Lignification and lignin topochemistry - an ultrastructural view.

This review discuses the ultrastructural aspects of cell wall lignification and lignin topochemistry. Lignification results from the enzyme mediated polymerization of monolignols initiated by unknown factors (initiation sites) located at the corners of cells and in the middle lamella. Lignification results in the filling of pores within the carbohydrate matrix following a sequence from the outer regions of the wall towards the lumen. The amount and chemical characteristics of lignin vary across the cell wall, with the presence of reaction wood, and among cell types.     

The Asymmetry of Organic Aerosol Fission and Prebiotic Chemistry

We examine the prebiotic applicability of our recent analysis of the fission of an atmospheric aerosol particle coated with an organic film. The fission is made possible by the free energy change upon compression of the exterior monolayer film on the parent particle, which overcomes the increase in surface area associated with the production of two spherical daughter particles. Asymmetric division into a larger and a smaller particle becomes possible following surfactant film collapse. The size of the airborne parent particle is determined by the balance between aerodynamics and gravity, while the ratio of the radii of the daughters is determined by the compression characteristics of the amphiphilic molecules comprising the parent film. For an Earth atmosphere of one bar surface pressure, the larger and smaller daughters have the sizes of a single-celled bacterium and of a virus respectively. Chemical differentiation between the daughters is possible.     

Maintaining excellence: the preservation and development of specialised services.

The advent of the Tomlinson inquiry draws attention to the need to strike a balance between market led and planned approaches to health care delivery. This is important not just for hospital rationalisation but also for the preservation and development of services which are provided in a smaller number of hospitals. Specialised services are often in the forefront of raising standards of care and introducing new developments and innovations. They are the only option for a small number of patients with serious illnesses. In the internal market for health care provision created by the 1990 NHS reforms more sophisticated and flexible mechanisms must be found to provide stability for specialised services while at the same time enabling the benefits of purchaser choice and provider competition to be realised.     

Peeled mammalian skeletal muscle fibers. Possible stimulation of Ca2+ release via a transverse tubule-sarcoplasmic reticulum mechanism

Single muscle fibers from rabbit soleus and adductor magnus and from semitendinosus muscles were peeled to remove the sarcolemma and then stimulated to release Ca2+ by (a) caffeine application or (b) ionic depolarization accomplished via substitution of choline chloride for potassium propionate at constant [K+] X [Cl-] in the bathing solution. Each stimulus, ionic or caffeine, elicited an isometric tension transient that appeared to be due to Ca2+ released from the sarcoplasmic reticulum (SR). The peak magnitude of the ionic (Cl- -induced) tension transient increased with increasing Cl- concentration. The application of ouabain to fibers after peeling had no effect on either type of tension transient. However, soaking the fibers in a ouabain solution before peeling blocked the Cl- -induced but not the caffeine-induced tension transient, which suggests that ouabain's site of action is extracellular, perhaps inside transverse tubules (TTs). Treating the peeled fibers with saponin, which should disrupt TTs to a greater extent than SR membrane, greatly reduced or eliminated the Cl- -induced tension transient without significantly altering the caffeine-induced tension transient. These results suggest that the Cl- -induced tension transient is elicited via stimulation of sealed, polarized TTs rather than via ionic depolarization of the SR.     

T lymphocyte heterogeneity in the rat. III. Autoreactive T cells are activated by B cells

Evidence has been presented to show that CD4+ autoreactive T cell lines (ATs)2 in the rat require periodic stimulation with syngeneic spleen cells for in vitro proliferation. This proliferation can be blocked by treatment of the stimulator (spleen) cells with mAb to Ia antigens. Although ATs are Ia+ and can activate the allogeneic MLR, they fail to be autostimulatory. Fractionation of the spleen cells revealed that ATs can be stimulated with B cells and not by macrophages, although the latter were efficient in several accessory cell functions, including antigen presentation, lectin-dependent T cell activation and allogenic MLR response. Moreover, B cells proliferated and differentiated in response to AT cells. These data are compatible with a model in which ATs respond to hitherto undetermined B cell membrane antigen(s) in association with MHC class II antigens. These results may have important implications in understanding autoimmune responses.     

Is there a floral parasite mutualism in cycad pollination? the pollination biology of Encephalartos villosus (Zamiaceae)

Observations and experiments were carried out over 5 yr to distinguish between wind and insect pollination in the cycad Encephalartos villosus Lemaire (Zamiaceae). They were also designed to determine whether a pollination mutualism exists between E. villosus and Antliarhinus zamiae (Thunberg) (Coleoptera: Brentidae), an obligate ovule parasite that routinely parasitizes a large proportion of the ovules. The percentage of fertilized ovules dropped slightly when wind was excluded from the megastrobilus. However, when insects were excluded by either net bags or insecticide there was a substantial decrease in the proportion of fertilized ovules. Five beetle species belonging to four families were found on the strobili at the time of pollination. Using data on the effectiveness of pollen transfer to the receptive ovule, as well as data on abundance and pollen loads, a pollinator importance value (PIV) was determined for each beetle species and a pollinator importance index (PII) was determined for each population. PII values showed that an undescribed weevil (Porthetes sp., Curculionidae) was consistently the most important pollinator. A. zamiae and an undescribed beetle species within the Xenoscelinae (Languriidae) played a minor role in pollination, and their contributions varied from year to year and between populations. Two additional beetle species, Metacucujus goodei Endrödy-Younga (Boganiidae) and a second species of Xenoscelinae, had very low PII values and probably had little or no effect on pollination. Low PIV scores for A. zamiae were a result of its low numbers on the microstrobilus and the tendency of the beetles to remain on the outside of the megastrobilus. In the interaction between E. villosus and A. zamiae, the cycad does not appear to benefit significantly from a pollination service and I interpret this to mean that the relationship is antagonistic rather than mutualistic. There is, however, a possible mutualism between Porthetes sp. and E. villosus.     

Iron and Aluminum Lakes of Gallo Blue E As Nuclear and Metachromatic Mucin Stains

Gallo blue E, C. I. No. 51040, Mordant Violet 54, furnishes a blue black nuclear stain when applied to tissue sections in the form of its moderately stable iron lakes. This adoring combined well with such counterstains as orange G and eosin B. The Van Gieson stain tends to decolorize mucins, cartilage, and mast ells previously stained with this dye. Its aluminum lake solutions tend to gel in a few minutes to 24 hours depending on the solvent wed and the amount of Al²⁺ present. Aluminum lake solutions give a moderately good blue to dark blue nuclear stain and a brilliant purplish red to dark purple stain to a variety of epithelial and connective tissue mucins. Acid dye counterstains are poorly tolerated. With either lake, nuclear staining is abolished by deoxyribonuclease digestion or relatively short mineral acid extraction of DNA.