The cytoskeleton of Cobaea seed hairs:

The cell wall of Cobaea scandens seed hairs developed in a characteristic sequence, with the deposition of a cellulose thread onto a pectic swelling layer was the final event. The cellulose thread was intracellularly accompanied by a band of 10–18 microtubules. During the formation of the swelling layer the microtubules were homogeneously distributed; they ran circumferentially normal to the cell axis. When cellulose-thread formation started, the microtubules became arranged in a helical band. The density of the microtubules varied during the different phases of development. The highest density was observed before cellulosethread formation and ranged from 6–15 m·m^-2. The length of the microtubules, 20–30 m, was determined by direct measurements, as well as estimated from the total microtubular length in a given area and the counted free ends. With the indirect immunofluorescence technique the microtubules of the band stained inhomogeneously. Those which were located at the edges of the band fluoresced more intensely than those of the central part. Attempts to visualize actin filaments in the hair cells with rhodaminyl-conjugated phalloidin resulted in a homogeneous staining of the area of the microtubular band, indicating that actin filaments may be present in this region. Though, in thin sections and dry-cleaved cells, filamentous structures were observed between the microtubules, caution is expressed that the observed fluorescence was, indeed, due to actin filaments. The role of the filamentous structures is discussed with respect to formation and maintenance of the microtubular band. Microtubules apparently did not cross coated pits which were visualized in the plasma membrane through the dry-cleaving technique.Abbreviations IFT indirect immunofluorescence technique - RP rhodaminyl-conjugated phalloidin - SEM scanning electron microscopy     

Pit organs in axolotls: a second class of lateral line neuromasts

The lateral line system of axolotls (Ambystoma mexicanum) consists of mechanoreceptive neuromasts and electroreceptive ampullary organs. All neuromasts in salamanders are located superficially and are organized into lines that are homologous to canal neuromasts in fishes. Ampullary organs are confined to the head and generally are located adjacent to the lines of superficial neuromasts. Axolotls, however, also possess a third class of receptors; these form restricted patches on the head and are possibly homologous to the superficial pit organs in fishes. In order to test this hypothesis the morphology of the suspected pit organs was examined with scanning electron microscopy, and a number of their physiological properties were determined. Pit organs are approximately half the size of neuromasts and have fewer hair cells, although these hair cells do possess kinocilia and stereocilia like those of neuromasts. Pit organs also possess cupulae and exhibit a pattern of innervation identical to that of neuromasts. Pit organs and neuromasts also exhibit similar rates of spontaneous activity, are excited by weak water currents but not weak electric stimuli, and are not inhibited by magnesium ions. Pit organs appear to have slightly lower rates of spontaneous discharge than neuromasts, however, and have slightly lower displacement thresholds to low frequency wave stimuli. These data support the contention that the pit organs of axolotls constitute a second class of neuromasts homologous to the pit organs of fishes.     

Cytoskeletal arrangement and its intercellular connec-tion in wheat young leaf cells

By using the techniques of partial digestion of cell wall and selective extraction,we examined the cytoskeleton of wheat yong leaf cells under scanning electron microscope(SEM).A 3-dimensional cytoskeletal system,showing some new features,was observed.The cortical network located beneath the cross wall was an anastomosing organization.The association of nucleus with the cell wall by some skeletal filaments was also found.It is notice able that there were cytoskeletal filaments,which passed through cell wall and connected together with cytoskeletal arrays of adjacent cells,Thus,it is possible that an integral skeletal network existed within the yong leaf tissue of wheat.     

人胚DA能神经元移植于帕金森氏病猴模型的TH免疫细胞化学的研究

为了对人胚黑质ＤＡ神经元移植治疗ＰＤ人的临床应用作出客观评估,将８－１２周人胚黑质细胞移植到用ＭＰＴＰ诱发的偏侧ＰＤ猴新纹状体内。实验动物分别存活２个月、５个月和１年后,用ＴＨ免疫细胞化学方法对被移植的人胚ＤＡ细胞的存活和与宿主间的突触联系进行检查。在光镜下可见被移植侧的新纹状体内有ＴＨ阳性细胞,它们成小群散在分布,每小群有３－１０个细胞。ＴＨ阳性细胞的轴突延伸到整个新纹状体,树突呈现出正常发育过     

Energetic approach to the folding of alpha/beta barrels

The folding of a polypeptide into a parallel (alpha/beta)8 barrel (which is also called a circularly permuted beta 8 alpha 8 barrel) has been investigated in terms of energy minimization. According to the arrangement of hydrogen bonds between two neighboring beta-strands of the central barrel therein, such an alpha/beta barrel structure can be folded into six different types: (1) left-tilted, left-handed crossover; (2) left-tilted, right-handed crossover; (3) nontilted, left-handed crossover; (4) nontilted, right-handed crossover; (5) right-tilted, left-handed crossover; and (6) right-tilted, right-handed crossover. Here "tilt" refers to the orientational relation of the beta-strands to the axis of the central beta-barrel, and "crossover" to the beta alpha beta folding connection feature of the parallel beta-barrel. It has been found that the right-tilted, right-handed crossover alpha/beta barrel possesses much lower energy than the other five types of alpha/beta barrels, elucidating why the observed alpha/beta barrels in proteins always assume the form of right tilt and right-handed crossover connection. As observed, the beta-strands in the energy-minimized right-tilted, right-handed crossover (alpha/beta)8-barrel are of strong right-handed twist. The value of root-mean-square fits also indicates that the central barrel contained in the lowest energy (alpha/beta)8 structure thus found coincides very well with the observed 8-stranded parallel beta-barrel in triose phosphate isomerase (TIM). Furthermore, an energetic analysis has been made demonstrating why the right-tilt, right-handed crossover barrel is the most stable structure. Our calculations and analysis support the principle that it is possible to account for the main features of frequently occurring folding patterns in proteins by means of conformational energy calculations even for very complicated structures such as (alpha/beta)8 barrels.     

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

Summary Holmsella australis Noble andKraft ms. is a colourless red algal parasite, forming whitish pustules on its photosynthetic red algal host,Gracilaria furcellata Harvey. 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.     

The control of cellulose microfibril deposition in the cell wall of higher plants

In maize (Zea mays L.) and pine (Pinus taeda L.) seedlings, cellulose microfibril impressions are present on freeze-fractured plasma membranes. It has been proposed that impressions of newly synthesized microfibrils are a record of the movement of terminal synthesizing complexes through the plasma membrane (Mueller and Brown, 1980, J. Cell Biol. 84, 315–326). The association of terminal complexes with the ends of microfibril impressions or with the ends of microfibrils torn through the membrane indicates the orientation of microfibril tips. Unidirectionally-oriented microfibril tips (all pointing in the same direction) are associated with the organized deposition of parallel arrays of microfibrils. Multidirectionally-oriented microfibril tips were observed in a cell in which microfibril deposition was unusually disorganized. Microfibril patterns around pit fields are asymmetric and resemble flow patterns. Unidirectionally-oriented tears are associated with these microfibrils. Although microfibril orientations are deflected around pit fields, the main axis of microfibril orientation is maintained across the surface of the cell. The hypothesis is proposed that the interaction of a flowing plasma membrane with microfibril synthesizing complexes in the plane of the membrane may result in unidirectional deposition and asymmetric microfibril impressions around pit fields.Some of this work has been published in preliminary form (Brown 1979)     

Direct studies of dikaryotization in Schizophyllum commune

The growth, duplication and fate of multikaryotic hyphae bearing true clamp connections, as derived from compatible matings of Schizophyllum commune, were studied by phase contrast microscopy. The nuclei (N) of multikaryotic apices maintained a near central position during hyphal growth. True clamp connection formation occurred with near synchronous mitosis followed by septal synthesis across the clamp neck and main hyphal axis. Nuclear progeny after mitosis in a hexakaryon included 6 N in the apex, 1 N in the clamp and 5 N in the penultimate cell; the solitary nucleus in the clamp later entered the penultimate cell. Similar events occurred for clamp connection formation and mitosis in the trikaryon, quadrikaryon or pentakaryon, whether in the apex or primary branches. Nuclear content of the multikaryotic apex (2 N through 10 N) had no apparent effect on the rate of individual hyphal growth. Reduction of the nuclear number in a trikaryon occurred by long-term entrappment of a solitary nucleus in the clamp and subsequent outgrowth of the dikaryotic penultimate cell. Occasionally, more than one nucleus became entrapped in the clamp cell. The ephemeral nature of the multikaryon was indicated by the fact that older cultures appeared to be exclusively dikaryotic hyphae at the colony periphery.     

Wolbachia host shifts: routes,mechanisms, constraints and evolutionary consequences

Wolbachia is one of the most abundant endosymbionts on earth, with a wide distribution especially in arthropods. Effective maternal transmission and the induction of various phenotypes in their hosts are two key features of this bacterium. Here, we review our current understanding of another central aspect of Wolbachia's success: their ability to switch from one host species to another. We build on the proposal that Wolbachia host shifts occur in four main steps: (i) physical transfer to a new species; (ii) proliferation within that host; (iii) successful maternal transmission; and (iv) spread within the host species. Host shift can fail at each of these steps, and the likelihood of ultimate success is influenced by many factors. Some stem from traits of Wolbachia (different strains have different abilities for host switching), others on host features such as genetic resemblance (e.g. host shifting is likely to be easier between closely related species), ecological connections (the donor and recipient host need to interact), or the resident microbiota. Host shifts have enabled Wolbachia to reach its enormous current incidence and global distribution among arthropods in an epidemiological process shaped by loss and acquisition events across host species. The ability of Wolbachia to transfer between species also forms the basis of ongoing endeavours to control pests and disease vectors, following artificial introduction into uninfected hosts such as mosquitoes. Throughout, we emphasise the many knowledge gaps in our understanding of Wolbachia host shifts, and question the effectiveness of current methodology to detect these events. We conclude by discussing an apparent paradox: how can Wolbachia maintain its ability to undergo host shifts given that its biology seems dominated by vertical transmission?     

Cartesian stiffness for wrist joints: analysis on the Lie group of 3D rotations and geometric approximation for experimental evaluation

This paper is concerned with the analysis and the numerical evaluation from experimental measurements of the static, Cartesian stiffness of wrist joints, in particular the human wrist. The primary aim is to extend from Euclidean spaces to so(3), the group of rigid body rotations, previous methods for assessing the end-point stiffness of the human arm, typically performed via a robotic manipulandum. As a first step, the geometric definition of Cartesian stiffness from current literature is specialised to the group so(3). Emphasis is placed on the choice of the unique, natural, affine connection on so(3) which guarantees symmetry of the stiffness matrix in presence of conservative fields for any configuration, also out of equilibrium. As the main contribution of this study, a coordinate-independent approximation based on the geometric notion of geodesics is proposed which provides a working equation for evaluating stiffness directly from experimental measurements. Finally, a graphical representation of the stiffness is discussed which extends the ellipse method often used for end-point stiffness visualisation and which is suitable to compare stiffness matrices evaluated at different configurations.