Fine structural and cytochemical studies on the hamster subcommissural organ

The subcommissural organ (SCO) of the golden hamster (Mesocricetus auratus) was studied by conventional electron microscopy, freeze-fracture technique, zinc-iodide-osmium (ZIO) and acid phosphatase cytochemical reactions. The ultrastructure of hamster SCO cells shows a few flattened cisternae of rough endoplasmic reticulum (ER) without dilated ones in the cytoplasm. The Golgi apparatus is very well developed. Freeze-fracture studies also indicate only short profiles of flattened ER in the cytoplasm endorsing the absence of dilated ER cisternae. After the treatment with ZIO mixture, reaction products were observed over flattened cisternae of the ER and the nuclear envelope. The Golgi apparatus was also reactive toward the ZIO mixture. Acid phosphatase activities are localized in the inner one or two saccules of the Golgi apparatus and dense bodies. From these results we suggest that (1) hamster SCO cells do not accumulate secretory material in the cytoplasm in the form of discrete secretory granules or dilated cisternae of ER, and (2) hamster SCO cells may possess extremely high secretory activity or may not be actively involved in secretory function at all as in rats or other rodents.     

Electron microscopic and cytochemical studies of the mouse subcommissural organ

Summary In mice most of the ependymal cells of the subcommissural organ (SCO cells) are densely packed with dilated cisternae of the endoplasmic reticulum (ER) containing either finely granular or flocculent materials. The well developed supra-nuclear Golgi apparatus consists of stacks of flattened saccules and small vesicles; the two or three outer Golgi saccules are moderately dilated and exhibit numerous fenestrations; occasional profiles suggesting the budding of coated vesicles and formation of membrane-bound dense bodies from the ends of the innermost Golgi saccules are seen. A few coated vesicles and membrane-bound dense bodies of various sizes and shapes are also found in the Golgi region.The contents of the dilated ER cisternae are stained with periodic acid-silver methenamine techniques. In the Golgi complex the two or three inner saccules are stained as deeply as the dense bodies, and the outer saccules are only slightly stained. The stained contents of ER cisternae are more electron opaque than those of the outer but less opaque than those of the inner Golgi saccules and the dense bodies.Acid phosphatase activities are localized in the dense bodies, some of the coated vesicles in the Golgi region, and in the one or two inner Golgi saccules.On the basis of these results the following conclusions have been reached: (1) In mouse SCO cells the finely granular and the flocculent materials in the lumen of ER cisternae contain a complex carbohydrate(s) which is secreted into the ventricle to form Reissner's fiber; (2) the secretory substance is assumed to be synthesized by the ER and stored in its cisternae, and the Golgi apparatus might play only a minor role, if any, in the elaboration of the secretory material; (3) most of the dense bodies in the mouse SCO cells are lysosomal in nature instead of being so-called dark secretory granules.Sponsored by the National Science Council, Republic of China.     

Osmium impregnation of the endoplasmic reticulum correlates with the functional status of prostatic secretory cells

The ultrastructure of prostatic secretory cells was studied with the osmium impregnation technique in order to determine if the ER reactivity, or its absence, and its three-dimensional organization correspond to specific functions possibly hormono-dependent. Thick sections (0.3 micron) of rat ventral prostate were made after a five-day impregnation with osmium tetroxide and examined by standard transmission electron microscopy at 80 kV. Studies were performed in normal adult rats, between the 3rd and 26th day following castration and in castrated rats treated with 5-alpha-dihydrotestosterone. In normal rats the impregnation technique delineated three secretory cell types (dark, greyish and clear), representing various degrees of reactivity in ER cisternae; however, despite this quantitative variation, they had similar morphological characteristics. In a longitudinal section, the ER network appeared to be made of saccules running parallel along the length of the cell and forming whorl-like patterns around the nucleus. Comparison of sections taken at various angles suggests that the ER network is made of concentric parallel saccules extending from the base to the apex of the cell and encircling the nucleus and the Golgi apparatus like a large multilayered cylinder. Whereas in dark cells the Golgi apparatus contained mostly clear vesicles, it was always heavily impregnated in clear cells. Noteworthy, osmium deposits were rarely observed on the nuclear envelope of secretory cells but were always present in basal cells. After castration, secretory cells became progressively cubic and the most conspicuous cytoplasmic change was observed in association with the ER. The Golgi apparatus decreased markedly in volume and became heavily stained with metallic osmium.(ABSTRACT TRUNCATED AT 250 WORDS)     

The structure of the Golgi apparatus: a sperm’s eye view in principal epithelial cells of the rat epididymis

 The Golgi apparatus of epididymal principal cells shares many structural features with other cell types. Saccular regions are arranged in a cis-Golgi network, eight flattened saccules, and several trans-Golgi networks (TGNs). Dilated tubules form intersaccular connecting regions which joint together saccules at the same or different levels between adjacent stacks. Wells exist as large perforations in register with the four cis-most saccules and serve as areas of vesicular interactions. TGNs are variable and can appear to peel off the stack or to be detached from it in the form of an anastomotic tubular network with pale dilated areas corresponding to prosecretory granules connected by short narrow bridges. Elongated or discoid dilated cisternae of endoplasmic reticulum (ER) (sparsely granulated) lie over the cis face of the stack, from which they are separated by an intermediate compartment filled with vesicles and tubules. The ER is also closely juxtaposed to the TGNs and the eighth saccule but interconnections are never seen between them. Vesicles of the COP variety reside at all levels of the stack and appear to bud off the cis-located ER and the edges of the saccules, while clathrin-coated vesicles appear mainly on the trans face of the stack and next to lysosomes. In the supranuclear cytoplasm, clusters of vesicles and tubules, at times budding off enveloping ER, appear to radiate toward the Golgi stacks where they fuse with cis Golgi elements. Taken together, these observations suggest dynamic functions and interactions for the various Golgi elements, associated vesicles, ER, and vesicular tubular clusters. Accepted: 29 January 1998     

Formation of secretion granules in the Golgi apparatus of plasma cells in the rat

The three-dimensional structure of the components of the Golgi apparatus was analyzed in plasma cells of rat duodenum. The spheroidal juxtanuclear Golgi apparatus was formed by a continuous ribbonlike structure composed of the following stacked elements. On the cis-face of the Golgi stack, there was a tubular membranous network referred to as the cis-element and/or a slightly dilated saccule perforated with small pores. The two or three subjacent saccules, which showed few pores, were slightly dilated and contained a fluffy granulofilamentous material. They were also perforated in register by cavities or wells containing 80-nm vesicles. The next one or two underlying elements were fenestrated saccules showing flattened portions as well as distended portions containing a homogeneous material denser than that seen in the overlying saccules. The last two or three elements of the stack showed a partially separated or "peeling off" configuration. These last elements consisted of prosecretory granules attached to flattened, empty-looking saccules showing buds at their surface; detached, more-or-less fenestrated, flattened saccules; and shrivelled residual trans-tubular networks. In the trans-region of the stack, in addition to numerous small vesicles, short membranous tubules, detached prosecretory granules, and denser fully formed secretion granules were also seen. These images were interpreted to indicate that secretory material present in the trans-saccules flows toward the dilated portions which become prosecretory granules. The trans-most elements seemingly peel off the stack to yield prosecretory granules and fragmenting trans-tubular networks.     

Connections between dictyosomes,ER and GERL in cotyledons of mung bean (Vigna radiata L.)

Summary The connections and structural inter-relations of dictyosomes and endoplasmic reticulum (ER) in cotyledons of germinating mung beans were studied using thick (0.3 m) sections of aldehyde fixed, zinc iodide-osmium tetroxide (ZIO) impregnated tissue. The sections were examined by conventional (100 kV), rather than high voltage, transmission electron microscopy.Continuity of cisternal ER with tubular ER was confirmed and a direct connection of tubular ER totrans dictyosome cisternae was observed as were GERL networks associated withtrans dictyosome cisternae.Dictyosomes also gave rise to an extensive system of very fine tubules (10–20 nm diam) which have not been described previously in plant tissue. These tubules, which originated at thetrans dictyosome face, extended throughout the cytoplasm and were found connected to cisternal ER and tubular ER.The implications of these observations are discussed with regard to present ideas concerning endomembrane flow and protein sorting by the Golgi apparatus.     

Glucose-6-phosphatase activity of endoplasmic reticulum and Golgi apparatus in spermatocytes and spermatids of the rat: an electron microscopic cytochemical study.

The glucose-6-phosphatase (G6Pase) activity of cytoplasmic components of spermatocytes and spermatids of the rat was examined by electron microscope cytochemistry using cerium chloride as a capture agent. G6Pase activity, a recognized ER-resident enzyme, was present in all ER cisternae of spermatocytes. In spermatids, while some ER cisternae were G6Pase-reactive, others were negative or only slightly reactive, indicating an unequal distribution of the enzymatic activity throughout the network of ER cisternae in these cells. In spermatocytes, the cis- and trans-elements of the stacks of Golgi saccules were slightly but significantly reactive for G6Pase. In the Golgi apparatus of spermatids, the cis-element, 4 or 5 underlying saccules, as well as one or two thick trans Golgi elements were G6Pase reactive. The G6Pase activity of the various Golgi elements, like that of the ER cisternae was not affected by the pH of the medium and was completely inhibited by Na-vanadate, a known G6Pase inhibitor. Sertoli and Leydig cells, submitted to the same cytochemical conditions, showed complete G6Pase reactivity of their ER; however in Sertoli cells, all Golgi components were consistently negative while in Leydig cells the cis- and trans-elements of the Golgi stacks were slightly reactive, as in spermatocytes. Thus, the G6Pase reactivity of Golgi elements, appeared variable from one cell type to another. The compact juxtanuclear Golgi apparatuses of spermatocytes and spermatids were both associated with numerous G6Pase reactive ER cisternae; some were present at their surface, others crossed their cortices between Golgi stacks and formed elaborate networks in their cores.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tridimensional structure of the Golgi apparatus of nonciliated epithelial cells of the ductuli efferentes in rat: an electron microscope stereoscopic study

The 3-dimensional structure of the Golgi apparatus has been analyzed in thin and thick sections of nonciliated epithelial cells of ductuli efferentes of rat by use of low- and high-voltage electron microscopes and a stereoscopic approach. In thick sections of tissue impregnated with osmium, the Golgi apparatus appeared at low magnification as a continuous network forming a corona at the apical pole of the nucleus. At higher magnification and in thin sections of tissue postfixed with reduced osmium and stained with lead citrate or treated to demonstrate phosphatase activity, the following structural features were observed. In the longitudinal axis of the Golgi network there were alternating compact and noncompact zones. The compact zones were composed of 6-8 flattened, poorly fenestrated saccules in close apposition to each other and forming stacks. The noncompact zones were composed of a number of highly fenestrated and slightly distended saccules, which were continuous with and bridged the saccules of the compact zones. In the cis-trans axis of the Golgi apparatus the following compartments were observed: (a) On the cis face there was a continuous osmiophilic tubular network referred to as the cis element; (b) a cis compartment composed of 3 or 4 NADPase-positive saccules perforated with pores in register forming wells that contained small vesicles; (c) a trans compartment composed of 1 or 2 TPPAse-positive elements underlying the NADPase ones, followed by 1 or 2 CMPase-positive elements that showed a flattened saccular part continuous with a network of anastomotic tubules. These tubular networks curved away from the overlying elements, giving these elements a 'peeling-off" configuration. These elements referred to as sacculotubular elements were discontinuous along the Golgi network. This compartment also included shriveled trans-tubular networks detached from the overlying sacculotubular elements and seemingly undergoing fragmentation into vesicles and tubules. The structural features of the elements of the trans compartment were indicative of continuous renewal.     

Interrelationships between Golgi, GERL and synaptic vesicles in the nerve cells of insect and gastropod ganglia.

In addition to demonstrating synaptic vesicles, staining with the zinc-iodide-osmium tetroxide (ZIO) method reveals the presence of positively reacting GERL membranes in association with the Golgi complex and lysosomes in the nerve cell bodies within ganglia from the locust Schistocerca gregaria and the gastropod molluscs, Limnaea stagnalis and Helix aspersa. A positive response to ZIO occurs in certain Golgi vesicles and saccules, in GERL (Golgi-endoplasmic-reticulum-lysosomes), in multivesicular bodies as well as residual bodies and in small vesicles and cisternae of axonal smooth endoplasmic reticllum (ER). The interrelationships between these organelles are considered in view of the similarity of the ZIO localization to phosphatase-rich sites in the neuronal perikarya and with respect to the possibility that components of the synaptic vesicles are formed in the Golgi region of the cell and migrate via the axonal smooth ER to the synaptic regions.     

Morphological study of cell organelles during development. I - The nuclear sac and the endoplasmic reticulum on the rat nephron

The maturation of the endoplasmic reticulum (ER) of rat kidney tubule cells was studied with an osmium impregnation technique. Thick sections (0.3-0.6 micron) of kidney tissue were made after a five-day impregnation with osmium tetroxide and examined by standard transmission electron microscopy at 80-100 kV. Studies were performed on rat foetuses from 18-21 days of gestation, on newborns, and on 2-20 day old animals. At the undifferentiated stage, only a small percentage of the tubule cells were impregnated; in these, the perinuclear sac was stained and a few nuclear pores were already seen. Rudimentary, but thick canalicular projections seemed to originate from the perinuclear sac and become more extensive with maturity. Flattened saccules appeared later and fenestrations were seen in proximal tubule cells only when they seemed to have reached their functional specialization. In some cells, only the Golgi apparatus was stained. In the distal tubule cells, there was also progressive formation of a network consisting first of canaliculi and later of saccules which were rarely fenestrated. The osmium impregnation technique appears to be useful as an index of the ER organization development.     

Virus-neuron interactions in the mouse brain infected with Japanese encephalitis virus

The virus-host interactions between Japanese encephalitis (JE) virus and mouse brain neurons were analyzed by electron microscopy. JE virus replicated exclusively in the rough endoplasmic reticulum (RER) of neurons. In the early phase of infection, the perikaryon of infected neurons had relatively normal-looking lamellar RER whose cisternae showed focal dilations containing progeny virions and characteristic endoplasmic reticulum (ER) vesicles. The reticular RER, consisted of rows of ribosomes surrounding irregular-shaped, membrane-unbounded cisternae and resembled that observed in JE-virus-infected PC12 cells, were also seen adjacent to the lamellar RER. The appearance of the reticular RER indicated that RER morphogenesis occurred in infected neurons in association with the viral replication. The fine network of Golgi apparatus was extensively obliterated by fragmentation and dissolution of the Golgi membranes and their replacement by the electron-lucent material. As the infection progressed, the lamellar RER was increasingly replaced by the hypertrophic RER which had diffusely dilated cisternae containing multiple progeny virions and ER vesicles. The Golgi apparatus, at this stage, was seen as coarse, localized Golgi complexes near the hypertrophic RER. In the later phase of infection, RER of infected neurons showed a degenerative change, with the cystically dilated cisternae being filled with ER vesicles and virions. Small, localized Golgi complexes frequently showed vesiculation, vacuolation, and dispersion. The present study, therefore, indicated that during the viral replication the normal lamellar RER which synthesized neuronal secretory and membrane proteins was replaced by the hypertrophic RER which synthesized the viral proteins. The hypertrophic RER eventually degenerated into cystic RER whose cisternae were filled with viral products. The constant degenerative change which occurred in the Golgi apparatus during the viral replication suggested that some of the viral proteins transported from RER to the Golgi apparatus were harmful to the Golgi apparatus and that increasing damage to the Golgi apparatus during the viral replication played the principal role in the pathogenesis of JE-virus-infected neurons in the central nervous system.     

Involvement of endoplasmic reticulum in the origins of dictyosome-like structures of guinea pig spermatocytes

Summary Dictyosome-like structures (DLS) are formed in early spermatocytes first as single saccules. These saccules occur in association with forms of endoplasmic reticulum (ER) characterized by a paucity of ribosomes and luminal content, by a constriction of the lumina, and by a tendency to fragment or form myelin figures during fixation. Nascent DLS and the unusual ER cisternae share many characteristics in common including a pattern of staining with fixatives containing tannic acid where the membranes appear thin due to the inner membrane leaflet being unstained or poorly stained. DLS also appear to form in the region conventional Golgi apparatus but always in association with ER forms that frequently occupy portions of the Golgi apparatus zone.An ability to stain with phosphotungstic acid at low pH exhibited by DLS is given also by the specialized ER forms. One possibility for DLS formation suggested by the present study is that DLS cisternae differentiate from ER membranes after which they ultimately associate into the stacked configurations that characterize mature DLS.     

Pre- and post-Golgi vacuoles operate in the transport of Semliki Forest virus membrane glycoproteins to the cell surface

The effect of reduced temperature on synchronized transport of SFV membrane proteins from the ER via the Golgi complex to the surface of BHK-21 cells revealed two membrane compartments where transport could be arrested. At 15 degrees C the proteins could leave the ER but failed to enter the Golgi cisternae and accumulated in pre-Golgi vacuolar elements. At 20 degrees C the proteins passed through Golgi stacks but accumulated in trans-Golgi cisternae, vacuoles, and vesicular elements because of a block affecting a distal stage in transport. Both blocks were reversible, allowing study of the synchronous passage of viral membrane proteins through the Golgi complex at high resolution by immunolabeling in electron microscopy. We propose that membrane proteins enter the Golgi stack via tubular extensions of the pre-Golgi vacuolar elements which generate the Golgi cisternae. The proteins pass across the Golgi apparatus following cisternal progression and enter the post-Golgi vacuolar elements to be routed to the cell surface.     

Three-dimensional architecture of the golgi apparatus in Sertoli cells of the rat.

Glutaraldehyde-fixed testes were stained "en bloc" with the Ur-Pb-Cu technique of Thiéry and Rambourg ('76) or post-fixed and stained with the osmium tetroxide-potassium ferrocyanide method of Karnovsky ('71). Thin or thick (up to 3 micron) sections were examined with the Philips (301 or 400) EM or the high voltage EM. Stereopairs were prepared with photographs of tilted specimens (+/- 7 degrees). At low magnification, in thick sections (0.5-3 micron) stained with Ur-Pb-Cu, the whole Golgi apparatus formed a single network of interconnected wavy ribbon or platelike structures extending from the juxtanuclear region toward the apex of the cell. At higher magnifications, with the two staining techniques, this Golgi network showed two distinct types of regions: the "saccular region" corresponding to the conventional stack of saccules and the "intersaccular connecting region" made up of anastomotic tubules which bridge adjacent stacks. In the saccurlar regions, there was, on the cis-face of the stack, a tight polygonal meshwork of anastomotic tubules (osmiophilic element). Underlying it there were three to seven closely apposed saccules perforated with pores of various diameters, and finally, on the trans-face, a network of tubules was usually connected to the last saccule of the stack, which seemed to peel off" from the pile. The intersaccular connecting regions showed proximal and distal zones with regard to the associated stacks. The proximal zone was made up of superimposed and parallel polygonal networks of membranous tubules which were continuous with corresponding saccules of the stack. In the distal zone they interdigitated, intertwined, anastomosed and bridged adjacent saccular regions; others turned at right angles and established connections with tubular extensions arising at various levels of the same stack. While cisternae of endoplasmic reticulum were contiguous with tubules or saccules located on the transface of the Golgi apparatus, a close association between the ER cisternae and the cis-face of the stacks was not usually observed.     

Formation of secretion granules in the Golgi apparatus of pancreatic acinar cells of the rat

The three-dimensional structure of the Golgi apparatus and its components has been analyzed in sections of pancreatic acinar cells by using stereopairs of electron microscope photographs. Pancreatic tissue fixed in glutaraldehyde was postfixed in reduced osmium, and the sections were stained with lead citrate. Tissues were also treated to demonstrate phosphatase activity (i.e., nicotinamide adenine dinucleotide phosphatase, NADPase; thiamine pyrophosphatase, TPPase; cytidine monophosphatase, CMPase). The following stacked components were observed along the branching, anastomotic, continuous, ribbonlike Golgi apparatus. 1) On the cis-face of the Golgi stack there was a tubular membranous network known to be osmiophilic and referred to as the cis-osmiophilic tubular network or cis-element. 2) A first, poorly fenestrated saccule, unreactive for the phosphatases tested, was slightly distended in places and contained a fluffy granulofilamentous material. 3) The subjacent three or four saccules, reactive for NADPase and/or TPPase, showed dilated portions containing a granulofilamentous secretory material similar to that filling the rest of the saccule. They also showed nondilated portions perforated with large fenestrations, some of which were in register and formed wells containing 80-nm vesicles. The dilated portions of these saccules were present at random along the length of the saccules and were not located exclusively at their edges. 4) The remaining one or two elements of the stack, CMPase positive, showed dilated spheroidal portions or prosecretory granules containing a homogeneous secretory material and flattened fenestrated regions free of secretory material and having the appearance of networks of narrow membranous tubules. 5) Lastly on the trans-aspect of the stack there were detached prosecretory granules reactive for CMPase and surrounded by a corona of small vesicles, and smooth-surfaced spherical CMPase-negative granules having a denser content that were identified as fully formed secretion granules; there were also occasional free trans-tubular networks strongly reactive for CMPase that appeared to undergo fragmentation and numerous small vesicles free from acid-phosphatase activity. These various images were interpreted as indicating that prosecretory granules formed in relation to two or three fenestrated saccules on the trans-side of the stack. Such granules, following their detachment from the trans-face of the stack, their separation from trans-tubular networks, and condensation of their content, yielded mature secretion granules.     

Structure of Golgi apparatus

Summary Golgi apparatus (GA) of eukaryotic cells consist of one or more stacks of flattened saccules (cisternae) and an array of fenestrae and tubules continuous with the peripheral edges of the saccules. Golgi apparatus also are characterized by zones of exclusion that surround each stack and by an assortment of vesicles (or vesicle buds) associated with both the stacks and the peripheral tubules of the stack cisternae. Each stack (sometimes referred to as Golgi apparatus, Golgi complex, or dictyosome) is structurally and functionally polarized, reflecting its role as an intermediate between the endoplasmic reticulum, the cell surface, and the lysosomal system of the cell. There is probably only one GA per cell, and all stacks of the GA appear to function synchronously. All Golgi apparatus are involved in the generation and movement of product and membrane within the cell or to the cell exterior, and these functions are often reflected as structural changes across the stacks. For example, in plants, both product and membrane appear to maturate from the cis to the trans poles of the stacks in a sequential, or serial, manner. However, there is also strong ultrastructural evidence in plants for a parallel input to the stack saccules, probably through the peripheral tubules. The same modes of functioning probably also occur in animal GA; although here, the parallel mode of functioning almost surely predominates. In some cells at least, GA stacks give rise to tubular-vesicular structures that resemble the trans Golgi network. Rudimentary GA, consisting of tubular-vesicular networks, have been identified in fungi and may represent an early stage of GA evolution.     

Pentachlorophenol affects mitochondria and induces formation of golgi apparatus-endoplasmic reticulum hybrids in tobacco pollen tubes

Summary The toxic effect of pentachlorophenol (PCP) on the growth and ultrastructure of tobacco pollen tubes was tested using a semivivo technique of tube culture. In this technique the pollen tubes were allowed to grow in the pistilin situ for 24 hr before they protruded from the cut end of the style and came into contact with the medium containing PCP. The inhibitory effect of different PCP concentrations was determined by measuring the length of tube bundles. The intracellular action of PCP was analysed by electron microscopy. This biocide caused four obvious alterations in the pollen tube ultrastructure: (1) swelling of the mitochondrial saccules; (2) enlargement of the dictyosomes by the increase of the cisternal diameter and the number of cisternae per stack; (3) formation of cup-shaped Golgi apparatus-endoplasmic reticulum hybrid structures (GER hybrids) showing continuities of ER and Golgi cisternae; (4) formation of stacked and/or concentric arrangements of rough ER cisternae. It is suggested that swelling of saccules was directly due to the uncoupling of oxidative phosphorylation whereas the changes of the endomembrane system were caused by energy depletion due to the inhibition of ATP synthesis. These changes are consistant with dynamic concepts of dictyosome and ER function when membrane formation exceeds membrane use in the production of secretory and transition vesicles. Thus, the enlargement of the dictyosomes and the formation of GER hybrids are thought to result from inhibition of budding of vesicles from the Golgi apparatus or from both the ER and the Golgi apparatus, respectively.     

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     

Morphology of endoplasmic reticulum and Golgi elements following microinjection of rat liver microsomes into Xenopus laevis oocyte cytoplasm

Fragments of rough endoplasmic reticulum and Golgi complex purified from rat liver homogenates were injected into Xenopus oocytes and the sites of microinjection analysed by electron microscopy at different times post-injection. The in vivo incubated fragments were located by their proximity to a microinjection vacuole, and identified by their association with specific morphological markers (peroxisomal cores associated with rough microsomes and lipoprotein particles with Golgi derivatives). Typical endoplasmic reticulum microsomes disappeared with time post-injection and seemed to be replaced by flattened cisternae of endoplasmic reticulum. Golgi fragments as defined by their content of lipoprotein particles became modified. Many were found associated with coated vesicles and some displayed membrane-coated regions. Furthermore lipoprotein particles were observed as integral components of Golgi stacks and were found within dilated rims in direct continuity with fenestrated Golgi saccules. The results suggest that the injected organelle fragments underwent transformation in vivo as a consequence of reconstitution.     

Effects of low temperature on the formation of secretion granules in the Golgi apparatus of granular cells of the frog urinary bladder.

The formation of secretion granules has been studied in the Golgi apparatus of granular epithelial cells of frog urinary bladders maintained at room temperature or cooled at 4 degrees C for various lengths of time. In control animals, the Golgi apparatus was composed of the following stacked elements: subjacent to the cis-element made up of anastomosed tubules, two elements in the mid-compartment consisted of flattened saccules interconnected by tubules. On the trans-face, two or three sacculo-tubular elements were slightly dilated by an electron dense granular material. In the trans-Golgi elements, this material was segregated into dilatations of various sizes and shapes which are continuous with flattened portions devoid of stained material. In the trans-Golgi region, free irregular progranules, seemingly formed by rupture of the trans-most Golgi elements. In granular cells examined after 4 h at 4 degrees C, all Golgi compartments were affected by the low temperature. The cis-half portion of the Golgi apparatus consisted mainly of anastomosed membranous tubules and the cis-element was no longer recognizable. The trans-compartment was reduced to a few flattened saccules with progranules hardly visible on their trans-aspect. At later time intervals, there was a progressive reconstitution of the cis-zone while saccular elements started to pile up in the trans-compartment. At 24 h, the trans-compartment comprised six to eight saccular elements which showed irregular dilatations filled with granular material separated by large flattened portions. These various observations were interpreted as indicating that the trans-compartment was a dynamic structure undergoing continuous renewal.