全文获取类型
收费全文 | 138篇 |
免费 | 3篇 |
出版年
2021年 | 4篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2015年 | 5篇 |
2012年 | 4篇 |
2011年 | 2篇 |
2010年 | 5篇 |
2007年 | 2篇 |
2006年 | 5篇 |
2005年 | 3篇 |
2001年 | 2篇 |
2000年 | 1篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1973年 | 1篇 |
1972年 | 4篇 |
1971年 | 1篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1967年 | 2篇 |
1966年 | 3篇 |
1965年 | 3篇 |
1964年 | 1篇 |
1962年 | 2篇 |
1961年 | 1篇 |
1960年 | 1篇 |
1958年 | 1篇 |
1956年 | 1篇 |
1952年 | 3篇 |
1951年 | 1篇 |
排序方式: 共有141条查询结果,搜索用时 15 毫秒
51.
V?Srinivasan GJM?Maestroni DP?Cardinali AI?Esquifino SR?Pandi?Perumal SC?MillerEmail author 《Immunity & ageing : I & A》2005,2(1):17
Aging is associated with a decline in immune function (immunosenescence), a situation known to correlate with increased incidence
of cancer, infectious and degenerative diseases. Innate, cellular and humoral immunity all exhibit increased deterioration
with age. A decrease in functional competence of individual natural killer (NK) cells is found with advancing age. Macrophages
and granulocytes show functional decline in aging as evidenced by their diminished phagocytic activity and impairment of superoxide
generation. There is also marked shift in cytokine profile as age advances, e.g., CD3+ and CD4+ cells decline in number whereas
CD8+ cells increase in elderly individuals. A decline in organ specific antibodies occurs causing reduced humoral responsiveness.
Circulating melatonin decreases with age and in recent years much interest has been focused on its immunomodulatory effect.
Melatonin stimulates the production of progenitor cells for granulocytes-macrophages. It also stimulates the production of
NK cells and CD4+ cells and inhibits CD8+ cells. The production and release of various cytokines from NK cells and T-helper
lymphocytes also are enhanced by melatonin. Melatonin presumably regulates immune function by acting on the immune-opioid
network, by affecting G protein-cAMP signal pathway and by regulating intracellular glutathione levels. Melatonin has the
potential therapeutic value to enhance immune function in aged individuals and in patients in an immunocompromised state. 相似文献
52.
Prof. Dr. M. Wolman 《Histochemistry and cell biology》1970,21(1):5-8
Summary In all microscopical studies based on fluorescence an artefact occurs which is due to reflection or diffraction of the primary light. Available filters do not completely eliminate visible light from the primary beam and this light can produce pseudo-fluorescence of the same order of magnitude as that of the true fluorescence. A method for estimating the contribution of pseudo-fluorescence to the total apparent fluorescence is described.on leave of absence from the Department of Pathology and of Cell Biology and Histology, Tel Aviv University Medical School, Government Hospital, Tel-Hashomer, IsraelThis investigation was supported in part by grant NB-016105 from the National Institute of Neurological Diseases and Stroke, National Institutes of Health. 相似文献
53.
MCWHINNIE MARY A.; CAHOON MARY ODILE SR.; JOHANNECK ROSEMARIE 《Integrative and comparative biology》1969,9(3):841-855
Cyclic shifts of calcium in the exoskeleton and soft tissues,as they are related to the intermolt cycle in crayfish, arereviewed. Regulatory factors, derived from the eyestalk, influencelevels of exoskeletal calcium; eyestalk extracts prepared fromanimals in premolt decrease shell calcium, while reciprocallyextracts from animals in intermolt increase it when these hormonalsources are injected into animals in the premolt stage (D0-D4). In addition, premolt eyestalk extract results in an increasein gastrolith calcium. In the exchange of calcium between theanimal and its environment there is evidence for differentialdepositionof recently available calcium in the exoskeleton. Further, intermoltand early premolt animals maintained in Ca45-labelled waterfor 15 days concentrate it 4 and 3fold in the exoskeletonand stomach, respectively. However, removal of a molt-inhibitingfactor through ablation of eyestalks results in a 20 and 40foldincrease in incorporation inthese same sites relative to environmentalconcentrations. Treatment with mammalian parathyroid extract mobilizes bothexoskeletal and gastric calciumand leads to a rise in bloodcalcium. However, there is little or no effect on levels ofexoskeletal citric acid. Further, citric acid is higher in thecrayfish carapace during stage C, the period of mineralization,than in stage D, the period of demineralization. There are both similarities and differences between the effectsof crustacean and mammalianregulating factors with respect tothe direction and extent of mineralization. Biochemical studiesshould elucidate the mechanisms regulated by these hormones. 相似文献
54.
The partition matrix: exploring variable phylogenetic signals along nucleotide sequence alignments 总被引:6,自引:2,他引:4
The partition matrix is a graphical tool for comparative analysis of
nucleotide sequences following alignment. It is particularly useful for
investigating the divergent phylogenies of sequence regions undergoing
reticulate evolution. A partition matrix is generated by determining the
consistency of the parsimoniously informative sites in a set of aligned
sequences with the binary partitions inferred from the sequences. Since the
linear order of sites is maintained, the matrix can be used to assess
whether the distribution of sites either supporting or conflicting with
particular partitions changes along the length of the alignment. The
usefulness of the matrix in allowing visual identification of differences
in evolutionary history among regions depends on the order in which
partitions are shown; several suitable ordering schemes are proposed. We
demonstrate the use of the partition matrix in interpreting the evolution
of the pseudoautosomal boundary region on the sex chromosome of catarrhine
primates. Its routine use should help to avoid attempts to derive single
phylogenies from sequences whose evolution has been reticulate and to
identify the gene conversion or recombination events underlying the
reticulation. The method is relatively fast. It is exploratory, and it can
form the basis for more formal analysis, which we discuss.
相似文献
55.
Positive selection and sequence rearrangements generate extensive polymorphism in the gamete recognition protein bindin 总被引:27,自引:12,他引:15
Bindin is a gamete recognition protein of sea urchins that mediates
species-specific attachment of sperm to an egg-surface receptor during
fertilization. Sequences of bindin from closely related urchins show fixed
species-specific differences. Within species, highly polymorphic bindin
alleles result from point substitution, insertion/deletion, and
recombination. Since speciation, positive selection favoring allelic
variants has generated diversity in bindin polypeptides. Intraspecific
bindin variation can be tolerated by the egg receptor, which suggests
functional parallels between this system and other flexible recognition
systems, including immune recognition. These results show that polymorphism
in mate recognition loci required for rapid evolution of sexual isolation
can arise within natural populations.
相似文献
56.
Several crystalline N-hydroxysuccinimide esters of short- and long-chain fatty acids have been synthesized. These compounds react with free amino acids to form preferentially N-acylamino acids. The reaction of the N-hydroxysuccinimide esters with hydroxylamine and the behavior of the N-acylamino acids on thin-layer chromatography are described. 相似文献
57.
Polarized light microscopy in the study of the molecular structure of collagen and reticulin 总被引:2,自引:0,他引:2
Although collagen structure has been studied by polarized light microscopy since the early 19th century and continued since, modern studies and reviews failed to correlate the conclusions based on data obtained by the techniques with those of polarized light microscopy. Collagen I is intensely positively birefringent in respect to length of the fibres; the positive intrinsic birefringence indicates a quasi-crystalline alignment parallel to the fibre and molecule axis of the amino acid residues of the polypeptide chains. This would not have been compatible with a helical structure but has been achieved by similar tilt angles and opposite directions of the coiling and supercoiling. Birefringence characteristics of collagen are also affected by chemical treatments, extractions and staining procedures. Attachment of chemical groups to the anionic charges present on the surface of collagen molecules results in increased positive birefringence in the case of bipolar molecules attached to two or more anionic residues. Unipolar attachment to the same groups, or to the cationic groups of the associated proteoglycans, as well as sulfation or acetylation of hydroxyls of the protein and/or the carbohydrate, reduced or reversed the sign of birefringence. Increased birefringence caused by stretching cannot be due to intramolecular events and is caused by intermolecular changes. The same applies to changes in collagen during aging. Reticulin is a group of different substances which mostly contain collagen III. The pliability and deformability of this collagen is related to its weakly negative birefringence due to large side chains and presence of different and greater amounts of interstitial proteoglycans and other molecules. The so-called reticulin of healing wounds differs in its constitution from other reticulins but is also rich in intermolecular carbohydrate components. 相似文献
58.
59.
60.
Danillo S Silva Susana SR Milhomem Julio C Pieczarka Cleusa Y Nagamachi 《BMC genetics》2009,10(1):1-8