An observation of fine tubules within the endoplasmic reticulum in a dog liver

Summary During a survey of liver tissue from 100 dogs, fine tubules were observed within the cisternae of the endoplasmic reticulum of hepatocytes in one dog. The tubules were 300 Å in diameter with electron dense walls of 100 Å thickness. Many cells contained the tubules but their significance is unknown. The tubules varied from other reported microtubules in their location, size and characteristics of fixation and staining.     

A new interpretation of plasmodesmatal ultrastructure

Summary It is shown that simple, unbranched, plasmodesmata between young xylem ray cells of willow have no direct intercellular continuity apart from the plasmalemma which limits the cytoplasm and lines the plasmodesmatal canal. Each plasmodesma is traversed by a 200 Å diameter tubule (the desmotubule) which has a wall with probably 11 subunits arranged around a central cavity through which runs a 40 Å diameter rod. This rod is connected to the inside of the tubule wall, by fine filaments. At the ends of each plasmodesma the plasmalemma and cell wall are closely appressed to the tubule, thus precluding direct continuity between the cytoplasm of adjacent cells. Through the central part of the plasmodesmata the tubule is separated from the plasmalemma by a 90–100 Å wide gap. Cytoplasmic microtubules in the same tissue have a diameter of approximately 250 Å and a wall probably composed of 13 subunits: both desmotubules and cytoplasmic microtubules therefore have a centre-to-centre subunit spacing of about 47 Å. It is suggested that the desmotubules are not microtubules but may be nuclear spindle fibres which become trapped in the wall during cell plate formation. The endoplasmic reticulum, while closely approaching the plasmodesmata, is not continuous across them. It is thought most unlikely that the endoplasmic reticulum traverses plasmodesmata, as the dimensions of the central tubule — found here as well as by other workers — are smaller than those which would be expected to allow a stable molecular configuration in a unit membrane. The plasmalemma, where it lines the plasmodesmatal canal, appears to have particulate subunits in the outer opaque layers and the presence of these subunits may be attributable to the need for stability in membranes arranged about so small a radius.     

The fine structure of differentiating interstitial cells in Hydra

Summary Interstitial cells of hydra are small undifferentiated cells containing an abundance of free ribosomes and few other cytoplasmic organelles. They are capable of differentiating into epitheliomuscular, digestive, glandular, nerve cells, and cnidoblasts. Developing epitheliomuscular and digestive cells acquire bundles of filaments, 50 Å in diameter, which later are incorporated into the muscular processes. Early gland cells develop an elaborate rough-surfaced endoplasmic reticulum and one or more Golgi apparatus. Secretory granules originate in the Golgi region eventually filling the apex of the cell. Neurons are recognized first by the presence of an elaborate Golgi apparatus, absence of a well-developed endoplasmic reticulum, and later the appearance of cytoplasmic processes. The most striking feature of nematocyst formation by cnidoblasts is the presence of a complex distribution system between protein synthesizing rough-surfaced endoplasmic reticulum and the nematocyst. This system consists of connections between cisternae of the endoplasmic reticulum with smooth Golgi vesicles which in turn are connected to minute tubules, 200 Å in diameter. The tubules extend from the Golgi region around the nematocyst finally entering the limiting membrane of the nematocyst. It is suggested that the interstitial cells of hydra represent a model system for the investigation of many aspects of cell differentiation.This work was supported by grants from the National Cancer Institute (TlCA-5055) and from the National Institute of Arthritis and Metabolic Diseases (AM-03688), National Institutes of Health, Department of Health, Education and Welfare.The author is indebted to Dr. Russell J. Barrnett for his guidance and interest throughout this investigation.     

Connections between mitochondria and endoplasmic reticulum in rat liver and onion stem

Summary Smooth-surfaced elements of endoplasmic reticulum contact and are attached to the outer membranes of mitochondria in rat liver and onion stem. Some connections appear as short, 150–300 Å diameter tubules that bridge the space between the conjoining elements. In liver, the smooth-surfaced endoplasmic reticulum cisternae connected to the outer mitochondrial membrane are shown to be continuous with rough-surfaced endoplasmic reticulum. Here, the smooth-surfaced endoplasmic reticulum is identified in negatively stained preparations of isolated cell fractions and in thin sections of tissues by the presence of lipoprotein particles characteristic of this cell component. In onion, the identification of endoplasmic reticulum is based on continuity with rough-surfaced endoplasmic reticulum.     

Membranous tubes in pseudoscorpion spermiogenesis

The highly complicated differentiation of the spermatid in the pseudoscorpion Diplotemnus sp. is accomplished without the presence of microtubules. Instead membranous tubes measuring approximately 50 nm in diameter and closely associated with endoplasmic reticulum are found from early to mid spermatids. The lumen of the tube is devoid of electron dense contents but a fluffy material is attached to the cytoplasmic side. They run straight or slightly bent and are in open connection with the cell membrane. First appearing near the cell bridge of the interconnected spermatids they form a bundle in the longitudinal axis during a transitory phase of elongation. When the cell rounds off again the tubules together with the endoplasmic reticulum disappear. The arrangement of the tubes and their presence during abortive elongation of the spermatid suggest a supportive function commonly attributed to microtubules. Moreover, the open connection with the cell membrane and their close association with the endoplasmic reticulum may indicate their participation also in transport.     

Filaments and rod-shaped tubulated bodies in the endothelia of anterior cerebral arteries in young rats

Summary Endothelia of the anterior cerebral arteries in rats aged 1 to 3 days were studied. Thin (about 50–90 Å) and thick (about 100–110 Å) filaments are present in the endothelia. Numerous spherical- or rod-shaped bodies, measuring approximately 0.07 to 0.3 m in diameter and up to 0.6 m in length occur in the endothelial cells. These bodies contain a tubular structure. The diameter of the individual tubules is about 200 Å. The present observations suggest that spherical- or rod-shaped inclusions are first synthesized in the rough endoplasmic reticulum and thereafter these materials are transported into the Golgi complex for maturation. A small number of the inclusions, however, may originate directly from the rough endoplasmic reticulum and not pass through the Golgi apparatus.A part of this study was demonstrated at the 70th Versammlung der Anatomischen Gesellschaft in Düsseldorf, April, 1–5, 1975The author thanks Mr. Tatsuro Fukushima for preparation of photographs     

Three-dimensional architecture of the tubular endocytic apparatus and paramembranous networks of the endoplasmic reticulum in the rat visceral yolk-sac endoderm

The three-dimensional architecture of the tubular endocytic apparatus and the endoplasmic reticulum in the rat yolk-sac endoderm was investigated after loading with horseradish peroxidase-conjugated concanavalin A by intrauterine administration. After 30 min, small vesicles (50–150 nm in diameter), small tubules (80–100 nm in diameter) and large vacuoles (0.2–1.0 m in diameter) in the apical cytoplasm were labeled with the tracer, but lysosomes (1.0–3.5 m in diameter) in the supranuclear cytoplasm were not labeled until 60 min after loading. Stereo-viewing of the labeled small tubules in thick sections revealed that they were not isolated structures but formed three-dimensional anastomosing networks, which were also confirmed by scanning electron microscopy after maceration with diluted osmium tetroxide. Their earlier labeling with the endocytic tracer, localization in the apical cytoplasm and three-dimensional network formation indicated that the labeled small tubules represented tubular endosomes (tubular endocytic apparatus). These well-developed membranous networks provided by the tubular endosomes are suggested to facilitate the receptor-mediated endocytosis and transcytosis of the maternal immunoglobulin in the rat yolk-sac endoderm. Scanning electron microscopy further revealed lace-like networks of the smooth endoplasmic reticulum near the lateral plasma membrane. Their possible involvement in transport of small molecules or electrolytes is discussed.     

Observations on the Fine Structure of the Gastric Parietal Cell of the Rat

The structure of the rat parietal cell was examined by electron microscopy. The intercellular and intracellular canalculi are lined by microvilli which are more numerous and larger than those of other gastric cells. The numerous mitochondria have closely packed cristae and a dense matrix containing opaque particles. The cytoplasmic vacuoles typical of parietal cells are part of a network of smooth surfaced tubules and vacuoles (the endoplasmic reticulum) which is intimately associated with the mitochondria and probably connected with the lumen of the canaliculi. Only a few dense particles are found attached to the surface of these tubules. The structure of the parietal cell is compared with that of other cells whose function also is transport of inorganic ions and water. Evidence is presented supporting the hypothesis that parietal cells differentiate from a less structurally specialized cell in the neck region of the gastric gland.     

Associations between microbodies and a system of cytoplasmic tubules in oil-body cells of Marchantia

Summary Preliminary observations on differentiating oil-body cells of Marchantia sp. revealed that the cytoplasm possesses conventional cytoplasmic microtubules as well as a greater number of other tubules, which average 35 nm in diameter and appear in cytoplasmic regions rich in endoplasmic reticulum. These tubules, at a stage of active synthesis of oil, increase in number, may form well-organized bundles traversing the cytoplasm close to the vacuole, and show a preferential spatial relationship to elongated microbodies. One layer of densely arrayed cytoplasmic tubules surrounds the microbodies partially or totally. Fine links bridge the tubules to one another or some of them to the microbody bounding membrane.     

Microtubules in some unicellular glands of two leeches

Summary What appear to be two types of unicellular glands are found in the integument of the leech, Helobdella stagnalis. Type I cells are characterized by a peripheral, subplasmalemmal sack of rough endoplasmic reticulum and accumulations of secretory product in the form of small membrane bound droplets. Type II cells are characterized by large numbers of closely opposed sacks of rough endoplasmic reticulum and secretory product in the form of large, evidently amorphous accumulations of secretory product.Both cell types attenuate into long, slender processes through which the secretory product passes to the surface of the leech. Each process is characterized by a subplasmalemmal sack of ER which runs the entire length of the process and is continuous, at the proximal end of the process, with sacks of rough ER. Associated with the inner member of the ER membrane pair are microtubules with a diameter of approximately 240 Å.A similar arrangement of a subplasmalemmal ER sack associated with microtubules also is found in secretory processes of the leech, Macrobdella decora.The possible source and functions of these microtubules are discussed.This investigation was supported by Public Health Service grant number GM 723-04 of the National Institutes of Health.The author is greatly indebted to Dr. David B. Slautterback for his advice and encouragement during the course of this investigation.     

The intranuclear mitosis of the ciliates Paracineta limbata and Ichthyophtirius multifiliis

Electron microscope studies on the premetaphase stages of micronuclear divisions of Paracineta limbata and Ichthyophtirius multifiliis showed that spindle material also exists during interphase. In the case of I. multifiliis scattered microtubule fragments persist in the nuclear space; in P. limbata the micronuclei contain a small paracrystalloid which is suggested to be microtubular protein. Wide microtubules, varying in diameter from 300 to 400 Å develop during intranuclear prophase near the nuclear envelope in both cases. There are good reasons to assume that they function as a kind of stem body during the enlargement of the surface area of the nuclear envelope. Later micronuclear prophase stages of both species show a some-what different development. In I. multifiliis, there are scattered groups of short microtubular segments, partly in parallel array, whereas in P. limbata the wide tubules are transformed into normal microtubules of 180–200 Å diameter. The nuclei of both species are similar at late prophase and prometaphase stages. Bundles of interpolar microtubules run between the chromosomes, and single microtubules, presumably induced by the chromosomes, cross them at different angles. The chromosome-induced microtubules appear a short time after the interpolars. At prometaphase stage all microtubules show a highly parallel arrangement and therefore it is suggested that chromosomal tubules reach their final polar orientation by the action of cross-bridges.     

Post-coital ultrastructural changes in neurons of the suprachiasmatic nucleus of the rabbit

Summary The effects of coitus on the ultrastructure of neurons in the suprachiasmatic nucleus of the rabbit were studied. Changes first became apparent 1/2 h after coitum in neurons located near capillaries. More pronounced ultrastructural changes were observed in large neurons removed at 1 h post-coitus. These changes, characterized by well developed Golgi systems and rough endoplasmic reticulum, the presence of large dense-core vesicles and a significant increase in both neuronal and nuclear size, were also evident in neurons observed at 2 to 10 h post coitum. Similar ultrastructural features were not observed in the neurons of the control animals. The post-coital ultrastructural changes observed within these neurons suggest high synthetic activity which may concern the production sites of the neurohormone LH-RF. Two populations of dense-core vesicles were observed: a) those with a mean diameter of 849 Å, and b) those with a mean diameter of 1542 Å. The small dense-core vesicle is probably monoamine in nature; the larger vesicle may contain the neurohormone LH-RF. A third vesicle type with a mean diameter of 1836 Å and characterized by a granular content of low electron density was also observed. That this vesicle represents the immature form of the large (1542 Å) dense-core vesicle is suggested; however, morphological evidence supporting this hypothesis is inconclusive. There is also no evidence for the storage of secreted materials within the soma of these neurons. Immediate transport toward the median eminence is suggested.This investigation was supported by grants to the two senior authors from the Medical Research Council of Canada.     

THE FINE STRUCTURE OF ACANTHAMOEBA CASTELLANII : I. The Trophozoite

The fine structure of the trophozoite of Acanthamoeba castellanii (Neff strain) has been studied. Locomotor pseudopods, spikelike "acanthopodia," and microprojections from the cell surface are all formed by hyaline cytoplasm, which excludes formed elements of the cell and contains a fine fibrillar material. Golgi complex, smooth and rough forms of endoplasmic reticulum, digestive vacuoles, mitochondria, and the water-expulsion vesicle (contractile vacuole) are described. A canicular system opening into the water-expulsion vesicle contains tubules about 600 A in diameter that are lined with a filamentous material. The tubules are continuous with unlined vesicles or ampullae of larger diameter. Centrioles were not observed, but cytoplasmic microtubules radiate from a dense material similar to centriolar satellites and are frequently centered in the Golgi complex. Cytoplasmic reserve materials include both lipid and glycogen, each of which amounts to about 10% of the dry weight.     

Ultrastructural analysis of transitional endoplasmic reticulum and pre-Golgi intermediates: a highway for cars and trucks

Cargo selection and export from the endoplasmic reticulum occurs at specialized sites in cells. Export complexes consist of transitional elements of the endoplasmic reticulum and pre-Golgi intermediates. It is generally assumed that 60 to 80 nm initially COPII-coated vesicles derived from the transitional endoplasmic reticulum are the main carriers for transport of cargo to the Golgi apparatus. We have analyzed on serial ultrathin sections the transitional endoplasmic reticulum and pre-Golgi intermediates of beta cells of islets of Langerhans in mouse pancreas. In addition to Golgi-associated complexes, others were observed in the periphery of the cells or close to the nuclear envelope. Upon three-dimensional reconstruction, non-coated ribosome-free tubules with an average diameter of 115 nm (range 60–195 nm) and a length of up to 500 nm were detected in the pre-Golgi intermediates in addition to small vesiculo-tubular elements. Furthermore, evidence was found that the large tubular elements may directly arise from transitional elements of the endoplasmic reticulum. In a given cell, pre-Golgi intermediates were found to be composed solely of small vesiculo-tubular elements or additionally of tubules or solely of tubules. Immunogold labeling for proinsulin indicated that the large tubular elements contained cargo and thus appear to take part in ER-to-Golgi transport.Presented at the 43rd Symposium of the Society for Histochemistry, Vienna, Austria, 27–29 September 2001     

Microtubule-like structures in the growing plastids or chloroplasts of two algae

Summary During oogenesis in Chara fibrosa, and in the enlarging, young daughter coenobia of Volvox spec., microtubule-like structures were found in growing plastids. These were appreciably bigger than the usual 240 Å cytoplasmic microtubules, measuring about 320 Å in diameter; a helical or banded organisation in the wall of these tubules was also evident. The tubules were generally present in greatest numbers when the plastids were elongating or enlarging.     

Electron microscopic study on the thyroid gland of the salamander Hynobius nebulosus in the breeding season

Summary The thyroid gland of adult salamanders, Hynobius nebulosus, in the breeding season was studied by electron microscopy. The follicular cells are different in cell height and fine structures; the taller cells with many cell organelles and granules and the lower cells with a few cell organelles and granules are both present in the same follicle. In the cytoplasm, three types of membrane-bounded granules, namely, cytosomes, colloid droplets, and vacuolar bodies and circular membrane complexes occur. The vacuolar bodies are subdivided into two types; the ordinary type having loosely distributed particles and the specific type containing tubules and/or closely packed filaments, crystalloid structures, except for the particles. The chromophobe colloids within the Bensley-cells correspond to extremely large, ordinary type vacuolar bodies, while the Langendorff-colloid cells possess increased numbers of granular cisternae of endoplasmic reticulum and a ribosome-rich, dense cytoplasmic matrix but not extremely large colloid. The intracytoplasmic circular membrane complexes appear in the Golgi area of cytosome-rich cells. It is suggested that they originate from the Golgi apparatus which was activated to produce many cytosomes. Intranuclear inclusions consisting of microtubules and filaments and tight junctions between two adjacent lateral plasma membranes are occasionally encountered.     

The ultrastructure of plasmodesmata in the filamentous green alga,Bulbochaete hiloensis (Nordst.) tiffany

Summary The ultrastructure of the plasmodesmata found in the green alga Bulbochaete hiloensis has been examined by electron microscopy of ultra-thin sections. Unlike most other plasmodesmata that have been described recently, there are no internal components such as a desmotubule or a derivative of the endoplasmic reticulum. Each plasmodesma consists of a cylindrical connection between the plasma membranes of adjacent cells. The cylinder is constricted at each end to orifices which may be less than 100 Å in diameter. Within the cylinder the cytoplasmic face of the plasma membrane is lined with material probably consisting of helically arranged particles. The lumen here is 400–450 Å in diameter.The observations are discussed in relation to possible functions in intercellular transport.     

General morphology and axonal ultrastructure of the olfactory nerve of the pike,Esox lucius

Summary The olfactory nerve of the European pike (Esox lucius) contains 5.1 × 10⁶ axons with an average diameter of 0.20 ± 0,04 m and a length of 5.5 cm in 1 meter long pike. Each axon contains an average of 4 microtubules as well as neurofilaments, smooth endoplasmic reticulum and about 500 mitochondria per centimeter. The number of neurofilaments ranges from zero in 15% of the cross sections to over 10 in 6%. Neurofilaments generally occur in clusters located opposite to microtubule regions. Smooth ER can not be identified in 14% of the cross sections suggesting that this structure may not be continuous. Microtubules often display annular regions (halos) of low electron density ranging in size from 800 to 1300 Å. Halos from adjacent tubules usually merge into regional halos. The ratio of axoplasm to glial cytoplasm is 4.4:1, while the ratio of axonal plasma membrane to glial plasma membrane exceeds 7:1. A 4 cm nerve contains 1280 cm² of axolemma. This nerve represents an extreme in high density axonal packing and is therefore exceptionally well suited for biochemical, biophysical and physiological investigations.     

Axonal agranular reticulum and synaptic vesicles in cultured embryonic chick sympathetic neurons

Cultured chick embryonic sympathetic neurons contain an extensive axonal network of sacs and tubules of agranular reticulum. The reticulum is also seen branching into networks in axon terminals and varicosities. The axonal reticulum and perikaryal endoplasmic reticulum resemble one another in their content of cytochemically demonstrable enzyme activities (G6Pase and IDPase) and in their characteristic membrane thicknesses (narrower than plasma membrane or some Golgi membranes). From the reticulum, both along the axon and at terminals, there appear to form dense-cored vesicles ranging in size from 400 to 1,000 Å in diameter. These vesicles behave pharmacologically and cytochemically like the classes of large and small catecholamine storage vesicles found in several adrenergic systems; for example, they can accumulate exogenous 5-hydroxydopamine. In addition, dense-cored vesicles at the larger (1,000 Å) end of the size spectrum appear to arise within perikaryal membrane systems associated with the Golgi apparatus; this is true also of very large (800–3,500 Å) dense-cored vesicles found in some perikarya.     

THE NORMAL FINE STRUCTURE OF OPOSSUM TESTICULAR INTERSTITIAL CELLS

The interstitial tissue of the opossum testis includes interstitial or Leydig cells, macrophages, and small cells which morphologically resemble mesenchymal cells. The latter are thought to give rise to mature interstitial cells. The most prominent feature of the interstitial cell cytoplasm is an exceedingly abundant agranular endoplasmic reticulum. This reticulum is generally in the form of a meshwork of interconnected tubules about 300 to 450 A in diameter, but occasionally it assumes the form of flattened, fenestrated cisternae resembling those of pancreatic acinar cells, except for the lack of ribonucleoprotein particles on the surface of the membranes. The interstitial cells vary considerably in their cytoplasmic density. The majority are quite light, but some appear extremely dense, and in addition usually have a more irregular cell surface, with numerous small pseudopodia. These differences may well reflect variations in physiological state. Cytoplasmic structures previously interpreted as "crystalloids" consist of long bundles of minute parallel tubules, each about 180 A in diameter, which seem to be local differentiations of the endoplasmic reticulum. The mitochondria are rod-shaped, and contain a moderately complex internal membrane structure, and also occasional large inclusions that are spherical and homogeneous. The prominent juxtanuclear Golgi complex contains closely packed flattened sacs and small vesicles. The results of the present study, coupled with biochemical evidence from other laboratories, make it seem highly probable that the agranular endoplasmic reticulum is involved in the synthesis of the steroid hormones produced by the interstitial cell. This finding therefore constitutes one of the first functions of the agranular reticulum for which there is good morphological and biochemical evidence.