全文获取类型
收费全文 | 10941篇 |
免费 | 859篇 |
国内免费 | 6篇 |
专业分类
11806篇 |
出版年
2023年 | 72篇 |
2022年 | 186篇 |
2021年 | 281篇 |
2020年 | 155篇 |
2019年 | 283篇 |
2018年 | 310篇 |
2017年 | 277篇 |
2016年 | 399篇 |
2015年 | 624篇 |
2014年 | 635篇 |
2013年 | 828篇 |
2012年 | 957篇 |
2011年 | 899篇 |
2010年 | 557篇 |
2009年 | 538篇 |
2008年 | 650篇 |
2007年 | 663篇 |
2006年 | 560篇 |
2005年 | 504篇 |
2004年 | 445篇 |
2003年 | 401篇 |
2002年 | 368篇 |
2001年 | 94篇 |
2000年 | 65篇 |
1999年 | 86篇 |
1998年 | 96篇 |
1997年 | 67篇 |
1996年 | 62篇 |
1995年 | 67篇 |
1994年 | 63篇 |
1993年 | 44篇 |
1992年 | 60篇 |
1991年 | 47篇 |
1990年 | 30篇 |
1989年 | 44篇 |
1988年 | 24篇 |
1987年 | 29篇 |
1986年 | 28篇 |
1985年 | 32篇 |
1984年 | 21篇 |
1983年 | 29篇 |
1982年 | 14篇 |
1981年 | 15篇 |
1980年 | 20篇 |
1979年 | 25篇 |
1978年 | 17篇 |
1977年 | 18篇 |
1976年 | 11篇 |
1974年 | 15篇 |
1972年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Marco Gessi Giovanni Monego Libero Lauriola Nicola Maggiano Franco O Ranelletti 《The journal of histochemistry and cytochemistry》2005,53(8):955-962
Parathyroid hormone-related peptide (hPTHrP) is expressed in human tissues and regulates cellular proliferation, differentiation, and apoptosis by an autocrine/paracrine loop. In rodent thymus, both parathormone and parathyroid hormone-related peptide (PTHrP) are expressed by thymic epithelial cells (TECs). The present study demonstrated by RT-PCR and immunohistochemistry that hPTHrP and parathyroid hormone-related peptide receptor type 1 (PTHR1) were expressed in human thymus at both RNA and protein levels. hPTHrP was expressed mainly in the thymic medulla by epithelial (cytokeratin-positive), mature dendritic (CD40+/86+) and plasmacytoid interleukin (IL)-3Ralpha1 cells. This protein was also present in some cells forming Hassall's bodies and a few subcapsular and cortical TECs. PTHR1 was expressed by scattered subcapsular and cortical TECs and by rare TECs in the medulla. Thymocytes did not express either hPTHrP or PTHR1. Primary cultures of human TECs revealed the presence of both hPTHrP and PTHR1 mRNAs, confirming the capacity of TECs to synthesize both peptides. Moreover, synthetic (1-39) hPTHrP peptide administered on cultured TECs induced the expression of IL-6 mRNA, suggesting that hPTHrP can regulate thymic functions by inducing in TECs the expression of IL-6, which is involved in the development and maturation of thymocytes. 相似文献
92.
A comparative analysis of microscopic alterations in modern and ancient undecalcified and decalcified dry bones 下载免费PDF全文
93.
Asymptotic distributions of kappa statistics and their differences with many raters,many rating categories and two conditions 下载免费PDF全文
Luca Grassano Guido Pagana Marco Daperno Enrico Bibbona Mauro Gasparini 《Biometrical journal. Biometrische Zeitschrift》2018,60(1):146-154
In clinical research and in more general classification problems, a frequent concern is the reliability of a rating system. In the absence of a gold standard, agreement may be considered as an indication of reliability. When dealing with categorical data, the well‐known kappa statistic is often used to measure agreement. The aim of this paper is to obtain a theoretical result about the asymptotic distribution of the kappa statistic with multiple items, multiple raters, multiple conditions, and multiple rating categories (more than two), based on recent work. The result settles a long lasting quest for the asymptotic variance of the kappa statistic in this situation and allows for the construction of asymptotic confidence intervals. A recent application to clinical endoscopy and to the diagnosis of inflammatory bowel diseases (IBDs) is shortly presented to complement the theoretical perspective. 相似文献
94.
Pia Gruber Filipe Carvalho Marco P. C. Marques Brian O'Sullivan Fabiana Subrizi Dragana Dobrijevic John Ward Helen C. Hailes Pedro Fernandes Roland Wohlgemuth Frank Baganz Nicolas Szita 《Biotechnology and bioengineering》2018,115(3):586-596
95.
Leishmania major, like all the other kinetoplastid protozoa, are unable to synthesize purines and rely on purine nucleobase and nucleoside acquisition across the parasite plasma membrane by specific permeases. Although, several genes have been cloned that encode nucleoside transporters in Leishmania and Trypanosoma brucei, much less progress has been made on nucleobase transporters, especially at the molecular level. The studies reported here have cloned and expressed the first gene for a L. major nucleobase transporter, designated LmaNT3. The LmaNT3 permease shows 33% identity to L. donovani nucleoside transporter 1.1 (LdNT1.1) and is, thus, a member of the equilibrative nucleoside transporter (ENT) family. ENT family members identified to date are nucleoside transporters, some of which also transport one or several nucleobases. Functional expression studies in Xenopus laevis oocytes revealed that LmaNT3 mediates high levels of uptake of hypoxanthine, xanthine, adenine and guanine. Moreover, LmaNT3 is an high affinity transporter with K(m) values for hypoxanthine, xanthine, adenine and guanine of 16.5 +/- 1.5, 8.5 +/- 0.6, 8.5 +/- 1.1, and 8.8 +/- 4.0 microM, respectively. LmaNT3 is, thus, the first member of the ENT family identified in any organism that functions as a nucleobase rather than nucleoside or nucleoside/nucleobase transporter. 相似文献
96.
Marco Schmidt Holger Kreft Adjima Thiombiano Georg Zizka 《Diversity & distributions》2005,11(6):509-516
A map of plant species diversity in Burkina Faso is presented based on field observations and specimen data from the Ouagadougou University Herbarium (OUA) and the Herbarium Senckenbergianum (FR). A map of collecting intensity and field observations illustrates centres of botanical research activities in Burkina Faso. To overcome problems associated with biased sampling intensity, distributions of species have been modelled and extrapolated to maps of vascular plant diversity, life forms and diversity of four selected families (Poaceae, Cyperaceae, Dioscoreaceae and Rubiaceae). The area of most intensive collection and observation is around Gorom‐Gorom and Fada N’Gourma. Modelled diversity generally increases towards the south, as does the proportion of phanerophytes, lianas and hemicryptophytes, while the opposite trend is observed for therophytes. Poaceae diversity is highly correlated with total vascular plant diversity, making the family especially suitable as an indicator for overall plant diversity. Cyperaceae are rather evenly distributed throughout the country, Dioscoreaceae are restricted to the Sudanian Zone. Rubiaceae have their highest diversities in the very south. Our approach can be transferred to areas with a similar database, certainly to other areas within West Africa. Future research should focus on distribution data for rare species, enabling our approach to evaluate the West African system of protected areas. 相似文献
97.
Elena Bassi Emanuela Donaggio Andrea Marcon Massimo Scandura Marco Apollonio 《Mammalian Biology》2012,77(5):369-376
Red fox (Vulpes vulpes) and wolf (Canis lupus) are two widespread opportunistic predators living in simpatry in many areas. Nonetheless, scarce information are available on their trophic interactions. We investigated food habits of these two carnivores in a mountain area in Italy and assessed the extent of their trophic niche overlap, focusing on the consumption of wild ungulates. Thereby we analyzed the content of 669 red fox scats and 253 wolf scats collected between May 2008 and April 2009. Red foxes resulted to have a more than three times higher niche breadth than wolves. Vegetables, small mammals, wild ungulates, and invertebrates were major items (altogether 92% of volume) of the red fox annual diet. On the contrary wolf annual diet relied on wild ungulates (94% of volume) with wild boar (Sus scrofa) being the main food item. The degree of trophic niche overlap between the two species was found to be low (Pianka's O = 0.356). Diet variation between the warm and the cold seasons was limited in both species, and higher in red fox than in wolf. The two canids appeared to use wild ungulates unevenly being the former more selective for younger preys, smaller in size (newborn piglets and roe deer Capreolus capreolus fawns), whereas the latter exhibited a preference for medium-sized and large ungulates (10–35 kg wild boar and adult roe deer). Even if wild ungulates represent the main shared food category, the different use of age/weight classes by the two predators, together with their possible consumption as carrions by red fox, suggests a very limited trophic competition between wolf and red fox.This study represents a contribution to the knowledge of trophic interaction in predator–prey systems where sympatric carnivores are present. 相似文献
98.
Barucca M Biscotti MA Olmo E Canapa A 《Journal of experimental zoology. Part B. Molecular and developmental evolution》2006,306(2):164-167
The ParaHox gene cluster contains three homeobox genes, Gsx, Xlox and Cdx and has been demonstrated to be an evolutionary sister of the Hox gene cluster. Among deuterostomes the three genes are found in the majority of taxa, whereas among protostomes they have so far been isolated only in the phylum Sipuncula.We report the partial sequences of all three ParaHox genes in the polyplacophoran Nuttallochiton mirandus, the first species of the phylum Mollusca where all ParaHox genes have been isolated. This finding has phylogenetic implications for the phylum Mollusca and for its relationships with the other lophotrochozoan taxa. 相似文献
99.
Genome physical mapping from large-insert clones by fingerprint analysis with capillary electrophoresis: a robust physical map of Penicillium chrysogenum 总被引:2,自引:0,他引:2 下载免费PDF全文
Xu Z van den Berg MA Scheuring C Covaleda L Lu H Santos FA Uhm T Lee MK Wu C Liu S Zhang HB 《Nucleic acids research》2005,33(5):e50
Physical mapping with large-insert clones is becoming an active area of genomics research, and capillary electrophoresis (CE) promises to revolutionize the physical mapping technology. Here, we demonstrate the utility of the CE technology for genome physical mapping with large-insert clones by constructing a robust, binary bacterial artificial chromosome (BIBAC)-based physical map of Penicillium chrysogenum. We fingerprinted 23.1× coverage BIBAC clones with five restriction enzymes and the SNaPshot kit containing four fluorescent-ddNTPs using the CE technology, and explored various strategies to construct quality physical maps. It was shown that the fingerprints labeled with one or two colors, resulting in 40–70 bands per clone, were assembled into much better quality maps than those labeled with three or four colors. The selection of fingerprinting enzymes was crucial to quality map construction. From the dataset labeled with ddTTP–dROX, we assembled a physical map for P.chrysogenum, with 2–3 contigs per chromosome and anchored the map to its chromosomes. This map represents the first physical map constructed using the CE technology, thus providing not only a platform for genomic studies of the penicillin-producing species, but also strategies for efficient use of the CE technology for genome physical mapping of plants, animals and microbes. 相似文献
100.