On some formulas in a partnership model from the perspective of a semi-Markov process

Many deterministic models of sexually transmitted diseases, as well as population models in general, contain elements of stochastic or statistical reasoning. An example of such a model is that of Dietz and Hadeler (1988) concerning sexually transmitted diseases in which there is partnership formation and dissolution. Among the interesting formulas in this paper, which enter into the analysis of the model, are those for the expected number of partners a male or female has during a lifetime. To a probabilist such formulas suggest the possibility that some stochastic process may be constructed so as to yield these formulas as well as others that may be of interest. The principal purpose of this paper is to demonstrate that such a stochastic process does indeed exist in the form of a three state semi-Markov process in continuous time with stationary laws of evolution and with a one-step density matrix determined by four parameters which were interpreted as constant latent risk functions in the classical theory of competing risks. This construction of a semi-Markov process not only provides a framework for the systematic derivation of the formulas of Dietz and Hadeler but also suggests pathways,for extensions to the age-dependent case.This research was partially supported by NATO Grant D.890350     

Kinetics of leucrose formation from sucrose by dextransucrase

Leucrose formation from sucrose and fructose by dextransucrase is of practical interest. It has been investigated at different experimental conditions, including the influence of temperature on reaction rate and selectivity. Under appropriate conditions high product yield can be obtained. Furthermore, a model is presented that allows interpretation of the experimental data.     

Formation and decomposition of vacuoles inBotryococcus in relation to the trans-Golgi network

Summary The formation and the decomposition of vacuoles in a member of Xanthophyceae,Botryococcus braunii, were examined by light and electron microscopy. Particles around the nucleus were identified as vacuoles from their stainability with neutral red. These particles disappeared during cell division. They reacted positively in an activity test for acid phosphatase. Electron microscopy revealed the presence of spherical vacuoles around the nucleus. During cell division, these vacuoles seemed to be decomposed by the ER which surrounded the vacuoles. Soon after this decomposition, many immature multivesicular bodies (MVBs) appeared to develop from the trans-Golgi network (TGN) and were pinched off from the TGN. These immature MVBs took up small vesicles in them as they grew into the mature MVBs. Mature MVBs took up and digested the surrounding cytoplasm, fused with one another, and eventually became the vacuoles.Abbreviations MVB multivesicular body - TGN trans-Golgi network     

Studies on plant regeneration from cotyledonary protoplasts in Brassica campestris

Protoplasts isolated from both 7-day-old light-grown and 4-day-old dark/dim light-grown cotyledons of four Brassica campestris varieties (Arlo, Sonja, Bunyip and Wonk Bok) were cultured in three liquid media: modified K8P, modified MS and modified Pelletier's B to compare the capacities for cell division and plant regeneration. Following cell wall regeneration the cultured protoplasts from dark/dim light-grown cotyledons of four varieties showed rapid division and high frequency of cell division compared with those isolated from light-grown cotyledons. The frequencies of cell division were significantly influenced by varieties and culture media but only in cultured protoplasts isolated from dark/dim light-grown cotyledons. The interaction between varieties and media was also significant. Cell colonies formed within 7–14 days in protoplast cultures from dark/dim light-grown cotyledons, and calli subsequently grown on a solid medium developed shoots when transferred onto a regeneration medium. Three of four tested varieties (Arlo, Sonja and Bunyip) showed shoot regeneration within 2–3 months after protoplast isolation, with a high degree of reproducibility in Arlo and Bunyip. Regenerated shoots, which were induced to root on half-strength MS medium with 0.1 mg.l^–1 IBA, survived in soil and grew to produce siliques and set viable seeds in the greenhouse. The present report is the first to document the production of regenerated plants that set seeds in Brassica campestris from cotyledonary protoplasts.Abbreviations BAP benzylaminopurine - CPW Composition of Protoplast Washing-solution - 2,4-D 2,4-dichlorophenoxyacetic acid - EDTA ethylenediamine-tetraacetic acid - GA₃ gibberellic acid - IBA indole-3-butyric acid - IAA indole-3-acetic acid - MS Murashige and Skoog medium - NAA -naphthaleneacetic acid - KT kinetin - FDA fluorescein diacetate - SDS sodium dodecyl sulfate     

Interaction of ethylene with indole-3-acetic acid in regulation of rooting in pea cuttings

Cuttings of pea (Pisum sativum L. cv Marma) were treated with 1-aminocyclopropane-l-carboxylic acid (ACC). This treatment caused increased ethylene production and reduction of root formation. The effect of 0.1 mM ACC on the level of endogenous indole-3-acetic acid (IAA) in the rooting zone and in the shoot apex was analyzed by gas chromatography-single ion monitoring mass spectrometry or by high pressure liquid chromatography with fluorimetric detection (HPLC). Concentrations of indole-3-acetylaspartic acid (IAAsp) in the stem bases were also determined using HPLC. The ACC treatment had little effect on the IAA level in the base measured after 24 h, but caused a considerable decrease during the 3 following days. IAAsp increased in the base on days 1, 2 and 3 and then declined. The build up of IAAsp in the base was not affected by ACC during the first two days of the treatment, but later this conjugate decreased more rapidly than in controls. No effect of the ACC treatment was found on the level of IAA in the apex. IAA (1 µM) applied to the cuttings during 24 h reduced the number of roots formed. The possibility that IAA-induced ethylene is involved in this response was investigated.Our results support earlier evidence that the inhibitory effect of ethylene on rooting in pea cuttings is due to decreased IAA levels in the rooting zone. The inhibitory effect of applied IAA is obtained if the internal IAA level is maintained high during the first 24 h, whereas stimulation of rooting occurs if the internal IAA level remains high during an extended period of time. Our results do not support the suggestion that ethylene mediates the inhibitory effect of applied IAA.     

Fine structure of the spermatozoa of Crassostrea gigas (Mollusca,Bivalvia)

We describe sperm ultrastructure and acrosome differentiation during spermiogenesis in Crassostrea gigas (Mollusca Bivalvia). The sperm cell is a uniflagellated cell of the primitive type. The head region contains a rounded or conical nucleus surmounted by small acrosome. This organelle consists of a membrane-bound acrosomal granule, the contents of which have a homogeneous density, except in the anterior region, which is positive for PTA. The acrosome also surrounds the perforatorium, which includes oriented fibrillar elements: this is the axial body. The middle piece contains four mitochondria encircling two perpendicular centrioles. The distal centriole is provided with a system of mechanical fixation to the plasma membrane, consisting of nine fibers in radial arrangement. The tail flagellum, about 50 m?m long, contains the usual microtubular axoneme. © 1993 Wiley-Liss, Inc.     

Relationship Between Spatial Pattern of Basal Bodies and Membrane Skeleton (Epiplasm) During the Cell Cycle of Tetrahymena: cdaA Mutant and Anti-Membrane Skeleton Immunostaining

Microtubular basal bodies and epiplasm (membrane skeleton) are the main components of the cortical skeleton of Tetrahymena. The aim of this report was to study functional interactions of basal bodies and epiplasm during the cell cycle. The cortex of Tetrahymena cells was stained with anti-epiplasm antibody. This staining produced a bright epiplasmic layer with a dark pattern of unstained microtubular structures. The fluorescence of the anti-epiplasm antibody disappeared at sites of newly formed microtubular structures, so the new basal body domains and epiplasmic layer could be followed throughout the cell cycle. Different patterns of deployment of new basal bodies were observed in early and advanced dividers. In advanced dividers the fluorescence of the epiplasmic layer diminished locally within the forming fission line where the polymerization of new basal bodies largely extincted. In wild type Tetrahymena, the completion of the micronuclear metaphase/anaphase transition was associated with a transition from the pattern of new basal body deployment and epiplasm staining of the early divider to the pattern of the advanced dividers. The signal for the fission line formation in Tetrahymena (absent in cdaA1 Tetrahymena mutationally arrested in cytokinesis) brings about 1) transition of patterns of deployment of basal bodies and epiplasmic layer on both sides of the fission line; and 2) coordination of cortical divisional morphogenesis with the micronuclear mitotic cycle.     

沙面结皮形成与微环境变化

研究结果表明沙坡头地区的大气年降尘(d相似文献   

Between Xylem and Phloem: The Genetic Control of Cambial Activity in Plants

Abstract: Post-embryonic development is controlled by two types of meristems: apical and lateral. There has been considerable progress recently in understanding the function of root and shoot apical meristems at the molecular level. Knowledge of analogous processes in the lateral, or secondary, meristems, i.e. the vascular cambium or cork cambium, is, however, rudimentary. This is despite the fact that much of the diversity in the plant kingdom is based on the differential functions of these meristems, emphasizing the importance of lateral meristems in the development of different plant forms. The vascular cambium is particularly important for woody plants, but it also plays an important role during the development of various herbaceous species, such as Arabidopsis thaliana. In this review, we focus on the two basic functions of cambial activity: cell proliferation and pattern formation.