Barbara A. Nichols

Endocytosis at the micropore ofToxoplasma gondii

Ultrastructural studies were undertaken to reexamine the structure and function of the micropore ofToxoplasma gondii. By incubating tachyzoites with the tracer horseradish peroxidase (HRP), we showed for the first time cytochemically that an extracellular tracer was internalized into vacuoles at the micropore. Our morphological observations also demonstrated that the base of the micropore in both tachyzoites and bradyzoites was sometimes covered by a clathrin-like bristle coat. A coated vesicle was observed in continuity with a bradyzoite micropore, and large (150-nm) coated vesicles were occasionally present just beneath the micropore. These results suggest that receptor-mediated endocytosis occurs at the micropore. In other micrographs, however, the micropore appeared uncoated. In some bradyzoites, the uncoated micropore was greatly dilated, and it contained vesicles like those found in the cyst matrix associated with debris from disintegrated parasites. We had previously observed such debris from fragmented organisms in cysts prepared in vivo. These results indicate that residues from dead bradyzoites may provide nutrients for younger, developing parasites in the same cysts. Our observations also suggest that either receptor-mediated or bulk endocytosis can occur at the micropore, perhaps depending upon the availability of specific ligands. Investigation of a receptor-mediated pathway may reveal a means for targeting therapy selectively to the parasites to benefit patients with disseminated toxoplasmosis.

Toxoplasma gondii: differentiation and death of bradyzoites.

The living parasites inToxoplasma cysts cannot be eradicated by current therapy and maintain latent infections for many years. Relatively little is known about encystedToxoplasma. We therefore undertook studies using mice infected with the avirulent ME 49 strain ofToxoplasma. The bradyzoites in young (12- to 17-day-old) cysts contained the same organelles as did tachyzoites. The bradyzoites of older cysts (4 weeks postinoculation) had differentiated, losing certain organelles and acquiring others. Our major new finding was that in animals inoculated ≥4 weeks previously, some bradyzoites were totally disrupted, splling their contents (perhaps including lytic substances) into the cyst matrix. Many older bradyzoites in the same cysts lacked internal membranes and their viability was questionable, but there were also occasional parasites resembling viable tachyzoites and mature bradyzoites, organisms that might possibly initiate daughter cyst formation after cyst rupture. The life span of an individual bradyzoite may be shorter than formerly appreciated despite the prolonged course of latent infections.

Glucose-6-Phosphatase as a Cytochemical Marker of Endoplasmic Reticulum in Human Leukocytes and Platelets'

Leukocytes and platelets, freshly isolated from normal human blood, were tested cytochemically for glucose-6-phosphatase (G-6-Pase) by a modified Wachstein-Meisel method. The enzyme was present in the endoplasmic reticulum (ER) and perinuclear cisternae of all five types of leukocytes and in the ER of platelets. The reaction product from the cytochemical test distinguished the ER from other intracellular membrane-limited cisternae (i.e., the smooth pinocytic tubules of monocytes and the surface-connected canalicular system of platelets) and thus is a valuable marker of the ER. The cytochemical test also showed that the ER of polymorphonuclear leukocytes (PMN), usually obscured by abundant granules in cells prepared for morphological examination, is more extensive than formerly appreciated. This is the first demonstration of G-6-Pase in human leukocytes. Its precise role in leukocyte metabolism can now be investigated.

Ultrastructure and peroxidase cytochemistry of normal human leukocytes at birth

Abstract At birth, differential and white blood cell counts of normal newborn infants are strikingly different from those of adults in that the number of leukocytes is increased and immature cells course through the circulation. In this study, our intent was to examine normal neonatal cord blood by electron microscopy and peroxidase cytochemistry to determine whether any detectable differences exist in the leukocytes of neonatal and adult blood. This investigation was undertaken because newborn infants have an increased susceptibility to infection, and alterations in phagocyte function have been implicated as the cause. Cord blood was found to contain mature leukocytes of all kinds, similar in ultrastructure and peroxidase localization to those of adults. Moreover, as indicated earlier by light microscopy, immature forms (normally found only in adult bone marrow) were present in the blood of newborns. We found that nearly all cell lines were represented in the neonatal circulation by such developmental forms as promyelocytes, myelocytes, promonocytes, erythroblasts, megakaryocytes, rare unidentifiable blasts, and dividing cells—all resembling their counterparts in adult bone marrow. With the techniques used here, neonatal leukocytes were similar to those of the adult in ultrastructure and peroxidase localization, although some had been mobilized into the blood in a remarkably immature state. This study, the first of its kind, will serve as a helpful background for future investigations of acquired, genetic or neoplastic leukocyte abnormalities which may be discovered at birth.

The Vacuolar Apparatus of Alveolar Macrophages and the Turnover of Surfactant

There are profound differences between alveolar macrophages and those elsewhere in the body. Structurally, a prominent distinguishing characteristic of alveolar macrophages is their generous complement of dense inclusions as compared with other populations of macrophages (Karrer 1960; Leake & Heise 1967; Pratt et al. 1971). There are metabolic differences as well, for alveolar macrophages rely primarily on aerobic respiration during phagocytosis, whereas peritoneal macrophages depend primarily upon glycolysis (Oren et al. 1963). The unique properties of alveolar macrophages may result from their location in the lung, since they alone among the phagocytes are continually exposed to noxious and infectious materials in the air.