Fine Structure of Developing Spores of Thelohania bracteata (Strickland, 1913) (Microsporida,Nosematidae) Emphasizing Polar-Filament Formation*

SYNOPSIS. In the microsporidian, Thelohania bracteata, the polar filament, as it starts to develop in the sporoblast, apparently receives material synthesized by the granular endoplasmic reticulum and Golgi vesicles. In immature spores many dilated sacs are observed in areas where there is less endoplasmic reticulum. These sacs, that persist into the almost mature spore, are probably Golgi-type vesicles and may be related to the formation of the spore coat. The polar filament of the mature spore possesses 8 coils and in cross section or cross-fractured face the electron-dense central portion of the polar filament contains a tubular structure, ringed by 12–14 cylindrical structures. In thin sections, an electron-lucid zone is observed between the core and membrane of the polar filament. The polar filament runs through the highly laminated polaroplast which occupies the anterior portion of the spore. In cross-fractured face the lamellae of the polaroplast are arranged like the petals of a flower. The basal portion of the polar filament is enlarged, appearing arrow-shaped in thin sections and pear-shaped in frozen-etched preparations. Frozen-etched membranes differ in the size and distribution of the surface particles.     

Description of Chytridiopsis Trichopterae N. Sp. (Microspora, Chytridiopsidae), A Microsporidian Parasite of the Caddis Fly Polycentropus Flavomaculatus (Trichoptera, Polycentropodidae), With Comments On Relationships Between the Families Chytridiopsidae and Metchnikovellidae

ABSTRACT. The microsporidium Chytridiopsis trichopterae n. sp., a parasite of the midgut epithelium of larvae of the caddis fly Polycentropus flavomaculatus found in southern Sweden, is described based on light microscopic and ultrastructural characteristics. All life cycle stages have isolated nuclei. Merogonial reproduction was not observed. the sporogony comprises two sequences: one with free spores in parasitophorous vacuoles, the other in spherical, 5.6-6.8 μm wide, sporophorous vesicles which lie in the cytoplasm. the free sporogony yields more than 20 spores per sporont. the vesicle-bound sporogony produces 8, 12 or 16 spores. the envelope of the sporophorous vesicle is about 82 nm thick and layered. the internal layer is the plasma membrane of the sporont; the surface layer is electron dense with regularly arranged translucent components. Both spore types are spherical. They have an ～ 35-nm thick spore wall, with a plasma membrane, an electron-lucent endospore, and an ～ 14-nm thick electron-dense exospore. the polar sac is cup-like and lacks a layered anchoring disc. the polar filament is arranged in two to three isofilar coils in the half of the spore opposite the nucleus. the coupling between the polar sac and the polar filament is characteristic. the surface of the polar filament is covered with regularly arranged membraneous chambers resembling a honeycomb. There is no polaroplast of traditional type. the cytoplasm lacks polyribosomes. the nucleus has a prominent, wide nucleolus. the two spore types have identical construction, but differ in dimensions and electron density. Free living spores are about 3.2 μm wide, the diameter of the polar filament proper is 102-187 nm, the chambers of the honeycomb are 70-85 nm high, and the polar sac is up to 425 nm wide. Living spores in the vesicle-bound sporogony are about 2.1 μm wide, the polar filament measures 69-102 nm, the chambers of the honeycomb are about 45 nm high, and these spores are more electron dense. Comparisons of cytology (especially the construction of the spore wall and the polar filament and associated structures) and life cycles reveal prominent differences among the Chytridiopsis-like microsporidia, and close relationships between the families Chytridiopsidae and Metchnikovellidae.     

Fine structure of the developing spore of Nosema apis zander

Summary The mature spore possesses a thick spore coat and a particle-bearing spore membrane. The highly laminated polaroplast membranes are located at the anterior pole of the spore. Close to its base, the polar filament is surrounded by the polaroplast membrane. The polar filament runs spirally towards the posterior pole of the spore. A large portion of the polar filament is arranged in two layers. A similar arrangement was also observed in immature spores and in the sporoblast stage, although it was not so orderly arranged in the latter. The developing polaroplast membrane was observed in the immature spore, but not in the sporoblast. The sporoblast wall is much thinner than the spore coat, but has the same texture. Endoplasmic reticulum is the most prominent cytoplasmic organelle in the developing stages of Nosema apis. Porous nuclear envelopes are also observed in developing stages. The role of the endoplasmic reticulum in the formation of the polar filament, polaroplast and spore coat, and the function of the spore membrane, are discussed.     

Light and Electron Microscope Observations on Nosema nelsoni Sprague, 1950 (Microsporida,Nosematidae) with Particular Reference to its Golgi Complex*

SYNOPSIS. A species of Nosema in the muscles of the North American white shrimp, generally known as Penaeus setiferus but also known as P. fluviatilis, appears identical with type specimens of N. nelsoni Sprague, 1950, in P. aztecus. Its Golgi apparatus, as seen in the sporoblast, is a complex system of cisternae, small vesicles and expanded sacs which plays a major role in spore morphogenesis. It transforms directly into the polaroplast complex, certain membranous investments of the polar filament, the polar sac and perhaps part of the posterior vacuolar system. Probably the polar sac contains the polar cap. The PAS-positive material in both the cap and the filament may be a component of the Golgi complex. This new concept of the Golgi complex supplements our earlier view of spore morphogenesis according to which the polar filament is of nuclear origin. It also reconciles the idea with Vávra's identification of Golgi vesicles associated with the developing polar filament.     

Further Observations on the Fine Structure of the Spores of Glugea weissenbergi (Microsporida,Nosematidae)*

SYNOPSIS. A Glugea xenoma sectioned and viewed with the electron microscope contained many spores with everting polar filaments. Several details not seen in previous studies of this species were observed. A specialized area with the appareance of a lattice was commonly present near the anterior end of the polaroplast. The external portion of a partially everted polar filament appeared to have about twice the diameter of the part remaining within the spore. No membrane was seen limiting the external surface of the everted portion. The everting filament had pushed thru the polar cap and the adjacent thin area of the spore wall, making the polar cap into a ring. The ring connected the proximal end of the everting filament to the inner spore membrane, thereby anchoring the filament to the spore. The electron density of some of the membranous organelles of the spore was enhanced by the use of ruthenium red.     

Development of the Polar Filament-Polaroplast Complex in a Microsporidian Parasite*

SYNOPSIS. The development of the polar filament in a microsporidian parasite was studied in the electron microscope. The polar filament is a peculiar and complex organelle with intricate anatomical relationships to other structures in the mature spore. The characteristic ultrastructure of the formative and mature stages of the polar filament made it possible to trace its development and study the interactions among various organelles during its formation. In sporoblasts the polar filament develops sequentially from 3 different regions. The base of the filament appears first and is derived from a dense body. The anterior part of the filament is formed from electron dense material located in the perinuclear cisterna and in agranular endoplasmic reticulum. The base and the anterior part of the filament move toward each other and fuse. Subsequently, the posterior part of the filament develops from the posterior part of the Golgi complex. The polar sac and the polaroplast surrounding the anterior segment of the filament are formed from the anterior region of the Golgi complex.     

A theory for the mechanism of polar filament extrusion in the microspora

A theory is presented which can explain the interaction of the major factors known to influence in vitro extrusion of the microsporidian polar filament. It is proposed that the pH, and concentration and species of cation in the external medium influence the activity of car?ylic ionophore molecules in spore membranes in the following manner: (1) Alkaline environmental conditions establish a proton gradient across the spore plasma membrane, and facilitate the activation of ionophore molecules in this membrane. (2) This proton gradient drives an ionophorically-mediated cation/proton exchange across the plasma membrane. (3) As protons are lost from the sporoplasm its alkalinity increases, so that ionophore molecules in organelle membranes (i.e. in the polaroplast and posterior vacuole) are activated. This initiates a cation/proton exchange between sporoplasm and organelles. (4) Continued movement of cations into organelles in the spore causes major osmotic imbalance across spore membranes. This leads to a rapid inflow of water into the spore and swelling of the polaroplast and posterior vacuole. The associated pressure increase in the spore causes the explosive discharge of the polar filament through the polar cap. This model is used to explain previously published results from the literature, and methods of testing predictions generated by this hypothesis are outlined.     

Ultrastructure of Triangulamyxa amazonica n. gen. and n. sp. (Myxozoa, Myxosporea), a parasite of the Amazonian freshwater fish, Sphoeroides testudineus (Teleostei, Tetrodontidae)

Triangulamyxa amazonica n. gen. and n. sp. (Myxozoa, Ortholineidae), found in the lumen of the intestine of the freshwater fish Sphoeroides testudineus, is described. The fish were collected from the Amazon River near the city of Algodoal, State of the Pará, Brazil. Numerous irregular plasmodia containing different stages of sporogony, including spores, were observed. The plasmodia were lying free in the lumen or had slender pseudopodia-like cytoplasmic processes in contact with intestinal epithelial cells with microvilli projections. Spores, which are equilaterally triangular in valvar view with rounded pointed ends and ellipsoidal in transverse section, are 8.5 μm long, 7.6 μm wide, and 3.8 μm thick. The anterior end of the spores contains two equal drop-shaped polar capsules measuring 2.6 μm in length, each having an isofilar polar filament with 5–6 turns. The characteristics of the spore shape, the spore wall structure and its ridge organization, the plasmodial characteristics and the identity of the host suggest that the parasite is a new genus and species, which is herein designated T. amazonica.     

Recherches Cytochimiques sur la Microsporidie Nosema bomycis au cours de son Développement chez le Ver à Soie (Bombyx mori)*

RESUME. La Microsporidie Nosema bombycis, Protozoaire parasite agent de la pébrine du ver à soie, a étéétudiée cytochimiquement à la fois en microscopie photonique et électronique. Les examens ont porté sur la détection et la localisation des acides nucléiques (ADN et ARN), des polysaccharides, de la phosphatase acide, au cours des différents stades du développement dans les cellules de I'hôte (du schizonte à la spore). Les principaux résultats concernent les observations relatives aux polysaccharides et à la phosphatase qui ne sont détectés qu'au stade de la spore et ne sont pas observés au stade du schizonte. Les polysaccharides sont présents au niveau du sac polaire, du filament polaire et sur la membrane cytoplasmique; la phosphatase acide est localisée au niveau du sac polaire, du filament polaire et dans la vacuole postérieure. SYNOPSIS. Nosema bombycis, agent of pebrine disease of silkworm, was studied cytochemically, using both light and electron microscopy. Presence of nucleic acids (DNA and RNA), polysaccharides, and acid phosphatases was demonstrated and localization of these substances was determined in various stages of the parasite (from the schizont to the spore). DNA and RNA were detected in all these stages. Polysaccharides and acid phosphatase were found in the spore but not in the schizogonic stages. Polysaccharides were detected in the polar cap, the polar filament, and the limiting membrane of the cytoplasm of the spore. Acid phosphatase was found in the polar cap, the polar filament, and the posterior vacuole.     

Pleistophora finisterrensis n. sp., a microsporidian parasite of blue whiting Micromesistius poutassou

Pleistophora finisterrensis n. sp. is a microsporidian parasite of the hypoaxial musculature of the blue whiting Micromesistius poutassou (Risso). Foci of infection are between 3 and 6 mm in length and have no evident effects on adjacent muscle fibres. We found only a single type of spore (uninucleate, with mean dimensions of 4×2 µm in fresh preparations), contained within sporophorous vesicles (mean diameter 19 µm in fresh preparations; 150–250 spores per vesicle). All of the development stages of this microsporidian are monokaryotic. The meronts are initially uninucleate and bounded by a plasmalemma. Towards the end of merogony, meronts are multinucleate plasmodia with a well-defined surface coat. Sporogony is polysporous, with multinucleate sporonts, which likewise have a well-defined surface coat (about 130 nm thick), dividing by plasmotomy to give rise to uninucleate sporoblasts. The polar tube is isofilar and consists of 8–9 turns in the posterior half of spore. The polaroplast is made up of an anterior lamellar part and a posterior vesicular part.     

An electron microscope study on the cytoplasmic and nuclear components of rat primary oocytes

Summary This paper reports on the structure of rat primary oocytes, as observed with the electron microscope. Four main components are described in the cytoplasm: Golgi apparatus, centrioles, mitochondria and multivesicular bodies.The components of the Golgi apparatus are forming a single mass confined to a limited region of the cytoplasm and the centrioles were found located in a clear zone sited in the middle of this mass. Mitochondria are scattered at random in the cytoplasm. Multivesicular bodies are elements integrated by an enveloping membrane containing a varied number of tiny vesicles. They are generally found associated with a short number of small free vesicles. Only one two groups of this kind are found per oocyte. This contrast with what has been observed previously in full-grown rat oocytes, where the groups are numerous and constituted by many units.Two components were described for the oocyte nucleus: nucleoli and chromosomes. Nucleoli are constituted by a tangled thread whose elemental component is a fine fibrous material of high electron density.At the age studied on this paper, primary oocytes are undergoing meiotic prophase, chromosomes have at this time the same components observed by different authors in primary spermatocytes. These are two thick ribbon-like threads helically twisted around a thinner medial filament. Each tripartite group is attached by one end to the nuclear membrane. It was actually seen tripartite groups incompletely organized; the images recorded of such groups suggest that the medial filament is the first to appear in the nucleoplasm. The possible significance of these filaments in respect to the meiotic phase called chromosome pairing is discussed.     

Fine Structure of a New Human Microsporidian, Encephalitozoon hellem, in Culture

Encephalitozoon hellem is a new human microsporidian isolated from corneal biopsies and conjunctival scrapings of three AIDS patients and cultured in Madin Darby canine kidney (MDCK) cells. Encephalitozoon hellem and Encephalitozoon cuniculi display different protein profiles with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and unique antibody binding patterns with murine antisera against Western blots of each organism. Developmental stages of E. hellem in culture are similar to E. cuniculi. Meronts are 1.3–2.7 μm in diameter, develop within a parasitophorous vacuole adjacent to the vacuolar membrane, divide by binary fission, and contain one or two discrete nuclei. Sporonts measure 2 × 3 μm, separate from the vacuolar membrane, and have a thickened outer membrane. Sporoblasts display a tri-layered wall and possess the earliest recognized polar filaments. Mature spores measure 1 × 1.5 μm and are more electron-dense than other stages. Each spore contains a single nucleus, a polar tubule with four to nine coils, thin electron-dense exospore and thick, electron-lucent endospore. Although E. hellem and E. cuniculi differ biochemically and immunologically, their fine structure and development are indistinguishable.     

Staining of microsporidian spores with diamidine phenylindole

A number of microscopic techniques and dyes are available to diagnose microsporidian infections in invertebrate and vertebrate hosts. Among these, DNA-specific fluorochrome DAPI is widely used to stain DNA in prokaryotic and eukaryotic cells, alone or in combination with other histochemical or fluorescent dyes. Moreover, this dye also binds to membraneous structures and protein complexes. In our studies, DAPI was used to stain spores of microsporidia infecting orthopteran, coleopteran, dipteran and lepidopteran insect hosts. DAPI staining of diplokarya helped to discriminate the Nosema-like microsporidian spores from spore-shaped bodies lacking this characteristic staining. It was found, moreover, that non-DNA staining occurred in many cases and other components of the spores were stained: the exospore, the cytoplasm, the extruded polar filament and the polaroplast. Staining of these structures was feeble as compared to DNA and in most cases did not interfere with nuclear apparatus staining. Feebly stained cytoplasm and exospore clearly indicated unstained zone of endospore, making it easier to diagnose both mono- and diplokaryotic spores. Staining of extruded polar filament allowed to demonstrate viability and to observe some stages of extrusion process of microsporidian spores.     

The spermiogenesis and the early spermatozoa of <Emphasis Type="Italic">Armorloricus elegans</Emphasis> (Loricifera,Nanaloricidae)

The spermiogenesis consisting of five spermatid stages and the early spermatozoon has been investigated in Armorloricus elegans (Loricifera) with the use of transmission electron microscopy. The male reproductive system consists of three parts; testes, vasa deferentia and seminal vesicles. Caudally, the two seminal vesicles merge together in a ciliated duct and the excretory/gonadal—and digestive systems continue through the recto-urogenital canal, which opens via the lateral gonopores and the temporarily closed anal system. Spermiogenesis mainly occurs in the testes, whereas further maturation of the late spermatids and early spermatozoa occurs in the vasa deferentia and seminal vesicles. A maturation gradient (from spermatocytes to spermatozoa) is found from the posterior peripheral part of the testes to the anterior periphery and then centrally. During spermiogenesis the round nucleus becomes more osmiophilic and condensation of chromatin occurs. Later the nucleus elongates until it becomes rod-shaped in the early spermatozoa. In the second spermatid stage, a large vesicle is formed by saccules developed from the Golgi complex. This vesicle develops further and consists of three different osmiophilic parts with some crystal-like structures inside and is on the outside almost entirely surrounded by thick striated filaments. In the mid-piece the flagellum has a typical 9 × 2 + 2 axoneme and the two mitochondria are fused into a single sheet surrounding the flagellum. In the early spermatozoon stage an acrosomal-like cap structure with an acrosome filament appears proximal to the protruded rod-shaped nucleus. This cap is not formed by the Golgi complex and therefore might not be a true acrosome. Comparing the early spermatozoa of A. elegans with other cycloneuralians has shown some similarities with especially Kinorhyncha and Priapulida. These similarities are thought to be plesiomorphic.     

Enterocytozoon hepatopenaei sp. nov. (Microsporida: Enterocytozoonidae), a parasite of the black tiger shrimp Penaeus monodon (Decapoda: Penaeidae): Fine structure and phylogenetic relationships

A new microsporidian species, Enterocytozoon hepatopenaei sp. nov., is described from the hepatopancreas of the black tiger shrimp Penaeus monodon (Crustacea: Decapoda). Different stages of the parasite are described, from early sporogonal plasmodia to mature spores in the cytoplasm of host-cells. The multinucleate sporogonal plasmodia existed in direct contact with the host-cell cytoplasm and contained numerous small blebs at the surface. Binary fission of the plasmodial nuclei occurred during early plasmodial development and numerous pre-sporoblasts were formed within the plasmodium. Electron-dense disks and precursors of the polar tubule developed in the cytoplasm of the plasmodium prior to budding of early sporoblasts from the plasmodial surface. Mature spores were oval, measuring 0.7 × 1.1 μm and contained a single nucleus, 5-6 coils of the polar filament, a posterior vacuole, an anchoring disk attached to the polar filament, and a thick electron-dense wall. The wall was composed of a plasmalemma, an electron-lucent endospore (10 nm) and an electron-dense exospore (2 nm). DNA primers designed from microsporidian SSU rRNA were used to amplify an 848 bp product from the parasite genome (GenBank FJ496356). The sequenced product had 84% identity to the matching region of SSU rRNA from Enterocytozoon bieneusi. Based upon ultrastructural features unique to the family Enterocytozoonidae, cytoplasmic location of the plasmodia and SSU rRNA sequence identity 16% different from E. bieneusi, the parasite was considered to be a new species, E. hepatopenaei, within the genus Enterocytozoon.     

Pseudoloma neurophilia n. g., n. sp., a new microsporidium from the central nervous system of the zebrafish (Danio rerio)

An unusual xenoma-forming microsporidium was discovered in the central nervous system of moribund zebrafish from a laboratory colony in Eugene, Oregon. Infected fish were often emaciated and lethargic, and histological examination commonly revealed severe myelitis and myositis associated with the infection. Based on its structure, development, and small subunit ribosomal DNA sequence it is unique among fish microsporidia. Spores are uninucleate, ovoid to pyriform, with a prominent posterior vacuole. Spores average 5.4 x 2.7 microm with 13-16 coils of the polar filament. The microsporidium produces xenomas within the spinal cord and hindbrain of fish, and xenomas contained sporophorous vesicles with up to 16 spores. Sporoblasts and presporoblast stages (probably sporonts) are found occasionally in small aggregates dispersed randomly throughout xenomas. It clustered in the "Ichthyosporidium group" along with other fish microsporidian genera based on rDNA sequence analysis. The rDNA sequence of the zebrafish microsporidium was most similar to that of Ichthyosporidium, but showed only 12.1% similarity and therefore this microsporidium can be considered a distinct genus and species, which we have named Pseudoloma neurophilia n. g., n. sp.     

On the Cytology and Taxonomic Position of Nudispora biformis N. G., N. Sp. (Microspora, Thelohaniidae), a Microsporidian Parasite of the Dragon Fly Coenagrion hastulatum in Sweden

The microsporidium Nudispora biformis n. g., n. sp., a parasite of a larva of the damsel fly Coenagrion hastulatum in Sweden, is described based on light microscopic and ultrastructural characteristics. Merogonial stages and sporonts are diplokaryotic. Sporogony comprises meiotic and mitotic divisions, and finally eight monokaryotic sporoblasts are released from a lobed plasmodium. Sporophorous vesicles are not formed. The monokaryotic spores are oval, measuring 1.4–1.8 × 2.8–3.4 μm in living condition. The thick spore wall has a layered exospore, with a median double-layer. The polaroplast has two lamellar parts, with the closest packed lamellae anteriorly. The isofilar polar filament is arranged in 6 (to 7) coils in the posterior half of the spore. Laminar and tubular extracellular material of exospore construction is present in the proximity of sporogonial stages. In addition to normal spores teratological spores are produced. The microsporidium is compared to the microsporidia of the Odonata; its possible relations to the genus Pseudothelohania and to the Thelohania-like microsporidia are discussed. The new genus is provisionally included in the family Thelohaniidae.     

Ormiersia carcini gen. n., sp. n., microsporidian of the Mediterranean crab, Carcinus mediterraneus Czerniavsky, 1844: developmental cycle and ultrastructural study]

A microsporidan parasite, Ormieresia carcini gen. n., sp. n., was found in the crab, Carcinus mediterraneus Czerniavsky. Its development and fine structure are the subject are the subject of the present study. The life cycle begins with a schizont surrounded by a unit membrane and containing a diplokaryon. The entire process of sporogony takes place in the host musculature. The sporogonic stages are enclosed in the pansporoblastic membrane. In each pansporoblast, sporogony gives rise to 8 sporoblasts; the octonucleate sporogonial plasmodium is lacking. In the course of schizogonic and sporogonic divisions, each kinetic center consists of 2 plaques, one located within and the other outside the nuclear envelope. The dividing sporonts and sporoblasts sevrets "metabolic" substances (granules, tubules) which are depostied in the pansporoblast. The uninucleate spore is long and cylindrical, measuring 19.1 X 2.4 micronm. A rectilinear manubrium traverses the spore. Its posterior end attenuates abruptly into a polar filament with 4 or 5 coils; its anterior end is attached to the polar cap, which is compressed by a double polar ring. The anterior part of the manubrium is surrounded by a polaroplast consisting of a "spongy" (vesicular) and a lamellar zone.     

Amazonspora hassar n. gen. and n. sp. (phylum Microsporidia,fam. Glugeidae), a parasite of the Amazonian teleost Hassar orestis (fam. Doradidae)

We describe the microsporidian Amazonspora hassar n. gen., n. sp. from the gill xenomas of the teleost Hassar orestis (Doradidae) collected in the estuarine region of the Amazon River. The parasite appeared as a small whitish xenoma located in the gill filaments near the blood vessels. Each xenoma consisted of a single hypertrophic host cell (HHC) in the cytoplasm of which the microsporidian developed and proliferated. The xenoma wall was composed of up to approximately 22 juxtaposed crossed layers of collagen fibers. The plasmalemma of the HHC presented numerous anastomosed, microvilli-like structures projecting outward through the 1-3 first internal layers of the collagen fibrils. The parasite was in direct contact with host cell cytoplasm in all stages of the cycle (merogony and sporogony). Sporogony appears to divide by plasmotomy, giving rise to 4 uninucleate sporoblasts, which develop into uninucleate spores. The ellipsoidal spores measured 2.69 +/- 0.45 x 1.78 +/- 0.18 microm, and the wall measured approximately 75 nm. The anchoring disk of the polar filament was subterminal, being shifted laterally from the anterior pole. The polar filament was arranged into 7-8 coils in a single layer in the posterior half of the spore, surrounding the posterior vacuole. The polaroplast surrounded the uncoiled portion of the polar filament, and it was exclusively lamellar. The spores and different life-cycle stages were intermingled within the cytoplasm of the HHC, surrounding the central hypertrophic deeply branched nucleus. The ultrastructural morphology of this microsporidian parasite suggests the erection of a new genus and species.     

Fine structure of a new human microsporidian, Encephalitozoon hellem, in culture

Encephalitozoon hellem is a new human microsporidian isolated from corneal biopsies and conjunctival scrapings of three AIDS patients and cultured in Madin Darby canine kidney (MDCK) cells. Encephalitozoon hellem and Encephalitozoon cuniculi display different protein profiles with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and unique antibody binding patterns with murine antisera against Western blots of each organism. Developmental stages of E. hellem in culture are similar to E. cuniculi. Meronts are 1.3-2.7 microns in diameter, develop within a parasitophorous vacuole adjacent to the vacuolar membrane, divide by binary fission, and contain one or two discrete nuclei. Sporonts measure 2 x 3 microns, separate from the vacuolar membrane, and have a thickened outer membrane. Sporoblasts display a tri-layered wall and possess the earliest recognized polar filaments. Mature spores measure 1 x 1.5 microns and are more electron-dense than other stages. Each spore contains a single nucleus, a polar tubule with four to nine coils, thin electron-dense exospore and thick, electron-lucent endospore. Although E. hellem and E. cuniculi differ biochemically and immunologically, their fine structure and development are indistinguishable.