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41.
42.
Mallomonas splendens (G. S. West) Playfair has a cell covering of siliceous scales and bristles. Interphase cells bear four anterior and four posterior bristles that each articulate, at their flexed basal ends via a complex of labile fibers (the fibrillar complex), on a specialized body scale (a base-plate scale). Body scales, base-plate scales and bristles are formed independently of each other and at different times in silica deposition vesicles (SDVs) that are associated with one of the two chloroplasts. The fine structure of scale and bristle morphogenesis in M. splendens agrees with that previously described for Synura and Mallomonas. Four new posterior bristles are formed at late interphase with their basal ends towards the cell posterior. The fibrillar complex is formed in situ on the bristle in the SDV. Mature bristles are secreted one by one onto the surface of the protoplast, beneath the layer of body scales, where the basal ends of the bristles adhere to the plasma membrane via the fibrillar complex. The extrusion of posterior bristles and their deployment onto the cell surface was monitored with video. A fine cellular protuberance accompanies the bristles as they are extruded from beneath the scale layer with their basal ends leading. When distant from the cell, the basal ends of the bristles appear attached to the protuberance, possibly by way of their fibrillar complexes. Once bristles are fully extruded, and their tips free in the surrounding environment, the bristle bases are drawn back to the posterior apex of the cell, apparently by the now shortening protuberance. Thus a 180° reorientation of the posterior bristles has been effected outside the cell. Thin-sections of cells that are extruding bristles show a threadlike, cytoplasmic extension of the cell posterior which may be analogous to the protuberance seen in live cells. Four new posterior base-plate scales are secreted after the bristles have reoriented. Scanning electron microscopy indicates that the fibrillar complex is involved in positioning the bristles onto their respective base-plate scales. Anterior bristles are formed in new daughter cells in the same orientation as the posterior bristles; thus they are extruded tip first and no reorientation is required. 相似文献
43.
Stanley A. Cohn Timothy P. Spurck Jeremy D. Pickett-Heaps Lesley A. Edgar 《Journal of phycology》1989,25(1):15-26
This paper describes the perizonium and initial valve formation in Navicula cuspidata Kütz., based on light microscope (LM) and scanning electron microscope (SEM) observations. The perizonium consists of concentric over-lapping bands, laid down sequentially at the tips of the expanding biconical auxospore during its elongation. The central perizonial band has fimbriate edges and is considerably more rigid than the more distal bands. During auxospore elongation and the band secretion, the chloroplasts continuously oscillate between the two ends of the cell; this oscillation ceases once the elongation is complete. The initial valves, formed within the perizonium, are molded into the basically biconical shape of the perizonium except for a central flattening of each valve face. In contrast to the raphes in gametangial and vegetative valves which are surrounded by a smooth axial area, the raphes in initial valves lie within a raised ridge running along the apical axis of the valve. The regular pattern of apically oriented ridges on the outer surface of vegetative valves is also lacking on initial valves. Comparison of pore–pore spacing within striae of gametangial valves, initial values and post-initial valves (first division and vegetative cells) reveals that the pore–pore distance within striae is conserved at all sexual stages. However, the distance between striae is considerably larger in initial valves than in gametangial and post-initial valves. Vegetative interstriae spacing as well as the planar morphology of the valve face is regained at the first division of the initial cell. This suggests that the spacing between striae is dependent on the sexual stage of the cell during valve formation (i.e. not directly dependent on the cell size) and can be altered independently of the pore–pore spacing. 相似文献
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Jeremy M LaMothe Russell T Hepple Ronald F Zernicke 《Journal of applied physiology》2003,95(4):1739-1745
Rodents are commonly used as models for human aging because of their relatively short life span, the ease of obtaining age-specific tissue samples, and lower cost. However, age-associated disease may confound inbred animal studies. For example, numerous physiologically significant lesions, such as chronic nephropathy, are more common in aged Fischer 344 (F344) rats than in other strains (Bronson RT, Genetic Effects of Aging, 1990). Conversely, F344 x Brown-Norway F1-hybrid (F344BN) rats, developed by the National Institute on Aging for aging research, live considerably longer and have fewer pathologies at any given age vs. inbred strains (Lipman RD, Chrisp CE, Hazzard DG, and Bronson RT, J Gerontol A Biol Sci Med Sci 51: 54-59, 1996). To our knowledge, there are no data regarding the effect of age on bone geometry and mechanics in this strain of rat. Furthermore, caloric restriction (CR) extends the mean and maximal life span of animals and significantly reduces age-associated disease but may have adverse consequences for bone growth and mechanics. Thus we investigated the effects of age and CR on bone geometry and mechanics in the axial and appendicular skeleton of F344 Brown-Norway rats. Ad libitum fed rats were assessed at 8 mo (young adult; n = 6), 28 mo (late middle age; n = 5), and 36 mo (senescence; n = 6). CR rats were assessed at 28 mo (n = 6). Tibiae and the sixth lumbar vertebrae (L6) were dissected, scanned (micro-computed tomography) to determine geometry, and tested mechanically. From 8 to 36 mo, there were no significant changes in L6 geometry, and only the cross-sectional moment of inertia changed (increased) with the tibia. CR-induced body mass reductions accounted for changes in L6 load at proportional limit, maximal load, and stiffness (structural properties), but altered tibial structural properties were independent of body mass. In tibiae, geometric changes dominated alterations in structural properties. Those data demonstrated that, whereas aging in ad libitum-fed animals induced minor changes in bone mechanics, axial and appendicular bones were adversely influenced by CR in late-middle-aged animals in different manners. 相似文献
46.
Chorusing Behaviour,a Density-dependent Alternative Mating Strategy in Male Common Toads (Bufo bufo)
Jacob Hglund Jeremy G. M. Robertson 《Ethology : formerly Zeitschrift fur Tierpsychologie》1988,79(4):324-332
Male chorusing behaviour was studied in a population of common toads (Bufo bufo) on the island of Öland south Sweden, and the functional role of male advertisement calling in this species was experimentally examined. Calling males were larger and heavier than non-calling males (t = 2.41, p < 0.025 and t = 2.36, p < 0.025, respectively). However, small males were also found to call. This indicates that large males persisted in calling for longer and/or called more often. The proportion of calling males decreased as population size increased during the breeding season, indicating that calling is a low density strategy. Females responded more readily to calls than males. There were insufficient data to determine if the dominant frequencies of advertisement calls were inversely correlated with male body size, however, this relationship was found for the similar release calls. Females were found not to discriminate between high and low frequency calls, but when given a choice between two calls of different sound pressure levels (SPL), females were attracted to the louder calls. Thus, the function of chorusing is to advertise the position of males to mate-seeking females when the population density is low. 相似文献
47.
Jeremy Minshull Aaron Straight Adam D. Rudner Abby F. Dernburg Andrew Belmont Andrew W. Murray 《Current biology : CB》1996,6(12):1609-1620
Background Mitosis is regulated by MPF (maturation promoting factor), the active form of Cdc2/28–cyclin B complexes. Increasing levels of cyclin B abundance and the loss of inhibitory phosphates from Cdc2/28 drives cells into mitosis, whereas cyclin B destruction inactivates MPF and drives cells out of mitosis. Cells with defective spindles are arrested in mitosis by the spindle-assembly checkpoint, which prevents the destruction of mitotic cyclins and the inactivation of MPF. We have investigated the relationship between the spindle-assembly checkpoint, cyclin destruction, inhibitory phosphorylation of Cdc2/28, and exit from mitosis.Results The previously characterized budding yeast mad mutants lack the spindle-assembly checkpoint. Spindle depolymerization does not arrest them in mitosis because they cannot stabilize cyclin B. In contrast, a newly isolated mutant in the budding yeast CDC55 gene, which encodes a protein phosphatase 2A (PP2A) regulatory subunit, shows a different checkpoint defect. In the presence of a defective spindle, these cells separate their sister chromatids and leave mitosis without inducing cyclin B destruction. Despite the persistence of B-type cyclins, cdc55 mutant cells inactivate MPF. Two experiments show that this inactivation is due to inhibitory phosphorylation on Cdc28: phosphotyrosine accumulates on Cdc28 in cdc55Δ cells whose spindles have been depolymerized, and a cdc28 mutant that lacks inhibitory phosphorylation sites on Cdc28 allows spindle defects to arrest cdc55 mutants in mitosis with active MPF and unseparated sister chromatids.Conclusions We conclude that perturbations of protein phosphatase activity allow MPF to be inactivated by inhibitory phosphorylation instead of by cyclin destruction. Under these conditions, sister chromatid separation appears to be regulated by MPF activity rather than by protein degradation. We discuss the role of PP2A and Cdc28 phosphorylation in cell-cycle control, and the possibility that the novel mitotic exit pathway plays a role in adaptation to prolonged activation of the spindle-assembly checkpoint. 相似文献
48.
49.
Weidong Jiang Moon-Young Lim Hye-Joo Yoon Jeremy Thorner G. Steven Martin John Carbon 《Molecular & general genetics : MGG》1995,246(3):360-366
We find that overexpression in yeast of the yeast MCK1 gene, which encodes a meiosis and centromere regulatory kinase, suppresses the temperature-sensitive phenotype of certain mutations in essential centromere binding protein genes CBF2 and CBF5. Since Mck1p is a known serine/threonine protein kinase, this suppression is postulated to be due to Mck1p-catalyzed in vivo phosphorylation of centromere binding proteins. Evidence in support of this model was provided by the finding that purified Mck1p phosphorylates in vitro the 110 kDa subunit (Cbf2p) of the multimeric centromere binding factor CBF3. This phosphorylation occurs on both serine and threonine residues in Cbf2p. 相似文献
50.