全文获取类型
收费全文 | 112125篇 |
免费 | 7962篇 |
国内免费 | 4510篇 |
专业分类
124597篇 |
出版年
2024年 | 226篇 |
2023年 | 1904篇 |
2022年 | 2729篇 |
2021年 | 3757篇 |
2020年 | 3645篇 |
2019年 | 4969篇 |
2018年 | 4226篇 |
2017年 | 3035篇 |
2016年 | 3107篇 |
2015年 | 3925篇 |
2014年 | 6913篇 |
2013年 | 8599篇 |
2012年 | 5152篇 |
2011年 | 6598篇 |
2010年 | 5007篇 |
2009年 | 5557篇 |
2008年 | 5627篇 |
2007年 | 5778篇 |
2006年 | 5111篇 |
2005年 | 4602篇 |
2004年 | 4113篇 |
2003年 | 3373篇 |
2002年 | 3144篇 |
2001年 | 2234篇 |
2000年 | 1817篇 |
1999年 | 1756篇 |
1998年 | 1714篇 |
1997年 | 1456篇 |
1996年 | 1335篇 |
1995年 | 1242篇 |
1994年 | 1143篇 |
1993年 | 971篇 |
1992年 | 961篇 |
1991年 | 847篇 |
1990年 | 680篇 |
1989年 | 630篇 |
1988年 | 566篇 |
1987年 | 474篇 |
1986年 | 404篇 |
1985年 | 573篇 |
1984年 | 841篇 |
1983年 | 546篇 |
1982年 | 631篇 |
1981年 | 537篇 |
1980年 | 418篇 |
1979年 | 397篇 |
1978年 | 317篇 |
1977年 | 247篇 |
1976年 | 223篇 |
1973年 | 148篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Einar Lystad Arne T. Høstmark Cecilie Kiserud Aage Haugen 《In vitro cellular & developmental biology. Animal》1994,30(9):568-573
Summary The protective influence of bovine serum albumin against growth inhibition caused by fatty acids was studied in human hepatoma
(HepG2) and immortalized human kidney epithelial (IHKE) cells. In general, growth inhibition by unsaturated fatty acids (0.15
mmol/liter) increased with increasing number of double bonds. For HepG2 cells crude albumin (1g/100 ml) did not greatly modify growth inhibition by arachidonic, eicosapentaenoic, and docosahexaenoic acid. With oleic,
linoleic, and linolenic acids, crude and defatted albumin stimulated cell growth. In contrast, for IHKE cells both albumins
counteracted growth inhibition by unsaturated fatty acids to approximately the same extent. When HepG2 cells were cultured
in the presence of saturated fatty acids (0.3 mmol/liter), C2, C6, and C8 had no or little inhibitory effect. C10 and C12
inhibited cell growth appreciably, whereas C14, and especially C16, had poor inhibitory effects. Crude albumin counteracted
growth inhibition by all these fatty acids. In contrast, defatted albumin had little or no effect (except against C10 and
C12), and even increased the growth inhibition by C14 and C16. With unsaturated fatty acids there seemed to be an inverse
relationship between cell growth and the concentration of thiobarbituric acid reactive substances (TBARS) in media. Vitamin
E abolished growth inhibition (and the increase in TBARS concentration) by unsaturated fatty acids. The complex interaction
between fatty acids and albumins calls for great caution when interpreting data on growth effects. 相似文献
952.
Y. Li M. M. Bhargava A. Joseph L. Jin E. M. Rosen I. D. Goldberg 《In vitro cellular & developmental biology. Animal》1994,30(2):105-110
Summary Using an automated cell analyzer system, the effect of hepatocyte growth factor/scatter factor (HGF/SF), epidermal growth
factor (EGF), basic fibroblast growth factor (bFGF), endothelial acidic fibroblast growth factor (a-FGF), platelet derived
growth factor (PDGF), and recombinant human insulinlike growth factor (IGF) on the motility and morphology of Madin-Darby
canine kidney (MDCK), rat hepatomas, C2, and H5–6 and murine mammary carcinoma (EMT-6) cells was investigated. Treatment of MDCK cells with HGF/SF, bFGF, EGF, and a-FGF resulted
in an increase in average cell velocity and in the fraction of moving cells. Cells treated with the PDGF and IGF did not show
significant alterations in velocity. MDCK cells treated with each growth factor were classified into groups of “fast” and
“slow” moving cells based on their average velocities, and the average morphologic features of the two groups were quantitated.
Fast-moving cells had larger average area, circularity, and flatness as compared to slow-moving cells. Factors that stimulated
cell movement also induced alterations in cell morphologic parameters including spreading, flatness, area, and circularity.
HGF/SF also scattered and stimulated motility of C2 and H5–6 hepatoma cells. In contrast to MDCK cells, there was no significant difference between the morphology of the fast moving
and slow moving C2 and H5–6 cells. These studies suggest that growth factor cytokines have specific effects on motility of normal and tumor cells. 相似文献
953.
Summary Procedures have been developed for the initiation and long-term maintenance of embryogenic suspension cultures of pickling
cucumber (Cucumis sativus) cultivar Endeavor and for the regeneration of normal plantlets. Embryogenic calluses from petiole explants plated on Murashige
and Skoog (MS) medium with 2,4-dichlorophenoxyacetic acid (2,4-D) and 6-benzylaminopurine (BA), both at 5μM, were used to initiate the embryogenic suspension cultures. Among various growth regulator combinations evaluated for initiation
and maintenance of these suspension cultures, only MS medium with 2,4-D and BA, both at 1μM, produced cultures that were yellow, friable, and still regenerable after repeated subculture (every two wk) over a 3- to
15-mo. period. The effects of various concentrations of auxin and cytokinin in the plating medium, the addition of AgNO3, and various plating procedures were also evaluated. The highest frequency of regeneration of shoots and plantlets was achieved
by plating aggregates onto filter paper overlaid on MS medium with naphthalene acetic acid (NAA)/BA at a concentration of
2:1 or 1:1μM. The addition of activated charcoal (0.5%) or AgNO3 (30μM) in the plating medium did not enhance the frequency of plantlet regeneration. The highest frequency of normal-appearing
plantlets recovered was 42 to 46% per petri dish. The procedures described in this study can be used to increase plantlet
recovery from individual embryogenic calluses of pickling cucumber. 相似文献
954.
S. Wright-Perkins M. R. Daniel C. Walker 《In vitro cellular & developmental biology. Animal》1994,30(7):450-459
Summary This study investigates the characteristics of two human cell lines—1PT and 1PT VARIANT A—both derived from the same histologically
undifferentiated, neuroendocrine positive, non-small cell lung carcinoma (NSCLC) and capable of growth in unsupplemented serum-free
minimum essential medium. In stationary culture, the cells of both lines grew both attached to a plastic substratum and in
suspension; the 1PT VARIANT A line formed three-dimensional clusters of loosely adherent cells. The cell lines differed in
their DNA content, the 1PT having 1.44 times and the 1PT VARIANT A having 2.39 times the normal human diploid DNA content.
Chromosome counts supported this observation, the ploidy of the 1PT and VARIANT A lines being 1.11 and 1.64, respectively.
On transmission electron microscopy the cells of both lines had dense core granules and immature desmosomes, whereas only
the 1PT VARIANT A line had mucin granules. Both lines formed, in nude mice, tumors that, like the original tumor from which
they were derived, were histologically undifferentiated and showed local invasion. The original tumor and both lines had demonstrable
neuroendocrine markers. Cytokeratins were apparent in the tumor but not the cell lines, and neurofilaments were present in
the cell lines only. Staining for epithelial membrane antigen, neural cell adhesion molecule, and desmoplakin differentiated
between the two lines. These lines provide a useful model for the investigation of the biology of the neuroendocrine positive
subgroup of NSCLC, which is clinically important because of the possible responsiveness of these tumors to chemotherapy. 相似文献
955.
Kumiko Ui Shoko Nishihara M. Sakuma S. Togashi R. Ueda Y. Miyata T. Miyake 《In vitro cellular & developmental biology. Animal》1994,30(4):209-216
Summary From the central nervous system ofDrosophila melanogaster 3rd instar larvae, eight continuous cell lines have been established (named ML-DmBG1 to 8). Using ML-DmBG2, single colony
isolation was carried out and six colonial clones were obtained. All reacted to the antibody to horseradish peroxidase, which
is a neuronal marker in insects. Acetylcholine, a known neurotransmitter inDrosophila, was detected in three of the colonial clones by high performance liquid chromatography. Therefore, it is concluded that
the established colonial clones are neural cells originating in the larval central nervous system. Among them, some variation
was observed with respect to morphology, acetylcholine content, and reactivity to anti-HRP. The variation may reflect the
heterogeneity of cells composing the central nervous system. 相似文献
956.
Stephen D. Bird Robert J. Walker Michael J. Hubbard 《In vitro cellular & developmental biology. Animal》1994,30(7):420-424
Summary The effect of a conventional antibiotic (penicillin/streptomycin) mixture on the widely used kidney epithelial cell line,
LLC-PK1, was investigated by measuring growth and intracellular free calcium. Free calcium concentration was the same in cells cultured
for 3 to 7 wk with (“plus”) and without (“minus”) antibiotics both at rest and when challenged with high (14 mM) external calcium. When exposed to vasopressin, minus cells exhibited significantly smaller calcium transients than plus
cells. A similar difference existed for transients elicited by a calcium ionophore, 4-br-A23187. After longer periods of culture
(>20 wk), minus cells grew slower than plus cells but on reaching confluence (minus cells took 1 day longer) the morphologies
and viabilities were indistinguishable. The finding that culture with penicillin/streptomycin reversibly modified some properties
of LLC-PK1 cells, at least partly through altered calcium homeostasis, is of importance for workers using this cell model to study drug
effects and raises the general possibility of similar effects on other cultured cells. 相似文献
957.
P. J. Hatt M. Moriniere H. Oberlander P. Porcheron 《In vitro cellular & developmental biology. Animal》1994,30(10):717-720
Summary During postembryonic development of insects, molting cycles affect epidermal cells with alternate periods of proliferation
and differentiation. Cells of the cell line established from imaginal discs of the Indian meal moth (IAL-PID2) differentiate
under the action of the molting hormone, 20-hydroxyecdysone, in a manner that is meaningful in terms of the development of
the tissue from which they were derived. In particular, the hormone caused an accumulation of the cells in the G2 phase of
their cycle and induced the formation of epithelial-like aggregates and the synthesis of specific proteoglycans. Recent discovery
of members of the insulin superfamily in insects and the role of growth factors played by this family of molecules in vertebrates
led us to check for their potential effects on IAL-PID2 cell cycle regulation. On the one hand, our results showed that insulin
was involved in partial resumption of the cell cycle after an arrest caused by serum deprivation, but that other growth factors
present in fetal calf serum were needed for full completion of mitosis. On the other hand, the cytostatic effect of 20-hydroxyecdysone
was reversible, and, prior exposure of the cells to the hormone allowed the cells to complete one cell cycle in serum-free
medium. These results suggest that the production of autocrine growth factors induced by ecdysteroids could circumvent the
absence of serum. This cell culture model provides potential for further study of interactions between ecdysteroids and growth
factor homologs during differentiation of insect epidermal cells. 相似文献
958.
959.
960.
Esterina Pascale Christine Liu Eulalia Valle Karen Usdin Anthony V. Furano 《Journal of molecular evolution》1993,36(1):9-20
Summary All modern mammals contain a distinctive, highly repeated (⩾50,000 members) family of long interspersed repeated DNA called
the L1 (LINE 1) family. While the modern L1 families were derived from a common ancestor that predated the mammalian radiation
∼80 million years ago, most of the members of these families were generated within the last 5 million years. However, recently
we demonstrated that modern murine (Old World rats and mice) genomes share an older long interspersed repeated DNA family
that we called Lx. Here we report our analysis of the DNA sequence of Lx family members and the relationship of this family
to the modern L1 families in mouse and rat. The extent of DNA sequence divergence between Lx members indicates that the Lx
amplification occurred about 12 million years ago, around the time of the murine radiation. Parsimony analysis revealed that
Lx elements were ancestral to both the modern rat and mouse L1 families. However, we found that few if any of the evolutionary
intermediates between the Lx and the modern L1 families were extensively amplified. Because the modern L1 families have evolved
under selective pressure, the evolutionary intermediates must have been capable of replication. Therefore, replicationcompetent
L1 elements can reside in genomes without undergoing extensive amplification. We discuss the bearing of our findings on the
evolution of L1 DNA elements and the mammalian genome. 相似文献