Extensive Golgi Complexes Research Articles

Histological, elemental, and ultrastructural analysis of melanin in mantle of Pacific oyster (Crassostrea gigas).

Colorful shell of bivalve is mainly because of the biological pigments, of which melanin plays an important role in shell color formation. More and more studies focus on the genes function involved in melanin synthesis, but relatively few studies address the biochemical character and ultrastructure of melanin in bivalve from microscopic perspective. Here, we investigated the histological structure of mantle of Crassostrea gigas with orange shell color. Distribution of melanin in mantle was verified with histochemical staining. In addition, immunofluorescence technique showed that strongly positive signal of CgTYR was specific to the mantle margin, which is consistence with the location of brown granules in H&E staining. The further result of elementary composition of melanin displayed that metal Ca, Fe, and Zn were detected using scanning transmission electron microscope and energy dispersive spectroscopy mapping methods. Next, based on TEM observations, it was speculated that the series of cellular events leading to the formation and release of melanin. Melanocyte in the primary stage showed many mitochondria and rough endoplasmic reticulum as well as an extensive Golgi complex with numerous vesicles intermingled with melanosome. Subsequently, melanosome was expended and their hue gradually intensified, and Golgi complex and mitochondria were still observed in the cytoplasm. Finally, after melanosome was discharged into intercellular spaces, the disintegration of membranes in some cells, and severe cellular vacuolization. These data enrich the understanding of ultrastructural characteristic and formation of melanin in mantle of bivalve and pave the way for further investigating shell coloration at the cellular level.

Ultrastructure of the Parotid Salivary Gland in the Greater Cane Rats (Thryonomys Swinderianus).

The parotid glands of adult male African greater cane rat (Thryonomys swinderianus) were examined by light microscopy (semi-thin sections) and transmission electron microscopy. Histologically, it consisted of acinar cells with vacuoles which corresponded to large oval electron-dense granules, intercalated, striated ducts, and myoepithelial cells which contacted the cells and intercalated ducts (IDs). The cytoplasmic organizations of acinar cells represented the features of serous secreting cells. Ultrastructurally, the acinar cells contained granules of low and moderately electron densities without substructures in their matrix. Lipid droplets were interspersed with the granules. Several coalesced low electron-dense granules were common in some of the acinar cells. The acinar cells also contained few dilated (vesicular) and abundant parallel arrays of tubular rough endoplasmic reticulum and extensive Golgi complex. IDs were lined by tall cuboidal cells interconnected by tight junctions. Secretory granules were absent in their cytoplasm. Striated ducts were composed of columnar cells with few basal cells, and secretory granules were absent as well. Apical blebbing was observed in these ducts. Myoepithelial cells were limited to the acinar-intercalated ductal system. Nerve terminals were observed among the adjacent acinar cells and the underlying basement lamina. The functional significance of these structures is discussed.

Age-related changes in blood capillary endothelium of human dental pulp: an ultrastructural study.

To describe the ultrastructural changes that occur in pulpal blood capillaries as a result of ageing. Thirty samples of healthy dental pulps were obtained from functional human permanent teeth. Two age groups were examined: young (10-17 years) and old (>60 years). The teeth were extracted under local anaesthesia using mepivacaine without adrenaline (Scandonest 3%, Septodont, Saint-Maur des Fossés, France) and split longitudinally in a bench press. The pulps were gently removed, immersed in fixative solution, sectioned and processed by conventional transmission electron microscopic techniques. Micrographs were taken from the endothelium, and the whole capillary area of each vessel was examined. In young pulps, the endothelial cell layer was characterized by the presence of numerous pinocytotic vesicles and microvesicles, RER cisterns, free ribosomes, a small Golgi complex, centrioles, microtubules, microfilaments and mitochondria. In the endothelial cell cytoplasm of older pulpal vessels, pinocytotic vesicles and microvesicles, as well as microfilaments, were more numerous. In addition, lipid-like vacuoles, monogranular glycogen granules and extensive Golgi complexes with dilated cisterns were also present. Weibel-Palade bodies were observed in both age groups without showing variations related with age. The results obtained in capillaries of aged pulpal tissue suggest that the endothelium experiences morphological changes that could be associated with advancing age.

Oogenesis in a placental viviparous onychophoran

This first ultrastruetural study of oogenesis in a placental viviparous onychophoran describes oocyte differentiation, cell interactions and reveals various unusual cellular features. The viviparous onychophoran Plicatoperipatus jamaicensis has paired ovaries medially located, attached to the dorsal body wall by muscular terminal filaments. The rest of the female reproductive tract consists of paired spermathecae oviduct/uteri (hereafter referred to as uterus). Bulbous spermathecae are joined to the oviducts by ducts. Also continuous with the oviduct lumen are two tubular structures whose lumina open to the hemolymph. The uteri contain a progression of developmental stages from implantation through stalked morulae, blastocysts, larvae and juveniles about to be born. Growing oocytes are characterized by large germinal vesicles showing synaptonemal complexes. Oocytes are surrounded by flattened follicle cells that possess extensive bundles of thick and thin filaments. Mature oocytes contain little or no yolk, but are unique among organisms in accumulating a large central reservoir of stored glycogen. The lack of yolk reflects the placental viviparous nature of the reproductive process. The glycogen reservoir provides a rapidly accessible energy source for early developmental stages. Particularly prominent also are unusually extensive and highly elaborate Golgi complexes in the cortical and peri-nuclear ooplasm. While extensive Golgi complexes have been described in oocytes of a variety of species, the particularly exaggerated size and amount of Golgi in these onychophorans suggests they may provide excellent material for the study of Golgi function. The features of the oocyte and placental viviparity show this is an ideal model to investigate the nature of the placental reproductive process analogous to mammals in an invertebrate and its implications to oogenesis.

Development and histological characteristics of neurosecretory cells in the cerebral ganglia of Achatina fulica (Bowdich)

Study of postembryonic development of neurosecretory cells in the cerebral ganglia of Achatina fulica showed that they first appear in two month-old snails. The cells are located at the periphery of the procerebrum of cerebral ganglion. The number and size of the neurosecretory cells increase with increasing age of snails, reaching a maximum in eight-month-old snails and thereafter remaining constant in nine-to twelve-month-old snails. Study of the ultrastructure of the cells showed that they each contain a round shaped nucleus with patches of heterochromatin and a conspicuous single large vacuole in the cytoplasm. In addition, electron-dense elementary granules with a mean diameter of 1600 A° are found to be associated with an extensive Golgi complex and rough endoplasmic reticulum.

Ultrastructural morphology and relaxin immunolocalization in giant trophoblast cells of the golden hamster placenta

Relaxin immunoreactivity was previously demonstrated in three cell types within the hamster placenta; fetal primary and secondary giant trophoblast cells (GTCs) and maternal endometrial granulocytes. The objectives of the present research were to examine the ultrastructure of the GTCs and identify the intracellular relaxin storage site. Primary GTCs, first present on day 8 of gestation, were characterized by numerous polyribosomes and large heterogeneous cytoplasmic inclusions suggesting phagocytic activity. Primary and secondary GTCs from days 10, 14, and 15 of gestation contained numerous polyribosomes, mitochondria with tubular cristae, and extensive Golgi complex, and abundant rough endoplasmic reticulum, all characteristics of a cell actively involved in protein synthesis. Membrane-bound secretory granules were not present. Relaxin was immunolocalized within the Golgi complex of primary and secondary GTCs using the avidin-biotin-peroxidase method. Following differential centrifugation of hamster placental homogenates and radioimmunoassay (RIA) of subcellular fractions, the majority of relaxin immunoactivity was detected in the postmicrosomal fraction; however, the majority of relaxin immunoactivity from similarly treated pig corpora lutea was present in the mitochondrial/granule fraction. These data indicate that hamster placental relaxin is not stored in membrane-bound secretory granules but is contained within the extensive Golgi complex of the GTC.

Ultrastructure of the excretory‐secretory system inToxocara canis(Nematoda, Ascarididae) infective larvae

Abstract The ultrastructure of the excretory‐secretory system of Toxocara canis infective larvae is described. The system is composed of a single uninucleate cell forming two elongate columns that extend throughout the length of the worm up to the anterior portion of the intestine. The cytoplasm shows an abundant endoplasmic reticulum and an extensive Golgi complex from which many vesicles of variable density originate. The material of the vesicles is discharged into the lumen of an intracellular duct crossing the two columns of the gland cell. These ducts appear as sinuous canals surrounded by a series of small, interconnected cavities. The excretory ducts join by means of transversal canals which, in turn, flow into a common duct opening outside through an excretory pore.

Ultrastructural analysis of the initiation and development of cytasters in sea-urchin eggs.

Two approaches were used to study the origin and overall development of cytasters in relation to the emergence and maturation of new centrioles in sea-urchin eggs. A continuous hypertonic treatment was used to gather information on cytoplasmic areas of potential cytaster formation and the subsequent development of these cytastral areas. A two-step parthenogenetic stimulation procedure was used to analyse various cytastral changes during and after mitosis. Potential cytastral areas are associated with extensive Golgi complexes and astral formation occurs only about newly induced centrioles. The construction of a single aster involves a large redistribution of local cytoplasmic structures, concentrating some components, excluding others and orienting more and more microtubules in an increasingly focussed arrangement. These events are correlated with an extensive accumulation of astral endoplasmic reticulum, an increase in the size of the astral area, and a gradual acquisition of a more typical radial configuration. The astral shape becomes more pronounced during mitosis, after which the cytasters regress, but retain the mature centrioles. The data reveal that cytaster formation is initiated after the activation and appearance of centriolar precursor bodies, and that both centrioles and associated cytasters are complementary structures, which develop in unison. The results suggest that developing centrioles, from the moment of their emergence, control and direct the events of cytastral formation.

Morphological changes in the hypothalamic arcuate nucleus and median eminence in the golden hamster during the neonatal period.

The purpose of the present investigation was to study the ultrastructure of the arcuate nucleus (ARC) and median eminence of hamsters on days 1-15 of the neonatal period. From days 1-6, the neurons of the ARC had large nuclei and a small amount of cytoplasm which contained polysomes, mitochondria, RER, lysosomes and Golgi complexes. From days 7-15 there was an increase in the amount of cytoplasm as well as more extensive Golgi complexes and RER. Astrocytes were the predominant glial component in both the ARC and median eminence. Astrocytic processes were in juxtaposition to unmyelinated axons, dendrites, and synapses. Axodendritic and axosomatic synapses containing clear vesicles were observed in the neuropil on day 1. There was an increase in the number of dense-core vesicles in the axonal endings beginning on day 4. Concomitantly, there were increasing numbers of clear and dense-core vesicles (64-70 nm) in terminals of the external layer of the median eminence, whereas larger dense-core vesicles (105-140 nm) were distinguishable by day 10 immediately dorsal to the external layer. The capillaries of the median eminence were composed of nonfenestrated endothelium from days 1-9. Fenestrae began to appear about day 10. Ependymal cells lining the third ventricle had pinocytotic vesicles, microvilli, and bleb-like protrusions on their apical surfaces. Ependymal processes were adjacent to nerve processes in the neuropil of the ARC and in the external layer of the median eminence, where they contacted the perivascular space. Two types of supraependymal cells were seen in animals throughout the neonatal period. One resembled a neuron which sent processes along the ependymal surface and often between cells. The second type was similar to a macrophage. The results of this study demonstrate the maturation of the neural elements in the ARC/median eminence area of the neonatal hamster.

Abnormal alveolar cells in monocrotaline induced pulmonary hypertension.

Monocrotaline given to rats causes lung injury which is followed by pulmonary hypertension. A large, abnormal intraalveolar cell has been repeatedly observed. The purpose of the present study was to define the nature of the cell and to determine how it related to the lung injury. As observed by electron microscopy, the intraalveolar cell contained numerous large, dense granules, as well as lipid whorls, extensive Golgi complexes, and many small vesicles. These cells appeared to be macrophages, possibly altered by ingestion of lipid-like material which was free in the alveolar space. Enlarged type II pneumonocytes and mast cells were seen in the alveolar walls but their appearance differed from that of the free alveolar cells. The electron microscopic findings plus the presence of Fc and C3b receptors on the cells indicated they were alveolar macrophages. When the severity of pulmonary hypertension was varied by a) giving monocrotaline to rats of varying age, b) by varying the dose of monocrotaline, or c) by varying the time interval after monocrotaline administration, the number of abnormal alveolar macrophages related to the severity of the pulmonary hypertension. When we reduced the number of circulating leukocytes by whole body radiation, monocrotaline administration was followed both by accelerated pulmonary hypertension and increased numbers of abnormal alveolar macrophages. The abnormal macrophage appeared to be a marker of the monocrotaline induced pulmonary hypertension. However, neither the abnormal macrophage nor the monocrotaline injury appeared to depend on circulating leukocytes.

Ultrastructure of Brachycoelium salamandrae Gastrodermis with Observations on the Effects of Starvation

The fine structure of the digestive tract of the digenetic trematode Brachycoelium salamandrae was studied. The digestive tract consisted of a foregut region whose lining was cytoarchitecturally similar to the general body tegument. Glands opened into the lumen of the foregut via microtubular-supported ducts. The junction between the foregut and the cellular gastrodermis was marked by a prominent desmosome. The apical plasma membranes of the gastrodermal cells were amplified by means of lamellae. The cells display an extensive rough endoplasmic reticulum, mitochondria, and numerous Golgi complexes. Two types of vesicles were observed. One type was a vesicle with a homogeneous matrix that appeared to be associated with the emitting face of Golgi complexes. Another structure was the multivesicular body that displayed acid phosphatase activity. This latter organelle was observed at times to be associated with autophagic vacuoles and is believed to be a part of the lysosome system of the gastrodermis. Starvation of the worms resulted in an increase in the disintegration of gastrodermal cells as well as an increase in the number of autophagic vacuoles in the cytoplasm. Ultrastructural organization of the adult diges- tive tract representative of nine families and 22 species of digenetic trematodes has been described to date (Fujino and Ishii, 1979). A number of cytochemical studies have supplemented the morphological studies and have provided invaluable information pertaining to the func- tion of portions of the digestive tracts of these organisms (see Erasmus, 1977, for a review). The objective of the present investigation was to study the ultrastructure of the digestive tract of Brachycoelium salamandrae, a trematode family whose ultrastructure has hitherto not been described. Preliminary cytochemical studies were made in order to compare distribution of acid phosphatase activity in both normal and starved worms. This aspect of the study was designed to provide greater understanding of the function of certain organelles associated with the digestive tract as well as provide a base for comparison of the distribution of the enzyme in B. salamandrae with that found in other digenea.

The ultrastructure of T and B lymphocytes in recurrent apthous ulceration of the buccal mucosa.

The purpose of the present study was to classify lymphocytes in the buccal mucosa of patients affected with recurrent apthous ulceration. Two lymphocytes were identified. The B lymphocyte was characterized by an extensive Golgi complex, an extensive tubular rough surfaced endoplasmic reticulum, and an extensive canalicular system. Usually the nucleus was electron lucent except for perinuclear condensation of chromatin. The T lymphocyte had a moderately developed Golgi complex, absent or poorly developed rough surfaced endoplasmic reticulum, lysosomes, and considerably more heterochromatin (perinuclear) than the B lymphocyte. Uropods were associated with the T lymphocyte and exerted cytotoxic effects on basal and suprabasal cells.

The chick embryo nodose ganglion: effects of nerve growth factor in culture.

Nodose ganglia from 8-day-old chick embryos were cultured in collagen gels for 2 days, with or without added nerve growth factor (NGF), in order to discover whether the nodose neurons, derived from an epidermal placode, are susceptible to this trophic factor. Neuronal survival and neurite outgrowth were stimulated only after addition of NGF, and the enhancing effects could be blocked by introducing antibodies to NGF. Stereological analysis of ganglia sectioned for light microscopy showed that the NGF-treated neurons increased their volume by about 50%, as did the nodose neurons in ovo from the eighth to the tenth day of incubation. The volume density, however, was significantly lower in vitro indicating a limited cell death during culture despite the presence of exogenously supplied NGF. The number of neurofilaments and microtubules increased in the cell centre of neurons treated with NGF; this region also showed numerous dense bodies and an extensive Golgi complex by electron microscopy. Ultrastructural similarities between neurons responding to NGF and neurons undergoing the axon reaction which follows axotomy are indicated. A role for a trophic factor resembling NGF in the normal development of placode-derived neurons of the sensory cranial ganglia is suggested.

Gross Morphology and Fine Structure of the Olfactory Organ of Rainbow Fish (Atheriniformes, Melanotaeniidae)

The olfactory organs of 6 rainbow fish species were studied, and were compared with those of closely related cyprinodonts. It has been found that, in contrast to cyprinodonts, the olfactory mucosa of rainbow fish is more conspicuously thrown up into folds. Their arrangement in parallel is considered a relatively primitive character of the organ. Electron microscopic investigations on Nematocentris maccullochi have shown that the sensory epithelium consists of basal, supporting, and two types of receptor cells, ciliated and microvillous. As in cyprinodonts the receptor cell cilia exhibit a basic 9 + 0 microtubular pattern. In Nematocentris the frequency of receptor cells, between 200,000 and 250,000 per mm2, is lower than in cyprinodonts. It may be suggested that the lower density is compensated for by enlargement of the sensory area due to the folding. In addition to broad-surfaced dendritic membrane appositions found in both Nematocentris and cyprinodonts, contacts by means of dendritic spines are found only in the rainbow fish. The olfactory supporting cells of the latter are distinguished from those in cyprinodonts by more prominent lateral interdigitations, centrioles, and the higher number of vesicles associated with extensive Golgi complexes.

Epithelioid cell cultures from rat small intestine. Characterization by morphologic and immunologic criteria.

Rat small intestinal epithelial cell lines have been established in vitro and subcultured serially for periods up to 6 mo. These cells have an epithelioid morphology, grow as monolayers of closely opposed polygonal cells, and during the logarithmic phase of growth have a population doubling time of 19--22 h. Ultrastructural studies revealed the presence of microvilli, tight junctions, an extensive Golgi complex, and the presence of extracellular amorphous material similar in appearance to isolated basement membrane. These cells exhibit a number of features characteristic of normal cells in culture; namely, a normal rat diploid karyotype, strong density inhibition of growth, lack of growth in soft agar, and a low plating efficiency when seeded at low density. They did not produce tumors when injected in syngeneic animals. Immunochemical studies were performed to determine their origin using antisera prepared against rat small intestinal crypt cell plasma membrane, brush border membrane of villus cells and isolated sucrase-isomaltase complex. Antigenic determinants specific for small intestinal epithelial (crypt and villus) cells were demonstrated on the surface of the epithelioid cells, but they lacked immunological determinants specific for differentiated villus cells. An antiserum specifically staining extracellular material surrounding the cells cultured in vitro demonstrated cross-reactivity to basement membrane in rat intestinal frozen sections. It is concluded that the cultured epithelioid cells have features of undifferentiated small intestinal crypt cells.

Cytochemical reaction for cationic proteins as a marker of primary granules during development in chick heterophils

The ammoniacal silver reaction (ASR) for cationic proteins was used as a cytochemical marker for the primary or A granules in the cytoplasm of developing heterophils of chick bone marrow. The presence of the electron-dense particulate reaction product of silver, which is localized in the fully formed rod-shaped A granules, provides a marker by which the A granules could be distinguished from the B granules of similar size and by which the formation and maturation of both granule types could be followed through the developmental stages. Progressive developmental stages were ascertained on the basis of decreasing cell size, increasing condensation and margination of the chromatin, and the number and morphology of the granules; the stages were divided into promyelocyte, myelocyte, metamyelocyte and heterophil. During the promyelocyte stage, the first appearance of the electron-dense, membrane-bound, spherical granules (0.3--1.0 micrometer in diameter) is observed in the vicinity of an extensive Golgi complex. They occur in a cytoplasm containing rough-surfaced endoplasmic reticulum, ribosomal clusters, centrioles, mitochondria, microtubules, as well as the membranes, saccules, vesicles and vacuoles of the Golgi complex. These granules are considered as primary but their presence as the only granule type appears very brief. The ASR reaction product is first detected on the surface of these primary granules in late promyelocytes or myelocytes. The secondary or B granule, devoid of reaction for cationic protein at all stages, appears as a condensing vacuole in promyelocytes, but after some A granules are already present. The vacuole contents condense to form the B granules which are 0.1--0.6 micrometer in diameter, often oval-shaped, and contain a loose filamentous material surrounded by a membrane. Tertiary C granules or lysosomes appear during the myelocyte stage as dense core vesicles (0.1--0.2 micrometer in diameter) negative for cationic protein.

Fine structure of the human secretory ameloblast

The fine structure of secretory ameloblasts in primary tooth germs from a human fetus was investigated. Various aldehyde fixatives produced varying qualities of preservation. The morphology of the best preserved ameloblasts differed from that earlier described, mainly in the following respects: the rough endoplasmic cisternae were lamellated and regularly arranged, the mitochondria were of regular shape and size, and the ground substance was dense and homogenous. The extensive Golgi complexes, the mitochondria and the rough endoplasmic cisternae in the apical (distal) part of the ameloblast were longitudinally arranged.

An ultrastructural study of mammalian cardiac ganglia

Atrial parasympathetic ganglia of rats, guinea pigs, kittens and monkeys were studied. In all species, the ganglia consisted of small clusters of nerve cells and nerve processes, together with supporting elements and blood vessels. Each ganglion was ensheathed in a connective tissue capsule. The nerve cells were large and each contained a large nucleus with one or more nucleoli. The cytoplasm contained much rough endoplasmic reticulum and extensive Golgi complexes. In the rat and guinea pig, and to a lesser extent in the cat, the surfaces of the perikarya were studded with short dendritic processes. The dendrites of monkey and cat neurons were much larger than those of the other species. Short and long dendrites and neuronal perikarya all were enclosed in sheath cell processes. Preganglionic nerves, interposed between the sheath cells and the surfaces of the neurons, made synaptic contact with the perikarya and the dendritic processes. Some synapses were made by axons en passant, while others involved nerve terminals. Typical cholinergic preganglionic terminals were numerous in all species. To ensure positive identification of adrenergic nerve terminals, some animals were pretreated with 5-hydroxydopamine. In these animals, preganglionic terminals containing many small dense-core vesicles were seen in synaptic contact with short dendrites of the intracardiac neurons. Other presynaptic terminals contained translucent vesicles with flattened and irregular profiles, a few large dense-core vesicles and much glycogen. These did not resemble adrenergic, cholinergic or purinergic endings. It seems to us, therefore, that the intracardiac neurons serve as a final common pathway, through which at least three types of input may simultaneously influence the effector tissues of the heart.

Studies of intestinal lymphoid tissue. I. Electron microscopic evidence of 'blast transformation' in epithelial lymphocytes of mouse small intestinal mucosa.

The morphology of epithelial lymphocytes in osmium-fixed, Epon-embedded jejunum of adult mice was studied by light and electron microscopy. Toluidine blue-stained 1 mum 'thick' plastic sections were compared with adjacent thin sections, thereby permitting precise ultrastructural identification and description of selected epithelial lymphocytes. Their size and appearances varied considerably, ranging from typical small lymphocytes through medium-sized lymphocytes to large immunoblasts. A high proportion of medium-sized epithelial lymphocytes (mean diameter 6-9 +/- 1-1 mum) contained several lysosomes, extensive Golgi complexes, prominent centrioles and abundant ribosomes. Their appearances, therefore, corresponded directly to mitogen-stimulated lymphocytes. In contrast, immunoblasts were big cells (mean diameter 11-0 +/- 0-8 mum) with large, euchromatic nuclei and prominent nucleoli. The majority had pale-staining, ribosome-studded cytoplasm and thus resembled type I, or T blasts. Very rarely, densely staining blasts containing ribosomes and well developed rough endoplasmic reticulum were observed; these corresponded to type II or B blasts. These observations indicate that transformation of lymphocytes occurs within the interepithelial cell spaces of the small intestinal mucosa, suggesting that epithelial lymphocytes are immunocompetent cells which may be responsive to local antigenic stimulation.

Study of Dientamoeba fragilis Jepps & Dobell. I. Electronmicroscopic observations of the binucleate stages. II. Taxonomic position and revision of the genus.

SYNOPSISLight‐ and electronmicroscopic observations on Dientamoeba fragilis strain A (Bi) 1 dealing primarily with the binucleate (arrested telophase) stages, predominant in all populations, revealed the microtubular nature of the extranuclear spindle which extends between the 2 polar complexes each adjacent to one of the nuclei. The spindle microtubules originate in paired, nonperiodic structures apparently homologous to the “atractophores” described from hypermastigotes. To the external surface of the atractophores are applied periodic elements, which extend laterally as the parabasal filaments. Extensive Golgi complexes overlie the filaments, these structures corresponding to the components of the parabasal apparatus known from trichomonads and hypermastigotes. The 2‐layered structures, consisting of the atractophores and periodic layers, together with the proximal parts of the Golgi complex and the spindle microtubules constitute the polar complex. No kinetosome‐ or centriole‐like organelles have been found in the polar complexes or elsewhere in the organism. The extranuclear spindle is composed of 2 microtubule bundles, each with ∼30‐40 microtubules. One of the bundles always appears at some distance from the nucleus; the other is juxtanuclear and is seen often to course within a groove of the nuclear envelope. A 3rd bundle of ∼35‐45 microtubules is seen on occasion to arise from the atractophores and to pass toward the nucleus at a wide angle to the other parts of the spindle. In some instances these microtubules traverse the nucleus within channels delimited by the nuclear envelope. The double‐layered nuclear envelope contains numerous pores. Two morphologic types of rounded inclusions, one microbody‐like, and the other with a more electron‐translucent matrix, as well as digestive vacuoles containing rice starch, bacteria, and/or myelin configurations are distributed in the cytoplasm, which abounds also in glycogen granules.The fine structure of Dientamoeba is compared with those of trichomonads and of Entamoeba spp. The taxonomic position of Dientamoeba is discussed and emended; in view of its affinities, this genus is placed among trichomonads in the family Dientamoebidae Grassé, emend.