Electron-dense Tubules Research Articles

Ultrastructural changes during the symbiotic seed germination of Gastrodia elata with fungi, with emphasis on the fungal colonization region

BackgroundGastrodia elata is a fully mycoheterotrophic orchid and has long been used in traditional Chinese medicine. The life cycle of G. elata requires an association with two different fungi-Mycena for seed germination and Armillaria for tuber growth. The association with Armillaria is representative of the phytophagous type of orchid mycorrhiza: the intracellular hyphae are lysed without forming condensed pelotons. However, whether the association with Mycena during seed germination belongs to the same type of orchid mycorrhiza is unknown.ResultsHistological and ultrastructural studies revealed several notable features in different developmental stages. First, a thickened cell wall with papillae-like structures appeared during fungal penetration in the suspensor end cell, epidermal cells and cortical cells of germinating embryos. In addition, the formation of two distinctive cell types in the colonized region of a protocorm (i.e., the passage canal cell filled with actively growing fungal hyphae) can be observed in the epidermal cell, and the distinctive digestion cell with a dense cytoplasm appears in the cortex. Finally, within the digestion cell, numerous electron-dense tubules form a radial system and attach to degrading fungal hyphae. The fungal hyphae appear to be digested through endocytosis.ConclusionsThe present study provides important structural evidence for the phytophagous type of orchid mycorrhiza in the symbiotic germination of G. elata with Mycena. This case demonstrates a particular nutrient transfer network between G. elata and its litter-decaying fungal partner.

The nuclear protein Waharan is required for endosomal-lysosomal trafficking in Drosophila.

Here we report Drosophila Waharan (Wah), a 170-kD predominantly nuclear protein with two potential human homologues, as a newly identified regulator of endosomal trafficking. Wah is required for neuromuscular-junction development and muscle integrity. In muscles, knockdown of Wah caused novel accumulations of tightly packed electron-dense tubules, which we termed 'sausage bodies'. Our data suggest that sausage bodies coincide with sites at which ubiquitylated proteins and a number of endosomal and lysosomal markers co-accumulate. Furthermore, loss of Wah function generated loss of the acidic LysoTracker compartment. Together with data demonstrating that Wah acts earlier in the trafficking pathway than the Escrt-III component Drosophila Shrb (snf7 in Schizosaccharomyces pombe), our results indicate that Wah is essential for endocytic trafficking at the late endosome. Highly unexpected phenotypes result from Wah knockdown, in that the distribution of ubiquitylated cargos and endolysosomal morphologies are affected despite Wah being a predominant nuclear protein. This finding suggests the existence of a relationship between nuclear functions and endolysosomal trafficking. Future studies of Wah function will give us insights into this interesting phenomenon.

Sperm structure and phylogeny of Astigmata

The Astigmata, a large and variable group, is still a subject of taxonomic dispute. Particularly, their origin from ancestors of the lower oribatid mites (e.g., Malaconothroidea) seems well documented by many lines of evidence. The structure of spermatozoa has been successfully applied to phylogenetic investigations in many animal groups. The aim of our study was to provide new data on spermatozoon structure in Astigmata and to consider its appropriateness in phylogenetic studies. The study reveals information on spermatozoa in 17 species of Astigmata (11 species studied for the first time) extending our knowledge to 18 species (one species known only from the literature) representing 12 families and 7 superfamilies. Spermatozoa have the same basic structure in all species: cells are multiform and the chromatin forms thin threads embedded directly in the cytoplasm; the acrosome is absent. The cytoplasm in most species contains electron-dense lamellae, varying in both number and arrangement within the cell. In Sarcoptoidea, electron-dense tubules in contact with lamellae margins were also observed in Psoroptidae (Psoroptes equi), whereas in two representatives of Sarcoptidae (Notoedres cati and Sarcoptes scabiei), only electron-dense tubules were found. In two species, Canestrinia sellnicki (Canestrinioidea: Canestriniidae) and Scutulanyssus obscurus (Analgoidea: Pteronyssidae), neither lamellae nor tubules were present. The mitochondria in a spermatozoon are usually gathered at the cell periphery and their structure is usually modified to form so-called mitochondrial derivatives. The chromatin threads are an autapomorphy strongly supporting the monophyly of Astigmata. As spermatozoa vary considerably between species in Astigmata, we deduce that sperm structure may be useful for phylogenetic analyses within the group. Several conclusions concerning the affinities within Astigmata are presented. Spermatology seems to be unhelpful, however, in questions on the origin of Astigmata (particularly for Astigmata-Oribatida relationships), since their sperm do not possess synapomorphies with sperm of the remaining groups of Acariformes, i.e., Endeostigmata, Prostigmata, and Oribatida.

CD1 and major histocompatibility complex II molecules follow a different course during dendritic cell maturation.

The maturation of dendritic cells is accompanied by the redistribution of major histocompatibility complex (MHC) class II molecules from the lysosomal MHC class II compartment to the plasma membrane to mediate presentation of peptide antigens. Besides MHC molecules, dendritic cells also express CD1 molecules that mediate presentation of lipid antigens. Herein, we show that in human monocyte-derived dendritic cells, unlike MHC class II, the steady-state distribution of lysosomal CD1b and CD1c isoforms was unperturbed in response to lipopolysaccharide-induced maturation. However, the lysosomes in these cells underwent a dramatic reorganization into electron dense tubules with altered lysosomal protein composition. These structures matured into novel and morphologically unique compartments, here termed mature dendritic cell lysosomes (MDL). Furthermore, we show that upon activation mature dendritic cells do not lose their ability of efficient clathrin-mediated endocytosis as demonstrated for CD1b and transferrin receptor molecules. Thus, the constitutive endocytosis of CD1b molecules and the differential sorting of MHC class II from lysosomes separate peptide- and lipid antigen-presenting molecules during dendritic cell maturation.

MHC class II compartments in human dendritic cells undergo profound structural changes upon activation.

Immature dendritic cells efficiently capture exogenous antigens in peripheral tissues. In an inflammatory environment, dendritic cells are activated and become highly competent antigen-presenting cells. Upon activation, they lose their ability for efficient endocytosis and gain capability to migrate to secondary lymphoid organs. In addition, peptide loading of MHC class II molecules is enhanced and MHC class II/peptide complexes are redistributed from an intracellular location to the plasma membrane. Using immuno-electron microscopy, we show that activation of human monocyte-derived dendritic cells induced striking modifications of the lysosomal multilaminar MHC class II compartments (MIICs), whereby electron-dense tubules and vesicles emerged from these compartments. Importantly, we observed that MHC class II expression in these tubules/vesicles transiently increased, while multilaminar MIICs showed a strongly reduced labeling of MHC class II molecules. This suggests that formation of the tubules/vesicles from multilaminar MIICs could be linked to transport of MHC class II from these compartments to the cell surface. Further characterization of endocytic organelles with lysosomal marker proteins, such as the novel dendritic cell-specific lysosomal protein DC-LAMP, HLA-DM and CD68, revealed differential sorting of these markers to the tubules and vesicles.

Ultrastructure of the Ookinetes of Haemoproteus meleagridis (Haemosporina: Haemoproteidae)

Ookinetes of Haemoproteus meleagridis were structurally similar to kinetes of other apicomplexan parasites and possessed a polar ring complex (PRC) composed of an electron-lucent polar ring with 25 supporting tines. Fifty subpellicular microtubules were anchored in a circle to the inner surface of the polar ring. A bilayered electron-dense canopy was continuous with the inner layer of the pellicle and formed a caplike cover over the PRC. Embedded rings of actin-sized microfilaments completely encircled each layer of the canopy. Numerous micronemes, 2 smaller preconoidal rings, and a conoid composed of approximately 6 spirally wound, electron-dense tubules were also present. Other organelles were similar to those reported in previous studies of haemosporidian ookinetes. Mature ookinetes of H. meleagridis developed in the midguts of engorged specimens of Culicoides edeni (Diptera: Ceratopogonidae) within 24 hr after a blood meal. Most parasites were found beneath, or embedded within, a peritrophic membrane composed of fine granules and fibrils. The observation of actin-sized microfilaments within the canopy is a previously unrecognized modification of the pellicle that probably supports the anterior end of ookinetes during penetration of the peritrophic membrane.

Fine structure of the terminal buds on the barbels of some fishes.

The terminal buds from three species of fresh water fishes (carp, cat-fish and loach) were examined electron microscopically. The buds consist of three types of cells and of nerve elements innervating the buds.1. The receptor cells are characterized first by the presence of specialized electron-lucid apical processes which show wide variety in dimension and form. The supranuclear cytoplasm contains numerous electron-dense tubules of 400-600A in diameter, sometimes running straight along the axis of cells, or sometimes presenting tortuous or spiral appearances. The infranuclear portion has abundant vesicles, 300-1000A in diameter. Some of these vesicles assemble to the particular site of contact of the cell with nerve element and resemble those in the synaptic regions in the central nervous system.2. The supporting cells contain abundant filaments through the whole extent of their cytoplasm, from just below the free surface with microvilli, to the fine sheets which are formed by branching of the infranuclear part of the cell and intervene the various elements in the basal portion of the bud.3. The basal cell is placed at the bottom of the bud directly on the basement membrane and contains numerous small and large vesicles. Some of the larger ones contain dense cores in them, while not a few of the smaller ones are accumulatad to the specialized site of contact with neural element, presenting seemingly the same situation as was found between the receptor cells and the neural element.4. The myelinated fibers in the core of the barbels branch into finer unmyelinated fibers which proceed into the raised papilla of the dermis underneath the bud to reach the base of the latter. After having entered the bud, the fibers seem to divide into finer branches, which, forming the intragemmal plexus, have a specified contact with either the receptor cells or the basal cells. On the bases of these findings, possible functional implications of the morphological constituents of the terminal buds were discussed.