Maria A. Rudzinska

An electron microscopic study of Babesia microti invading erythrocytes

SummaryIntracellular sporozoan parasites invade the host cell through the invagination of the plasma membrane of the host and a vacuole is formed which accommodates the entering parasite. The vacuole may disappear and the invaginated membrane of the host then becomes closely apposed to that of the parasites own membrane. As a result the parasite is covered by two membranes. Members of the class Piroplasmea differ from other Sporozoa in that their trophozoites are covered by a single membrane. By screening numerous sections of intraerythrocytic Babesia microti belonging to the class Piroplasmea, it was found that merozoites of Babesia enter the erythrocytes of hamsters in the same way as those of other Sporozoa. When a merozoite touches the red blood cell with its anterior end it becomes attached to the membrane of the host, which starts to invaginate and a parasitophorous vacuole is formed. The vacuolar space disappears rapidly and the membrane of the vacuole and that of the parasite become closely adjacent. At this stage the parasite is surrounded by two plasma membranes. The outer membrane derived from the invaginated host membrane disintegrates quickly and the parasite is left with a single membrane throughout its life span.

Inhibition of lysosomal fusion with symbiont-containing vacuoles in Paramecium bursaria.

Abstract Paramecium bursaria harbors several hundred intracellular Chlorella symbionts which remain undigested at the same time that the host cell phagocytizes and digests other organisms. Using electron microscopy and thorotrast labelling, we have shown that secondary lysosomes fuse with food vacuoles, but do not fuse with vacuoles containing symbiotic algae. From these and other data we suggest that the symbiotic algae alter the membrane of the vacuole which surrounds them, thus inhibiting fusion with secondary lysosomes.

The Fine Structure of Malaria Parasites

Publisher Summary Plasmodium, the malaria parasite of mammals, birds, and reptiles, is among the most complex intracellular parasites. For its full development and survival in nature it requires two hosts, an invertebrate, in which its sexual reproduction takes place, and a vertebrate, in which it multiplies asexually. In the vertebrate host, plasmodium first invades cells of tissues of different organs before it enters erythrocytes, the main host cell; thus the latter phase is termed intraerythrocytic, and the former, exoerythrocytic. The difference between the mitochondrial systems of intraerythrocytic species parasitized in mammals and those in birds is highly significant and serves as an example of the influence of the host on the fine structure of the parasite. Bird malaria parasites have typical protozoan mitochondria, whereas mammalian plasmodia are deprived of such mitochondria and have a structure composed of concentric double membranes which presumably performs mitochondrial functions. The concentric double-membraned structures in plasmodium are derived from the plasma membrane.

Penetration of the peritrophic membrane of the tick by Babesia microti

SummaryA peritrophic membrane (PM) has been demonstrated in the gut of feeding larvae, nymphs, and adults of the tick Ixodes dammini. This is the first report of a PM in ticks. This temporary structure divides the lumen of the gut into two compartments, an endoperitrophic space, the lumen proper, and an ectoperitrophic space located between the PM and the epithelial cells of the gut wall. The PM is a mechanical barrier and even such small particles as ribosomes derived from ingested reticulocytes are retained in the lumen proper; they are never found in the ectoperitrophic compartment. In Ixodes dammini fed on hamsters infected with Babesia microti some of the parasites are found in the ectoperitrophic space. This passage is accomplished by a highly specialized organelle, the arrowhead, which develops in some Babesia during their metamorphosis in the gut of the vector. The arrowhead, while passing through the PM, changes its fine structure and loses its internal organization as if releasing some of its contents. Its disintegration continues and it disappears shortly after the Babesia have entered the epithelial cells. Only Babesia equipped with the arrowhead structure are able to cross the PM. This is the first documented case of a parasite traversing a solidified PM.

Ultrastructural studies on sporogony of Babesia microti in salivary gland cells of the tick Ixodes dammini.

SummaryTick larvae were permitted to feed on infected hamsters and then allowed to molt. Nymphs were examined just prior to feeding on uninfected hamsters or at timed intervals thereafter. Invasion of the salivary gland by B. microti occurs before feeding of the nymph begins, and development of the parasite is further stimulated by feeding. The sporoblast forms a massive multinucleated meshwork which ramifies throughout the large host cell. No separation of the meshwork into multiple subdivisions, termed “cytomeres” by other workers, has been detected. Instead the specialized organelles characteristic of sporozoites, namely micronemes, rhoptries, and segments of double membrane appear in the meshwork itself and gradually become organized into sporozoite anlagen which protrude from its surface. At the same time the meshwork shortens and thickens giving rise to large compact undifferentiated bodies whose surface is also studded with sporozoite anlagen. Sporozoites thus originate either from the meshwork or from the undifferentiated bodies. In either case large lobate nuclei send projections into the anlagen as they protrude from the surface of the sporoblast. In a final step the mature sporozoites arise by simultaneous nuclear and cytoplasmic divisions. There is no separate stage of schizogony and the process is one of true budding.

The ultrastructure of nuclear division in a suctorian, Tokophrya infusionum

SummaryUltrastructural changes in the micro- and macronucleus throughout division were followed in synchronized cultures of the suctorian, Tokophrya infusionum. After an initial swelling, the micronucleus elongates enormously; microtubules within the micronucleus proliferate and lengthen as the micronucleus elongates. Changes in the macronucleus become visible only after micronuclear division is well underway. The chromatin bodies fuse into long chromatin strands, and the large bundles of microtubules present in the resting macronucleus break up into small groups which parallel the chromatin strands. Colchicine, which prevents reproduction in Tokophrya, seems to block division at a very early stage. The macronucleus appears the same as the resting nucleus of untreated organisms, with numerous microtubules and distinct chromatin bodies. The chromatin in the micronucleus aggregates into large clumps, however, and proliferation of microtubules does not occur.

Electron microscope study of intact tentacles and disc inTokophrya infusionum

Elektronenmikroskopische Untersuchungen am sessilen Süsswasser-ProtistenTokophrya infusionum zeigen besonders komplizierte Strukturen der «Tentakel» und der «Haftscheibe». Die Tentakel sind von zwei Membranen umgeben und umschliessen eine Anzahl von Längselementen. Ihre Spitzen sind aus einer grösseren Zahl von «Papillen» zusammengesetzt. Die Haftscheibe besteht aus einem reichen Fibrillennetz.

Formation of merozoites in intraerythrocytic Babesia microti: an ultrastructural study

Babesia microti used in this study derives from a human infection and has been maintained in hamsters by intraperitoneal injections of infected blood. The parasite is covered by a single membrane and lies free in the cytoplasm of erythrocytes. It forms numerous pseudopods extending in all directions and to follow the events of merozoite formation it was necessary to resort to serial sections. Through this method it was found that the number of merozoites produced simultaneously by a single parasite is not more than four, and that a large chunk of the nucleus and the cytoplasm remains still available for further reproduction. Each merozoite after pinching off from the parent becomes an independent organism able to leave the host cell. The nucleus divides by budding and the buds retain their connection with the main body of the nucleus until the very end when the merozoite pinches off from the parent cell. This is budding sensu stricto. Since nuclear division does not precede cytoplasmic fission, Babesia is...

Babesia microti: Prolonged survival of salivarian piroplasms in nymphal Ixodes dammini

We determined how long Babesia microti survive in the salivary glands of nymphal Ixodes dammini. Of those ticks held at 21 C, the proportion with demonstrable piroplasms decreased from 95%, prior to 20 weeks post-larval-feeding (p-l-f), to less than 80% at 42 weeks p-l-f. Similarly, the number of infected acini decreased significantly. Nymphal I. dammini were kept alive for as long as 1 year, by transferring them to 4 C, at 20 weeks p-l-f. The proportion of infected ticks at 52 weeks p-l-f was less than half of the proportion of infected nymphs examined prior to 20 weeks p-l-f, and only 1/6 as many acini were infected. Ultrastructural observations of salivary glands from ticks at 44 weeks p-l-f revealed that B. microti parasites in older ticks remain in the sporoblast meshwork phase; such parasites rarely differentiated into sporozoites. Degenerating parasites containing autophagic vacuoles were also observed in older ticks.

Cellular and clonal aging in the suctorian protozoan Tokophrya infusionum

Quantitative methods for the study of aging in single organisms of the suctorian protozoan Tokophrya infusionum are described. New cell lines are initiated by metamorphosis of a ciliated embryo to form a sessile adult. The life history of adult cells consists of a sequence of age-related stages, culminating in cessation of reproduction and feeding, and eventual death. Lifespans of single cells were measured and were found to range rather widely about a mean, even when the cells compared were closely related within a single lineage. Variation appears to be inherent in the aging process in Tokophrya. Clones of Tokophrya undergo a gradual deterioration on a scale many times longer than the lifespan of individual cells. Lifespans of individual cells were determined when each of two clones were relatively young and later when their reproductive vigor had begun to decline. In both cases, the lifespan of individual cells were strikingly reduced in the old, as opposed to the young clones. The two types of senescence are thus experimentally separable, but nonetheless coupled phenomena. The similarity of aging in Tokophrya to that of other protozoa, fungi, and tissue culture cells is described and possible mechanisms are discussed.