[Electron microscopy studies on the developmental stages of Eimeria maxima (Sporozoa, Coccidia). I. Ultrastructure of macrogametes]

Recent achievements and doors opened for coccidian parasite research and development through transcriptomics of enteric sexual stages

The Coccidia is the largest group of parasites within the Apicomplexa, a phylum of unicellular, obligate parasites characterized by the possession of an apical complex of organelles and structures in the asexual stages of their life cycles, as well as by a sexual reproductive phase that occurs enterically in host animals. Coccidian sexual reproduction involves morphologically distinct microgametes and macrogametes that combine to form a diploid zygote and, ultimately, following meiosis and mitosis, haploid, infectious sporozoites, inside sporocysts within an oocyst. Recent transcriptomic analyses have identified genes involved in coccidian sexual stage development and reproduction, including genes encoding for microgamete- and macrogamete-specific proteins with roles in gamete motility, fusion and fertilization, and in the formation of the resilient oocyst wall that allows coccidians to persist for long periods in the environment. Transcriptomics has also provided important clues about the regulation of gene expression in the transformation of parasites from one developmental stage to the next, a complex sequence of events that may involve transcription factors such as the apicomplexan Apetala2 (ApiAP2) family, alternative splicing, regulatory RNAs and MORC (a microrchida homologue and regulator of sexual stage development in Toxoplasma gondii). The molecular dissection of coccidian sexual development and reproduction by transcriptomic analyses may lead to the development of novel transmission-blocking strategies.

Use of fluorescent nanoparticles to investigate nutrient acquisition by developing Eimeria maxima macrogametocytes

The enteric disease coccidiosis, caused by the unicellular parasite Eimeria, is a major and reoccurring problem for the poultry industry. While the molecular machinery driving host cell invasion and oocyst wall formation has been well documented in Eimeria, relatively little is known about the host cell modifications which lead to acquisition of nutrients and parasite growth. In order to understand the mechanism(s) by which nutrients are acquired by developing intracellular gametocytes and oocysts, we have performed uptake experiments using polystyrene nanoparticles (NPs) of 40 nm and 100 nm in size, as model NPs typical of organic macromolecules. Cytochalasin D and nocodazole were used to inhibit, respectively, the polymerization of the actin and microtubules. The results indicated that NPs entered the parasite at all stages of macrogametocyte development and early oocyst maturation via an active energy dependent process. Interestingly, the smaller NPs were found throughout the parasite cytoplasm, while the larger NPs were mainly localised to the lumen of large type 1 wall forming body organelles. NP uptake was reduced after microfilament disruption and treatment with nocodazole. These observations suggest that E. maxima parasites utilize at least 2 or more uptake pathways to internalize exogenous material during the sexual stages of development.

Eimeria maxima: isolation of gametocytes and their immunogenicity in mice, rabbits, and chickens

Eimeria maxima gametocytes were isolated from infected chicken intestinal tissue by treatment with hyaluronidase and subsequent filtration through polymon filters. The isolated gametocytes were analyzed by microscopical and biochemical methods and shown to be highly enriched. The antigenicity of the gametocytes was analyzed in mice, rabbits, and chickens by ELISA and indirect immunofluorescence. Contrary to published results, we have found gametocytes to be highly immunogenic in all animals tested.

Ultrastructural evaluation of the effects of diclazuril on the endogenous stages of Eimeria maxima and E. brunetti in experimentally inoculated chickens

The ultrastructural morphology of the different endogenous stages of Eimeria maxima and E. brunetti was evaluated after oral treatment of inoculated chickens with a single dose of 5 mg/kg diclazuril. The drug induced no ultrastructural change in the growth or differentiation of the various schizont stages of both Eimeria spp. In E. maxima, the micromorphological appearance of micro- and macrogamonts developing from the blast from to maturation also remained unaffected by drug treatment. However, in all fertilized macrogamonts the normal pattern of oocyst wall establishment was completely disturbed, resulting in the formation of an abnormally thickened, incomplete oocyst wall and the necrosis of the zygote. In E. brunetti, the growth and nuclear division during microgametogenesis were not affected but differentiation was clearly abnormal. In comparison with the controls, this abnormal differentiation was characterized by a less extensive enlargement of the parasite surface area, aberrant morphological configurations of condensed heterochromatin, intracytoplasmic flagella formation, and glycogen accumulation. Finally, the complete degeneration of all microgamonts ensued. The growth and differentiation leading to mature macrogamonts was not disturbed; however, subsequent oocyst wall formation was largely precluded and the macrogamonts proceeded to degenerate completely. We conclude that diclazuril treatment primarily affected particular stages in the sexual development of both Eimeria spp., resulting in the complete eradication of these coccidian species.

The effects of sym. Triazinones on developmental stages of Eimeria tenella, E. maxima and E. acervulina: a light and electron microscopical study

The development of three chicken coccidia (Eimeria tenella, E. acervulina and E. maxima) was studied by means of light and electron microscopy. One group of chickens infected with 6000-20,000 oocysts received a single dose of 5 mg/kg body weight (comparable to approx. 25 ppm in the feed) Bay Vg 7183 or Bay Vi 9142 orally (on day 3 or 4 p.i.), whereas others received two doses (on days 3 and 4 or on days 4 and 5 p.i.). The animals were killed on days 3, 4, 5, 6 and 7 p.i. and parts of the mucosa were dissected from the caecum (E. tenella), the ileum (E. maxima) and the duodenum (E. acervulina). Significant damage was observed in comparison to the controls, affecting nearly all of the parasites in those animals that had been treated twice, whereas some of the parasites remained microscopically unchanged after only one treatment. In general, the perinuclear space, mitochondria and the endoplasmic reticulum were found to be considerably enlarged. Nuclear divisions were disturbed in schizonts and microgamonts, thus resulting in a greatly reduced production of parasites. The most important damage occurring in the macrogamonts concerned the wall-forming bodies II. As they burst, the formation of intact oocyst-walls was hindered, even if fertilization had taken place.

Fine structure of zygotes and oocysts of Eimeria nieschulzi

Mature macrogamonts were present in the small intestine of rats 5.5 to 7.5 days postinoculation with Eimeria nieschulzi oocysts; oocysts were present at 6 to 7.5 days. Types I and II wall-forming bodies in macrogamonts began to undergo ultrastructural changes within zygotes to form the outer and inner layers of the oocyst wall. Before and during oocyst wall formation a total of 5 membranes (M1-5) were formed at or near the surface of the zygote. The outer and inner oocyst wall layers formed between M2 and M3, and M4 and M5, respectively. The mature oocyst was loosely surrounded by M1 and M2, had an electron-dense outer layer, 100-275 nm thick, and an electron-lucent inner layer, 160-180 nm thick. It also contained an electron-lucent line consisting of M3 and M4 interposed between the outer and inner layers of the oocyst wall. The micropyle, measuring 935 x 47 nm, was located in the outer layer of the oocyst wall and consisted of 10-14 alternating layers of electron-dense and lucent material. The sporont of mature oocysts was covered by M5, immediately beneath which were M6 and M7. The sporont contained a nucleus and nucleolus, lipid and amylopectin bodies, mitochondria, ribosomes, as well as smooth and rough endoplasmic reticulum. Canaliculi, Golgi complexes, and types I and II wall-forming bodies were absent.

The ultrastructural development of the oocyst wall of Eimeria maxima

Oocyst wall formation in Eimeria maxima was studied during the macrogamete stage in intestinal cells of the chick and in unsporulated oocysts isolated from faeces. The outer of the 2 membranes bounding the mature macrogamete separated from the surface but remained as a veil surrounding the developing oocyst throughout the whole intracellular process. Wall-forming bodies Type I were initially applied to the limiting membrane of the zygote cytoplasm; a layer of material similar to their contents was then formed around the zygote. As this occurred a new double membrane was formed surrounding the oocyst cytoplasm. The outer wall layer was initially homogenous in appearance but later developed into 2 zones, an outer amorphous region and an inner osmiophilic region. The inner layer of the oocyst wall was formed from the contents of the wall-forming bodies Type II which dispersed between the outer wall and the limiting membranes of the oocyst cytoplasm. There was evidence of an additional membrane formed external to the outer wall. The outer membranes were not present around the wall of oocysts passed in the faeces of chicks, but the same wall zonation was evident, although the inner osmiophilic zone of the outer wall layer was markedly thinner in comparison with the same zone seen in the tissues.

Ultrastructural observations on macrogametogenesis and the structure of the macrogamete of Isospora felis

The ultrastructural changes occurring during macrogametogenes of Isospora felis were studied in the small intestine of the cat at 8 and 9 days post-infection. The parasites were situated in parasitophorous vacuoles within the host epithelial cells. The early macrogamont could be identified by its ellipsoidal shape and by the presence of a large nucleus with a distinct nucleolus. Throughout the development of the macrogamont the organism remained limited by a pellicle. At an early stage of macrogametogenesis the formation of the wall-forming bodies of types I and II (WFB I and WFB II) was initiated simultaneously. The formation of the WFB I appeared to involve the rough endoplasmic reticulum (rER) and the Golgi bodies, whereas the WFB II developed within cisternae of the rER. Early in the developmental process vacuoles were observed which budded off from the nucleus and formed multimembranous vacuoles. During gametogenesis both lipid globules and polysaccharide granules developed within the cytoplasm. The mature macrogamete was spherical in appearance and was limited by a pellicle. It possessed a large nucleus with a nucleolus and the cytoplasm was filled with numerous WFB I, WFB II, lipid globules and polysaccharide granules. A few multimembranous vacuoles were also present together with canaliculi, mitochondria, Golgi bodies and some rER.

The fine structure of the developing macrogamete of Eimeria maxima

The fine structure of the developing macrogamete of Eimeria maxima was studied from chicks killed at intervals from 138 to 147 h after inoculation. The macrogamete developed within a parasitophorous vacuole. Lying within the vacuole and extending for some distance around the periphery of the macrogamete were intravacuolar tubules, grouped in certain areas, and in some cases they were seen to make direct connexions with the cytoplasm of the parasite. During development, electron-pale vesicles were pinched off externally from the surface of the macrogamete. There appeared to be 2 forms of wall-forming bodies of the Type I during development, one form being less osmiophilic than the other. Other organelles present, such as wall-forming bodies of Type II, granular endoplasmic reticulum, mitochondria, canaliculi, lipid inclusions and intravacuolar folds, were similar in structure to those of other Eimeria species.

Electron microscopical study on gamogony of Sarcocystis suihominis in human tissue cultures

Sexual stages and oocysts of Sarcocystis suihominis were developed in human tissue cultures and studied with the electron microscope. This development was extremely rapid, being completed about 18--22 h post infection and there were no preceding schizogonic processes, thus confirming the earlier observations that schizogony is obligatorily restricted to the intermediate host in the sarcosporidian life cycle. Micro- and macrogamonts could be distinguished about 12 h post infection and were situated in a parasitophorous vacuole bounded by two membranes. These gamonts reached diameters of up to 10 micrometer. The large nucleus of every microgamont gave rise simultaneously to about 20--30 microgametes. Only dense projections of the nucleus were used as nuclei of microgametes. The microgametes were slender, about 4--5 micrometer long, and several were found to have three flagella, one of which was attached to the body for some distance. Besides these flagella additional microtubules were found and in several cases the attached flagellum was not complete and contained various numbers of single or paired microtubules. The macrogametes were bounded by two membranes and contained two types of inclusions similar to the wall-forming bodies known from the genus Eimeria. The oocysts were bounded by a wall consisting of a dense outer layer and four membranes, under which two other membranes covered the cytoplasm. Beginning from the 22nd h post infection a development similar to sporulation was noted inside these oocysts. This sporulation, i.e., the formation of two sporocysts inside an oocyst, was, however, not completed, probably due to the rapid degeneration of the parasitized host cell. The oocyst itself even appeared intact five days later.

The ultrastructure and distribution of micropores in the various developmental forms of Eimeria brunetti

The structure and distribution of micropores in the various developmental stages of Eimeria brunetti was examined. Micropores were observed in all the endogenous forms with the exception of the microgamete. Oocysts from chicken faeces were also examined at various stages of sporulation and micropores were demonstrated in zygotes, sporoblasts, sporozoites, and the residual cytoplasmic masses. The number of micropores per organism appeared to be correlated with the surface area of the organisms irrespective of whether these were endogenous or sporulating forms. The increase in the number of micropores did not appear to be related to micropore activity because semmingly active micropores were observed only in the trophozoites, in the early multinucleate forms (early shizonts and microgamonts), and in the early macrogamonts. All these forms, however, possessed relatively few micropores. No active micropores were ever observed within the sporulating oocysts.

Ultrastructural studies on the endogenous development of eimeria brunetti. III. Macrogametogony and the macrogamete

The ultrastructural changes occurring during macrogametogony in Eimeria brunetti were studied in tissue from the small intestine of infected young domestic fowls. As the macrogametocyte developed, an increase in the volume of the cytoplasm and the nucleus occurred. At this early stage of development the organism was limited by a single unit membrane which possessed a number of micropores. The organism was situated in a parasitophorous vacuole which contained intra-vacuolar folds and intra-vacuolar tubules. The wall forming bodies of type II (WFB II) started to appear before the wall forming bodies of Type I (WFB I). The WFB I developed in the cytoplasmic matric whereas the WFB II were formed within the cisternae of the rough endoplasmic reticulum apparently in association with Golgi bodies. The mature WFB I were homogeneous, limited by a unit membrane, and larger than WFB II which had no limiting membrane but remained surrounded by a membrane of the rough endoplasmic reticulum. Polysaccharide granules were formed in the cytoplasm between strands of the rough endoplasmic reticulum. During maturation the WFB I & II and the polysaccharide granules increased in size and number, and as this occurred a number of electron translucent vacuoles were ejected from the surface of the macrogametocyte. The mature macrogamete possessed a large nucleus with a nucleolus, a number of multi-membranous vacuoles, mitochondria, and canaliculi. The WFB I & II were located at the cell periphery and the polysaccharide granules further towards the interior of the organism. The organism was limited by a unit membrane but during macrogametogony a homogeneous layer had developed which extensively coated the macrog...

Ultrastructure of macrogametogenesis of Eimeria mivati

The development of the macrogamete of Eimeria mivati Edgar and Seibold 1964 was studied with the electron microscope. Development of the young gamont was characterized by a loss of organelles such as the apical complex, subpellicular microtubules, rhoptries and micronemes, followed by an increase in micropores, mitochondria, rough endoplasmic reticulum (rER), and Golgi complexes. Nuclear detachment bodies and canaliculi were present in maturing macrogamonts. Amylopectin was first observed as small electron-dense rod-like bodies that eventually became large electron-transparent bodies. Type II wall-forming bodies developed in the cisternae of the rER. Type I wall-forming bodies appeared shortly thereafter in close association with numerous Golgi complexes. Many small vesicles located between the cisternae of the rER and the Golgi complexes formed what appeared to be a secretory pathway whereby protein formed in the cisternae and, modified by the Golgi complex, may produce the type I wall body material. The outer wall of the oocyst developed between two distal membranes on the surface of the macrogamete. Although the actual mechanism of deposition of the wall material was not seen, it was probably by some secretory process. Wall-forming bodies did not fuse.

The fine structure of the endogenous stages of Eimeria labbeana. 3. Feeding Organelles

The fine structure of the feeding organelles of the endogenous developmental stages of Eimeria labbeana from the ileal mucosa of the common Pigeon, Columba livia, is described and compared with similar structures of other species of Eimeria. Intra-cytoplasmic, membrane-bound vesicles of varying shapes and dimensions, and pinocytotic vesicle, were seen in association with cup-shaped or v-shaped invaginations in early schizonts, early macrogamonts, and macrogametes. Deep invaginations, averaging 1.7 x 0.5 mu in size, and found on the surface of early schizonts, early and young macrogamonts, and developing microgamonts, apparently function as organelles of ingestion and breakdown of host-cell cytoplasm. Micropores were rarely seen in schizonts and never in microgametes. The merozoite had one typical micropore (850 x 680 A) and a number of micropore-like invaginations. Micropores of the microgamonts averaged 610 x 580A, and those of macrogamonts and macrogametes averaged 1,220 x 780 A. A typical micropore was observed in an early oocyst. Intravacuolar tubules, each 580 A in diameter and composed of nine microtubule-like subunits, were observed only in about 1 per cent of the more than 4,000 macrogametes studied. This paper establishes that E. labbeana is a species that possesses all the known organelles associated with feeding, expect the intravacuolar folds.

The ultrastructure of macrogametes of Eimeria ferrisi Levine and Ivens 1965 in Mus musculus

Macrogametes of Eimeria ferrisi occurred in epithelial cells of the cecum and colon of Mus musculus and were studied by electron microscopy. Young stages were identified as macrogamonts by the presence of wall-forming bodies. At first an outerlimiting membrane and remnants of the inner membrane complex of the former merozoite pellicle were present; the latter was later lost but in mature macrogametes 3 limiting membranes were observed. Type II wall-forming bodies appeared before type I; the former developed in expanded cisternae of the endoplasmic reticulum whereas the latter were smaller in size and appeared in the ground substance of the cytoplasm. After formation of the oocyst wall the bodies of the 2 types were no longer visible. The presenceodies of the 2 types were no longer visible. The persistence of micronemes in mature macrogametes and the presence of numerous layers of rough endoplasmic reticulum during wall formation have not been previously reported.

The ultrastructural development of the macrogamete and formation of the oocyst wall of Toxoplasma gondii

The macrogametes of Toxoplasma gondii develop within the epithelial cells of the cat ileum. As they develop the nucleus enlarges and dense patches of chromatin which were present in the nucleoplasm, disappear. Polysaccharide granules and lipid globules appear in the cytoplasm and increase in number during development. The wall-forming bodies of Type I (WFB I) appear before the wall-forming bodies of Type II (WFB II); WFB I are smaller, more osmiophilic and more numerous than the WFB II. The WFB I appear to form from vesicles produced by the smooth endoplasmic reticulum, and the WFB II form within the lacunae of the rough endoplasmic reticulum. Double membraned vacuoles appear to form from the nuclear membranes but the function of these is unknown. Throughout development the macrogamete retains a normal pellicle which possesses numerous micropores. The first evidence of oocyst wall formation is the appearance of participate matter in the parasitophorous vacuole which precipitates to form Layer 1 of the oocyst wall. Layers 2 and 3 are unit membranes which form between Layer 1 and the pellicle. During this development the organism has an organelle complement similar to that of the macrogamete. Layers 4 and 5 form between Layer 3 and the pellicle. Layer 4 is osmiophilic and its formation is accompanied by the disappearance of WFB I. Layer 5 is less osmiophilic than Layer 4 and its formation is accompanied by the disappearance of WFB II. The two innermost layers (Layers 4 & 5) in the oocyst wall of Toxoplasma are similar to those found in Isospora spp. and Eimeria spp. Toxoplasma seems to be unusual in that, firstly, it possesses an additional 3 layers and, secondly, all 5 layers are formed outside the pellicle of the original macrogamete.

The fine structure of the endogenous stages of Eimeria labbeana: 2. Mature macrogamonts and young oocysts

The fine structure of the mature macrogamonts and intracellular oocysts of Eimeria labbeana from the ileal mucosa of experimentally infected Pigeons (Columbia livia) was investigated and described. The macrogamont reached a maximum size of 12.0 x 9.5 mum (average equals 10.8 x 8.8 mum), and was located within a narrow parasitophorus vacuole. Most of the macrogamonts were limited by two membranes. Intravacuolar tubules, 1.2 mum long and 58 nm in diameter, established direct connections between the parasite and the host cell. Each tabule was composed of 9 subunits arranged around the central lumen. Cytoplasmic canaliculi were composed of bundles of microtubule-like structures (8-10 nm wide). Type 1 wall-forming bodies reached a maximum size of 1.8 x 1.5 mum, and many had centric or eccentric electron transparent portions within them. They were frequently seen lodged within peripherally-located mitochondria. Type 2 wall-forming bodies averaged 1.5 mum in diameter. The role of the two types of wall-forming bodies in forming the outer and inner layers of the wall of the oocyst was similar to that in other species of Eimeria. The oocyst wall was 0.2 mum thick and composed of a limiting membrane (20 nm thick), an outer layer (75 nm thick), and an inner layer (100 nm thick).

Structure and mode of function of the organelles associated with nutrition of the macrogametes of Eimeria acervulina

The macrogamete of Eimeria acervulina, lay and developed within the host cell in a parasitophorous vacuole. The cytoplasmic membrane of the host cell bordering the vacuole was not smooth, but it had numerous folds extending into the vacuole. These "intravacuolar folds" varied in depth and number in different sections. In some, the majority of the folds were disconnected from the host cell. Once disconnected, they evidently disintegrated forming the amorphous, particulate material present in the parasitophorous vacuole. The pellicle of the young macrogamete consisted of a single unit membrane with an osmiophilic material representing the second membrane. Two unit membranes were apparent at a later stage of development when the wall-forming bodies had been formed and amylopectin granules deposited. Two kinds of organelles were present on the surface of the macrogamete, typical micropores and invaginations of the pellicle. The micropores arose from an invagination of the outer membrane, which continued through the invagination without interruption. Irrespective of whether an inner membrane was present in the pellicle or not, a thickened cylindrical wall around the inner portion of the invagination was always present. Micropores appeared in large numbers in both micro- and macrogametocytes. As many as three micropores were seen in a surface area of 2 mu2. Invaginations arose in a similar manner by infolding of the pellicle. They differed from micropores in that the thickened cylindrical wall present around the inner portion of the micropore was absent, and also in that invaginations had no uniform appearance. They were of varying shapes, and lengths, varying from very short V-shaped to long and narrow. Micropores and invaginations take in nutrients in the form of particulate matter present in the parasitophorous vacuole, this material having been derived from the host-cell membranous "intravacuolar folds". The micropores function as cytostomes and the invaginations take in material by means of pinocytosis. Large numbers of intravacuolar tubules were seen at the surface of the macrogamete. They were present only at certain areas of the macrogamete and in groups and were connecting the parasite with the host cell. They were about 80-110 nm in diameter, and were seen to attain a length of up to 6 mu. Evidence was obtained indicating that the tubules transport free ribosomes from the host cell to the parasite. The ribosomes were seen to accumulate in "pockets" within the cytoplasm of the host cell, at the area where the tubules were connected.