Changes in cell wall structure during the protonema development on organic medium in conditions of light and darkness in Funaria hygrometrica

The cell walls of a protonema of <i>Funaria hygrometrica</i> cultivated in glucose containing medium were considerably thicker in dark than in light. After a prolonged time of dank culture, a considerable reduction of the wall thicknes was observed, simultaneously with the occurrence of vesicles and plasmalemma invaginations containing fibrillar material. It is suggested that in conditions unfavourable for growth, the sugar taken up from the medium can be accumulated in cell walls, from which it can be mobilized again in conditions of starvation. The authors also think that similar mechanism and cell structures can be involved in both building and decomposition of the cell wall.

Ultrastructural response of cabbage outer leaf mesophyll cells (Brassica oleracea L.) to excess of nickel

Changes in the structure and in the ultrastructure of cabbage outer leaf mesophyll cells [<em>Brassica oleracea</em> L.] cv. Sława from Enkhouizen were examined by means of light and electron microscopy. The examined plants were grown on the basic Murashige and Skoog medium with addition of excesive concentrations of nickel (added as NiSO₄ x 7H₂O),i.e. Ni 5, Ni 10 and Ni 20 mg/dm³. In Ni 5 mg samples mainly adaptation changes to the conditions of stress were observed. These changes were manifested by the increase of cytoplasm content and by cytoplasm vacuolization, by the increase of nucleus and nucleous volume, nucleolus vacuolization, the increase of plasmalemma invaginations and of the amount of rough ER, by the central arrangement of smooth ER and of the thylakoids of chloroplasts; it was also shown by the growth of the number of mitochondria and of peroxisomes in the cell. In Ni 10 mg samples, apart from adaptation changes, such as the increase of the nucleus volume, increase of plasmalemma invaginations, cytoplasm and nucleolus vacuolization, degeneration changes were also observed. They concerned mainly the nucleus (the increasing amount of condensed chromatin), ER (swelling and fragmentation of rER and sER), mitochondrium (swelling and reduction of cristae), Golgi apparatus (disintegration and decay) and chloroplasts (changes of shape, swelling and reduction of thylakoids, disappearance of starch and presence of big plastoglobuli). In Ni 20 mg samples cell protoplasts were in different stages of degeneration and the cell organelles that were identifiable, were usually damaged.

Ultrastructure and oil secretion in Hiptage sericea Hook

The oil secreting glands of <i>Hiptage sericea</i> Hook. consist of three regions: epithelial, sub-epithelial and sub-glandular. In early stages, the oil secreting cells are characterized by the presence of plastids with starch grains and electron translucent vesicles, mitochondria, rER, polysomes, small vacuoles, numerous lipid bodies and well-defined nucleus with nucleolus. Later, the accumulation of plastoglobuli and inclusion bodies occur in the matrix of the plastid. Tubular, smooth endoplasmic reticulum begins to appear in the cytoplasm. With the onset of secretion, the osmiophilic contents of plastids which appear as electron dense, round droplets move-into cytoplasm and often occur in the region of the plasmalemma invaginations. However, in matured glands the lipid bodies disappear from the cytoplasm. The size of the vacuoles increases and are filled with electron opaque substance. Similar substances are also found in the sub-cuticular spaces as well as outside the cuticle.

Pit Membrane Structure and Development in Ginkgo Biloba

Pit membrane ontogeny in radial walls of Ginkgo biloba tracheids was followed using transmission electron microscopy. Torus initiation occurs prior to initiation of the pit border and without benefit of a microtubule plexus. The developing pit membrane is associated with masses of wall material located within plasmalemma invaginations. Wall material is added in such a manner as to form a torus with highly irregular surfaces. Margo and torus are traversed by plasmodesmata, whose channels are connected by extcnsive median cavities. Matrix material is removed from both margo and torus shortly after hydrolysis of the adjacent cytoplasms. Matrix removal begins at the pit membrane surface and is not preferentially associated with the plasmodesmata. Tori in aspirated pit membranes have compacted fibrils, and their fibrillar compaction might reduce permeability to air embolisms.

Changes in the membrane components of nondividing cambial cells

Horse-chestnut cambial cells are characterized by the formation of numerous plasmalemma invaginations and the accumulation of membrane whorls in the vacuoles during the transition from activity to rest. This suggests an active membrane trafficking which was investigated with conventional electron microscopy methods combined with selective cytochemical staining. After the cessation of meristematic activity, cell wall thickening is accompanied by increased dictyosome activity. The incorporation of dictyosome vesicles into the plasma membrane produces an increase in plasmalemma surface area in these nongrowing cells. This increase is compensated for by endocytosis accomplished by the formation of saclike plasmalemma invaginations into the peripheral cytoplasm. These invaginations often contain vesicles and tubules. When these invaginations come in contact with a vacuole, they appear to push the tonoplast into the vacuole and form double-membrane protrusions which may eventually separate from the plasmalemma. ER cisternae situated in the intermembrane zone also appear to be transported into the vacuole. Other cisternae may be directly sequestered into the vacuole or take part in the formation of the myelinlike structures which were observed in the cytoplasm. Thus, the vacuoles appear to fill progressively with complex membranous structures of various origins (plasmalemma, tonoplast, ER). It is suggested that their subsequent disappearance during the winter is a consequence of the hydrolytic action of vacuolar contents.

Beginning of exocytosis captured by rapid-freezing of Limulus amebocytes.

Structural changes underlying exocytosis evoked by the application of endotoxin to Limulus amebocytes were studied at the level of detail afforded by freeze-fracture and freeze-substitution techniques combined with the time resolution of direct rapid-freezing. The results with amebocytes prepared in this manner differed from those with other secretory cells prepared by conventional means. Exocytosis begins within seconds of endotoxin treatment when the plasmalemma invaginates to form pedestallike appositions with peripheral secretory granules. The juxtaposed membranes at these pedestal appositions form several punctate pentalaminar contacts, but examination of freeze-fractured pedestals failed to reveal any corresponding changes in the intramembrane particle distribution. Small secretory granule openings or pores, which are very infrequent, appear within the first 5 s after endotoxin treatment. These pores rapidly widen and this widening is immediately followed by the sequential dissolution of the granule contents, which then move into the surrounding extracellular space. Cytoplasmic filaments connecting the plasmalemma with the granule membrane are suitably deployed to be responsible for the plasmalemma invaginations. How pores begin is not certain, but the appearance of clear spaces between the granule core and the granule membrane at this point in exocytosis supports the possibility of a role of osmotic forces.

Plasmalemma invaginations of Physarum dependent on the nutritional content of the plasmodial environment

Quantitative estimates of plasmalemma invaginations in plasmodial veins of Physarum polycephalum were made under different conditions of nutrition. Pronounced differences were observed dependent on the nutritional content of the substrate. There was a decided increase in the number of plasmalemma invaginations in plasmodial veins grown on substrates containing absorbable food substances compared to veins migrating on non-nutrient substrates. This observation supports the proposition that the nutritional content, rather than the physical properties, of the substrate is the decisive factor for the formation of plasmalemma invaginations. The invaginations are believed to be concerned with the uptake of non-particulate food substances.

Lytic activity of the plasmalemma and its derivatives in plant cells

Ultrastructural, cytochemical and densitometrical investigation carried out on suspension culture cells of Haplopappus gracilis, protoplasts isolated from leaf parenchyma cells of Nicotiana tabacum, root meristem, root cap cells and young leaves of Hordeum seedlings revealed a pronounced lytic capacity of plasmalemma evaginations and invaginations. In cultivated cells plasmalemma outgrowths into the cell walls exhibited high acid phosphatase activity; presumably, they are important for separation of the aggregated cells. The lytic activity of plasmalemma infoldings seen in cultivated cells increased if the cells were transferred to a medium lacking carbohydrate. In starved cells plasmalemma evaginations protruding into the wall isolated wall fragments which underwent lysis (fig. 1). The degraded wall material captured by plasmalemma invaginations was then transferred to the central vacuole. It is suggested that in this way wall polysaccharides become mobilized allowing a partial survival of the cell population even after 4-day's carbon starvation. During the endocytic process the cytoplasmic islets are included into plasmalemmasomes; so these structures may be identified as auto-heterophagosomes (ambilysosomes by de Duve).

Plasmalemma Invaginations as Characteristic Constituents of Plasmodia of Physarum Polycephalum

Plasmodia of Physarum polycephalum grown on agar or filter paper and fed with rolled oats as food or with a partially defined medium were morphologically analysed in the living state and after fixation. Observation of the living plasmodium growing on agar reveals plasmalemma indentations in the outer regions of protoplasmic strands, which were studied in more detail by phase-contrast microscopy of unstained 1-µm sections. Plasmodia fixed and embedded in situ, i.e. in close contact to their substrate, exhibit an extensive system of plasmalemma invaginations as characteristic constituents throughout all regions. In plasmodial strands measuring between 40 µm and 1.5 mm in diameter and involved in shuttle streaming, the plasmalemma invaginations are found within the outer ectoplasmic wall. Rounded-up parts of this branched extracellular labyrinth limit the endoplasmic core engaged in the mass transport of protoplasm by shuttle streaming. Despite this clearcut borderline, the central endoplasmic core and the ectoplasmic cortex are connected by occasional protoplasmic bridges. The extracellular phase within the ectoplasmic regions of the strands can be interpreted either as a result of plasmalemma invaginations from the outer border of the strand, or as a consequence of pseudopodial-like processes originating from the central core and extending into the surrounding medium. The invagination system provides an extensive enlargement of the surface area within the multinucleate protoplasmic mass, probably important for food absorption, excretion processes and motility phenomena. In thick protoplasmic strands with diameters between 0.2 and 1.5 mm, there is an intimate connexion between the actomyosin fibrils and the invagination system. The fibrils are attached to the plasmalemma invaginations and/or run parallel to the invaginated plasmalemma sheets. The close relations between the invagination system and actomyosin fibrils will be described in detail in a subsequent paper.