全文获取类型
收费全文 | 6167篇 |
免费 | 556篇 |
国内免费 | 305篇 |
出版年
2024年 | 14篇 |
2023年 | 138篇 |
2022年 | 185篇 |
2021年 | 267篇 |
2020年 | 285篇 |
2019年 | 257篇 |
2018年 | 229篇 |
2017年 | 197篇 |
2016年 | 202篇 |
2015年 | 202篇 |
2014年 | 372篇 |
2013年 | 423篇 |
2012年 | 266篇 |
2011年 | 323篇 |
2010年 | 272篇 |
2009年 | 274篇 |
2008年 | 267篇 |
2007年 | 274篇 |
2006年 | 256篇 |
2005年 | 213篇 |
2004年 | 184篇 |
2003年 | 157篇 |
2002年 | 141篇 |
2001年 | 114篇 |
2000年 | 99篇 |
1999年 | 99篇 |
1998年 | 76篇 |
1997年 | 100篇 |
1996年 | 85篇 |
1995年 | 87篇 |
1994年 | 92篇 |
1993年 | 67篇 |
1992年 | 61篇 |
1991年 | 69篇 |
1990年 | 62篇 |
1989年 | 60篇 |
1988年 | 49篇 |
1987年 | 33篇 |
1986年 | 25篇 |
1985年 | 52篇 |
1984年 | 59篇 |
1983年 | 49篇 |
1982年 | 54篇 |
1981年 | 56篇 |
1980年 | 46篇 |
1979年 | 26篇 |
1978年 | 17篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1973年 | 19篇 |
排序方式: 共有7028条查询结果,搜索用时 31 毫秒
31.
Summary In the pelagic larvacean Oikopleura dioica, the epithelium lining the alimentary tract consists of ciliated and unciliated cell types. The ciliated cells also exhibit an apical border of long microvilli. Between the microvilli, the cellular membrane often projects deeply down into the cytoplasm; the membranes of these invaginations and those of apicolateral interdigitations may be associated with one another by tight junctions. Some of these junctions may be autocellular. The tight junctions are seen by freeze-fracture to be very simple in construction, composed of a single row of intramembranous particles, which may be fused into a P-face ridge. There is a dense cytoplasmic fuzz associated with these tight junctions which may extend into adjoining zonula adhaerens-like regions. The invaginations of the apical membranes are, in addition, associated by gap junctions which may also be autocellular. More conventional homocellular and heterocellular tight and gap junctions occur along the lateral borders of ciliated cells and between ciliated and unciliated cells. These gap junctions possess a reduced intercellular cleft and typical P-face connexons arranged in macular plaques, with complementary E-face pits. Both cell types exhibit extensive stacks of basal and lateral interdigitations. The tight junctions found here are unusual in that they are associated with a dense cytoplasmic fuzz which is normally more characteristic of zonulae adhaerentes. 相似文献
32.
Green islands/infection sites recorded higher cytokinin activity than surrounding tissue as well as non-inoculated tissue. This activity in infected areas increased with time of incubation while in tissue surrounding the green islands and non-inoculated tissue, cytokinin activity decreased with time of incubation. The culture filtrate extracts of H. maydis had cytokinin activity which increased with growth of the fungus. Cytokinin activity of thin-layer Chromatographic fractions from tissue and culture filtrate extracts revealed that a major portion of the activity was confined to Rf zone 0.6 to 0.8 which co-chromatographed with zeatin and zeatin riboside. Presence of zeatin and zeatin riboside in tissue and culture filtrates was confirmed by high performance liquid chromatography. Cytokinin substances, such as zeatin and zeatin riboside, increase at infection sites with growth of the pathogen suggesting they may be involved in the pathogenicity of H. maydis on maize. 相似文献
33.
34.
Howard D. Rees Robert W. Bonsall Dr. Richard P. Michael 《Cell and tissue research》1988,254(1):139-146
Summary Target cells for testosterone, dihydrotestosterone, and estradiol in the pituitary gland and genital tract of the male primate were localized by thaw-mount autoradiography, and high performance liquid chromatography was used to identify the metabolites of these steroids in cell nuclei. Castrated rhesus monkeys were injected with 3H-testosterone, 3H-dihydrotestosterone, or 3H-estradiol and killed 60 min later. In the anterior pituitary gland, fewer cells were labeled and less radioactivity was taken up by cell nuclei following the administration of either 3H-testosterone (4% of pars distalis cells and 5 dpm/g DNA) or 3H-dihydrotestosterone (5% of cells and 13 dpm/g DNA) than following the administration of 3H-estradiol (43% of cells and 214 dpm/g DNA). Most of the radioactivity in nuclei was in the form of the unmetabolized parent compound (78–94%). In prostate, seminal vesicles, and penis, 3H-dihydrotestosterone was the predominant form of nuclear radioactivity following both 3H-testosterone (67–90%) and 3H-dihydrostestosterone (94–97%) administration, and both androgens labeled epithelial and smooth muscle cells. In contrast, 3H-estradiol was taken up in unchanged form, by cell nuclei of the genital tract and it labeled connective tissue fibroblasts, but not epithelial cells. Thus, the distributions of target cells for androgens and estrogens were clearly different in all these tissues, and the uptake of testosterone resembled that of its androgenic rather than that of its estrogenic metabolite. 相似文献
35.
Cryptococcus neoformans was instilled intranasally into mice which were periodically sacrificed to determine the course of infection. Cryptococci persisted within the nasal passages throughout the 90 day study. Extranasal dissemination began 14–28 days after instillation and was still demonstrable 90 days post-exposure. Ten percent mortality was observed in mice receiving 106 cryptococci, while no mortality was observed in mice exposed to 103 or 104 cryptococci. Our research suggests that nasal colonization with C. neoformans can precede pulmonary and systemic cryptococcosis by weeks or months. 相似文献
36.
Substance P binding sites in the nucleus tractus solitarius were visualized with receptor autoradiography using Bolton-Hunter [125I]substance P. Substance P binding sites were found to have distinct patterns within the cat nucleus tractus solitarius. The majority of substance P binding sites were present in the medial, intermediate and the peripheral rim of the parvocellular subdivisions. Lower amounts of substance P binding sites were present in the commissural, ventrolateral, interstitial and dorsolateral subdivisions. No substance P binding sites were present in the central region of the parvocellular subdivision or the solitary tract. The localization of substance P binding sites in the nucleus tractus solitarius is very similar to the patterns of substance P immunoreactive fibers previously described for this region. Results of this study add further support for a functional role of substance P in synaptic circuits of the nucleus tractus solitarius. 相似文献
37.
Calcium-Dependent Evoked Release of N[3 H]Acetylaspartylglutamate from the Optic Pathway 总被引:2,自引:0,他引:2
Guochuan Tsai Gianluigi Forloni Michael B. Robinson Barbara L. Stauch Joseph T. Coyle 《Journal of neurochemistry》1988,51(6):1956-1959
N-Acetylaspartylglutamate (NAAG) is a neuropeptide localized to several putative glutamatergic neuronal systems, including the rodent optic pathway. To determine whether the peptide is released by depolarization, the superior colliculus of the rat was perfused with 2 microCi of [3H]NAAG, then with Krebs-bicarbonate buffer for 1 h, using a microdialysis system. Subsequently, 10-min fractions were collected and analyzed by HPLC for [3H]NAAG. Addition of 100 microM veratridine resulted in a several-fold increase in the evoked release of [3H]NAAG that was virtually abolished by coperfusion with Ca2+-free Krebs buffer containing 1 mM EGTA. When [3H]glutamate was used as the precursor, veratridine depolarization resulted in only an 80% increase in the release of [3H]NAAG. Prior enucleation of the right eye reduced the spontaneous release of [3H]NAAG by 50%, and the veratridine-evoked release by greater than 85%, from the left superior colliculus. These results suggest that NAAG is released upon depolarization and may serve as a neurotransmitter/neuromodulator in the optic tract. 相似文献
38.
我国沙漠中部地区主要不同生态类型植物脯氨酸的累积、光合、呼吸和叶绿素含量 总被引:21,自引:0,他引:21
中生植物脯氨酸含量为0.42mg/g.dw, 低于少浆旱生植物(1.73mg/g.dw)。多浆旱生植物脯氨酸含量最高(7.22mg/g.dw),为前两者的17倍和4倍,两类旱生植物在干旱条件下(非灌溉)脯氨酸含量均高于灌水处理。少浆与多浆旱生植物的光合强度(16.74,14.04CO2mg/g.dw.h)差异不大,而中生植物(37.57mg/g.dW.h)略高于多浆旱生植物(4.73CO2Mg/g.dw.h),与中生植物(7.60Co2mg/g.dw.h)接近,光合/呼吸值,少浆,多浆与中生植物分别为2.50,3.09和4.59,说明中生植物的合成明显大于消耗,季节动态中,中生植物显著高于两类旱生植物,叶绿素总量三类植物差异甚微。 相似文献
39.
Richard C. Woodman John T. Curnutte Bernard M. Babior 《Free radical biology & medicine》1988,5(5-6):355-361
We examined the effects of the recombinant human colony stimulating factors GM-CSF and G-CSF, cycloheximide (a protein synthesis inhibitor) and dihydrocytochalasin B (a microfilament disrupting agent) upon FMLP (N-formyl-methionyl-leucylphenylalanine)-stimulated O2 − production by neutrophils. We confirmed a time dependent augmentation of O2 − production following preincubation of neutrophils either alone or with colony stimulating factors. Furthermore, we found that GM-CSF, but not G-CSF, increased O2 − production at some concentrations of the stimulus. Preincubation of neutrophils with cycloheximide in the absence of CSF caused a marked fall in O2−-production that was first evident at 2 hours. The fall in O2−-forming capacity caused by cycloheximide was much less pronounced if dihydrocytochalasin B was also included in the preincubation buffer. These findings suggest a previously unrecognized role for de novo protein synthesis in maintaining the ability of neutrophils to manufacture O2−, and support earlier studies indicating that the cycling of FMLP receptors between the cell membrane and an intracellular compartment is important in determining the magnitude of the respiratory burst in FMLP-stimulated neutrophils. 相似文献
40.
F Puvion-Dutilleul E Pichard 《Biology of the cell / under the auspices of the European Cell Biology Organization》1992,76(2):139-150
Formation of progeny viruses in the nuclei of HeLa cells infected with adenovirus type 5 was studied at the ultrastructural level by in situ hybridization techniques allowing specific detection of either viral double-stranded DNA (dsDNA) or single-stranded DNA (ssDNA). Prior to the initiation of replication of viral genomes, infective DNA molecules which entered the nucleus of the target cell were randomly distributed among host chromatin fibers including nucleolus-associated chromatin. They were double-stranded, that is, without single-strand breaks. Such association of viral DNA with host condensed chromatin also occurred in mitosis. The initiation of viral genome replication occurred simultaneously with the appearance in the nucleoplasm of small fibrillar regions containing intermingled viral dsDNA and ssDNA. Later, at the intermediate stage of nuclear transformation, viral dsDNA and ssDNA molecules were almost entirely separated into two contiguous substructures. At this stage, viruses were observed occasionally in the vicinity of viral ssDNA accumulation sites. Still later, an additional substructure developed in the centre of the nucleus which consisted of large quantities of viral dsDNA, traces of viral ssDNA and abundant viruses. Portions of viral ssDNA were attached to some viruses even at late stage of nuclear transformation, an association which strongly suggests the occurrence of encapsidation of at least some of the viral genomes while they are still engaged in replication. 相似文献