Vacuolar Elements Research Articles

The pollen tube clear zone: clues to the mechanism of polarized growth.

Pollen tubes usually exhibit a prominent region at their apex called the "clear zone" because it lacks light refracting amyloplasts. A robust, long clear zone often associates with fast growing pollen tubes, and thus serves as an indicator of pollen tube health. Nevertheless we do not understand how it arises or how it is maintained. Here we review the structure of the clear zone, and attempt to explain the factors that contribute to its formation. While amyloplasts and vacuolar elements are excluded from the clear zone, virtually all other organelles are present including secretory vesicles, mitochondria, Golgi dictyosomes, and the endoplasmic reticulum (ER). Secretory vesicles aggregate into an inverted cone appressed against the apical plasma membrane. ER elements move nearly to the extreme apex, whereas mitochondria and Golgi dictyosomes move less far forward. The cortical actin fringe assumes a central position in the control of clear zone formation and maintenance, given its role in generating cytoplasmic streaming. Other likely factors include the tip-focused calcium gradient, the apical pH gradient, the influx of water, and a host of signaling factors (small G-proteins). We think that the clear zone is an emergent property that depends on the interaction of several factors crucial for polarized growth.

Role of SNX16 in the Dynamics of Tubulo-Cisternal Membrane Domains of Late Endosomes

In this paper, we report that the PX domain-containing protein SNX16, a member of the sorting nexin family, is associated with late endosome membranes. We find that SNX16 is selectively enriched on tubulo-cisternal elements of this membrane system, whose highly dynamic properties and formation depend on intact microtubules. By contrast, SNX16 was not found on vacuolar elements that typically contain LBPA, and thus presumably correspond to multivesicular endosomes. We conclude that SNX16, together with its partner phosphoinositide, define a highly dynamic subset of late endosomal membranes, supporting the notion that late endosomes are organized in distinct morphological and functional regions. Our data also indicate that SNX16 is involved in tubule formation and cholesterol transport as well as trafficking of the tetraspanin CD81, suggesting that the protein plays a role in the regulation of late endosome membrane dynamics.

Myosin V orientates the mitotic spindle in yeast.

Coordination of spindle orientation with the axis of cell division is an essential process in all eukaryotes. In addition to ensuring accurate chromosomal segregation, proper spindle orientation also establishes differential cell fates and proper morphogenesis. In both animal and yeast cells, this process is dependent on cytoplasmic microtubules interacting with the cortical actin-based cytoskeleton, although the motive force was unknown. Here we show that yeast Myo2, a myosin V that translocates along polarized actin cables into the bud, orientates the spindle early in the cell cycle by binding and polarizing the microtubule-associated protein Kar9 (refs 7-9). The tail domain of Myo2 that binds Kar9 also interacts with secretory vesicles and vacuolar elements, making it a pivotal component of yeast cell polarization.

Confocal Light Microscopy and Scanning Electron Microscopy of the Human Eye Lens

The potential of confocal light microscopy (CLM) forin vivoobservation of pathology in the anterior pole of the eye lenses was evaluated by performing anin vitrostudy of human lenses comparing this type of microscopy with scanning electron microscopy (SEM).In vitroCLM showed high resolution images of the epithelium which would enable early detection of pathology and easily allows cell counting and estimating cell size. Superficial lens fibres are well visualized and low and high frequency bands as well as vacuolar elements were easily detected. SEM observations fully supported the CLM observations. This study shows that CLM has the potential to become a useful tool for detecting lens changes, after suitable adaptation for clinical use.

Delivery of internalized ricin from endosomes to cisternal golgi elements is a discontinuous, temperature-sensitive process

Galactose-terminating membrane glycoproteins and glycolipids on two established human breast carcinoma cell lines were tagged at 4 °C with a ricin-horseradish peroxidase conjugate (Ri-HRP). The cells were then incubated for various periods of time at 37 or 18 °C. After fixation and diaminobenzidine cytochemistry, the compartments reached by Ri-HRP were studied by analyzing thin serial sections. In both cell types a highly pleomorphic endosomal system comprising vacuolar elements as well as smaller, sometimes branched, tubular elements (tubular endosomes) was revealed at both 37 and 18 °C. At 37 °C Ri-HRP was consistently observed in flattened cisterns of the Golgi region in 30–40% of the Golgi complexes examined after 30–60 min of incubation. However, no Ri-HRP reached such Golgi elements at 18 °C, even after incubation for 180 min. Moreover, at 18 °C the ability of ricin to inhibit protein synthesis was virtually abolished, whereas the effect of diphtheria toxin was reduced much less. Following incubation with a monovalent transferrin-HRP conjugate or with unconjugated HRP, no labeling of cisternal Golgi elements was detected. These data indicate that delivery of galactose-terminating membrane molecules from endosomes to the Golgi complex is a discontinuous, temperature-sensitive process and that this process may be required for optimal ricin A-chain translocation.

Localization of the epidermal growth factor (EGF) receptor within the endosome of EGF-stimulated epidermoid carcinoma (A431) cells.

We have followed the internalization pathway of both epidermal growth factor (EGF) and its receptor in human epidermoid carcinoma (A431) cells. Using EGF conjugated with horseradish peroxidase and anti-receptor monoclonal antibodies (TL5 and EGFR1) coupled either directly or indirectly to colloidal gold we have identified an extensive elaboration of endosomal compartments, consisting of a peripheral branching network of tubular cisternae connected to vacuolar elements that contain small vesicles and a pericentriolar compartment consisting of a tubular cisternal network connected to multivesicular bodies. Immunocytochemistry on frozen thin sections using receptor-specific antibody-gold revealed that at 4 degrees C in the presence of EGF, receptors were mainly on the plasma membrane and, to a lesser extent, within some elements of both the peripheral and pericentriolar endosomal compartments. Upon warming to 37 degrees C there was an EGF-dependent redistribution of most binding sites, first to the peripheral endosome compartment and then to the pericentriolar compartment and lysosomes. Upon warming only to 20 degrees C the ligand-receptor complex accumulated in the pericentriolar compartment. Acid phosphatase cytochemistry identifies hydrolytic activity only within secondary lysosomes and trans cisternae of the Golgi stacks. Together these observations suggest that the prelysosomal endosome compartment extends to the pericentriolar complex and that the transfer of EGF receptor complexes to the acid phosphatase-positive lysosome involves a discontinuous, temperature-dependent step.

The ultrastructure of an anuran trypanosome, Trypanosoma andersoni Reilly and Woo, 1982 (Kinetoplastida)

The blood trypomastigote of Trypanosoma andersoni Reilly and Woo has an amorphous surface coat and trilaminar pellicle with a series of subpellicular tubules. A long, tubular mitochondrion with many platelike cristae is present. The cytoplasm is rich in ribosomes and vacuolar elements. The surface coat of T. andersoni is similar to those of the stercorarian mammalian trypanosomes, whereas the undulating membrane and mitochondrion resembled those of avian trypanosomes.

Studies on the rat liver following iron overload. 1. Fine structural appearance.

Iron overload of the rat liver following parenteral administration of Jectofer (an iron sorbitol citric acid complex) was studied in the electron microscope. Abundant ferritin-like granules were present in parenchymal and Kupffer cells, partly free in the cell sap and partly concentrated in 3 types of membrane-bound organelles, with characteristic appearances. In the parenchymal cells these organelles consisted of lysosome-like structures, apparent autophagic vacuoles, and vacuoles lacking features linking them to specific cytoplasmic elements. Organelle-bound ferritinlike granules in the Kupffer cells were demonstrated in lysosomelike structures, in phagocytic vacuoles, and in tubular and vacuolar elements referred to as "type 1" and "type 2" bodies. No ferritin-like granules were observed in other cell types than parenchymal and Kupffer cells.

Giant cell tumor of bone: fine structural localization of acid phosphatase.

The fine structural localization of acid phosphatase in the different cells in a benign giant cell tumor of bone has been studied. Stromal cells type 1 and 2 (fibroblast-like and macrophage-like, respectively) showed the presence of lead phosphate precipitate following incubation in a Gomori-type lead salt medium only in conventional lysosomes. In the multinucleated giant cells, the final product was deposited over lysosome-like organelles, and also over Golgi cisternae, vesicles, and vacuoles. Furthermore, evidence for presence of acid phosphatase was obtained in smooth-surfaced tubular, sausage-, horse-shoe-, and ring-shaped structures and over digestive vacuoles of autophagic or heterophagic origin. Finally, in these cells, many of the tubular and vacuolar elements located subjacent to areas of the plasma membrane with microvillous specializations (abortive brush borders?) were shown to carry acid phosphatase.

Capillitial Formation in Arcyria Cinerea

The capillitial system in Arcyria cinerea originates from an anastomosing system of elongated vacuolar elements. This system forms very early in the development of the sporangium and assumes the po...

THE FINE STRUCTURE OF BRUNNER'S GLANDS IN THE MOUSE

Examined with the electron microscope, the secretory cells of the submucosal glands of Brunner in the mouse present a curious combination of the fine-structural features of both serous and mucus-secreting cells. The cells have numerous mitochondria, abundant basal ergastoplasm, dense secretory granules that bear a superficial resemblance to pancreatic zymogen granules, and an unusually extensive Golgi apparatus. The prominence of the lamellar, vesicular, and vacuolar elements of the Golgi complex facilitates detailed observation of these components. More evident than in other glandular cells, aggregates of small vesicles appear to represent the transitional elements and are vehicles for transport of the product between the ergastoplasm and the Golgi complex. The numerous vesicular evaginations of smooth-surfaced regions on cisternae of the rough-surfaced endoplasmic reticulum and accumulations of innumerable vesicles of similar size in the area between the nearest profiles of the ergastoplasm and the Golgi complex support this contention. The cytological characteristics and physiologic properties of Brunner's glands in various species are discussed briefly. It is concluded that the submucosal glands of the mouse are excellent material for exploration of the ultrastructural correlates of both protein and carbohydrate secretion, and it is suggested that their secretion may have functions other than those generally attributed to them, namely, chemical and mechanical protection of the duodenal surface epithelium.