首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   276077篇
  免费   30202篇
  国内免费   258篇
  306537篇
  2016年   3079篇
  2015年   4295篇
  2014年   4850篇
  2013年   7378篇
  2012年   7938篇
  2011年   8192篇
  2010年   5429篇
  2009年   5008篇
  2008年   7419篇
  2007年   7526篇
  2006年   7161篇
  2005年   6913篇
  2004年   6819篇
  2003年   6604篇
  2002年   6392篇
  2001年   11751篇
  2000年   11794篇
  1999年   9327篇
  1998年   3388篇
  1997年   3621篇
  1996年   3518篇
  1995年   3126篇
  1994年   3108篇
  1993年   3103篇
  1992年   7868篇
  1991年   7852篇
  1990年   7514篇
  1989年   7474篇
  1988年   6852篇
  1987年   6601篇
  1986年   6020篇
  1985年   6207篇
  1984年   5146篇
  1983年   4338篇
  1982年   3373篇
  1981年   3144篇
  1980年   2944篇
  1979年   4950篇
  1978年   3811篇
  1977年   3724篇
  1976年   3432篇
  1975年   3785篇
  1974年   4175篇
  1973年   4092篇
  1972年   3666篇
  1971年   3435篇
  1970年   3086篇
  1969年   2995篇
  1968年   2745篇
  1967年   2370篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Several products derived from processed maca hypocotyls (Lepidium peruvianum Chacón, previously known asL. meyenii Walp.) were surveyed for glucosinolate content and quantified by HPLC analysis. These included pills, capsules, flour, liquor, tonic and mayonnaise. Different plant organs such as fresh hypocotyls and leaves, seeds, dry hypocotyls, and sprouts were also included in the survey. The most abundant glucosinolates detected in fresh and dry hypocotyls and leaves were the aromatic glucosinolates, benzylglucosinolate (glucotropaeolin) and p-methoxybenzylglucosinolate. Maca seeds and sprouts differed in profile from hypocotyls and leaves due to the modification of benzylglucosinolate. No glucosinolates were detected in liquor and tonic, while mayonnaise had only trace amounts of those glucosinolates. It had instead allylglucosinolate (sinigrin), which is an aliphatic glucosinolate. The pills, capsules and flour had the same glucosinolates as those observed in hypocotyls, but in variable amounts. The richest sources of glucosinolates were seeds, fresh hypocotyls and sprouts, in that order.  相似文献   
942.
943.
Hybrid cells were isolated by fusing primary chicken myoblasts to HPRT-deficient rat L6 myoblasts and incubating the cells in medium containing HAT and ouabain. All hybrid clones contained both rat and chicken chromosomes and expressed a number of gene products characteristic of both species. Although all clones were capable of fusing spontaneously to form myofibers, immunofluorescence and isoenzyme analysis revealed only the rat forms of skeletal muscle myosin and MM-creatine kinase. No differentiated gene products of chicken origin were detected. Analysis of the expression of chicken HPRT revealed that some hybrid clones were capable of modulating this enzyme activity when switched from HAT medium into thioguanine medium and back into HAT, even though HPRT is normally a constitutively expressed enzyme. Parental control cells were incapable of this modulation phenomenon.  相似文献   
944.
945.
Human bone marrow contains a population of haemopoietic progenitor cells that can be distinguished by their ability to adhere to preformed stromal layers (cultured in the presence of methylprednisolone [MP+] and form blast cell colonies. The stromal layers function in the colony assay after they have been heavily irradiated but not after they have been passaged. The binding of the progenitor cells to the stromal cells is complete after 2 hours of coincubation, and stromal layers of 9.6 cm2 can provide adhesion sites for at least 2,000 blast colony-forming cells. The blast colony-forming cells were shown by micromanipulation to self-renew as well as to give rise to multipotential and lineage-committed colony-forming progenitor cells.  相似文献   
946.
947.
The brain of the lizard, Ctenosauria pectinata, was studied light microscopically using an immunocytochemical staining method that is specific for neurohypophysial hormones and somatostatin. It was shown that the telencephalon and particularly the diencephalon contain somatostatin-producing perikarya, while somatostatinergic fibers occur in the entire brain. Similar to the situation in other vertebrates, somatostatin neurons in Ctenosauria pectinata form a population distinct from the neurohypophysial hormone-producing neurons. The small-sized somatostatin neurons were found in the cortex and the diencephalon: (1) ventral from, and partially overlapping with, the classical neurosecretory paraventricular nucleus; and (2) in the region of the infundibular (tuberal) nucleus. Somatostatin fibers were found among the classical neurosecretory fibers of the supraoptico-paraventricular system (tract, median eminence, neural lobe), near to and within the epiphysis, in the septum, in the vicinity of the tectum opticum and the cerebellum, and in the tegmentum.  相似文献   
948.
Treatment in a group of 19 patients with congenital adrenal hyperplasia (CAH) has been monitored by frequent, serial measurements of saliva 17OH-progesterone (17OHP) concentrations. Detailed 17OHP profiles were obtained during consecutive weekend days and every 1-2 h over a separate 24-hour period. Patients showed a marked diurnal rhythm in 17OHP levels, particularly when treated with hydrocortisone. In some patients, 10 mg/m2/day of hydrocortisone was sufficient glucocorticoid replacement to produce adequate control, although there was considerable individual variation. Saliva 17OHP profiles provided valuable information on the efficacy of hydrocortisone, cortisone acetate, prednisolone and dexamethasone as glucocorticoid suppressive regimes in the treatment of CAH. Preliminary results suggest that hydrocortisone given in two divided doses during the day, supplemented by a small dose of prednisolone at bedtime, is suitable treatment for CAH patients who are still growing. In the patient who has completed statural growth, single daily dose dexamethasone therapy ensures adequate adrenal suppression and is convenient in the longterm.  相似文献   
949.
The importance of calcium in lymphocyte activation is well recognized, but the levels of extracellular ionized free calcium (Ca++) necessary for lymphocyte proliferation via various pathways have not been investigated in detail. We studied the ability of a lectin mitogen (PHA) and a calcium ionophore (ionomycin) to induce interleukin 2 receptors, interleukin 2 (IL2) production, and proliferation over various concentrations of extracellular Ca++. Reducing the Ca++ levels from the normal 200 microM to 10 microM in PHA-stimulated cultures partially inhibited IL2 receptor expression, IL2 production, and subsequent proliferation. At 1 microM Ca++, both IL2 activity and proliferation were eliminated, but partial IL2 receptor expression was still observed. Ionomycin did not induce any of these events in cultures where the extracellular Ca++ concentration was below 100 microM. Restoring calcium in the medium resulted in normal levels of IL2 receptor expression, IL2 activity, and proliferation when PBL were stimulated with either mitogen. Exogenous magnesium partially restored these events in PHA-stimulated cultures, but had no effect when ionomycin was used as the mitogen. These data indicate that stimulation by ionomycin is much more dependent upon the levels of extracellular Ca++ than is PHA. Extracellular calcium also appears to be necessary subsequent to IL2 receptor acquisition, since the latter was seen without IL2 activity or proliferation at very low extracellular Ca++, and IL2 failed to restore the proliferative response under these conditions. The data also suggest that PHA, but not ionomycin, can activate lymphocytes via a magnesium-dependent pathway, or that PHA has a lower specificity for divalent cation cofactors.  相似文献   
950.
The structure of a small-molecule, non-peptide chemotactic factor has been determined from activity purified to apparent homogeneity from Helicobacter pylori supernatants. H. pylori was grown in brucella broth media until one liter of solution had 0.9 absorbance units. The culture was centrifuged, and the bacteria re-suspended in physiological saline and incubated at 37 degrees C for 4 h. A monocyte migration bioassay revealed the presence of a single active chemotactic factor in the supernatant from this incubation. The chemotactic factor was concentrated by solid phase chromatography and purified by reverse phase high pressure liquid chromatography. The factor was shown to be indistinguishable from diethyl phthalate (DEP) on the basis of multiple criteria including nuclear magnetic resonance spectroscopy, electron impact mass spectroscopy, UV visible absorption spectrometry, GC and high pressure liquid chromatography retention times, and chemotactic activity toward monocytes. Control experiments with incubated culture media without detectable bacteria did not yield detectable DEP, suggesting it is bacterially derived. It is not known if the bacteria produce diethyl phthalate de novo or if it is a metabolic product of a precursor molecule present in culture media. DEP produced by H. pylori in addition to DEP present in man-made products may contribute to the high levels of DEP metabolites observed in human urine. DEP represents a new class of chemotactic factor.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号