Gymnodinium aeruginosum (Dinophyta): A blue-green dinoflagellate with a vestigial,anucleate, cryptophycean endosymbiont

Gymnodinium aeruginosum has the usual fine structure of a dinoflagellate but does not seem to contain a well elaborated peduncle or a microtubular basket. Naked cells are surrounded by a single large amphiesmal vesicle. It houses an endosymbiont with typical blue-green cryptophycean chloroplasts (generally only one), cryptophycean starch grains in the periplastidal cytoplasm without a nucleomorph, and two membranes separating the periplastidal cytoplasm from the cryptophycean cytoplasm which contains mitochondria, ER, vesicles and ribosomes, but no eukaryotic nucleus. The endosymbiont is surrounded by a single membrane. Possible ways of the acquisition of the endosymbiont and the problem of the existence of ribosomes within a compartment without nucleus are discussed.Devoted to Prof. Dr.L. Geitler, the Nestor of phycology and endosymbiosis research, on the occasion of the 90th anniversary of his birthday.     

Light and electron microscopy of virus inclusions inAmaranthus lividus cells

Summary Amaranthus plants infected with a virus of rod-shaped particles showed under the light microscope intracytoplasmic amorphous and crystalline inclusions.The submicroscopic organization of mesophyll cells from infectedAmaranthus leaves by electron microscopy is described. Besides big crystalline inclusions, long dark inclusions correspondent to needle-like inclusions observed by light microscopy are definable in the cytoplasm. The amorphous inclusion bodies were formed by an overgrown protrusion of vacuolate cytoplasm containing virus particles, long very dark stained inclusions forming dense bands and rings, normal elements of the cytoplasm such as mitochondria, endoplasmic reticulum and ribosomes, and some spherosomes. Inclusions and virus particles were not found in chloroplasts, mitochondria or nuclei of infected cells.     

Ultrastructure of the mesophyll in scots pine and Norway spruce: Seasonal variation and molarity of the fixative buffer

Summary Resolution of the ultrastructure of the needles of both Scots pine (Pinus silvestris L.) and Norway spruce [Picea abies (L.) Karst.] is strongly influenced by the molarity of the buffer used in fixation. When 0.2 M or 0.1 M buffer is used in fixation during the summer, the constituents of the cytoplasm are precipitated, resulting in poor resolution of the membranes and lamellae and often in negative staining. The tannin in the central vacuole appears as a thick ribbon. By using correct molarities of buffer during each season (0.1 M for autumn and winter and ca. 0.05 M for the growing season), the best possible resolution will be achieved. With good resolution the tannin in the central vacuole appears in granular form throughout the year, and the cytoplasm and its organelles are clearly distinguishable during every season. During the growing season, the chloroplasts in the needles of Scots pine are spread to the cell walls and have large starch grains; the stroma and grana lamellae are well developed; the stroma and cytoplasm are rich in polysomes. Mitochondria and microbodies can be clearly resolved. During hardening and afterwords throughout the winter, the chloroplasts, which at this time contain no starch, and other cytoplasmic organelles aggregate in the corners of the cells. The chloroplast envelopes and the stroma and grana lamellae stay intact. The cytoplasm is netlike and rich in ribosomes, mitochondria and microbodies, all of which are intact and clearly distinguishable. During spring activation the structure returns to that described for the growing season.     

Structural alterations inEuglena gracilis exposed to the herbicide isopropyl-N-phenylcarbamate (IPC)

Summary Ultrastructural abnormalities of various kinds and severity appeared inEuglena gracilis green cells after a 48-hour exposure to 4 × 10^–4M of isoprophyl-N-phenylcarbamate (IPC), a widely used herbicide thought to affect microtubules and/or microtubule organizing centers in susceptible plant and animal species.A high proportion of cells contained nuclei in the G 2 phase of the cycle; in a significant percentage of organisms, however, structural aberrations of the nucleolus and chromosomes were evident. The pellicle outline, chloroplasts, mitochondria, and dictyosomes were also affected. The cytoplasm was rich in dense bodies which sometimes showed granular, fibrillar or tubular materials. Furthermore IPC partially inhibited flagellum regeneration after mechanical amputation.The mechanism by which IPC causes these responses remains unclear. Nevertheless, some indications suggest that the herbicide acts primarily on microtubule organizing centers. However, mitochondria, chloroplasts and nuclear constituents appear as possible additional targets of the compound.The study was supported by a grant from Italian Research Council (CNR), contract No. 80.00420.04.     

NaCl胁迫对宁夏枸杞叶和幼根显微及超微结构的影响

以宁夏枸杞为材料,采用超薄切片技术制备样品,应用光学显微镜和透射电镜分析了不同浓度NaCl胁迫条件下宁夏枸杞叶和幼根显微及超微结构的变化。结果表明：随着NaCl胁迫的加重,(1)叶片上表皮细胞增厚,栅栏组织细胞出现缩短现象,排列疏松且紊乱;幼根的初生结构无明显变化。(2)叶片栅栏组织中叶绿体不再紧靠在细胞膜上,叶绿体双层膜破坏,基粒片层松散排列,杂乱无章,出现膨胀和空泡现象,淀粉粒和嗜锇颗粒增多,叶肉细胞中线粒体发生轻微变化;幼根中皮层薄壁细胞线粒体形状发生改变,结构破坏,内膜和外膜模糊甚至破裂,大多数嵴模糊,出现空泡现象;细胞核解体,基质外溢。研究表明, 不同浓度的NaCl胁迫对宁夏枸杞叶片和幼根细胞的显微及超微结构影响不同,NaCl浓度大于200 mmol/L时,宁夏枸杞叶片和幼根细胞的显微及超微结构发生了明显变化,且叶肉细胞中线粒体的变化没有叶绿体的变化显著,推测叶肉细胞中线粒体的耐盐性比叶绿体强。     

On the ultrastructure of the ovary of the liver fluke (Fasciola hepatica L.)

Summary The ovary of the liver fluke has been studied with light and electron microscopy. The organ consisted of germ cells and a layer of peripheral cells suggested to be nurse cells, and was surrounded by a capsule containing muscular tissue. The peripheral cells rested on a thick basement membrane and were irregular in outline. Their nuclei were of irregular shape, the mitochondria were dark with few cristae and the endoplasmic reticulum was tubular or vesicular and partly studded with ribosomes. The germ cells were rounded or polyhedral except in the outer part of the ovary where some of them showed irregular processes. The germ cells of the outer region (oogonia) were relatively small and in close contact with the cells suggested to be nurse cells. The inner germ cells (oocytes) were large and loosely packed. Their nuclei were irregular and contained round distinct nucleoli. The nuclear envelopes showed numerous pores. The endoplasmic reticulum was very sparse, but free ribosomes were abundant in the cytoplasm. This corresponded with a strong basophilia removable with RNase. In addition round basophilic bodies formed by densely packed ribosomes and membraneous material occurred in close spatial relation to mitochondria. The latter contained dense granules and few cristae. Groups of vesicles and membraneous lamellae were found in the cytoplasm, but they were considerably smaller than vertebrate Golgi complexes. Numerous dense spherical granules were found mainly in the periphery of the large germ cells. The granules were strongly osmiophilic except in the terminal part of the ovary. They were PAS-positive, but negative to Sudan dyes.Supported by a grant from Jordbrukets Forskningsråd, Stockholm.     

Ultrastructure of endopolyploid antipodals inAconitum vulparia Rchb.

Summary The ultrastructure of the antipodals ofAconitum vulparia Rchb. was studied in mature embryo sacs. Antipodal cell wall thickness varies in different parts of the cells. The antipodals resemble transfer cells with distinctly marked wall ingrowths which are particularly well developed in the chalazal part and between the antipodals. A few plasmodesmata occur in the cell wall between the antipodals and the central cell. The cytoplasm is rich in ribosomes which occur free or bound to the membranes of the well developed endoplasmic reticulum. Only in the micropylar region of the cells are some larger vacuoles found. The antipodals contain numerous mitochondria, plastids and apparently active dictyosomes. Vesicles with electron dense contents, microbodies, multivesicular bodies as well as lipid droplets and small multiple concentric cisternae are also present in the cytoplasm. The giant endopolyploid nuclei have lobed outlines, especially at the chalazal side of the nuclei.Ultrastructural features, especially the occurrence of numerous free ribosomes and the development of extensive rough endoplasmic reticulum, suggest high metabolic activity in the growing and differentiating antipodals of this species.     

Isolation of active polyribosomes from the cytoplasm, mitochondria and chloroplasts of Euglena gracilis

1. A procedure is described for the isolation of intact polyribosomes from the cytoplasm, chloroplasts and mitochondria of Euglena gracilis. 2. All three polyribosomal preparations incorporated labelled amino acids in a system in vitro. The cytoplasmic system was inhibited by chcloheximide but not by chloramphenicol. Both the chloroplast and the mitochondrial systems, however, were inhibited by chloramphenicol but not by cycloheximide. It is shown that mitochondrial polyribosomes, like the polyribosomes from cytoplasm and chloroplasts, can participate directly in protein synthesis without supplementary mRNA being added to the synthesizing system, as in previously reported instances. 3. Sedimentation coefficients were measured for the ribosomes, ribosomal subunits, and rRNA of the cytoplasm, chloroplasts and mitochondria. 4. The G+C content was 55% for cytoplasmic rRNA, 50% for chloroplast rRNA, and 29% for mitochondrial rRNA. 5. The cytoplasmic ribosomal subunits contained a ribonuclease activity that was inhibited by heparin.     

The cytoplasmic organization of hyphal tip cells in the fungusAllomyces macrogynus

Summary Light and transmission electron microscopy were used to examine hyphal tip cells of the fungusAllomyces macrogynus (Chytridiomycetes). A well defined apical body, i.e., Spitzenkörper, was observed at the extreme apex of hyphal cells. This distinctive, spherical cytoplasmic region consisted of a granular matrix devoid of ribosomes and most organelles. To our knowledge this is the first report describing such a structure in hyphae of an aseptate fungus. Vesicles (45–65 nm diameter) were concentrated in the peripheral cytoplasm of the apex, while relatively few were observed within the Spitzenkörper. Filasomes, spherical patches of dense fibrillar material containing a microvesicle core, were abundant in the apical regions near the plasma membrane. Microtubules traversed the Spitzenkörper at various angles and were in close association with the plasma membrane. Microfilaments were observed as individual elements in the cytoplasm or were organized into bundles. Individual microfilaments were frequently in close association with the plasma membrane, vesicles and microtubules. In the immediate subapical region mitochondria, multivesicular bodies, microbodies, Golgi equivalents and nuclei were abundant.Abbreviations CW cell wall - F filasome - M mitochondria - N nucleus - PM plasma membrane - TEM transmission electron microscopy     

Ultrastructural changes in the spore and mycelia of Ascosphaera apis after treatment with benomyl (Benlate 50 W)

In Ascosphaera apis, after 8 days growth in darkness at 28° C, numerous sporocysts were observed, within which mature spores were seen aggregated into a spore ball. The mature spore of A. apis had a thick spore wall with an electron-opaque outer layer, a spore membrane with many depressions, and sporoplasm containing numerous ribosomes and mitochondria. In the cytoplasm of the mycelium, mitochondria with well-defined cristae and numerous ribosomes were observed. At a concentration of 1 g/ml of culture medium, benomyl appeared to inhibit colony growth of A. apis, but some sporocysts containing deformed spores were found. Deformed spores possessed a thick spore wall with a grainy matrix, and depressions were no longer detected in the spore membrane. Ribosomes were lacking in the sporoplasm and mitochondria appeared degenerate. The mycelium from the treated culture contained mitochondria with an electron-lucid matrix and no well defined cristae, while ribosomes were completely depleted. The significance of these observations in relation to the use of benomyl to control chalkbrood disease in the honey bee is discussed.     

SPERMATOGENESIS IN A TRICHUROID NEMATODE,TRICHURIS MURIS. II. FINE STRUCTURE OF PRIMARY SPERMATOCYTE AND FIRST MEIOTIC DIVISION

Ultrastructural observations indicate that the primary spermatocyte of Trichuris muris is larger than the spermatogonial stage with an increased cytoplasm to nucleus ratio. The cytoplasm contains an extensive reticular system, mitochondria, numerous free ribosomes and prominent Golgi complexes which may contribute to the formation of a sub-surface, vesicular complex. Although only two spermatocytes were seen to be linked by a cytoplasmic bridge it is suggested that the number of conjoined cells is probably greater. The rearrangement of mitochondria in a ring around the nucleus and the indentation and vesiculation of the nuclear envelope preceeded its disappearance and indicated the onset of meiosis. Centrioles were frequently resolved at this stage. They were composed of nine peripheral doublets surrounded by a dense pericentriolar sheath. Three dense chromatin areas indicative of haploid chromosomes were present in later meiotic stages. Each chromosome was surrounded by a number of mitochondria and there was a clear separation of the chromosome-mitochondrial clusters from the remainder of the cytoplasm. This was particularly evident at telophase when two daughter cells were partially separated by membrane infoldings. This reflects incomplete cytokinesis in the dividing spermatocyte of T. muris and is similar to that described in other trichuroid species. A close association with processes of the non-germinal, sustentacular cells was noted throughout the spermatocyte stage.     

Low temperature-induced modifications in cell ultrastructure and localization of phenolics in winter oilseed rape (Brassica napus L. var. oleifera L.) leaves

Acclimation of winter oilseed plants in the cold (i.e. at temperatures >0 degrees C) followed by short exposure to sub-lethal freezing temperatures resulted in pronounced ultrastructural changes of leaf epidermal and mesophyll cells. The following major changes were observed upon acclimation at 2 degrees C: increased thickness of cell walls; numerous invaginations of plasma membranes; the appearance of many large vesicles localized in the cytoplasm in close proximity to the central vacuole; the occurrence of abundant populations of microvesicles associated with the endoplasmic reticulum (ER) cisternae or located in the vicinity of dictyosomes; and the occurrence of paramural bodies and myelin-like structures. In addition, large phenolic deposits were observed in the vicinity of the plasma membrane and membrane-bound organelles such as chloroplasts, large vesicles or cytoplasm/tonoplast interfaces. Transient freezing (-5 degrees C for 18 h) of the cold-acclimated leaves led to reversible disorganization of the cytoplasm and to pronounced structural changes of the cellular organelles. Chloroplasts were swollen, with the stroma occupying one half of their volume and the thylakoid system being displaced to the other half. Large phenolic aggregates disappeared but distinct layers of phenolic deposits were associated with mitochondrial membranes and with chloroplast envelopes. In frost-thawed cells recovered at 2 degrees C for 24 h, dictyosomes and dictyosome- or ER-derived small vesicles reappeared in the ribosome-rich cytoplasm. Aberrations in the structure of chloroplasts and mitochondria were less pronounced. Few phenolic deposits were seen as small grains associated with chloroplast envelopes and vesicle membranes. These observations demonstrate that plants undergo different changes in cell ultrastructure depending on whether they are subjected to chilling or freezing temperatures. Results are discussed in relation to membrane recycling and the possible role of phenolics during the first and second stages of plant acclimation at low temperature.     

Ultrastructure of sperm cells and the male germ unit in pollen tubes ofNicotiana tabacum

Summary The structure of sperm cells and their association with the vegetative nucleus in pollen tubes ofNicotiana tabacum grown in styles were observed with the electron microscope, demonstrating the existence of a male germ unit. The two sperm cells are arranged in tandem and are closely associated with the vegetative nucleus, which always takes the lead. The leading sperm cell (SC 1) has a long and narrow cytoplasmic projection which lies within the enclaves of the much lobed vegetative nucleus, thus forming a physical association. The trailing sperm cell (SC 2) and the SC 1 are not only joined by a common transverse cell wall but also are surrounded by a periplasm bounded by the plasma membrane of the sperm cells and that of the vegetative cell, thus forming a structural connection. The sperm cells are elongated, with cytoplasmic projections at the anterior end of the SC 1 and at both ends of the SC 2. The cytoplasm of both sperm cells includes mitochondria, endoplasmic reticulum, dictyosomes, ribosomes, small vacuoles and axially oriented microtubules. No plastids were observed.Abbreviations DAPI 4,6-diamino-2-phenylindole - MGU male germ unit - MT microtubule - SC 1 the leading sperm cell physically associated with the vegetative nucleus - SC 2 the trailing sperm cell     

Possible association of actin filaments with chloroplasts of spinach mesophyll cells in vivo and in vitro

Summary. In palisade mesophyll cells of spinach (Spinacia oleracea L.) kept under low-intensity white light, chloroplasts were apparently immobile and seemed to be surrounded by fine bundles of actin filaments. High-intensity blue light induced actin-dependent chloroplast movement concomitant with the appearance of a couple of long, straight bundles of actin filaments in each cell, whereas high-intensity red light was essentially ineffective in inducing these responses. The actin organization observed under low-intensity white light has been postulated to function in anchoring chloroplasts at proper intracellular positions through direct interaction with the chloroplasts. Intact chloroplasts, which retained their outer envelopes, were isolated after homogenization of leaves and Percoll centrifugation. No endogenous actin was detected by immunoblotting in the final intact-chloroplast fraction prepared from the leaves kept under low-intensity white light or in darkness. In cosedimentation assays with exogenously added skeletal muscle filamentous actin, however, actin was detected in the intact-chloroplast fraction precipitated after low-speed centrifugation. The association of actin with chloroplasts was apparently dependent on incubation time and chloroplast density. After partial disruption of the outer envelope of isolated chloroplasts by treatment with trypsin, actin was no longer coprecipitated. The results suggest that chloroplasts in spinach leaves can directly interact with actin, and that this interaction may be involved in the regulation of intracellular positioning of chloroplasts. Correspondence and reprints: Department of Biology, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan. Present address: Tsukuba Research and Development Center, Fuji Oil Co., Ltd., Tsukuba-gun, Ibaraki, Japan.     

Chlorophyll biosynthesis from glutamate or 5-aminolevulinate in intact Euglena chloroplasts

Chloroplasts observed, by electron microscopy, to be intact and uncontaminated, with high rates of light-dependent protein synthesis and CO₂ fixation were isolated from cells grown on low-vitamin-B₁₂ medium in the light or from cells grown in the same medium in the dark and then exposed to light for 36 h. Both types of chloroplasts were active but less variability was encountered with developing chloroplasts from 36-h cells. The 36-h chloroplasts showed good light-dependent incorporation of 5-amino-levulinic acid (ALA) or l-glutamic acid into chlorophyll (Chl) a which was linear for approx. 1 h. The specific activity of the Chl a remained the same after conversion to pheophytin a, methylpheophorbide a or pyromethylpheophorbide a and rechromatography, indicating that the label was in the tetrapyrrole. Incorporation of ALA was inhibited by levulinic acid, and by chloramphenicol and other inhibitors of translation of 70S-type chloroplast ribosomes at concentrations which did not appreciably inhibit photosynthesis but which blocked plastid protein synthesis nearly completely. Cycloheximide, an inhibitor of translation on 87S cytoplasmic ribosomes of Euglena, was without effect. The 70S inhibitors did not block uptake of labeled ALA. Although labeled glycine was taken up by the plastids, no incorporation into Chl a was observed. Thus the developing chloroplasts appear to contain all of the enzymatic machinery necessary to convert glutamic acid to Chl via the C5 pathway of ALA formation but the Shemin pathway from succinyl coenzyme A and glycine to ALA appears to be absent. The requirement for plastid protein synthesis concomitant with Chl synthesis indicates a regulatory interaction and also indicates that at least one protein influencing Chl synthesis is synthesized on 70S-type plastid ribosomes and is subject to metabolic turnover.Abbreviations ALA 5-aminolevulinic acid - Chl chlorophyll     

C4-Dicarboxylic acid metabolism in bundle-sheath chloroplasts,mitochondria and strands of Eriochloa borumensis Hack., a phosphoenolpyruvate-carboxykinase type C4 species

C₄-acid metabolism by isolated bundlesheath chloroplasts, mitochondria and strands of Eriochloa borumensis Hack., a phosphoennolpyruvate-carboxykinase (PEP-CK) species, was investigated. Aspartate, oxaloacetate (OAA) and malate were decarboxylated by strands with several-fold stimulation upon illumination. There was strictly light-dependent decarboxylation of OAA and malate by the chloroplasts, but the chloroplasts did not decarboxylate aspartate in light or dark. PEP was a primary product of OAA or malate decarboxylation by the chloroplasts and its formation was inhibited by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea or NH₄Cl. There was very little conversion of PEP to pyruvate by bundle-sheath chloroplasts, mitochondria or strands. Decarboxylation of the three C₄-acids by mitochondria was light-independent. Pyruvate was the only product of mitochondrial metabolism of C₄-acids, and was apparently transaminated in the cytoplasm since PEP and alanine were primarily exported out of the bundle-sheath strands. Light-dependent C₄-acid decarboxylation by the chloroplasts is suggested to be through the PEP-CK, while the mitochondrial C₄-acid decarboxylation may proceed through the NAD-malic enzyme (NAD-ME) system. In vivo both aspartate and malate are considered as transport metobolites from mesophyll to bundle-sheath cells in PEP-CK species. Aspartate would be metabolized by the mitochondria to OAA. Part of the OAA may be converted to malate and decarboxylated through NAD-ME, and part may be transported to the chloroplasts for decarboxylation through PEP-CK localized in the chloroplasts. Malate transported from mesophyll cells may serve as carboxyl donor to chloroplasts through the chloroplastic NAD-malate dehydrogenase and PEP-CK. Bundle-sheath strands and chloroplasts fixed ¹⁴CO₂ at high rates and exhibited C₄-acid-dependent O₂ evolution in the light. Studies with 3-mercaptopicolinic acid, a specific inhibitor of PEP-CK, have indicated that most (about 70%) of the OAA formed from aspartate is decarboxylated through the chloroplastic PEP-CK and the remaining (about 30%) OAA through the mitochondrial NAD-ME. Pyruvate stimulation of aspartate decarboxylation is discussed; a pyruvate-alanine shuttle and an aspartate-alanine shuttle are proposed between the mesophyll and bundle-sheath cells during aspartate decarboxylation through the PEP-CK and NAD-ME system respectively.Abbreviations CK carboxykinase - -Kg -ketoglutarate - ME malic enzyme - 3-MPA 3-mercaptopicolinic acid - OAA oxaloacetate - PEP phosphoenolpyruvate - R5P ribose-5-phosphate     

Structure and Development of the Endophyte in the Parasitic Angiosperm <Emphasis Type="Italic">Cuscuta japonica</Emphasis>

The endophyte, that is, the haustorial part within the tissues of the host plant Impatiens balsamina, of the parasitic angiosperm Cuscuta japonica was studied with light and electron microscopy. The endophyte consisted mainly of vacuolated parenchymatous axial cells and elongate, superficial (epidermal) cells. Then the elongate, epidermal cells separated from each other and transformed into filamentous cells, called searching hyphae. The hyphae grew independently either intercellularly or intracellularly in the host parenchyma. The apical end of the hyphal cells was characterized by conspicuous, large nuclei with enlarged nucleoli and very dense cytoplasm with abundant organelles, suggesting that the hyphal cells penetrating host tissue were metabolically very active. Numerous osmiophilic particles and chloroplasts were noted in the hyphae. The osmiophilic particles were assumed to be associated with elongation of the growing hyphe. Plasmodemata connections between the searching hyphal cells of the parasite and the host parenchyma cells were not detected. Hyphal cells that reached the host xylem differentiated into water-conducting xylic hyphae by thickening of the secondary walls. A xylem bridge connecting the parasite and the host was confirmed from serial sections. Some hyphal cells that reached the host phloem differentiated into nutrient-conducting phloic hyphae. Phloic hyphae had a thin layer of peripheral cytoplasm with typical features of sieve-tube members in autotrophic angiosperms, i.e., parallel arrays of smooth endoplasmic reticulum, mitochondria, and plastids with starch granules. Interspecific open connections via the sieve pores of the host sieve elements and plasmodesmata of the parasite phloic hyphae were very rarely observed, indicating that the symplastic translocation of assimilate to the parasite from the host occurred.     

Ultrastructure of the trophic chamber and nutritive cord of Aspidiotus hederae (Homoptera,coccoidea)

Summary Each ovariole of the coccidian Aspidiotus hederae contains a single oocyte connected by means of a nutritive cord to the trophic chamber. The trophic chamber consists of three nurse cells characterized by an enlarged, ramified nucleus with a prominent nucleolus. The perinuclear cytoplasm contains nuage material, large amounts of free ribosomes, and scattered mitochondria. Occasional cisternae of the rough endoplasmic reticulum and bacteroids are found in trophocyte cytoplasm. The nutritive cord contains many microtubules in parallel array interspersed with numerous free ribosomes and a few mitochondria. The nutritive cord is strengthened by trophocyte projections which surround it. Microtubules in the projections are oriented perpendicular to the long axis of the cord.     

The discovery of polyribosomes

By the early 1960s, evidence had accumulated that proteins were synthesized from special RNA copies of genes, named "messenger RNAs" (mRNAs), not directly from the stable RNAs found in the ribosomes of the cytoplasm. Yet, precisely how the protein chains were assembled along the RNA and, in particular, the relationship between the mRNAs and the ribosomes during protein synthesis, was obscure. In this account, I discuss how my laboratory found that multiple ribosomes traverse each mRNA, yielding the structures known as polysomes. This work led on to the first physical determination of the coding ratio, new insights into how protein chains are initiated, and an early suggestion that chloroplasts and mitochondria in eukaryotic cells might ultimately have been derived from symbiotic bacteria.     

Seasonal ultrastructural variations in pinealocytes of the woodchuck,Marmota monax

The ultrastructure of the pinealocyte in the woodchuck, Marmota monax, was studied during the four seasons of the year. Fall cells have a fairly uniform cytoplasmic density, organelles consistent with synthetic and/or secretory activity and rather extensive pericapillary and intercellular spaces. Many winter pinealocytes are nearly devoid of ribosomes and granular endoplasmic reticulum but contain lipid droplets associated with mitochondria. Pericapillary and intercellular spaces are minimal. Spring glands have the greatest variation in cytoplasmic density with intercellular and pericapillary spaces similar to that seen in fall glands. Cells containing electron dense cytoplasm have Golgi zone associated, secretory granules, free ribosomes, short sections of granular endoplasmic reticulum and dense bodies. Cells with a more electron lucent cytoplasm are similar to the most frequently observed summer pinealocytes which have numerous Golgi zones but few associated secretory granules. Microtubules are prominent in the cytoplasm of these cells, the plasma membranes are smooth and intercellular and pericapillary spaces are minimal. A yearly rhythm or cyclic activity of the pinealocyte is suggested.