The Cell Surface of Trypanosoma musculi Bloodstream Forms. I. Fine Structure and Cytochemistry*†

SYNOPSIS. An extracellular surface coat was observed at the fine-structural level on the outer lamina of the pellicular and flagellar membranes of intact Trypanosoma musculi bloodstream forms. The surface coat had a mean width of 9.2 nm, and was composed of a somewhat electron dense, uneven, fibrillar-like matrix. Brief trypsin treatment of living blood forms completely removed the cell surface coat. Several cytochemical methods applicable to electron microscopy were used to detect the presence and distribution of carbohydrates in the trypanosome's surface coat and pellicular membrane. The polycationic dye compounds employed were: ruthenium red, ruthenium violet, Alcian blue chloride, and lanthanum nitrate. Electron-dense stain reaction products, indicative of polysaccharides, were evident in the surface coat of cells treated with these dyes, which also agglutinated both living and glutaraldehyde fixed cells. Like the surface coat, the pellicular membrane of trypsinized cells gave strong positive staining reactions with the several dyes, indicating the presence of membrane bounded carbohydrates, and living and glutaraldehyde-fixed trypsinized cells were agglutinated with the polycationic stains. Bloodstream forms were treated with the enzymes, α-amylase, dextranase, and neuraminidase. No obvious morphologic difference, however, was apparent between the surface coat of untreated cells and those subjected to treatment with any of the various glycoside hydrolase enzymes. Further, these enzymes had no apparent gross effect on the staining affinity of the surface coat for the several polycationic dyes. Cationized ferritin was used to visualize the negative cell surface charge of T. musculi bloodstream forms. Large quantities of cationized ferritin were bound in the surface coat matrix. Glycoside hydrolase enzyme treatments had no apparent effect on the amount of ferritin bound in the surface coat. Cationized ferritin was bound also to the outer lamina of the pellicular membrane in trypsinized cells, which had quantitatively less ferritin bound per surface unit area than bloodstream forms untreated by the enzyme. Living and glutaraldehyde-fixed cells were agglutinated with cationized ferritin. The results obtained in the various experiments indicated that polyanionic polysaccharides were constituent terminal ligands of the surface coat matrix and pellicular membrane in T. musculi bloodstream forms.     

The fine structure of the kidney of Achatina achatina (L.)

Summary The kidney sac of Achatina achatina, the site of primary urine formation, seems to contain no direct structural analogue of the vertebrate glomerular podocytes. The nephrocytes which line the kidney sac and separate the blood from the primary urine are supported by a basal lamina which is permeable to ferritin but impermeable to colloidal gold particles (ca. 100 Å, and 80–240 Å respectively). The blood capillaries within the kidney sac are of two types, fenestrated and unfenestrated. The basal lamina which surrounds them is impermeable to haemocyanin. The nephrocytes are then, bathed apically by primary urine and basally by an ultrafiltrate of the blood. It is proposed that fluid enters the urinary space from the connective tissue by passing between the nephrocytes, perhaps through pores in the septate junctions. Other possible mechanisms of primary urine formation are discussed. The nephrocytes contain peroxisomes which may be involved in urate metabolism.The cells of the ureteral epithelium bear a lumenal microvillous border. Their lateral and basal plasma membranes are elaborately folded. These cytoplasmic folds enclose extracellular channels through which fluid is transported from the urine back into the blood.     

An electron microscopic investigation of the surface coat of the electrocyte of Electrophorus electricus

Summary The surface coat of the electrocyte of the main electric organ of Electrophorus electricus was studied using cytochemical methods (periodic acid-silver methenamine, periodic acid-chromic acid-silver methenamine, periodic acid-thiosemicarbazide-silver proteinate, Concanavalin A — horseradish peroxidase, ruthenium red, Alcian-blue lanthanum nitrate, colloidal iron hydroxide and cationized ferritin). The surface of the electrocyte presents perpendicularly oriented tubular invaginations of the cell membrane. The fibrous coat 50–100 nm thick, penetrates into the lumen of the invaginations. It is also observed in the synaptic clefts existent in the posterior face of the electrocyte. The coating of the surface membrane gives a positive reaction with all techniques used. Binding of colloidal iron hydroxide particles was observed only in the outer layer of the coat. With the Alcian-blue lanthanum nitrate technique, microtubules were observed in the cytoplasm of the electrocyte.The results indicate that the surface coat of the electrocyte contains mucopolysaccharides, glycoproteins, acid mucopolysaccharides and anionic sites detected at low (colloidal iron hydroxyde) and neutral (cationized ferritin) pH.This work has been supported by Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq), Conselho de Ensino e Pesquisa da UFRJ (CEPG) and Banco Nacional de Desenvolvimento Econômico     

Regeneration of the cell wall in protoplasts of Candida albicans

To assess the dynamics of synthesis of the wall by regenerating Candida albicans protoplasts deposition of chitin and mannoproteins were investigated ultrastructurally using wheat germ agglutinin conjugated with either horseradish peroxidase or colloidal gold, and Concanavalin A coupled to ferritin respectively.Freshly prepared protoplasts lacked wheat germ agglutinin receptor sites but after 1–2 h of regeneration, they were detected. After 4–5 h of regeneration, the cell wall showed a discrete structure which was only labelled with wheat germ agglutinin in thin sections. At this stage of regeneration the outermost layer of the wall was labelled with clusters of Concanavalin A-ferritin particles.After 8 h regeneration, the cell wall appeared compact, and homogenously marked with wheat germ agglutinin whereas only the surface layers appeared consistently labelled with Concanavalin A-ferritin.From these observations we conclude that C. albicans protoplasts are able to regenerate in liquid medium a cell wall consisting of a network of chitin fibrils and mannoproteins at least (glucan polymers were not determined in the present cytological study). The former are the fundamental component of the inner layers at early stages of regeneration, whereas the latter molecules are predominant in the outer layers of the wall.Abbreviations WGA-HRP wheat germ agglutinin conjugated with horseradish peroxidase - WGA-Au wheat germ agglutinin conjugated with colloidal gold - Con A-ferritin Concanavalin A coupled to ferritin     

Iron loaded ferritin secretion and inhibition by CI-976 in Aedes aegypti larval cells

Ferritin is a multimer of 24 subunits of heavy and light chains. In mammals, iron taken into cells is stored in ferritin or incorporated into iron-containing proteins. Very little ferritin is found circulating in mammalian serum; most is retained in the cytoplasm. Female mosquitoes, such as Aedes aegypti (yellow fever mosquito, Diptera), require a blood meal for oogenesis. Mosquitoes receive a potentially toxic level of iron in the blood meal which must be processed and stored. We demonstrate by ⁵⁹Fe pulse-chase experiments that cultured A. aegypti larval CCL-125 cells take up iron from culture media and store it in ferritin found mainly in the membrane fraction and secrete iron-loaded ferritin. We observe that in these larval cells ferritin co-localizes with ceramide-containing membranes in the absence of iron. With iron treatment, ferritin is found associated with ceramide-containing membranes as well as in cytoplasmic non-ceramide vesicles. Treatment of CCL-125 cells with iron and CI-976, an inhibitor of lysophospholipid acyl transferases, disrupts ferritin secretion with a concomitant decrease in cell viability. Interfering with ferritin secretion may limit the ability of mosquitoes to adjust to the high iron load of the blood meal and decrease iron delivery to the ovaries reducing egg numbers.     

Permeability of gentamicin and polymyxin B into the inside of Bacillus subtilis spores

The penetration of gentamicin and polymyxin B into the inside of Bacillus subtilis spores was examined by an immunoelectron microscopy method with colloidal gold--immunoglobulin G (IgG) complex. The colloidal gold particles were located predominantly in the coat region of both gentamicin-treated and polymyxin B-treated spores and were hardly observed in the other regions, i.e., the cortex and core regions. When these antibiotic-treated spores were subsequently treated with CaCl2, the number of gold particles bound to the coat region was greatly decreased. These results suggest that these two antibiotics are able to penetrate into the spore coat but not into the cortex or core, that is, the primary permeability barrier to them exists between the coat and the cortex regions.     

Structure and development of the surface coat of erythrocytic merozoites of Plasmodium knowlesi

Summary The surface of extracellular merozoites of P. knowlesi is covered with a coat 15–20 nm thick, made up of clusters of filaments standing erect on the plasma membrane. Filaments have stems 2 nm thick, the peripheral ends of which are complex, branching or ending in long trailing threads. Coat filaments occur on the surface of the parasite in regular rows at an early schizont stage, and persist until well after merozoite release. They are sensitive to trypsin and papain, and bind ethanolic phosphotungstate, indicating a proteinaceous nature. They are also removed by exposure to phosphate-buffered saline. Filaments bear negative charges, binding cationised ferritin throughout the depth of the coat and staining with ruthenium red. They cover the whole merozoite surface and mediate intercellular adhesion at distances of 15–150 nm, membrane to membrane. It is suggested that these filaments correspond to a major merozoite surface protein, and are important in the initial capture of red cells.     

Quantitative Analysis of Polymyxin B Released from Polymyxin B-Treated Dormant Spores of Bacillus subtilis and Relationship between Its Permeability and Inhibitory Effect on Outgrowth

Polymyxin B, one of the cyclic polypeptide antibiotics, binds to the coat of Bacillus subtilis dormant spores and inhibits them from growing after germination. When about 2.8 × 10⁸ cells/ml of polymyxin B-treated dormant spores were incubated in heart infusion broth, 3.6 μg/ml of polymyxin B were released into the liquid medium during germination. Incubation of the same concentration of polymyxin B-treated ones in 100 mM CaCl₂ solution released 4.0 μg/ml of the antibiotic. The effect of various concentrations of polymyxin B on germination, outgrowth and vegetative growth of the dormant spores was investigated; the results showed that concentrations of 4.0 μg/ml and higher of the antibiotic inhibited their outgrowth and vegetative growth after germination. Young vegetative cells were less sensitive to the antibiotic than germinated spores. In addition to these results, immunoelectron microscopy with colloidal gold particles indicated that polymyxin B permeated into the core of the germinated spores and inhibited them from outgrowing.     

Morphological study of lectin binding in the rabbit temporomandibular joint disc

Summary Glycoconjugates of the extracellular matrix are important for the normal mechanical functions of connective tissue structures such as the temporomandibular joint disc. Since lectins are known to bind to sugar residues with high affinity, a variety of lectins were used to study the presence and distribution of glycoconjugates in the temporomandibular joint disc. Discs were removed from 6 to 8-month-old rabbits and either sectioned in a cryostat and processed for light microscopy or fixed in 2% glutaraldehyde and processed for electron microscopy. The frozen sections were incubated with fluorescein- or peroxidaseconjugated lectin solutions. Ultrathin sections mounted on grids were incubated with lectins combined with a colloidal gold marker system for electron microscopical study. Our results indicate thatCanavalia ensiformis agglutinin (ConA) showed little or no binding to the discal tissue.Triticum vulgaris agglutinin (WGA) andMacluras pomifera (MPA) were bound strongly to both the synovium and the extracellular matrix and WGA also bound to the territorial matrix of chondrocyte-like cells.Glycine max andArachis hypogoea agglutinins (SBA and PNA), were localized in the synovium and extracellular matrix but to a lesser degree than WGA and MPA. WGA, MPA,Griffonia simplicifolia II andUlex europaeus were bound by discal fibroblasts. WGA was also localized in lysosomes of synovial A-cells (macrophages). The electron microscopical studies with lectins and colloidal gold marker systems indicated that some areas of the disc may be fibrocartilagenous as had been suggested by earlier immunohistochemical studies using monoclonal antibodies to characteristic glycosaminoglycans (GAGs) in cartilage.     

Transient holes in the erythrocyte membrane during hypotonic hemolysis and stable holes in the membrane after lysis by saponin and lysolecithin

Ferritin and colloidal gold were found to permeate human erythrocytes during rapid or gradual hypotonic hemolysis. Only hemolysed cells contained these particles; adjacent intact cells did not contain the tracers. Ferritin or gold added 3 min after the onset of hypotonic hemolysis did not permeate the ghost cells which had, therefore, become transiently permeable. By adding ferritin at various times after the onset of hemolysis, it was determined that for the majority of the cells the permeable state (or interval between the time of development and closure of membrane holes) existed only from about 15 to 25 sec after the onset of hemolysis. It was possible to fix the transient "holes" in the open position by adding glutaraldehyde only between 10 and 20 sec after the onset of hemolysis. The existence of such fixed holes was shown by the cell entry of ferritin and gold which were added to these prefixed cells. Membrane defects or discontinuities (of the order of 200–500 A wide) were observed only in prefixed cells which were permeated by ferritin subsequently added. Adjacent prefixed cells which did not become permeated by added ferritin did not reveal any membrane discontinuities. Glutaraldehyde does not per se induce or create such membrane defects since cells which had been fixed by glutaraldehyde before the 10-sec time point or after the 180-sec time point were never permeable to added ferritin, and the cell membranes never contained any defects. It was also observed that early in hemolysis (7–12 sec) a small bulge in one zone of the membrane often occurred. Ghost cells produced by holothurin A (a saponin) and fixed by glutaraldehyde became permeated by ferritin subsequently added, but no membrane discontinuities were seen. Ghosts produced by lysolecithin and fixed by glutaraldehyde also became permeated by subsequently added ferritin, and many membrane defects were seen here (about 300 A wide).     

Nervous system of the snail Helix aspersa

Summary The fine structure of vascular channels and amebocytes associated with the sheath of the infraesophageal ganglion of Helix aspersa, is described. The extracellular stroma of the sheath, together with the hemocoel and blood vessels, forms an interconnected system of pathways which appears to be involved in the transport of metabolites, amebocytes, hemocyanin and experimentally introduced opaque tracers. The hemocoel, blood capillaries and precapillaries are lined by a discontinuous layer of single muscle cells whose luminal aspect is covered by a lamina of extracellular material named the vascular coat. This coat consists of a ground substance that forms a basement membrane and filamentous elements some of which are collagenous. Gaps in the blood vessel wall seem to provide the main routes for the movement of cells and large molecules to the hemocoel. Tracer experiments have given support to the idea that a diffusion barrier may be absent at the sheath-ganglion junction. Amebocytes have phagocytic properties; they appear associated in groups or scattered singly within the extracellular space of the sheath and the lumen of blood vessels. Single amebocytes have features of mobile cells and may function in the transport of hemocyanin as well as other proteins.This work has been supported by the Rockefeller Foundation and grants NB 06662 (from the U.S. Public Health Service) and N-105 (from Conicyt, Santiago, Chile). The continuous advice and encouragement of Drs. R. W. Guillery and D. B. Slautterback are gratefully acknowledged.     

Development and cell surface of a non-syncytial invasive tapetum inCanna: Ultrastructural,freeze-substitution,cytochemical and immunofluorescence study

Summary The anther ofCanna indica L. ×C. sp. hybrid contains a hitherto uncharacterized non-syncytial, invasive category of tapetum. With the onset of prophase I the tapetal walls are dissolved and the released protoplasts migrate into the loculus, where they stay discrete. Concomitant with the dissolution of walls the tapetal protoplasts develop a 17 nm thick extracellular granulo-fibrillar cell coat. This feature develops in the synchronous phase of tapetal development. The cell coat reacts positively with ruthenium red, potassium ferrocyanide, ConA-FITC and in the Thiéry reaction. Immunofluorescence microscopy using anti-tubulin revealed that even after the migration of tapetal cells into the loculus, the microtubules retain a predominant orientation in the cell cortex, probably derived from that in the original tapetal walled cells. This order is lost during late post-meiotic stages when the cells distort and can produce amoeboid processes. The microtubule orientation is correlated with that of the cell coat fibrils. Tapetal cells vary in ultrastructure and the density of cell coat fibrils after their migration into the loculus, but the cell coat persists until the cells degenerate. It is surmised that development of the cell coat relates to the lack of cell fusion and that the cortical microtubules help to sustain cell form. During post-meiotic stages the free tapetal cells develop massive peripheral arrays of interconnected ER cisternae, probably as part of a secretory apparatus which matures when the spores are producing their ornamented walls. Buds grown in colchicine solution showed accumulation of sporopolleninlike granules in all extracellular spaces of the anther cavity.     

Some Properties of Discharged Glugea hertwigi (Microsporida) Sporoplasms

Glugea hertwigi spores were activated to discharge sporoplasms in Medium 199 with 3% gelatin at pH 9.0; the liberated sporoplasms were transferred to a maintenance medium with 6% gelatin (pH 7.0) supplemented with 2 mM ATP and 10% (v/v) fetal calf scrum. The spherical sporoplasms (measuring 3.5-4 m?m in diameter) had single nuclei and had a cytoplasm rich in free ribosomes. Each G. hertwigi sporoplasm was initially bounded by an external (0.1-0.2 m?m) satellite body adjoining the plasma membrane. The satellites displayed ordered membrane and appeared to merge with the sporoplasm 15-30 min after spore discharge. The external location of the satellite (in reference to the discharged sporoplasm) seems to be part of the normal sequence of events under the in vitro conditions provided. The surface of G. hertwigi sporoplasms does not bear an obvious surface coat; however, our cytochemical observations indicate the plasma membrane of the sporoplasm was somewhat responsive to concanavalin A-peroxidase, colloidal iron, and native ferritin. During the short term incubations of sporoplasms with ferritin, the particles permeated membrane channels extending into the sporoplasm cytoplasm.     

P. Falciparum infected erythrocytes are capable of endocytosis

Summary P. falciparum, an intraerythrocytic parasite, obtains nourishment primarily through phagocytosis of the host cytosol but also through the incorporation of extracellular small molecules which enter through the parasitized red cell's membrane via pores. Normal mature erythrocytes are incapable of endocytosis. Several lines of evidence suggest that extracellular large molecules may be taken up when the mature red cell is parasitized byP. falciparum, but direct evidence has been lacking. We now report the use of ferritin, an electron dense protein, to demonstrate endocytosis inP. falciparum infected red cells. Parasitized red cells incubated with ferritin internalize that macromolecule as demonstrated by electron microscopy. While normal red cells incubated with ferritin took up none of the tracer molecule, parasitized red cells internalized substantial amounts. In addition both ferritin and apoferritin inhibited the growth ofP. falciparum in a dose dependent fashion, again indicating endocytosis of a macromolecule. These data indicate thatP. falciparum can somehow stimulate the mature erythrocyte to engage in endocytosis. We also note that both infected and non-infected red cells in a culture in whichP. falciparum is growing become abnormally sticky for ferritin. Moreover, parasitized red cells bind I¹²⁵-transferrin while non-parasitized erythrocytes do not. These observations suggest that a soluble parasite product alters the red cell membrane in a non-global manner, causing selective effects in relation to different proteins.     

AN ELECTRON MICROSCOPIC STUDY OF PINOCYTOSIS IN AMEBA : I. The Surface Attachment Phase

The attachment to the surface of the ameba (Chaos chaos L. (Pelomyxa carolinensis, Wilson)) of two proteins, ribonuclease and ferritin, and two colloidal suspensions, thorium dioxide and gold, was studied in the electron microscope. The initial step in the pinocytosis of ferritin and thorium dioxide particles by amebas is shown to be the attachment of these substances to the "hairlike" extensions of the plasmalemma. Ribonuclease caused alterations in the structure of the plasmalemma, but on account of its relative lack of density, it could not be definitely localized. Colloidal gold did not appear to be active with respect to pinocytosis in amebas. Since molecules in solution and particles in suspension are taken up by the same mechanism, the first step of which is their attachment to the cell surface, it is suggested that a single mechanism underlies phagocytosis, pinocytosis, ropheocytosis, cytopempsis, and potocytosis.     

Ultrastructural studies of the binding of concanavalin A to the plasmalemma of higher plant protoplasts

Summary The binding of concanavalin A to the plasmalemma of higher plants has been studied using protoplasts of two species. The lectin aggregates both tobacco (Nicotiana tabacum L.) leaf protoplasts and protoplasts prepared from a suspension cell culture of grapevine (Vitis vinifera L.). Differences in lectin binding have been investigated using concanavalin A conjugated to ferritin or bound to colloidal gold. Tobacco protoplasts exhibit continuous and saturated labelling of the plasmalemma surface with gold-concanavalin A mixtures. Vine protoplasts under the same conditions show a discontinuous and patchy distribution of label. These results are discussed in terms of a possible binding mechanism.Abbreviations ConA concanavalin A - PBS Phospholi Buffered Saline - PEG polyethylene glycol     

Phagocytized Intracellular Microsporidian Blocks Phagosome Acidification and Phagosome-Lysosome Fusion

This study examines the relationship between phagosome acidification and phagosome-lysosome fusion events using phagocytized Glugea hertwigi spores. The incidence of lysosome fusion with Glugea spores in phagosomes of mouse peritoneal macrophages and of Tetrahymena was monitored using colloidal gold and acridine orange as labels for secondary lysosomes. Over 80% of the Glugea phagosomes remained segregated from the labeled compartments in macrophages after 60 min; this inhibition of fusion was still evident after 4 h. In Tetrahymena, Glugea spores also showed a high capacity to block fusion with secondary lysosomes (67%); however, spores coated with cationized ferritin showed an 80% fusion rate with labeled acidic compartments (i.e. lysosomes) after 60 min with both Tetrahymena and macrophages. The pH of phagosome compartments was monitored by measuring the emissions of fluorescein isothiocyanate (FITQ-labeled Glugea ingested by Tetrahymena. Tetrahymena phagosomes with FITC-Glugea did not acidify within the first hour after phagocytosis; however, phagosomes with cationized ferritin-labeled Glugea underwent acidification during this time period. This acidification took place although the capability of the host cells' lysosomes to fuse was blocked by pretreatment with poly-D-glutamic acid. The cationized ferritin bound to Glugea spores was uncoupled from the spore wall prior to fusion with colloidal gold-labeled compartments. In vitro testing showed that ferritin dissociation requires an acid pH, indicating that phagosomes acidify prior to lysosome fusion.     

Cell to cell recognition between the ciliatePseudomicrothorax dubius and its food organisms: the role of surface charges

Summary The importance of charged groups during phagocytic recognition of filamentous Cyanobacteria (Oscillatoria formosa andAnabaena spp.) by the stenophagic ciliatePseudomicrothorax dubius has been studied. Anionic and cationic domains are evenly and randomly distributed over the cyanobacterial surface, as demonstrated with scanning electron microscopy following labeling with colloidal gold (–) and colloidal gold coupled with poly-L-lysine (+). The phagocytosis ofOscillatoria was inhibited when filaments were treated with cationic reagents such as poly-L-lysine (pLL), FeCl₃ and carbodiimide. In contrast elimination of cationic charges on theOscillatoria surface by treatment with poly-L-glutamic acid (pLGa) or colloidal gold did not affect phagocytosis. The effects of sequential treatment with pLL and pLGa demonstrated that pLL reduced phagocytosis of pLGa-pretreatedOscillatoria, whereas the pLGa restored phagocytosis of pLL-pretreated filaments. Scanning electron microscopy showed that pLL- or pLGa- treated filaments can still adsorb the oppositely charged colloidal gold particles on their surface. However, the treatment of filaments with pLL followed by pLGa prevented subsequent labeling with gold as well as with pLL-gold particles. Filaments ofAnabaena spp., which are not normally ingested byPseudomicrothorax, were also treated individually or sequentially with pLL and pLGa. None of these treatments, however, provoked phagocytosis ofAnabaena byPseudomicrothorax. We suggest that the surface charge alone does not play a crucial role in phagocytic recognition inPseudomicrothorax and that phagocytosis-specific molecules are implicated.     

Tim‐2 is the receptor for H‐ferritin on oligodendrocytes

Oligodendrocytes stain more strongly for iron than any other cell in the CNS, and they require iron for the production of myelin. For most cell types transferrin is the major iron delivery protein, yet neither transferrin receptor protein nor mRNA are detectable in mature oligodendrocytes. Thus an alternative iron delivery mechanism must exist. Given the significant long term consequences of developmental iron deficiency and the iron requirements for normal myelination, identification of the iron delivery mechanism for oligodendrocytes is important. Previously we have reported that oligodendrocytes bind H‐ferritin and that H‐ferritin binds to white matter tracts in vivo. Recently, T cell immunoglobulin and mucin domain‐containing protein‐2 (Tim‐2) was shown to bind and internalize H‐ferritin. In the present study we show that Tim‐2 is expressed on oligodendrocytes both in vivo and in vitro. Further, the onset of saturable H‐ferritin binding in CG4 oligodendrocyte cell line is accompanied by Tim‐2 expression. Application of a blocking antibody to the extracellular domain of Tim‐2 significantly reduces H‐ferritin binding to the differentiated CG4 cells and primary oligodendrocytes. Tim‐2 expression on CG4 cells is responsive to iron; decreasing with iron loading and increasing with iron chelation. Taken together, these data provide compelling evidence that Tim‐2 is the H‐ferritin receptor on oligodendrocytes suggesting it is the primary mechanism for iron acquisition by these cells.