全文获取类型
收费全文 | 4534篇 |
免费 | 365篇 |
国内免费 | 2篇 |
专业分类
4901篇 |
出版年
2023年 | 16篇 |
2022年 | 39篇 |
2021年 | 71篇 |
2020年 | 39篇 |
2019年 | 55篇 |
2018年 | 66篇 |
2017年 | 68篇 |
2016年 | 118篇 |
2015年 | 163篇 |
2014年 | 213篇 |
2013年 | 277篇 |
2012年 | 305篇 |
2011年 | 296篇 |
2010年 | 236篇 |
2009年 | 186篇 |
2008年 | 267篇 |
2007年 | 265篇 |
2006年 | 221篇 |
2005年 | 227篇 |
2004年 | 245篇 |
2003年 | 250篇 |
2002年 | 219篇 |
2001年 | 73篇 |
2000年 | 55篇 |
1999年 | 62篇 |
1998年 | 79篇 |
1997年 | 42篇 |
1996年 | 37篇 |
1995年 | 36篇 |
1994年 | 32篇 |
1993年 | 35篇 |
1992年 | 53篇 |
1991年 | 52篇 |
1990年 | 37篇 |
1989年 | 39篇 |
1988年 | 34篇 |
1987年 | 24篇 |
1986年 | 19篇 |
1985年 | 19篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 22篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1974年 | 16篇 |
1973年 | 11篇 |
1972年 | 13篇 |
1970年 | 11篇 |
排序方式: 共有4901条查询结果,搜索用时 15 毫秒
81.
Denis A. Magoffin Gregory F. Erickson 《In vitro cellular & developmental biology. Plant》1988,24(9):862-870
Summary Although luteinizing hormone (LH) alone stimulates ovarian interstitial cells cultured in serum-free medium to synthesize
large amounts of androgens, there seem to be additional factors in vivo that modulate the time course and magnitude of the
cellular responses to LH. In an attempt to develop a more nearly physiologic cell culture model, lipoproteins, insulin, and
insulinlike growth factor-I (IGF-I) were added to the serum-free medium. The effects of these modifications on androgen biosynthesis
by dispersed cells from ovaries of hypophysectomized immature rats cultured in 96-well tissue culture plates were examined.
A saturating dose of LH stimulated a 25-fold increase in androsterone synthesis at 2 d, which decreased at 4 and 6 d. Addition
of human high density (hHDL) or human low density lipoprotein (hLDL) caused a 2.5-fold increase in LH-stimulated androsterone
synthesis. Cells were approximately twice as sensitive to hHDL (ED50=5.5±0.5 μg cholesterol/ml) compared to hLDL (ED50=9.1±1.1 μg cholesterol/ml). Surprisingly, rat HDL caused only a 40% increase in LH-stimulated androsterone synthesis. When
insulin alone was added to cells cultured with a saturating dose of LH, there was a 2.8-fold increase in androsterone synthesis.
Addition of hHDL and insulin together caused a synergistic increase in LH-stimulated androsterone synthesis. In contrast to
hHDL, which did not change the time course of LH-stimulated androsterone production, insulin prolonged maximal LH-stimulated
androsterone synthesis at 4 and 6 d. Inasmuch as the ED50 for insulin action (1.3±0.1 μg/ml) was supraphysiologic, the effects of IGF-I on LH-stimulated androgen synthesis were examined.
IGF-I mimicked all of the effects of insulin, but at a physiologic concentration (ED50=2.5±0.3 ng/ml). Ovarian cells cultured in serum-free medium supplemented with hHDL and insulin or IGF-I exhibit responses
that closely approximate the physiologic responses observed in vivo. These results suggest that lipoproteins and IGF-I are
important physiologic stimulators of ovarian theca-interstitial cell androgen biosynthesis which, when added to the serum-free
medium, make the cellular responses in this in vitro model more nearly approximate the responses in vivo.
This research was supported by research center grant HD 12303 from the National Institute of Child Health and Human Development,
Bethesda, MD, and USCD Academic Senate grant RM-169M 相似文献
82.
Vujanovic Vladimir; St-Arnaud Marc; Barabe Denis; Thibeault Genevieve 《Annals of botany》2000,86(1):79-86
This study reports the ability of Fusarium to induce orchidseed colouration and germination. The in vitro bioassay germinationtest, using a Fusarium isolate from the protocorm of Cypripediumreginae, was compared with standard chemical procedures of triphenyltetrazolium chloride (TTC) and acid fuchsin (AC) for testingseed viability. With Cypripedium reginae, Cypripedium parviflorumand Platanthera grandiflora, the efficiency of the bioassaywas similar to that of the TTC and AC procedures. However, thebioassay was more appropriate for estimating embryo viabilityafter a prolonged seed pretreatment (more than 2 h) in 10% sodiumhypochlorite, a surface sterilant often used to enhance germinationof terrestrial species. We also obtained in vitro Cypripediumreginae seed germination induction and protocorm formation bythe same Fusarium isolate. This is the first confirmation ofBernard's early reports that orchid fusaria could stimulateseed germination (Bernard N. 1990.Révue Généralede Botanique12 : 108120). However, the importance ofthe non-mycorrhizal Fusarium fungus in promoting germinationseems to be relatively minor compared to that of specificRhizoctoniaorchid mycorrhizas. Our results are discussed in light of thecurrent North American strategy on orchid conservation methodswhich proposes the use of symbiotic germination.Copyright 2000Annals of Botany Company Orchid, Cypripedium, Platanthera, seed, Fusarium, bioassay, staining, viability, germination, protocorm, mycorrhiza 相似文献
83.
Pacheco-Oliver M McDonald IR Groleau D Murrell JC Miguez CB 《FEMS microbiology letters》2002,209(2):313-319
Tundra soil samples from the Canadian Arctic community, Kuujjuaq, were analyzed for the presence of the soluble (sMMO) and particulate (pMMO) methane monooxygenase genes. Total genomic DNA extracted from these soils was used as template for PCR using sMMO- and pMMO-specific primers, mmoX1-mmoX2 and A189-A682, respectively. pMMO and sMMO genes were detected in the Kuujjuaq soil samples. Isolation of sMMO-possessing methanotrophic microorganisms from the three soils, as determined by the colony naphthalene oxidation assay, was carried out using direct plating (5 degrees C) and methane enrichment studies (5 degrees C and 25 degrees C). Direct plating did not yield sMMO-possessing methanotrophic bacteria, whereas methane enrichments yielded isolates possessing and expressing sMMO activity. Analysis of derived amino acid sequences of pmoA genes and partial 16S rRNA genes obtained by PCR, using DNA isolated directly from this environment and from isolates, revealed the presence of highly divergent PmoA/AmoA sequences and 16S rRNA sequences that cluster closely with but are distinct from the genes from the genera Methylosinus and Methylocystis. 相似文献
84.
Cancer development is a multistep process often starting with a single cell in which a number of epigenetic and genetic alterations have accumulated thus transforming it into a tumor cell. The progeny of such a single benign tumor cell expands in the tissue and can at some point progress to malignant tumor cells until a detectable tumor is formed. The dynamics from the early phase of a single cell to a detectable tumor with billions of tumor cells are complex and still not fully resolved, not even for the well-known prototype of multistage carcinogenesis, the adenoma-adenocarcinoma sequence of colorectal cancer. Mathematical models of such carcinogenesis are frequently tested and calibrated based on reported age-specific incidence rates of cancer, but they usually require calibration of four or more parameters due to the wide range of processes these models aim to reflect. We present a cell-based model, which focuses on the competition between wild-type and tumor cells in colonic crypts, with which we are able reproduce epidemiological incidence rates of colon cancer. Additionally, the fraction of cancerous tumors with precancerous lesions predicted by the model agree with clinical estimates. The correspondence between model and reported data suggests that the fate of tumor development is majorly determined by the early phase of tumor growth and progression long before a tumor becomes detectable. Due to the focus on the early phase of tumor development, the model has only a single fit parameter, the time scale set by an effective replacement rate of stem cells in the crypt. We find this effective rate to be considerable smaller than the actual replacement rate, which implies that the time scale is limited by the processes succeeding clonal conversion of crypts. 相似文献
85.
86.
87.
88.
89.
90.