The tubular network of the Golgi apparatus

 Golgi apparatus of both plant and animal cells are characterized by an extensive system of approximately 30 nm diameter peripheral tubules. The total surface area of the tubules and associated fenestrae is thought to be approximately equivalent to that of the flattened portions of cisternae. The tubules may extend for considerable distances from the stacks. The tubules are continuous with the peripheral edges of the stacked cisternae, but the way they interconnect differs across the stack. In plant cells, for example, tubules associated with the near-cis and mid cisternae often begin to anastomose close to the peripheral edges of the stacked cisternae, whereas the tubules of the trans cisternae are less likely to anastomose and are more likely to be directly continuous with the peripheral edges of the stacked cisternae. Additionally, the tubules may blend gradually into fenestrae that surround some of the stack cisternae. Because of the large surface area occupied by tubules and fenestrae, it is reasonable to suppose that these components of the Golgi apparatus play a significant role in Golgi apparatus function. Tubules clearly interconnect closely adjacent stacks of the Golgi apparatus and may represent a communication channel to synchronize stack function within the cell. A feasible hypothesis is that tubules may be a potentially static component of the Golgi apparatus in contrast to the stacked cisternal plates which may turn over continuously. The coated buds associated with tubules may represent the means whereby adjacent Golgi apparatus stacks exchange carbohydrate-processing enzymes or where resident Golgi apparatus proteins are introduced into and out of the stack during membrane flow differentiation. The limited gradation of tubules from cis to medial to trans offers additional possibilities for functional specialization of Golgi apparatus in keeping with the hypothesis that tubules are repositories of resident Golgi apparatus proteins protected from turnover during the flow differentiation of the flattened saccules of the Golgi apparatus stack. Accepted: 3 November 1997     

Immunocytochemical evidence for the involvement of golgi apparatus in the deposition of seed lectin ofBauhinia purpurea (Leguminosae)

Summary The seed lectin of the tree legume,Bauhinia purpurea alba, was localized by electron microscopic immunocytochemistry. The pattern of lectin deposition and site of intracellular localization was examined in mid- to late-maturation seeds. The seed tissue was embedded in Lowicryl K4M, the use of which with seed tissues is discussed. Immunocytochemical labeling was accomplished with colloidal gold coupled to a second antibody. The immunocytochemical reaction was specific and sensitive. Protein bodies, Golgi apparatus and Golgi secretion vesicles were densely labeled. Golgi apparatus was oriented such that Golgi secretion vesicles were in close proximity to the protein bodies. The entire Golgi apparatus was labeled with no concentration gradient across the Golgi stack. These observations indicate that the final site of lectin deposition is the protein body, and that the Golgi apparatus plays an essential role in the deposition process.     

OBSERVATIONS ON THE RELATIONSHIP OF THE GOLGI APPARATUS TO WALL FORMATION IN THE MARINE CHRYSOPHYCEAN ALGA, PLEUROCHRYSIS SCHERFFELII PRINGSHEIM

The role of the Golgi apparatus in wall formation of vegetative cells of a marine chrysophyte, Pleurochrysis scherffelii, is described. Wall fragments are synthesized within the cisternae of the Golgi apparatus. A single Golgi apparatus is always located at the cell periphery, and the distended cisternae are oriented toward the cell surface. A highly-ordered body found near the inflated cisternae is associated with spherical, membrane-bounded bodies which may be involved in the progressive degeneration of cisternal membranes which release wall fragments. Protoplast movement has been detected by time-lapse cinephotomicrography and is correlated at the ultrastructural level with change in positions of the Golgi cisternae. Wall-synthesizing capacity is greatest during transverse wall formation. Senescent cells lack a Golgi apparatus with inflated cisternae. In addition, wall fragments are not present in the Golgi cisternae at this stage. Zoosporogenesis results in a temporary loss of the wall-forming capacity of the Golgi apparatus; this activity then resumes with the formation of a different morphological entity, the scale. Preliminary quantitative measurements of the turnover capacity of the Golgi apparatus have been made. From these data it has been determined that between 41 and 82 Golgi generations are required to synthesize the cell wall of an actively growing cell; this estimate indicates that approximately one cisterna is produced every 2 min, provided the cell generation time is 3 days. The time-lapse cinephotomicrographic data confirm that the rate of production of Golgi cisternae is at least one cisterna every 2 min.     

Identification and characterization of AtCASP,a plant transmembrane Golgi matrix protein

Golgins are a family of coiled-coil proteins that are associated with the Golgi apparatus. They are necessary for tethering events in membrane fusion and may act as structural support for Golgi cisternae. Here we report on the identification of an Arabidopsis golgin which is a homologue of CASP, a known transmembrane mammalian and yeast golgin. Similar to its homologues, the plant CASP contains a long N-terminal coiled-coil region protruding into the cytosol and a C-terminal transmembrane domain with amino acid residues which are highly conserved across species. Through fluorescent protein tagging experiments, we show that plant CASP localizes at the plant Golgi apparatus and that the C-terminus of this protein is sufficient for its localization, as has been shown for its mammalian counterpart. In addition, we demonstrate that the plant CASP is able to localize at the mammalian Golgi apparatus. However, mutagenesis of a conserved tyrosine in the transmembrane domain revealed that it is necessary for ER export and Golgi localization of the Arabidopsis CASP in mammalian cells, but is not required for its correct localization in plant cells. These data suggest that mammalian and plant cells have different mechanisms for concentrating CASP in the Golgi apparatus.†These authors have contributed equally to the work     

MG160, a Membrane Protein of the Golgi Apparatus Which Is Homologous to a Fibroblast Growth Factor Receptor and to a Ligand for E-selectin, Is Found Only in the Golgi Apparatus and Appears Early in Chicken Embryo Development

While over 20 intrinsic proteins of the Golgi apparatus have been identified and sequenced, there is no information on their developmental history, i.e., whether all Golgi proteins are expressed simultaneously or whether there is a hierarchical or stage-specific order of their expression during embryonic development. In this study we have examined the emergence and distribution of MG160 during the development of chicken embryos. MG160 is a conserved membrane sialoglycoprotein of the Golgi apparatus of most cells displaying over 90% amino acid sequence identities with two apparently unrelated molecules, namely CFR, a chicken fibroblast growth factor receptor, and ESL-1, a ligand for E-selectin (Gonatas et al., J. Biol. Chem. 1989, 264, 646-653; Burrus and Olwin, J. Biol. Chem. 1989, 264, 18647-18653; Burrus et al., Mol. Cell Biol. 1992, 12, 5600-5609; Gonatas et al., J. Cell Sci. 108, 457-467; Steegmaier et al., Nature 1995, 373, 615-620). This study was carried out by in situ hybridization, using a 56-mer antisense probe for the chicken homologue of MG160 which differs only by four bases from the corresponding segment of the rat cDNA and by immunocytochemistry and Western blotting using a polyclonal antiserum against MG160. The protein was ubiquitously and exclusively localized in the Golgi apparatus and appeared early in development within the ectoblast and primitive endoblast prior to the formation of the primitive streak. At 2 to 3 days, MG160 was particularly prominent in the notochord, neural tube, somites, and cartilage cells. In organs with central lumens, such as the neural tube, the Golgi apparatus, visualized by immunostaining for MG160, was elongated and it was located at the apical pole of cells. In 6-day-old embryos, the ongoing physiologic degeneration of the notochord was accompanied by fragmentation of the immunostained Golgi apparatus and decreased labeling of the mRNA for MG160. In order to gain information on possible interactions between MG160 and basic fibroblast growth factor (bFGF), the localization of both molecules was studied by immunocytochemistry in 3-day-old chicken embryos. While MG160 was ubiquitous in the Golgi apparatus of all cells and tissues, endogenous bFGF was not detected while exogenous bFGF bound only to basement membranes. These results indicate that MG160 is a primordial protein of the Golgi apparatus and are consistent with the hypothesis that the binding of MG160 to fibroblast growth factors and E-selectin is not related to the still unknown principal function of MG160 in the Golgi apparatus.     

The Golgi apparatus of the scaly green flagellate Scherffelia dubia: uncoupling of glycoprotein and polysaccharide synthesis during flagellar regeneration

The flagella of the green alga Scherffelia dubia are covered by scales which consist of acidic polysaccharides and glycoproteins. Experimental deflagellation results in the regeneration of flagella complete with scales. During flagellar regeneration, scales are newly synthesized in the Golgi apparatus, exocytosed and deposited on the growing flagella. Flagellar regeneration is dependent upon protein synthesis and N-glycosylation, as it is blocked by cycloheximide and partially inhibited by tunicamycin. Metabolic labeling with [³⁵S]methionine/cysteine demonstrated that scale-associated proteins were not newly synthesized during flagellar regeneration, suggesting that the proteins deposited on regenerating flagella were drawn from a pool. Quantitative immunoelectron microscopy using a monospecific antibody directed against a scale-associated protein of 126 kDa (SAP126) revealed that the pool of SAP126 was primarily located at the plasma membrane, with minor labeling of the scale reticulum and trans-Golgi cisternae, both before deflagellation and during flagellar regeneration. Since SAP126 was sequestered during flagellar regeneration into secretory vesicles together with newly synthesized scales, it is concluded that the persistent presence of SAP126 in the trans-Golgi cisternae during scale biogenesis requires retrograde transport of the protein from the plasma membrane to the Golgi apparatus. Received: 3 July 1999 / Accepted: 21 August 1999     

The Golgi apparatus in the endomembrane-rich gastric parietal cells exist as functional stable mini-stacks dispersed throughout the cytoplasm

Background information. Acid‐secreting gastric parietal cells are polarized epithelial cells that harbour highly abundant and specialized, H⁺, K⁺ ATPase‐containing, tubulovesicular membranes in the apical cytoplasm. The Golgi apparatus has been implicated in the biogenesis of the tubulovesicular membranes; however, an unanswered question is how a typical Golgi organization could regulate normal membrane transport within the membrane‐dense cytoplasm of parietal cells. Results. Here, we demonstrate that the Golgi apparatus of parietal cells is not the typical juxta‐nuclear ribbon of stacks, but rather individual Golgi units are scattered throughout the cytoplasm. The Golgi membrane structures labelled with markers of both cis‐ and trans‐Golgi membrane, indicating the presence of intact Golgi stacks. The parietal cell Golgi stacks were closely aligned with the microtubule network and were shown to participate in both anterograde and retrograde transport pathways. Dispersed Golgi stacks were also observed in parietal cells from H⁺, K⁺ ATPase‐deficient mice that lack tubulovesicular membranes. Conclusions. These results indicate that the unusual organization of individual Golgi stacks dispersed throughout the cytoplasm of these terminally differentiated cells is likely to be a developmentally regulated event.     

Transport of rosettes from the golgi apparatus to the plasma membrane in isolated mesophyll cells ofZinnia elegans during differentiation to tracheary elements in suspension culture

Summary Rosettes of six particles have been visualized by freeze-fracture in the protoplasmic fracture (PF) faces of: a) the plasma membrane, b) Golgi cisternae, and c) Golgi-derived vesicles in mesophyll cells ofZinnia elegans that had been induced to differentiate synchronously into tracheary elements in suspension culture. These rosettes have been observed previously in the PF face of the plasma membranes of a variety of cellulose-synthesizing cells and are thought to be important in cellulose synthesis. InZinnia tracheary elements, the rosettes are localized in the membrane over regions of secondary wall thickening and are absent between thickenings. The observation of rosettes in the Golgi cisternae and vesicles suggests that the Golgi apparatus is responsible for the selective transport and exocytosis of rosettes in higher plants, as has been previously indicated in the algaMicrasterias (Giddings et al. 1980). The data presented indicate that the Golgi apparatus has a critical role in the control of cell wall deposition because it is involved not only in the synthesis and export of matrix components but also in the export of an important component of the cellulose synthesizing apparatus. The rosettes are present in the plasma membrane and Golgi vesicles throughout the enlargement of the secondary thickening, suggesting that new rosettes must be continually inserted into the membrane to achieve complete cell wall thickening.Abbreviations EF Golgi vesicles, exoplasmic fracture; the plasma membrane, extracellular fracture - PF protoplasmic fracture     

Targeting of proteins to the Golgi apparatus

The Golgi apparatus maintains a highly organized structure in spite of the intense membrane traffic which flows into and out of this organelle. Resident Golgi proteins must have localization signals to ensure that they are targeted to the correct Golgi compartment and not swept further along the secretory pathway. There are a number of distinct groups of Golgi membrane proteins, including glycosyltransferases, recyclingtrans-Golgi network proteins, peripheral membrane proteins, receptors and viral glycoproteins. Recent studies indicate that there are a number of different Golgi localization signals and mechanisms for retaining proteins to the Golgi apparatus. This review focuses on the current knowledge in this field.     

The Yeast Golgi Apparatus

The Golgi apparatus is an organelle that has been extensively studied in the model eukaryote, yeast. Its morphology varies among yeast species; the Golgi exists as a system of dispersed cisternae in the case of the budding yeast Saccharomyces cerevisiae, whereas the Golgi cisternae in Pichia pastoris and Schizosaccharomyces pombe are organized into stacks. In spite of the different organization, the mechanism of trafficking through the Golgi apparatus is believed to be similar, involving cisternal maturation, in which the resident Golgi proteins are transported backwards while secretory cargo proteins can stay in the cisternae. Questions remain regarding the organization of the yeast Golgi, the regulatory mechanisms that underlie cisternal maturation of the Golgi and transport machinery of cargo proteins through this organelle. Studies using different yeast species have provided hints to these mechanisms.      

Sulfated glycosaminoglycans in guinea pig basophils studied by means of cationic colloidal gold

 Bone marrow embedding in the hydrophilic resin, Lowicryl K4M, followed by cationic colloidal gold (CCG, pH 1.0) staining was used to study the sulfated glycosaminoglycans (GAGs) and their sites of sulfation ultrastructurally in various maturational stages of both basophil granulocytes and basophil granules in the guinea pig. CCG at pH 1.0 is specific for sulfated GAG staining. Basophil granulocytes and granules reacted positively to CCG with a variety of staining according to the stage of maturation. The formation of basophil granules takes place throughout the myelocyte stage. Early basophil myelocytes contain a large Golgi apparatus with active granulogenesis, while late myelocytes contain a small and less active Golgi apparatus as judged by CCG staining. All the immature granules and some of the granules with characteristic ultrastructure stained positively. However, some of the mature granules had lost their affinity for CCG upon maturation. Interestingly, strongly positive CCG staining was also observed in the trans to transmost Golgi apparatus. This indicates that sulfation of GAGs occurs in the trans to transmost Golgi apparatus in all maturational stages of basophil granulocytes. Treatment with chondroitinase ABC or heparinase I abolished the majority of CCG staining. Accepted: 17 July 1997     

Fractionation of suspension cultures of wild carrot and kinetics of membrane labeling

Summary Wild carrot (Daucus carota L.) cells, grown in suspension culture, were labeled with radioactive precursors and fractionated into constituent membranes to be analyzed for specific radioactivity. Results show rapid incorporation of [³H] leucine into endoplasmic reticulum (ER)-, Golgi apparatus-, and plasma membrane/tonoplast-enriched fractions. The time lag between incorporation into ER and its appearance in Golgi apparatus or plasma membrane/tonoplast were less than 5 minutes. With an average time of 3–4 minutes for cisternal formation estimated from studies with monensin, and an average of 5 cisternae per dictyosome (total transit time of 15–20 minutes), it was not possible to account for early incorporation of radioactivity into plasma membranes by passage of proteins from ER to plasma membrane via the Golgi apparatus. To account for the findings, it would appear that at least some proteins were delivered to the plasma membrane via the first membranes that exited (i.e., mature face vesicles) from the Golgi apparatus post-pulse and that some of these proteins had been translated and inserted into membranes at or near the mature face of the Golgi apparatus.     

Cell-free analysis of Golgi apparatus membrane traffic in rat liver

 Cell-free systems for the analysis of Golgi apparatus membrane traffic rely either on highly purified cell fractions or analysis by specific trafficking markers or both. Our work has employed a cell-free transfer system from rat liver based on purified fractions. Transfer of any constituent present in the donor fraction that can be labeled (protein, phospholipid, neutral lipid, sterol, or glycoconjugate) may be investigated in a manner not requiring a processing assay. Transition vesicles were purified and Golgi apparatus cisternae were subfractionated by means of preparative free-flow electrophoresis. Using these transition vesicles and Golgi apparatus subfractions, transfer between transitional endoplasmic reticulum and cis Golgi apparatus was investigated and the process subdivided into vesicle formation and vesicle fusion steps. In liver, vesicle formation exhibited both ATP-independent and ATP-dependent components whereas vesicle fusion was ATP-independent. The ATP-dependent component of transfer was donor and acceptor specific and appeared to be largely unidirectional, i.e., ATP-dependent retrograde (cis Golgi apparatus to transitional endoplasmic reticulum) traffic was not observed. ATP-dependent transfer in the liver system and coatomer-driven ATP-independent transfer in more refined yeast and cultured cell systems are compared and discussed in regard to the liver system. A model mechanism developed for ATP-dependent budding is proposed where a retinol-stimulated and brefeldin A-inhibited NADH protein disulfide oxidoreductase (NADH oxidase) with protein disulfide-thiol interchange activity and an ATP-requiring protein capable of driving physical membrane displacement are involved. It has been suggested that this mechanism drives both the cell enlargement and the vesicle budding that may be associated with the dynamic flow of membranes along the endoplasmic reticulum-vesicle-Golgi apparatus-plasma membrane pathway. Accepted: 26 January 1998     

Osmium Zinc Iodide staining of Golgi elements in oocytes of Triturus cristatus

Summary Developing oocytes of the newt Triturus cristatus were studied in order to clarify the role played by the Golgi apparatus in the formation of yolk. The cytochemical method used for this purpose was that of Maillet (1968) which employs an Osmium Zinc Iodide (OZI) complex.Previtellogenic oocytes reveal a pattern of OZI staining only after hormonal (HCG) stimulation, following which both the Golgi apparatus and the multivesicular bodies are stained.Vitellogenic oocytes taken from non-hormonally stimulated females reveal OZI deposits in a number of vesicles peripheral to the Golgi apparatus as well as within the superficial layer of the forming yolk platelets. Following hormone stimulation, many of the Golgi apparatus located in the central ooplasm of vitellogenic oocytes have all their cisternae blackened by the OZI deposits; other apparatuses, more peripherally located, remain essentially unchanged in their staining pattern. Further, a large number of OZI stained vesicles becomes visible in the vicinity of the Golgi apparatus and within the superficial layer of the forming yolk platelets.The present findings are interpreted as indicating the occurrence of fusion between Golgi derived vesicles and forming yolk platelets. It is also suggested that the vesicles in question function as carriers of Golgi produced enzymes which are presumably required to accomplish the final elaboration of the yolk material.Supported by a grant from the Consiglio Nazionale delle RicercheWe acknowledge the valuable help received from Prof. G. Mancino throughout this investigation     

Relationships between membranous organelles in amoebae studied by electron microscopic cytochemical staining

Summary The intracellular location of a variety of enzymes was studied in Amoeba proteus with the use of electron microscopic cytochemical methods, in an attempt to assess the relationships between different membranous organelles. One group of enzymes, including nucleoside diphosphatases (IDPase, UDPase, GDPase, ADPase), carbamoyl phosphatase, alkaline phosphatase, and BAXD oxidase was localized mainly in the rough endoplasmic reticulum, nuclear envelope, and convex side of the Golgi apparatus. Esterase activity had a similar localization except that the Golgi apparatus was "stained" throughout most of its extent. A second group of enzymes was found in Golgi cisternae and vesicles, and in some vacuoles. This group included acid phosphatase, thiamine pyrophosphatase, and aryl sulfatase. Some enzymes previously detected in cytoplasmic membranes of other cells, including glucose-6-phosphatase, showed little or no activity in amoebae. The results suggest that there are chemical similarities and probable functional relationships between the rough endoplasmic reticulum, the nuclear envelope, and the convex side of the Golgi apparatus. On the other hand, the concave pole of the Golgi apparatus, aggregates of smooth tubules and vesicles, and the cell surface appear more closely related to one another than to the endoplasmic reticulum and the convex side of the Golgi apparatus. The cytochemical similarity between the Golgi apparatus and certain vacuoles such as food vacuoles may reflect the role of the Golgi apparatus in the formation of lysosomes. The locations of reaction products of the various enzymes in amoebae are compared with observations reported for other cell types.Supported by a research grant (VC-169) from the American Cancer SocietyThe author is indebted for technical assistance to Mrs. Sue Thompson and Mrs. Christine Folsom-Kovarik     

Distribution and morphological changes of the Golgi apparatus during Drosophila spermatogenesis

In spermatogenesis, the Golgi apparatus is important for the formation of the acrosome, which is a sperm‐specific organelle essential for fertilization. Comprehensive examinations of the spatiotemporal distribution and morphological characterizations of the Golgi in various cells during spermatogenesis are necessary for functional analyses and mutant screenings in the model eukaryote Drosophila. Here, we examined the distribution and morphology of the Golgi during Drosophila spermatogenesis with immunofluorescence and electron microscopy. In pre‐meiotic germ cells, the Golgi apparatuses were distributed evenly in the cytoplasm. In contrast, they were located exclusively in two regions near the poles during the meiotic metaphase, where they were segregated prior to the chromosomes. In cells in anaphase to telophase, the Golgi were predominantly left behind in the equatorial region between the separating daughter nuclei. After completion of meiosis, the dispersed Golgi were assembled at the apical side of the spermatid nucleus to form the acrosome. Further investigation of the Golgi distribution in β2‐tubulin mutants showed aberrant and uneven distributions of the Golgi among sister cells in the meiotic spermatocytes and in the post‐meiotic spermatids. At the ultrastructural level, the Golgi apparatus in pre‐meiotic spermatocytes comprised a pair of stacks. The two stacks were situated adjacent to each other, as if they had duplicated before entering into meiotic division. These results highlight the dynamic nature of the Golgi during spermatogenesis and provide a framework for analyzing the correlations between the dynamics of the Golgi and its function in sperm development.     

The ceramide-enriched <Emphasis Type="Italic">trans</Emphasis>-Golgi compartments reorganize together with other parts of the Golgi apparatus in response to ATP-depletion

In this study, the ceramide-enriched trans-Golgi compartments representing sites of synthesis of sphingomyelin and higher organized lipids were visualized in control and ATP-depleted hepatoma and endothelial cells using internalization of BODIPY-ceramide and the diaminobenzidine photooxidation method for combined light-electron microscopical exploration. Metabolic stress induced by lowering the cellular ATP-levels leads to reorganizations of the Golgi apparatus and the appearance of tubulo-glomerular bodies and networks. The results obtained with three different protocols, in which BODIPY-ceramide either was applied prior to, concomitantly with, or after ATP-depletion, revealed that the ceramide-enriched compartments reorganize together with other parts of the Golgi apparatus under these conditions. They were found closely associated with and integrated in the tubulo-glomerular bodies formed in response to ATP-depletion. This is in line with the changes of the staining patterns obtained with the Helix pomatia lectin and the GM130 and TGN46 immuno-reactions occurring in response to ATP-depletion and is confirmed by 3D electron tomography. The 3D reconstructions underlined the glomerular character of the reorganized Golgi apparatus and demonstrated continuities of ceramide positive and negative parts. Most interestingly, BODIPY-ceramide becomes concentrated in compartments of the tubulo-glomerular Golgi bodies, even though the reorganization took place before BODIPY-ceramide administration. This indicates maintained functionalities although the regular Golgi stack organization is abolished; the results provide novel insights into Golgi structure–function relationships, which might be relevant for cells affected by metabolic stress.     

Elektronenmikroskopische Untersuchungen von Differenzierungsvorgängen bei Moosen

Summary In meristematic cells of the gemma of Riella helicophylla and in young bud cells from the protonema of Funaria hygrometrica the cell plate is formed by fusion of small vesicles originating from the Golgi apparatus. These spherical vesicles of about 0.1 m diameter have an electron dense centre, probably consisting of pectic substances or their precursors. The endoplasmic reticulum producing multivesicular bodies participate in cell plate formation too. Another cytoplasmic component forming the cell plate are coated vesicles, the origin of which is the Golgi apparatus and perhaps also the endoplasmic reticulum. In view of these observations the question of whether the endoplasmic reticulum or the Golgi apparatus forms the cell plate must be answered in this way: both endoplasmic reticulum and Golgi apparatus supply material for growth of the cell plate. Multivesicular bodies, coated vesicles and other small vesicles of unknown nature participate in the formation of the primary wall.

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Zum Teil finanziert mit Sondermitteln des Landes Niedersachsen an Prof. Dr. M. Bopp.     

Origin of the Golgi system in amoebae

Summary An electron microscopic study of the Golgi apparatus in the giant amoeba, Pelomyxa illinoisensis, has been presented. Studies of normally feeding, dividing, starving, and refeeding amoebae were made. The major finding is that plasmalemma vesicles, formed via pinocytosis and phagocytosis, either flatten or invaginate and form the cisternae of the Golgi apparatus. Plasmalemma vesicles are also a source of new cisternae during the lifetime of a given Golgi apparatus. The cisternae migrate through the Golgi system, but before being released they either inflate, or segment into smaller vesicles. It is postulated that they later empty into the contractile vacuole and into certain other vacuoles. No evidence was found for the fusion of smooth Golgi vesicles or fringed vesicles of any kind with the plasmalemma.Dedicated to Professor Friedrich Wassermann with admiration and affection on the occasion of his eightieth birthday.Work supported by U. S. Atomic Energy Commission. A part of the work was reported at the XVI International Congress of Zoology, Washington, D. C., in 1963.     

An Ultrastructural Study of Mitosis in a Marine Dinoflagellate: Prorocentrum minimum1

An ultrastructural study on mitosis in the marine dinoflagellate Prorocentrum minimum is described. Early in mitosis the nuclear membrane invaginates in the area of the Golgi apparatus. Additional membrane-lined channels form within the nucleus as the Golgi apparatus separates and moves toward opposite spindle poles. Microtubules appear within the channels and make contact with distinct kinetochore-like structures on the cytoplasmic side of the channels adjacent to the site of chromosome attachment. By mid-mitosis two or three parallel channels dissect the nucleus perpendicular to the suture plane of the cell. Chromosome separation thus occurs perpendicular to the suture plane. An additional group of microtubules extends posteriorly from the flagellar apparatus towards the nucleus but has no apparent role in mitosis. Mitosis in P. minimum is compared to that of P. micans and to other dinoflagellates.