全文获取类型
收费全文 | 1627291篇 |
免费 | 153552篇 |
国内免费 | 1727篇 |
出版年
2021年 | 18282篇 |
2019年 | 16281篇 |
2018年 | 19162篇 |
2017年 | 18023篇 |
2016年 | 29254篇 |
2015年 | 43276篇 |
2014年 | 51398篇 |
2013年 | 77614篇 |
2012年 | 47330篇 |
2011年 | 38431篇 |
2010年 | 47164篇 |
2009年 | 47340篇 |
2008年 | 35268篇 |
2007年 | 33971篇 |
2006年 | 37092篇 |
2005年 | 38139篇 |
2004年 | 37299篇 |
2003年 | 34583篇 |
2002年 | 32416篇 |
2001年 | 51783篇 |
2000年 | 49625篇 |
1999年 | 44871篇 |
1998年 | 27568篇 |
1997年 | 27385篇 |
1996年 | 26537篇 |
1995年 | 24708篇 |
1994年 | 24330篇 |
1993年 | 23561篇 |
1992年 | 37806篇 |
1991年 | 35873篇 |
1990年 | 34488篇 |
1989年 | 34923篇 |
1988年 | 32177篇 |
1987年 | 30207篇 |
1986年 | 28584篇 |
1985年 | 29998篇 |
1984年 | 28131篇 |
1983年 | 24347篇 |
1982年 | 23104篇 |
1981年 | 21975篇 |
1980年 | 20477篇 |
1979年 | 23760篇 |
1978年 | 21253篇 |
1977年 | 20119篇 |
1976年 | 18845篇 |
1975年 | 18917篇 |
1974年 | 19686篇 |
1973年 | 19952篇 |
1972年 | 17113篇 |
1971年 | 15628篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
D. Stavrou 《Histochemistry and cell biology》1973,34(1):85-96
Zusammenfassung Die prä- bzw. postnatale enzymtopochemische Differenzierung des subependymalen Glialagers in der lateralen Wand des Vorderhornes wurde bei der SpragueDawley-Ratte untersucht. Während der pränatalen Entwicklungsphase des Rattenhirns zeichnet sich die Subependymschicht, beim Nachweis der wichtigsten hydrolytischen und oxydativen Enzyme, im allgemeinen durch eine geringe enzymatische Aktivität aus. Die Ergänzung der enzymatischen Ausstattung dieser Zellschicht erfolgt erst während der postnatalen Hirnentwicklung und zwar in den ersten 2 Lebenswochen. Eine Ausnahme von dieser Regel wurde beim Nachweis der Phosphomonoesterase-I festgestellt, welche in der prä- bzw. perinatalen Entwicklungsphase die höchste Aktivität zeigte. Die erhobenen enzymtopochemischen Befunde deuten darauf hin, daß in der subependymalen Zellschicht eine vom übrigen Hirn vor allem zeitlich abweichende topochemische Differenzierung stattfindet, welche im wesentlichen während der ersten 2 Lebenswochen abgeschlossen ist.
Mit Unterstützung durch die Deutsche Forschungsgemeinschaft (SFB 51). 相似文献
Enzyme topochemical findings of the subependymal cell layer in the developing rat brain
Summary The pre- and postnatal enzyme topochemical differentiation of the subependymal glial layer of the lateral ventricle was examined in the Sprague-Dawley rat. During the prenatal development phase of the subependymal layer, weak enzymatic activity of the most important hydrolytic and oxidative enzymes is demonstrated. The completion of the enzyme activity of this cell layer follows in the postnatal brain development period, and certainly within the first two weeks of life. An exception to this rule has been found in the case of phosphomonoesterase-I which showed the greatest activity in the pre- and perinatal development phases. The observed enzyme topochemical findings indicate that the subependymal cell layer, in contrast to other parts of the brain, shows temporally deviating topochemical differentiation, which is essentially completed in the first two weeks of life.
Mit Unterstützung durch die Deutsche Forschungsgemeinschaft (SFB 51). 相似文献
942.
943.
944.
Biosynthesis of immunoglobulin A (IgA). Secretion and addition of carbohydrate to monomer and polymer forms of a mouse myeloma protein 总被引:9,自引:3,他引:6
Cell suspensions of mouse plasma-cell tumour MOPC 315 secreting predominantly IgA (immunoglobulin A) monomer and dimer were incubated with radioactive leucine, mannose, galactose and fucose for various periods of time. The amounts of secreted and intracellular immunoglobulins were measured by co-precipitation with specific antibody, and the molecular species present were assessed by electrophoresis in polyacrylamide gels. Analysis of the secreted myeloma protein demonstrated that monomer and dimer IgA molecules are identical with respect to carbohydrate composition and rate of secretion. Within the cell, the myeloma protein is almost entirely accounted for by monomer units which either leave the cell as such or are polymerized with the addition of J chain close to the time of secretion. The results support the concept of a stepwise addition of carbohydrate residues to IgA immunoglobulin during the process of secretion. Similar patterns of carbohydrate assembly were found for the monomer or dimer molecules. Mannose residues are added at an early stage, whereas fucose is added close to the time of secretion. Galactose is also added early, but some may also be incorporated at a later stage. Control of IgA polymerization is considered unlikely to reflect regulation at the level of carbohydrate addition, and it is suggested that the critical controlling factor is the J chain. 相似文献
945.
Abstract— The effect of excess leucine in the diet on serotonin metabolism in the brain was investigated in experimental animals. It was found that:
(1) Animals receiving diets containing 3 % and 8 % leucine and those receiving jowar diets had significantly lower levels of serotonin in the brain.
(2) Intraperitoneal administration of the precursor amino acid 5-HTP increased the serotonin concentration in brain in both control and leucine-fed animals. However, the serotonin concentration in leucine-fed animals was significantly lower than that of pairfed controls. Larger amounts of the synthesized serotonin were found to be catabolized in 3 hr in leucine-fed animals than in control animals.
(3) The in vitro uptake of [14 C]5-HTP by brain slices of animals fed leucine was found to be similar to that of control animals.
(4) The basal concentration of 5-HIAA in brain was higher in leucine-fed animals, suggesting a higher rate of catabolism of serotonin.
(5) Administration of nicotinic acid resulted in a further fall of serotonin concentration in the brains of leucine-fed animals but not in control animals. 相似文献
(1) Animals receiving diets containing 3 % and 8 % leucine and those receiving jowar diets had significantly lower levels of serotonin in the brain.
(2) Intraperitoneal administration of the precursor amino acid 5-HTP increased the serotonin concentration in brain in both control and leucine-fed animals. However, the serotonin concentration in leucine-fed animals was significantly lower than that of pairfed controls. Larger amounts of the synthesized serotonin were found to be catabolized in 3 hr in leucine-fed animals than in control animals.
(3) The in vitro uptake of [
(4) The basal concentration of 5-HIAA in brain was higher in leucine-fed animals, suggesting a higher rate of catabolism of serotonin.
(5) Administration of nicotinic acid resulted in a further fall of serotonin concentration in the brains of leucine-fed animals but not in control animals. 相似文献
946.
LESZEK KUICKI THEODORE L. JAHN JAMS R. FONSECA 《The Journal of eukaryotic microbiology》1970,17(1):16-24
SYNOPSIS. Phase and interference cinemicrographs of cilia of Paramecium multimicronucleatum, immersed 3–24 hours in 1.0% methyl cellulose, revealed that 1) in swimming Paramecium the cilia beat with a traveling helical wave from base to tip rather than with the back and forth movement usually assumed, 2) during ciliary reversal the cilia merely change direction, but continue to beat with a traveling helical wave, and 3) in stationary Paramecium the beat is conicoidal. The traveling wave appears as an undulatory wave about 1 1/4 wave lengths long in both surface and profile views, and therefore must be helical. Envelope of the wave is cylindrical except near the base. Observations were confirmed in media without methyl cellulose by means of high speed cinemicrography, up to 4000 frames/sec. The back and forth movement, as described in all textbooks and monographs, is based mostly on 1) analogy to the abfrontal cilia (cirri) of Mytilus, which do beat with a back and forth movement, and 2) conclusions drawn from fixed preparations which do not represent what actually happens in a living animal. In a stationary Paramecium the envelope of the beat is conicoidal as seen in profile, but probably is a spiral wave, i.e., similar to a helix but increasing in diameter from base to tip. This change in wave form could be caused by the increase in resistance of the water in a stationary organism over one that is moving. Cilia and flagella (also ciliates and flagellates) are usually distinguished on the basis of wave form, but the present observations, together with previous data on flagella, show that such distinctions are untenable. 相似文献
947.
948.
Cells from clones of anti-hapten murine cytotoxic T lymphocytes (CTL) can act as both target and effector cells, but will not lyse members of the same clone. The effect of haptenation on the cytolytic activity of anti-fluorescein (FL) and anti-trinitrophenol (TNP) CTL clones was examined. Treatment of anti-FL clones with fluorescein isothiocyanate or anti-TNP clones with trinitrobenzene sulphonic acid induces these clones to kill in an antigen-independent fashion. Targets killed by the haptenated CTL included syngeneic and allogeneic B lymphocyte blast cells, P815, YAC-1 and in one case human GM 4072 tumor cells. The importance of CD8 and T cell receptor (TCR) occupancy is demonstrated by the ability to block autotriggering by antibody directed against Ly 2 and the TCR. The results demonstrate that effects other than antigen recognition of the target play a role in the final outcome of effector-target cell interactions and provide a mechanism which could lead to autodestruction and immunosuppression particularly in some types of viral infection. 相似文献
949.
Dr. F. Tüttelmann 《Medizinische Genetik》2011,23(2):259-266
The cause for infertility which affects about 10–15% of all couples may be found in approximately half of the cases in the male partners who usually exhibit reduced sperm counts in the ejaculate (i.e. oligozoospermia or azoospermia). The clinically most relevant genetic causes of spermatogenic failure are chromosomal aberrations including Klinefelter’s syndrome and Y chromosomal microdeletions of the AZF loci. Aside from the full clinical picture of cystic fibrosis, mutations in the CFTR gene can cause an isolated obstructive azoospermia without spermatogenic impairment. Genetic investigations should depend on the results of andrological examinations. Chromosomal aberrations are detected more frequently with decreasing sperm counts, where autosomes (e.g. translocations) are predominantly involved in men with oligozoospermia whereas in 10–15% azoospermia is caused by Klinefelter’s syndrome. Classical AZF deletions are found only in men with severe oligospermia or azoospermia and have a prognostic value. In contrast to men with AZFc deletions, carriers of complete AZFa and AZFb deletions have virtually no chance for testicular sperm extraction and a testicular biopsy is not advised. Rare cases of male infertility may be caused by specific syndromes or sperm defects (e.g. globozoospermia and disorders of ciliary structure). 相似文献
950.