全文获取类型
收费全文 | 8803篇 |
免费 | 1524篇 |
国内免费 | 1652篇 |
专业分类
11979篇 |
出版年
2024年 | 105篇 |
2023年 | 339篇 |
2022年 | 432篇 |
2021年 | 579篇 |
2020年 | 512篇 |
2019年 | 534篇 |
2018年 | 453篇 |
2017年 | 427篇 |
2016年 | 455篇 |
2015年 | 388篇 |
2014年 | 541篇 |
2013年 | 605篇 |
2012年 | 420篇 |
2011年 | 430篇 |
2010年 | 336篇 |
2009年 | 383篇 |
2008年 | 453篇 |
2007年 | 506篇 |
2006年 | 457篇 |
2005年 | 372篇 |
2004年 | 356篇 |
2003年 | 330篇 |
2002年 | 293篇 |
2001年 | 230篇 |
2000年 | 169篇 |
1999年 | 198篇 |
1998年 | 177篇 |
1997年 | 155篇 |
1996年 | 152篇 |
1995年 | 157篇 |
1994年 | 137篇 |
1993年 | 129篇 |
1992年 | 96篇 |
1991年 | 112篇 |
1990年 | 69篇 |
1989年 | 61篇 |
1988年 | 53篇 |
1987年 | 46篇 |
1986年 | 42篇 |
1985年 | 41篇 |
1984年 | 36篇 |
1983年 | 27篇 |
1982年 | 38篇 |
1981年 | 30篇 |
1980年 | 38篇 |
1979年 | 17篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1976年 | 8篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Early mouse embryos produce and release factors with transforming growth factor activity 总被引:2,自引:0,他引:2
Angie Rizzino 《In vitro cellular & developmental biology. Plant》1985,21(9):531-536
Summary Previous studies have shown that extracts from mouse embryos at mid and late stages of development contain factors that exhibit
transforming growth factor activity. The work reported here demonstrates that cultured mouse embryos at significantly earlier
stages of development produce and release factors that exhibit the characteristic property of transforming growth factors.
Specifically, the data demonstrate that embryos cultured from the blastocyst stage in serum-containing medium or in serum-free
medium release factors that promote the anchorage-independent growth of normal rat kidney fibroblasts. It is shown that these
factors are produced and released by cells derived from the inner cell mass and by trophoblasts. The precise developmental
stage when production of these factors first begins has not been determined but our findings suggest that these factors are
produced by cell types associated with early postimplatation embryos.
This work was supported by the Laboratory of Viral Carcinogenesis at the National Cancer Institute and by grants from the
National Cancer Institute (CA-36727) and the University of Nebraska Medical Center (22-271-732).
Editor's Statement This paper presents evidence that, in an in vitro assay system, early embryonic cells are capable of both
synthesizing and secreting TGF-like growth factors, implicating the production of these factors in the events of early development.
David W. Barnes 相似文献
52.
Meiofauna of a sewage-polluted sandy beach, where sand alone constituted > 90%, was surveyed. Nematodes dominated the fauna
numerically at all stations, followed by harpacticoid copepods. Most of the animals were confined to the top 5 cm of the sediment.
A seasonal pattern was observed in the distribution of the fauna. There were significant spatial and temporal variations in
mean meiofauna density, attributed to organic discharge via sewage and prevailing environmental conditions in the study area. 相似文献
53.
Phytoplankton ecology in an unusually stable environment (Montezuma Well,Arizona, U.S.A.) 总被引:1,自引:0,他引:1
Relatively minor annual amplitudes of change in certain major nutrients, and especially pH and water temperature were measured in the spring-fed system of Montezuma Well, Arizona during a four year study. phytoplankton diversity was low but for the most part, composition was spatially and temporally constant; total seasonal phytoplankton density was significantly correlated with regional incident light. Phytoplankton species composition changed briefly during and for a short period following the summer monsoon. Ultraplankton (<5 µm diam.) numerically comprised nearly 80% of the phytoplankton community throughout most of the year. The limited residence time of water in the Well may have provided a competitive advantage for cells with high surface area:volume ratios and correspondingly rapid division rates. Nannochloris bacillaris Naum. and Coccomyxa minor Skuja were perennial dominants. Diatom populations did not increase with annual increases in vernal solar radiation. Low pH, high dissolved CO2, and limited residence time for metabolic inhibitors are considered to be largely responsible for the reduced blue-green populations in the Well. The only flagellated photosynthetic group present in Montezuma Well was the Cryptophyta. Desmid populations were minimal, even though pH was consistently below circumneutral (6.5) and free CO2 concentrations high. The role of grazing by an amphipod, Hyalella montezuma, on annual phytoplankton abundance is examined. 相似文献
54.
Hiroyoshi Hoshi Mikio Kan Wallace L. McKeehan 《In vitro cellular & developmental biology. Plant》1987,23(10):723-732
Summary Hepatocytes were isolated from human fetal liver in order to analyze the direct effects of growth factors and hormones on
human hepatocyte proliferation and function. Mechanical fragmentation and then dissociation of fetal liver tissue with a collagenase/dispase
mixture resulted in high yield and viability of hepatocytes. Hepatocytes were selected in arginine-free, ornithine-supplemented
medium and defined by morphology, albumin production and ornithine uptake into cellular protein. A screen of over twenty growth
factors, hormones, mitogenic agents and crude organ and cell extracts for effect on the stimulation of hepatocyte growth revealed
that EGF, insulin, dexamethasone, and factors concentrated in bovine neural extract and hepatoma cell-conditioned medium supported
attachment, maintenance and growth of hepatocytes on a collagen-coated substratum. The population of cells selected and defined
as differentiated hepatocytes had a proliferative potential of about 4 cumulative population doublings. EGF and insulin synergistically
stimulated DNA synthesis in the absence of other hormones and growth factors. Although neural extracts enhanced hepatocyte
number, no effect on DNA synthesis of neural extracts or purified heparin-binding growth factors from neural extracts could
be demonstrated in the absence or presence of defined hormones, hepatoma-conditioned medium or serum. Hepatoma cell-conditioned
medium had the largest impact on both hepatocyte cell number and DNA synthesis under all conditions. Dialyzed serum protein
(1 mg/ml) at 10 times higher protein concentration had a similar effect to hepatoma cell-conditioned medium (100 μg/ml). The
results suggest that hepatoma cell conditioned medium may be a concentrated and less complicated source than serum for purification
and characterization of additional normal hepatocyte growth factors.
This work was supported by NIH grant DK35310.
Editor’s statement Many investigators have struggled with the special problems associated with culture of differentiated hepatocytes.
In this paper attention is given to the specific growth factor requirements for fetal human hepatocytes. The observation that
factors from hepatoma conditioned medium or neural extracts enhanced the growth of the cells may indicate that additional
growth factors are to be identified that are important in the survival and proliferation of hepatocytes, and may also indicate
that the malignant transformation of these cells may involve the production of autocrine growth stimulators. 相似文献
55.
Different hormonal requirements for androgen-independent growth of normal and tumor epithelial cells from rat prostate 总被引:4,自引:0,他引:4
Wallace L. McKeehan Pamela S. Adams Danna Fast 《In vitro cellular & developmental biology. Plant》1987,23(2):147-152
Summary The proliferation of isolated normal prostate epithelial cells from rat and man is androgen-independent and requires cholera
toxin, insulin, dexamethasone, epidermal growth factor (EGF) and one or more polypeptide factors that are concentrated in
bovine neural tissue. The active agents in the neural tissue extract are heparin-binding polypeptides (prostatropins), the
predominant form of which has a molecular weight of 17400 and an acetylalanine at the aminoterminus. Prostatropins supported
a half-maximal increase in normal prostate epithelial cell number at 50 picomolar. The proliferation of primary and serially-cultured
epithelial cells from androgen-responsive Dunning R3327 rat prostate tumors was also androgen-independent, but exhibited dramatic
alterations in response to hormones that stimulated normal cell proliferation. At low cell density, androgen-independent growth
of isolated tumor-derived epithelial cells was independent on cholera toxin, was stimulated by dexamethasone, required insulin
andeither EGFor prostatropin. The presence of either EGF or prostatropin masked the response to the other factor. In the absence of EGF,
purified prostatropins supported a half-maximal increase in tumor cell number at 7 picomolar. Endogenous production of EGF-like
and prostatropin-like factors or both was suggested by the reduced requirement for EGF and prostatropin at high prostate tumor
cell density. These results suggest that anti-hormonal therapies against prostate tumor growth should be based on intervention
with the activity of insulin (or insulin-like factors) or simultaneous intervention with both EGF and prostatropin (or their
homologues).
This work was supported by NIH grants CA 37589 and HL 33847, and grant 1718 from the Council for Tobacco Research.
Editor’s Statement This paper is the first report of the comparison of the hormone requirements of primary cultures of normal
and tumor prostate epithelial cells from the same system. 相似文献
56.
本文根据沉积物成因、古气候和古生物特征,把禄丰古猿化石产地新第三纪地层划分为晚中新世石灰坝组和早上新世庙山坡组。据岩性和岩相特征,可划分出五个不同的沉积阶段,每个阶段代表不同的沉积环境。古猿和其他脊椎动物化石主要埋在石灰坝组(四段)湖泊沼泽化沉积环境,次之为庙山坡组(五段)河流相沉积环境。 相似文献
57.
The postribosomal particle of rabbit liver contains protein-synthesis factors and serum albumin mRNA
As demonstrated by indirect immunoprecipitation and polyacrylamide gel electrophoresis, an 85S particle separated by sucrose density-gradient centrifugation from the postribosomal pellet of rabbit liver, is able to synthesize serum albumin if supplemented with both ribosomal subunits and sources of energy. It is retained on heparin bound to Sepharose 4B, contains translatable mRNA and apparently all protein factors required for translation. This particle may represent a highly organized protein synthesizing machinery, the combination of which with ribosomes results in formation of new protein molecules. 相似文献
58.
Abstract: The higher-molecular-weight elongation factor-1 (EF-1H) of the chick brain was observed to contain three subunits (denominated α, β, and γ), contrary to a previous report that the brain EF-1H consisted of aggregates of low-molecular-weight elongation factor- 1 (EF-1L). Crude EF-1H, obtained from 20-day embryonic brain, was treated with 0.4 M ammonium chloride and 0.1 mM GTP, and EF-1βγ, was obtained using a DEAE-Sephadex column equilibrated in 0.025 mM GTP. Both EF-1β, and EF-1γ, were isolated by means of a DE-52 column equilibrated in 6 M urea and were found to have molecular weights of 2.8 and 4.8 × 104, respectively. EF-1β and EF-1γ were also obtained from young rat and calf brains by the same procedures. The molecular weight of the isolated EF-1α was 5 × 104. It was found that EF-1β stimulated the two EF-1α-dependent reactions, i.e., phenylalanyl-tRNA binding (reaction 1) and polyphenylalanine synthesis (reaction 2), and also stimulated the nucleotide exchange reaction in the EF- 1α-guanine nucleotide binary complex (reaction 3). The degrees of stimulation of reactions 1, 2, and 3 by the addition of EF-1β were 2 to 3 times, about 18 times, and 2 to 3 times as much as with EF-1α alone, respectively. The amino acid compositions of EF-1α -1β, and -1γ and EF-2 were very similar to those of other eukaryotic tissues. Thus the constituents and properties of EFs of the brain were found to be basically similar to those of other tissues of eukaryotes, although EF-1β, and EF-1, had not been reported in the brain. A possible physiological significance of EF-1β during brain development is also discussed. 相似文献
59.
Michael Ready Sandra Bird Gail Rothe J.D. Robertus 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,740(1):19-28
It has been known for some time that pokeweed antiviral protein acts by enzymatically inhibiting protein synthesis on eucaryotic ribosome systems. The site of this action is known to be the ribosome itself. In this paper we show that the pokeweed antiviral protein reaction against ribosomes is a strong function of salt concentrations, where 160 mM K+ and 3 mM Mg2+ retards the reaction, while 20 mM K+ and 2 mM Mg2+ allows maximum reaction rate. It is also shown, however, that an unidentified protein in the postribosomal supernatant solution, together with ATP, allows the ribosome to be attacked even in the presence of high salt. Kinetic analysis of the antiviral protein reaction has been carried out under both sets of conditions, and reveals that the turnover number for the enzyme is about 300–400 mol/mol per min. in each case. The Km for ribosomes is 1 μM in the presence of low salt and 0.2 μM at higher salt in the presence of postribosomal supernatant factors plus ATP. The antiviral protein reaction is also shown to be pH dependent and is controlled by a residue with pKa value of approx. 7.0, apparently a histidine. Stoichiometric reaction of the enzyme with iodoacetamide results in a significant loss of antiribosomal activity. 相似文献
60.
Yuhsi Matuq Pamela S. Adams Nozomu Nishi Hidetaro Yasumitsu John W. Crabb Robert J. Matusik Wallace L. McKeehan 《In vitro cellular & developmental biology. Plant》1989,25(6):581-584
Summary Rat prostate extracts contain an abundant 20–22 kilodalton heparin-binding protein with near identical chromatographic properties,
but only 0.2–1% of the mitogenic activity, of bovine brain heparin-binding growth factor-1 (acidic fibroblast growth factor).
Amino terminal amino acid sequence (met-met-thr-asp-lys-asn-leu-lys-lys-lys-ile-glu-gly-asn-trp-arg-thr-val-tyr-leu-ala-ala-ser-?-val-glu-lys-ile-asn-glu-gly-ser-pro)
and immunochemical analysis revealed that the protein is identical to the androgen-dependent protein “probasin”.
This work was supported in part by NCI grant CA37589 (W. L. M., J. W. C.) and the Medical Research Council of Canada (R. J.
M.). 相似文献