotes, because they undergo a transformation process leading ultimately (in 3 days) to the formation of a motile stage, the kinete, the fine structure of which is very similar to that of the ookinetes of the hemosporidia. The kinete leaves the gut cells of the tick and penetrates the salivary gland cells where it produces infective stages (the sporozoites). These stages may be transmitted to sheep during the next blood meal of the tick. The developmental processes of T. ovis are compared to those of Hemosporina.     

Immunocytochemical study on photoreceptor cell differentiation in the cultured retina of the chick

Photoreceptor cell differentiation was investigated in a dissociated monolayer culture of chick embryonic retinas with electron microscopic immunohistochemistry. The antibody was raised against bovine rhodopsin purified on SDS-polyacrylamide gel electrophoresis. In the developing retina, immunoreactivity was first recognized on the 14th day of incubation and was localized on the plasma membrane of the growing inner segment. On the 16th day, immunoreactivity was observed on some differentiating outer segments but not on inner segments. In the culture from 6 1/2-day-old embryonic retinas, immunoreactivity was found on the 7th day of culturing on the plasma membrane of large-sized neurons. Electron microscopic observations confirmed that such stained cells showed reaction product on the plasma membrane, and that they displayed fine structures characteristic of intact photoreceptor cells. They had a number of microvillous processes and often one thick process, both of which were intensely stained. Outer segment formation, however, was not observed in the present monolayer culture. These results indicate that opsin synthesis and its transport to the plasma membrane begins prior to and probably independently of outer segment formation.     

CHANGES OF THE PRESPORE SPECIFIC STRUCTURE DURING DEDIFFERENTIATION AND CELL TYPE CONVERSION OF A SLIME MOLD CELL

In the slug of the cellular slime mold, Dictyostelium discoideum , are differentiated the anterior prestalk cells and the posterior prespore cells, whose differentiation is characterized by formation of the prespore specific vacuole (PSV). The ultrastructural changes of the PSV were investigated during dedifferentiation of a prespore cell disaggregated from a slug and also during conversion of the cell type, caused by fragmentation of a slug, between the prespore and the prestalk cells.
During the dedifferentiation, the PSV first lost its lining membrane which subsequently congregated, together with the inner filamentous material, to form some electron dense granules. Finally, the vacuole membrane was punctured, and the granules were released into cytoplasm. During conversion of the prespore to the prestalk cell, the PSV was degraded through the same process as in dedifferentiation, but the degradation proceeded much more synchronously in a converting cell. When a prestalk fragment was isolated from a slug, formation of the PSV was detected in no cell until 2 hr of incubation. After a lag, the PSV was formed in a converting cell through the process which is not a simple reversal of its degrading process.     

Origin and function of the stalk-cell vacuole in Dictyostelium

Large vacuoles are characteristic of plant and fungal cells, and their origin has long attracted interest. The cellular slime mould provides a unique opportunity to study the de novo formation of vacuoles because, in its life cycle, a subset of the highly motile animal-like cells (prestalk cells) rapidly develops a single large vacuole and cellulosic cell wall to become plant-like cells (stalk cells). Here we describe the origin and process of vacuole formation using live-imaging of Dictyostelium cells expressing GFP-tagged ammonium transporter A (AmtA-GFP), which was found to reside on the membrane of stalk-cell vacuoles. We show that stalk-cell vacuoles originate from acidic vesicles and autophagosomes, which fuse to form autolysosomes. Their repeated fusion and expansion accompanied by concomitant cell wall formation enable the stalk cells to rapidly develop turgor pressure necessary to make the rigid stalk to hold the spores aloft. Contractile vacuoles, which are rich in H⁺-ATPase as in plant vacuoles, remained separate from these vacuoles. We further argue that AmtA may play an important role in the control of stalk-cell differentiation by modulating the pH of autolysosomes.     

Morphogenesis of the antenna of the male silkmoth. Antheraea polyphemus, III. Development of olfactory sensilla and the properties of hair-forming cells

During adult development of the male silkmoth Antheraea polyphemus, the anlagen of olfactory sensilla arise within the first 2 days post-apolysis in the antennal epidermis (stage 1-3). Approximately on the second day, the primary dendrites as well as the axons grow out from the sensory neurons (stage 4). The trichogen cells start to grow apical processes approximately on the third day, and these hair-forming 'sprouts' reach their definite length around the ninth day (stages 5-6). Then the secretion of cuticle begins, the cuticulin layer having formed on day 10 (stage 7a). The primary dendrites are shed, the inner dendritic segments as well as the thecogen cells retract from the prospective hair bases, and the inner tormogen cells degenerate around days 10/11 (stage 7b). The hair shafts of the basiconic sensilla are completed around days 12/13 (stage 7c), and those of the trichoid sensilla around days 14/15 (stage 7d). The trichogen sprouts retract from the hairs after having finished cuticle formation, and the outer dendritic segments grow out into the hairs: in the basiconic sensilla directly through, and in the trichoid sensilla alongside, the sprouts. The trichogen sprouts contain numerous parallel-running microtubules. Besides their cytoskeletal function, these are most probably involved in the transport of membrane vesicles. During the phase of cuticle deposition, large numbers of vesicles are transported anterogradely from the cell bodies into the sprouts, where they fuse with the apical cell membrane and release their electron-dense contents (most probably cuticle precursors) to the outside. As the cuticle grows in thickness, the surface area of the sprouts is reduced by endocytosis of coated vesicles. When finally the sprouts retract from the completed hairs, the number of endocytotic vesicles is further increased and numerous membrane cisterns seem to be transported retrogradely along the microtubules to the cell bodies. Here the membrane material will most probably be used again in the formation of the sensillum lymph cavities. Thus, the trichogen cells are characterized by an intensive membrane recycling. The sensillum lymph cavities develop between days 16-20 (stage 8), mainly via apical invaginations of the trichogen cells. The imago emerges on day 21.     

The human homolog of Saccharomyces cerevisiae Apg7p is a Protein-activating enzyme for multiple substrates including human Apg12p, GATE-16, GABARAP, and MAP-LC3

Autophagy is a process that involves the bulk degradation of cytoplasmic components by the lysosomal/vacuolar system. In the yeast, Saccharomyces cerevisiae, an autophagosome is formed in the cytosol. The outer membrane of the autophagosome is fused with the vacuole, releasing the inner membrane structure, an autophagic body, into the vacuole. The autophagic body is subsequently degraded by vacuolar hydrolases. Taking advantage of yeast genetics, apg (autophagy-defective) mutants were isolated that are defective in terms of formation of autophagic bodies under nutrient starvation conditions. One of the APG gene products, Apg12p, is covalently attached to Apg5p via the C-terminal Gly of Apg12p as in the case of ubiquitylation, and this conjugation is essential for autophagy. Apg7p is a novel E1 enzyme essential for the Apg12p-conjugation system. In mammalian cells, the human Apg12p homolog (hApg12p) also conjugates with the human Apg5p homolog. In this study, the unique characteristics of hApg7p are shown. A two-hybrid experiment indicated that hApg12p interacts with hApg7p. Site-directed mutagenesis revealed that Cys(572) of hApg7p is an authentic active site cysteine residue essential for the formation of the hApg7p.hApg12p intermediate. Overexpression of hApg7p enhances the formation of the hApg5p.hApg12p conjugate, indicating that hApg7p is an E1-like enzyme essential for the hApg12p conjugation system. Cross-linking experiments and glycerol-gradient centrifugation analysis showed that the mammalian Apg7p homolog forms a homodimer as in yeast Apg7p. Each of three human Apg8p counterparts, i.e. the Golgi-associated ATPase enhancer of 16 kDa, GABA(A) receptor-associated protein, and microtubule-associated protein light chain 3, coimmunoprecipitates with hApg7p and conjugates with mutant hApg7p(C572S) to form a stable intermediate via an ester bond. These results indicate that hApg7p is an authentic protein-activating enzyme for hApg12p and the three Apg8p homologs.     

The T3SS Effector EspT Defines a New Category of Invasive Enteropathogenic E. coli (EPEC) Which Form Intracellular Actin Pedestals

Enteropathogenic Escherichia coli (EPEC) strains are defined as extracellular pathogens which nucleate actin rich pedestal-like membrane extensions on intestinal enterocytes to which they intimately adhere. EPEC infection is mediated by type III secretion system effectors, which modulate host cell signaling. Recently we have shown that the WxxxE effector EspT activates Rac1 and Cdc42 leading to formation of membrane ruffles and lamellipodia. Here we report that EspT-induced membrane ruffles facilitate EPEC invasion into non-phagocytic cells in a process involving Rac1 and Wave2. Internalized EPEC resides within a vacuole and Tir is localized to the vacuolar membrane, resulting in actin polymerization and formation of intracellular pedestals. To the best of our knowledge this is the first time a pathogen has been shown to induce formation of actin comets across a vacuole membrane. Moreover, our data breaks the dogma of EPEC as an extracellular pathogen and defines a new category of invasive EPEC.     

Localization of perlecan and heparanase in Hertwig's epithelial root sheath during root formation in mouse molars.

During cementogenesis, dental follicular cells penetrate the ruptured Hertwig's epithelial root sheath (HERS) and differentiate into cementoblasts. Mechanisms involved in basement membrane degradation during this process have not been clarified. Perlecan, a heparan sulfate (HS) proteoglycan, is a component of all basement membranes. Degradation of HS of perlecan by heparanase cleavage affects a variety of biological processes. We elucidated immunolocalization of perlecan and heparanase in developing murine molars to clarify their roles in cementoblast differentiation. At the initial stage of root formation, perlecan immunoreactivity was detected on the basement membrane of HERS. Weak heparanase immunoreactivity was detected in HERS cells. HERS showed intense staining for heparanase as root formation progressed. In contrast, labeling for perlecan disappeared from the basement membrane facing the dental follicle, and weak immunoreactivity for perlecan was detected on the inner side of the basement membrane of HERS. These findings suggest that perlecan removal is an important step for root and periodontal tissue formation. Heparanase secreted by the cells of HERS may contribute to root formation by degrading perlecan in the dental basement membrane.     

Investigations into the fire structure of the asexual developmental stages of Frenkelia in the liver of the bank vole (author's transl)]

Bank voles (Clethrionomys glareolus) were infected by stomach tube with Frenkelia sporocysts from the faeces of buzzards (Buteo buteo). The voles were sacrificed at regular intervals and their livers examined electronmicroscopically. Seven days p.i. developmental stages of Frenkelia could be detected in liver parenchymal cells. The youngest schizonts detected are enveloped by a pellicle consisting of two membranes. This pellicle, which is in direct contact with the host cell mitochondria, shows marked invaginations which increase with the development of the schizont. A parasitophorous vacuole is not detectable. In developing schizonts numerous sections through nuclei with nucleic spindles and merozoite anlagen (dome-shaped) structures) are visible. It is not clear whether there are several nuclei or a section through one large and lobed nucleus. Within the merozoite anlagen the conoid and the subpellicular microtubules are formed first. By the prolongation of the dome-shaped structures towards the posterior pole, the nucleus and the other newly formed cell organelles are incorporated into the forming merozoite. The posterior pole of the merozoite still remains open at this stage of development. With increasing differentiation the merozoites become lancet-shaped, their apical poles bing always directed towards the periphery of the schizont. The outer membrane of the pellicle of the schizont forms the outer part of the pellicle of the merozoites by invaginating around them. At this stage of development the inner membrane of the pellicle of the schizont is no longer detectable. Thus the typical pellicle of the motile stages of sporozoaonsisting of three membranes is formed. In the centre of the merozoites which lie freely in the liver cell a residual body is present. The host cell reacts against the parasites by forming a thick border of mitochondria and distinct endoplasmic reticulum.     

Microtubules, but not actin microfilaments, regulate vacuole motility and morphology in hyphae of Pisolithus tinctorius

While it is now recognised that transport within the endomembrane system may occur via membranous tubules, spatial regulation of this process is poorly understood. We have investigated the role of the cytoskeleton in regulating the motility and morphology of the motile vacuole system in hyphae of the fungus Pisolithus tinctorius by studying (1) the effects of anti-microtubule (oryzalin, nocodazole) and anti-actin drugs (cytochalasins, latrunculin) on vacuolar activity, monitored by fluorescence microscopy of living cells; and (2) the ultrastructural relationship of microtubules, actin microfilaments, and vacuoles in hyphae prepared by rapid-freezing and freeze-substitution. Anti-microtubule drugs reduced the tubular component of the vacuole system in a dose-dependent and reversible manner, the extent of which correlated strongly with the degree of disruption of the microtubule network (monitored by immunofluorescence microscopy). The highest doses of anti-microtubule drugs completely eliminated tubular vacuoles, and only spherical vacuoles were observed. In contrast, anti-actin drugs did not reduce the frequency of tubular vacuoles or the motility of these vacuoles, even though immunofluorescence microscopy confirmed perturbation of microfilament organisation. Electron microscopy showed that vacuoles were always accompanied by microtubules. Bundles of microtubules were found running in parallel along the length of tubular vacuoles and individual microtubules were often within one microtubule diameter of a vacuole membrane. Our results strongly support a role for microtubules, but not actin microfilaments, in the spatial regulation of vacuole motility and morphology in fungal hyphae.     

Ultrastructure of vacuolar inclusions in root tips

Summary Vacuoles in plant cells often contain inclusions which at early stages of development are bounded by a single membrane. The inclusion bodies (IBs) comprise a diversity of forms and various stages of differentiation are recognizable. IBs are divided into two categories: those which have a matrix without internal membranes, and those which contain cytoplasmic organelles and other membranous material. The internal membranes may be tightly coiled or in the form of vesicles. IBs develop from invaginations of the tonoplast which become detached into the vacuole. They are initiated mainly during active cell growth but may remain within the vacuole in differentiated cells. Various components contribute to the contents of IBs: endoplasmic reticulum, nuclear envelope, Golgi vesicles, extruded portions of mitochondria and plastids, ribosomes and groundplasm. In most IBs the limiting membrane and contents eventually disappear within the vacuole. Some IBs prior to their breakdown within the vacuole also function as sites for the formation of material not found elsewhere in the cell. The disappearance of IBs from vacuoles suggests that such vacuoles behave as lysosomes.     

Studies on the Fine Structure of Microorganisms : V. Morphogenesis of Nuclear and Membrane Structures during Ascospore Formation in Yeast

The fine structure of cells of Saccharomyces cerevisiae engaged in the formation of ascospores was studied in electron micrographs of ultrathin sections. Although the mode of the first reduction division could not be clearly determined, the second nuclear division appeared to proceed in a manner similar to that observed previously during vegetative division. That is, division by constriction of the existing nucleus occurs without dissolution of the nuclear membrane and without involvement of discrete chromosomes. Variously shaped areas of low electron density were discerned within the nucleoplasm; these had not been previously seen in the vegetative nucleus. The significance of this nuclear differentiation and its possible similarity to nuclear structures reported in bacteria and an imperfect fungus are discussed. The cytoplasmic membrane appears first in the developing ascospore. The formation of an outer coat and an inner coat then follows. The cytoplasmic vacuole was observed not to be incorporated into the spore. An unusual intracytoplasmic membrane was observed in the spore and appeared to be at least temporarily continuous with the nuclear membrane.     

家蚕V型ATP酶B亚基的克隆及表达特征

V型ATP酶(Vacuolar-type ATPase)是一种定位于细胞膜和细胞器膜上的氢离子转运酶。它利用ATP水解的能量将氢离子转运到液泡、囊泡或者胞外,从而维持细胞内正常的酸碱环境。V型ATP酶B亚基(V-ATPase B)作为ATP的催化位点,也有着非常重要的作用。为了探讨家蚕V-ATPase B(Bm V-ATPase B)的功能,首先从家蚕五龄幼虫的中肠c DNA中克隆了Bm V-ATPase B基因并构建原核表达载体进行原核表达,获得了重组蛋白,经质谱鉴定正确后,通过镍柱亲和层析的方法纯化了该蛋白并制备了多克隆抗体;最后分析了该蛋白在家蚕丝腺中的表达特征并利用免疫荧光对其在丝腺中的表达位置进行了定位。结果显示Bm V-ATPase B基因序列全长1 473 bp,预测蛋白分子量55 k Da,预测等电点5.3。通过Western blotting对家蚕5龄第3天和上蔟第1天幼虫丝腺的不同区段进行Bm V-ATPase B蛋白的表达特征分析,发现在两个时期该蛋白均在前部丝腺高量表达,而在中部丝腺和后部丝腺表达量相对较低。进一步对两个时期丝腺的不同区段进行免疫荧光定位,发现该蛋白在两个时期的前部丝腺、中部丝腺和后部丝腺均定位于细胞层。利用激光共聚焦显微镜对该蛋白进行进一步的定位,发现该蛋白主要在丝腺的细胞膜表达。研究结果明确了该蛋白在丝腺中的表达模式,为深入研究该蛋白在蚕丝纤维形成中的作用奠定了基础。     

Bases moléculaires et cellulaires de l'invasion des cellules épithéliales intestinales par Shigella flexneri

A key step in the pathogenesis of shigellosis is the capacity of the causative bacteria, shigellae, to invade colonic and rectal epithelial cells in humans. This invasive process encompasses several steps: entry into epithelial cells by induction of a macropinocytic event caused by secreted Ipa proteins. The bacterium then escapes from the vacuole and reaches the cytoplasmic compartment in which it divides rapidly and becomes motile via the expression of a surface protein, IcsA, whose polar localization achieves directed polymerization of actin filaments that push the bacterial body forward. Bacteria then engage the inner face of the cellular membrane in the junctional area and form protrusions allowing their passage into the adjacent cell. Lysis of the double membrane eventually allows access to the cytoplasmic compartment of the adjacent cell, thus providing the bacterium with a very efficient mechanism of epithelial colonization.     

Structure and differentiation of the inner egg membrane of Trichocephaloides megalocephala (Cestoda, Dilepididae)

Electron microscope studies of the inner membrane of developing eggs of T. megalocephala were carried out. At early developmental stages the inner membrane is a syncytial cytoplasmatic layer lying on the basal plate of the embryo. At the preoncosphere stage the division of the membrane into two zones (external and internal ones) takes place. Initially the differentiation manifests itself in the cytoplasm polarisation; at the end of the middle preoncosphere stage the zones are divided by the "oncosphere membrane". The formation of the "oncosphere membrane" is accomplished by the external part of the internal zone. Embryophore is a derivative of the external zone, at the final stages of the formation the embryophore material is transformed from granular into thin-fibrillary. The origin of the external integument of oncospheres of cyclophillids, which, as it has been shown for T. megalocephala, is a derivative of the inner membrane rather than of specialized epithelial oncosphere cells, is considered.     

Theileria annulata promotes Src kinase-dependent host cell polarization by manipulating actin dynamics in podosomes and lamellipodia

Theileria annulata is an intracellular protozoan parasite that infects B cells and macrophages of ruminants. Macrophages infected with T. annulata are de-differentiated and display tumour cell properties and a metastatic behaviour. How parasitized cells adapt their morphology, motility and invasive behaviour has not yet been addressed in detail. In this study, I investigated the regulation of host cell actin dynamics in T. annulata-transformed macrophages and how this affects host cell morphology and motility. T. annulata was found to promote the formation of filamentous-actin-rich podosome-type adhesions (PTAs) and lamellipodia, and to establish a polarized morphology of the infected cell. Characteristic for parasite-dependent host cell polarization is that infected cells display a single, persistent lamellipodium. Src kinases--in particular Hck--are required for the polar extension of this lamellipodium. Hck does so by promoting the clustered assembly of PTAs and accumulation of proteins of the Ezrin, Radixin, Moesin (ERM) family in lamellipodia. Polar accumulation of PTAs and ERM proteins correlates with focal matrix degradation underneath lamellipodia. These findings suggest that T. annulata equips its host cell with properties to adhere and invade. These properties are likely to promote the motile behaviour required for dissemination of infected cells in vivo.     

Phagocytosis of bacteria by polymorphonuclear leukocytes: a freeze-fracture, scanning electron microscope, and thin-section investigation of membrane structure

The changes in membrane structure of rabbit polymorphonuclear (PMN) leukocytes during bacterial phagocytosis was investigated with scanning electron microscope (SEM), thin-section, and freeze-fracture techniques. SEM observations of bacterial attachment sites showed the involvement of limited areas of PMN membrane surface (0.01-0.25μm(2)). Frequently, these areas of attachment were located on membrane extensions. The membrane extensions were present before, during, and after the engulfment of bacteria, but were diminished in size after bacterial engulfment. In general, the results obtained with SEM and thin-section techniques aided in the interpretation of the three-dimensional freeze-fracture replicas. Freeze-fracture results revealed the PMN leukocytes had two fracture faces as determined by the relative density of intramembranous particles (IMP). Membranous extensions of the plasma membrane, lysosomes, and phagocytic vacuoles contained IMP's with a distribution and density similar to those of the plasma membrane. During phagocytosis, IMPs within the plasma membrane did not undergo a massive aggregation. In fact, structural changes within the membranes were infrequent and localized to regions such as the attachment sites of bacteria, the fusion sites on the plasma membrane, and small scale changes in the phagocytic vacuole membrane during membrane fusion. During the formation of the phagocytic vacuole, the IMPs of the plasma membrane appeared to move in with the lipid bilayer while maintaining a distribution and density of IMPs similar to those of the plasma membranes. Occasionally, IMPs were aligned to linear arrays within phagocytic vacuole membranes. This alignment might be due to an interaction with linearly arranged motile structures on the side of the phagocytic vacuole membranes. IMP-free regions were observed after fusion of lysosomes with the phagocytic vacuoles or plasma membrane. These IMP-free areas probably represent sites where membrane fusion occurred between lysosomal membrane and phagocytic vacuole membrane or plasma membrane. Highly symmetrical patterns of IMPs were not observed during lysosomal membrane fusion.