Cap Membrane Research Articles

The fine structure and cytology of the association between the parasitic red algaHolmsella australis and its red algal hostGracilaria furcellata

Holmsella australisNoble andKraft ms. is a colourless red algal parasite, forming whitish pustules on its photosynthetic red algal host,Gracilaria furcellataHarvey. In the infected region, host cortical tissue continues to grow and enclose the expanding pustule. Filaments of both host and parasite grow apically, the cells being connected by primary pit connections (PCs). Secondary PCs form between cells of the same species, and in addition,H. australis initiates the formation of secondary PCs with cells ofG. furcellata. All three types of secondary PC are morphologically distinct. In hostparasite PCs the surface adjoining the host cell is similar in structure to a host-host PC, while that adjoining the parasite cell has the structure of a parasite-parasite PC. The plasma membrane is continuous between the cells of the unrelated host and parasite. In addition, a cap membrane is typically produced only on the host surface, though occasionally the parasite side is enclosed by a cap membrane as well. Cap membranes are absent from parasite-parasite PCs (making them intracellular), while host-host PCs are typically extracellular, both cells producing cap membranes. The presence or absence of a cap membrane in certain positions appears to vary, and suggests that cells may be able to regulate its presence. Since transport of nutrients would be expected to occur from host to parasite cells, and between parasite cells, the morphological evidence presented here suggests the PCs may be the pathway.

An acrosome reaction in natantian sperm

AbstractSperm of the natantian Sicyonia ingentis are highly polarized with a conical cap and single spike extending from a spherical main body. The acrosome consists of a spiraled spike and two glycoprotein cap components: the anterior granule and the membrane pouches. During sperm‐egg interaction, the spike is lost and the cap membrane folds back, exposing the two glycoprotein acrosomal components. At this point, the anterior granule lies directly adjacent to the oolemma and has replaced the spike as the anterior end of the sperm. Treatment of sperm with ionophore A23187 initiates the acrosome reaction, thereby facilitating the experimental sequencing of the reaction. The ionophore‐induced reaction begins with the spike's loss of its spiraled nature, the appearance of filaments in the spike core, and the immediate depolymerization of the spike into a granular mass. As the spike retracts, the membrane pouches swell. The cap membrane in the area of the retracted spike ruptures and begins to fold back. As seen during natural sperm‐egg interaction, the anterior granule becomes the new anterior end of the sperm after loss of the spike. Several known artificial activators of sperm were tested, but only the divalent cation ionophore A23187 successfully induced the acrosome reaction. The use of artificial seawater with different ionic properties showed that the acrosome reaction is dependent on the influx of Ca++ ions.

The meiospore ofPhysoderma maydis. The causal agent ofPhysoderma disease of maize

The meiospore ofPhysoderma maydis (Phycomycetes, Chytridiales, Physodermataceae) has a nuclear cap enclosing the cellular ribosomes within a double membrane, and double membranes traversing the nuclear cap. Aggregates of ribosomes not incorporated into the nuclear cap are also enclosed by double membranes. A vesicular network is observed in the anterior portion of the spore in direct connection with the nuclear cap membrane and with a stacked parallel array of membranes, which itself is connected with the nuclear cap membrane.

Evidence for concanavalin a-receptors and their redistribution on lumbricid leukocytes

Fluorescent and radioactive concanavalin A (Con A) were used to demonstrate the existence of Con A-receptors on earthworm leukocyte membranes. Inhibition of binding by methyl-α- d-mannose, methyl-α- d- glucose and d-fructose suggested that Con A can bind to specific sites. Differences of labelling between two types of leukocytes support the existence of physiological heterogeneity among the leukocytic population. Native distribution of Con A-receptors was uniform on the whole cell membrane. After 30 minutes at room temperature, patches of Con A-binding site complexes were formed. Capping occurred in 30 % of the small-spheric-cells which represented 20 % of the non-adherent leukocytes, after 1 hour at room temperature. The con A molecules were then internalized by endocytosis of the cap membrane.

A freeze-etch study of the ultrastructure of red algal pit plugs

Freeze-etch studies indicate that the fully-formed pit plug ofPalmaria palmata consists of a homogeneous plug core, a pair of plug caps, and associated membranes. A single cap membrane separates the two layers of each plug cap and is the only membrane found between the cytoplasm and the plug core. Due to the apparent junction of the plasmalemma and cap membranes, the plug core, along with the adjacent plug cap layer on either face of the core, is entirely membrane-bounded and essentially extracellular. Fracture faces of the cap membrane bear large populations of particles which are inconspicuous due to their low profile. The plasmalemma lining the plug core from cell to cell has numerous, very obvious particles on its fracture faces, more even than does the cytoplasmic plasmalemma.

A COMPLEMENTARY FREEZE‐FRACTURE STUDY OF CHRYSOCHROMULINA CHITON (HAPTOPHYCEAE)

Complementary freeze‐fracture replicas and high resolution tantalum‐tungsten shadowing have been used in a study of the membanes of the marine alga Chrysochromulina chiton. Membrane particle populations range from 38/100 nm2 in the plastid to 2/100 nm2 in the pyrenoid cap membrane. Membrane asymmetry was evident in all membranes, but was most obvious in those with higher particle numbers. In all complementary replica pairs, particles were always more numerous on protoplasmic fracture faces. Small, particle‐free areas with bordering particles were also seen as recurring membrane features. Complementarity of matching fracture faces was seen for very small background granularity patterns and for large membrane components, but not for particles. Complementarity can also be seen in non‐membranous fracture faces both within and external to the cell, suggesting the presence of polymeric materials in these areas that produce “particles” due to plastic deformation.

The fine structure of campaniform sensilla on the halteres of Drosophila melanogaster

AbstractThe campaniform sensilla on halteres of Drosophila were studied by electron microscopy in order to establish the relationships of functional elements in the sensory system. The surface of the sensillum consists of an oval cuticular cap membrane which may contain resilin, the rubberlike protein. A border of denser cuticle rings the cap membrane, and extending down around the neural process is a third type of cuticle filled with a fourth light fibrous type. The four cuticular components form a system for displacement of the neural process. The neural process is differentiated into a terminal fan‐shaped structure projecting from a bulbous dilatation which tapers to a neck region ending proximally with two basal bodies. The neural process is packed with microtubules. Surrounding the dendrite is an inner enveloping cell, attached to the basal body region by septate desmosomes and by desmosomes to which microtubules of the enveloping cell are applied. An outer enveloping cell surrounds the inner one. The tip of the neural process is covered with a dense secretion which is tightly bound to the cap membrane. The dense secretion is surrounded by an extracellular fluid which might be compressed hydraulically by the cuticular system. The stimulus of cuticular distortion could thus be transmitted to the neural process which may be displaced between its fixed ends.

Regulation der UDP-glucose 4-epimerase synthese in kernhaltigen und kernlosen acetabularien

1. 1. The activity of uridine-diphosphateglucose 4-epimerase (EC 5.1.3.2) was investigated during the differentiation (stalk and cap formation) of the unicellular and uninucleate green alga Acetabularia mediterranea. 2. 2. The activity of this enzyme is related to the development of the cap. There was a substantial increase in the activity of the enzyme during cap formation. 3. 3. The increase of enzyme activity during cap formation also took place in anucleate cells. 4. 4. These findings are in agreement with the fact that the galactose content of the cap membrane is much higher than that of the stalk membrane. 5. 5. The increase in enzyme activity in nucleate and anucleate cells was reversibly stopped by puromycin. Therefore, the increase in the epimerase activity must be due to synthesis of this enzyme. 6. 6. These findings indicate that the synthesis of uridine-diphosphateglucose 4-epimerase in Acetabularia does not involve a direct regulatory function of nuclear genes. It rather suggests that this regulation takes place in the cytoplasm and probably at the level of translation.