全文获取类型
收费全文 | 10013篇 |
免费 | 930篇 |
出版年
2024年 | 3篇 |
2023年 | 53篇 |
2022年 | 72篇 |
2021年 | 280篇 |
2020年 | 136篇 |
2019年 | 191篇 |
2018年 | 232篇 |
2017年 | 195篇 |
2016年 | 310篇 |
2015年 | 537篇 |
2014年 | 594篇 |
2013年 | 643篇 |
2012年 | 996篇 |
2011年 | 993篇 |
2010年 | 576篇 |
2009年 | 497篇 |
2008年 | 762篇 |
2007年 | 736篇 |
2006年 | 659篇 |
2005年 | 589篇 |
2004年 | 539篇 |
2003年 | 488篇 |
2002年 | 420篇 |
2001年 | 84篇 |
2000年 | 41篇 |
1999年 | 49篇 |
1998年 | 65篇 |
1997年 | 45篇 |
1996年 | 17篇 |
1995年 | 18篇 |
1994年 | 10篇 |
1993年 | 14篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1987年 | 4篇 |
1985年 | 3篇 |
1984年 | 10篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 5篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1972年 | 3篇 |
1965年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Jason Yang Raphael Guzman James Richards S. Nandi 《In vitro cellular & developmental biology. Plant》1980,16(6):502-506
Summary Mammary tumor epithelial cells from BALB/cfC3H mice were dispersely embedded inside the collagen gels in Ham's F-12 medium
containing horse serum. A sustained cell growth leading to a 5- to 10-fold increase in cell number over initial level was
observed in less than 2 weeks. The extent of this growth was found to be dependent on serum concentration. However, addition
of various protein and steroid hormones, both singly and in combination, to low-serum-containing medium failed to achieve
a comparable level of growth to that promoted by higher serum concentration. Mammary tumor cells can now be consistently propagated
in primary culture.
This investigation was supported by Grants CA05388 and CA09041 awarded by the National Cancer Institute, Department of Health,
Education and Welfare, and by cancer research funds of the University of California. 相似文献
43.
The Rhizobium--legume symbiosis. 总被引:6,自引:0,他引:6
J E Beringer N Brewin A W Johnston H M Schulman D A Hopwood 《Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain)》1979,204(1155):219-233
The rhizobia are soil microorganisms that can interact with leguminous plants to form root nodules within which conditions are favourable for bacterial nitrogen fixation. Legumes allow the development of very large rhizobial populations in the vicinity of their roots. Infections and nodule formation require the specific recognition of host and Rhizobium, probably mediated by plant lectins. Penetration of the host by a compatible Rhizobium species usually provokes host root cell division to form the nodule, and a process of differentiation by both partners then ensues. In most cases the rhizobia alter morphologically to form bacteroids, which are usually larger than the free-living bacteria and have altered cell walls. At all stages during infection, the bacteria are bounded by host cell plasmalemma. The enzyme nitrogenase is synthesized by the bacteria and, if leghaemoglobin is present, nitrogen fixation will occur. Leghaemoglobin is a product of the symbiotic interaction, since the globin is produced by the plant while the haem is synthesized by the bacteria. In the intracellular habitat the bacteria are dependent upon the plant for supplies of energy and the bacteroids, in particular, appear to differentiate so that they are no longer able to utilize the nitrogen that they fix. Regulation of the supply of carbohydrate and the use of the fixed nitrogen thus appear to be largely governed by the host. 相似文献
44.
Micronuclear elongation is the first major event in a series of nuclear changes occurring during the sexual stage of the life cycle of Tetrahymena. Beginning at about one hour after cells of complementary mating types have conjugated, the micronucleus leaves its recess in the macronucleus and swells slightly. This is accompanied by a reorganization of its chromatin from a reticular to a solid body. In the next stage the micronucleus assumes an egg shape, a development concomitant with the appearance of microtubules. While the chromatin spins out from the dense body, and microtubules increase in number, the nucleus assumes a spindle shape. During the elongation, which increases the length of the nucleus some fifty fold, microtubules are prominent in clusters just internal to the nuclear membrane, and parallel to the longitudinal axis of the nucleus. When elongation is completed the nucleus is curved around the macronucleus. Internally, partially condensed strands of chromatin are located off-center, towards the macronuclear side, and the density of the microtubules is diminished. At all the stages, DNA is located throughout the nucleus; neither discrete chromosomes nor synaptonemal complexes are seen. Occasionally cytoplasmic membrane systems are seen fused to the nuclear envelope which retains the typical appearance of a double membrane with pores. 相似文献
45.
46.
Summary
R. leguminosarum trp alleles mapped by R68.45-mediated recombination were located in three distinct chromosomal regions. We isolated three derivatives of R68.45 that carried different trp genes of R. meliloti. Each of the plasmids suppressed all of the R. leguminosarum trp alleles in a particular region. The R-primes were transferred to strains of P. aeruginosa carrying mutations in different trp genes. The plasmid pAJ24JI suppressed trpA, B and F mutants, pAJ73JI suppressed trpC and D and pAJ88JI suppressed a trpE mutant. When the R-primes were transferred to E. coli trp strains they failed to suppress any trp mutants. A derivative of pAJ24JI was isolated which was able to suppress trpA and F mutants of E. coli. 相似文献
47.
Genetic mouse models are an important tool in the study of mammalian neural tube closure (Gray & Ross, 2009; Ross, 2010). However, the study of mouse embryos in utero is limited by our inability to directly pharmacologically manipulate the embryos in isolation from the effects of maternal metabolism on the reagent of interest. Whether using a small molecule, recombinant protein, or siRNA, delivery of these substances to the mother, through the diet or by injection will subject these unstable compounds to a variety of bodily defenses that could prevent them from reaching the embryo. Investigations in cultures of whole embryos can be used to separate maternal from intrinsic fetal effects on development.Here, we present a method for culturing mouse embryos using highly enriched media in a roller incubator apparatus that allows for normal neural tube closure after dissection (Crockett, 1990). Once in culture, embryos can be manipulated using conventional in vitro techniques that would not otherwise be possible if the embryos were still in utero. Embryo siblings can be collected at various time points to study different aspects of neurulation, occurring from E7-7.5 (neural plate formation, just prior to the initiation of neurulation) to E9.5-10 (at the conclusion of cranial fold and caudal neuropore closure, Kaufman, 1992). In this protocol, we demonstrate our method for dissecting embryos at timepoints that are optimal for the study of cranial neurulation. Embryos will be dissected at E8.5 (approx. 10-12 somities), after the initiation of neural tube closure but prior to embryo turning and cranial neural fold closure, and maintained in culture till E10 (26-28 somities), when cranial neurulation should be complete. 相似文献
48.
Recruitment and derecruitment (R/D) of air spaces within the lung is greatly enhanced in lung injury and is thought to be responsible for exacerbating injury during mechanical ventilation. There is evidence to suggest that R/D is a time-dependent phenomenon. We have developed a computer model of the lung consisting of a parallel arrangement of airways and alveolar units. Each airway has a critical pressure (Pcrit) above which it tends to open and below which it tends to close but at a rate determined by how far pressure is from Pcrit. With an appropriate distribution of Pcrit and R/D velocity characteristics, the model able to produce realistic first and second pressure-volume curves of a lung inflated from an initially degassed state. The model also predicts that lung elastance will increase transiently after a deep inflation to a degree that increases as lung volume decreases and as the lung becomes injured. We conclude that our model captures the time-dependent mechanical behavior of the lung due to gradual R/D of lung units. 相似文献
49.
50.
Differential Effects of AGS3 Expression on D2L
Dopamine Receptor-Mediated Adenylyl Cyclase Signaling
Activator of G protein signaling 3 (AGS3) binds Gαi subunits in the GDP-bound state, implicating AGS3 as an important regulator of Gαi-linked receptor (e.g., D2 dopamine and μ-opioid) signaling. We examined the ability of AGS3 to modulate recombinant adenylyl cyclase (AC) type 1 and 2 signaling in HEK293 cells following both acute and persistent activation of the D2L dopamine receptor (D2LDR). AGS3 expression modestly enhanced the potency of acute quinpirole-induced D2LDR modulation of AC1 or AC2 activity. AGS3 also promoted desensitization of D2LDR-mediated inhibition of AC1, whereas desensitization of D2LDR-mediated AC2 activation was significantly attenuated. Additionally, AGS3 reduced D2LDR-mediated sensitization of AC1 and AC2. These data suggest that AGS3 is involved in altering G protein signaling in a complex fashion that is effector-specific and dependent on the duration of receptor activation. 相似文献