全文获取类型
收费全文 | 1710篇 |
免费 | 169篇 |
出版年
2023年 | 5篇 |
2021年 | 27篇 |
2020年 | 19篇 |
2019年 | 40篇 |
2018年 | 39篇 |
2017年 | 40篇 |
2016年 | 62篇 |
2015年 | 102篇 |
2014年 | 101篇 |
2013年 | 105篇 |
2012年 | 147篇 |
2011年 | 143篇 |
2010年 | 87篇 |
2009年 | 54篇 |
2008年 | 94篇 |
2007年 | 102篇 |
2006年 | 84篇 |
2005年 | 72篇 |
2004年 | 76篇 |
2003年 | 65篇 |
2002年 | 74篇 |
2001年 | 19篇 |
2000年 | 18篇 |
1999年 | 24篇 |
1998年 | 19篇 |
1997年 | 22篇 |
1996年 | 16篇 |
1995年 | 17篇 |
1994年 | 14篇 |
1993年 | 5篇 |
1992年 | 16篇 |
1991年 | 18篇 |
1990年 | 19篇 |
1989年 | 19篇 |
1988年 | 16篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 5篇 |
1979年 | 3篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 5篇 |
1972年 | 3篇 |
1970年 | 4篇 |
1967年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有1879条查询结果,搜索用时 15 毫秒
1.
Morten Colding-Jorgensen 《Journal of theoretical biology》1976,57(2):373-383
A simplified model of excitation is introduced in which the membrane capacity is ignored. It is shown that: (1) Threshold, action potentials, and strength-duration relation can be reproduced by a membrane without a capacity, even for a very simplified model. (2) The delayed build up of the sodium conductance can mimic a membrane capacity. (3) A constant potential stimulus can be used to reveal the influence of the membrane capacity, eventually combined with a feed back mechanism which reduces the effect of the capacity. (4) The effect of the membrane capacity depends on the ratio between the membrane time constant and the time constant for the fast conductance changes. 相似文献
2.
3.
Andrew A. Lackner Morten Schidt Gary C. Armitage Peter F. Moore Robert J. Munn Preston A. Marx Murray B. Gardner Linda J. Lowenstine 《Journal of medical primatology》1989,18(3-4):195-207
Simian acquired immune deficiency syndrome (SAIDS) caused by the type D retrovirus SRV-1 results in opportunistic infections and a spectrum of oral lesions similar to those seen in humans with AIDS. To better understand the pathogenesis of these oral lesions we have retrospectively examined the oral mucosa from ten rhesus monkeys that died with SAIDS and prospectively examined the oral mucosa of ten additional animals inoculated with SRV-1 to determine at what time, and in what cells SRV-1 infection of the oral mucosa occurs. Using single and double label immunohistologic techniques, and electron microscopy we detected SRV-1 in clusters of oral epithelial cells and rare Langerhans cells as early as 1 month postinoculation. 相似文献
4.
Genetic convergence during serial in vitro passage of a polyclonal squamous cell carcinoma 总被引:1,自引:0,他引:1
A cell line was established from an in situ squamous cell carcinoma of the skin (Bowen's disease), and its in vitro karyotypic evolution was cytogenetically analyzed. Initially, considerable genetic heterogeneity was evident. Nine cytogenetically abnormal clones, eight of which were apparently unrelated, were found among the 83 metaphases analyzed from the primary culture and the first passage. With increasing time in culture this complexity was reduced, so that a single clone dominated passages 7-11. The clone that emerged from this genetic convergence had a t(12;17)(p13;q21) as the sole abnormality. Our findings indicate that the cytogenetic multiclonality that has been repeatedly detected in short-term cultures of squamous cell carcinomas is not caused by the in vitro conditions. Instead, the principles of Darwinian selection apply: the altered, but stable, selection pressure facing a newly established and initially multiclonal cell line will lead to a reduction of genetic heterogeneity until the one clone that now has the proliferative advantage outgrows the other subpopulations. 相似文献
5.
Mef Nilbert Sverre Heim Nils Mandahl Ulla-Maria Flodérus Helena Willén Felix Mitelman 《Human genetics》1990,85(6):605-611
Summary The cytogenetic analysis of 224 leiomyomas from 138 patients is presented. An insufficient number of mitoses was found in 35 tumors, normal karyotypes in 145, and clonal chromosome aberrations were detected in 44. The three previously identified cytogenetic subgroups were all represented in this series: del(7) (q21.2q31.2) was found in 11, trisomy 12 in five, and t(12;14)(q14-15;q23-24) in one leiomyoma. Rearrangements of 6p, including deletions, inversions, and various translocations, were found in eight tumors, thus delineating a new cytogenetic subgroup of uterine leiomyoma. The remaining 21 karyotypically abnormal tumors had nonrecurrent changes. One leiomyoma had two cytogenetically unrelated clones characterized by del(7)(q21.2 q31.2) and +12. Karyotypic changes in two separate leiomyomas from the same uterus were identified in five patients; in three of them, different anomalies were found in the two tumors, whereas cytogenetically identical aberrations – del(7q) and dic(21;22) – were detected in two macroscopically discrete tumors. These findings suggest that whereas some multiple leiomyomas originate independently, others may be derived from the same neoplastic clone. 相似文献
6.
Morten Glasø Olav Hilmar Iversen Torstein Hovig 《Virchows Archiv. B, Cell pathology including molecular pathology》1988,56(1):221-235
The nature and significance of so-called dark keratinocytes in the epidermis during chemical carcinogenesis is still a matter
of concern and debate. Based on ultrastructural observations it has been suggested that dark cells most often are shrunken
cells. Reports on skin carcinogenesis, however, claim that dark cells are a sign of ongoing tumor promotion and represent
those stem cells in the epidermis from which the tumors originate. It is therefore important to find out whether these cells
are simply injured and shrunken cells, or vital cells of great importance for carcinogenesis. Dark cells are assumed to be
rich in ribosomes. There is evidence, however, that the observed number of dark cells is highly dependent on tissue fixation.
In the present ultrastructural study, morphometric methods were used to compare the effects of two different fixation procedures
on the amount of cytoplasmic ribosomes in dark cells from both untreated and carcinogen-treated hairless mouse epidermis.
The results show that the ultrastructural features of both dark and clear cells vary considerably with different fixation
procedures. In acetone-treated controls typical dark cells are only observed when the fixative has a lower osmotic activity
than the plasma. With iso-osmolal fixation typical dark cells are not observed. After an abortive two-stage carcinogenesis
treatment, in which a single application of 9,10-dimethyl-l,2-benzanthracene (DMBA) in acetone was followed by a single application
of 12-O-tetradecanoyl-13-acetate (TPA) in acetone, signs of cell injury could be found after both fixation procedures. With
DMBA/TPA and hypo-osmolal fixation the number of dark cells seemed to increase, whereas only signs of cell injury with occurrence
of some heavily altered “clear cells” dominated the picture with iso-osmolal fixation. Morphometry showed that both the numerical
and the volumetric densities of cytoplasmic ribosomes in basal keratinocytes varied most significantly with the fixation procedure
used. The cytoplasmic volumes did not vary in a way that could explain these differences. One might therefore assume that
the number of ribosomes depends on the fixative. Large swelling artifacts occurred when a fixative with low osmotic activity
was used, leading to compression of neighboring cells. Hence, an increased ribosomal density reported previously in dark cells
is probably related to such cell volume artifacts and does not reflect an actually increased quantity of ribosomes. With both
fixation procedures, a single application of DMBA followed by one of TPA appeared to produce an increased number of ribosomes
in basal keratinocytes. When hypo-osmolal fixation was used, however, treatment with DMBA/TPA did not influence the cytoplasmic
volume or the numerical density of ribosomes, in dark cells. This might indicate that so-called dark keratinocytes following
DMBA/TPA treatment are functionally inactive cells that appear more vulnerable than active cells to compression during hypo-osmolal
fixation. 相似文献
7.
The swi4+ gene of Schizosaccharomyces pombe encodes a homologue of mismatch repair enzymes. 总被引:5,自引:5,他引:0 下载免费PDF全文
The swi4+ gene of Schizosaccharomyces pombe is involved in termination of copy-synthesis during mating-type switching. The gene was cloned by functional complementation of a swi4 mutant transformed with a genomic library. Determination of the nucleotide sequence revealed an open reading frame of 2979 nucleotides which is interrupted by a 68 bp long intron. The putative Swi4 protein shows homology to Duc-1 (human), Rep-3 (mouse), HexA (Streptococcus pneumoniae) and MutS (Salmonella typhimurium). The prokaryotic proteins are known as essential components involved in mismatch repair. A strain with a disrupted swi4+ gene was constructed and analysed with respect to the switching process. As in swi4 mutants duplications occur in the mating-type region of the swi4 (null) strain, reducing the efficiency of switching. 相似文献
8.
Summary The Saccharomyces cerevisiae KEX2 gene encodes the membrane-bound endoprotease yscF, which is responsible for the site-specific endoproteolytic cleavages at pairs of basic amino acid residues in the -factor precursor. In order to obtain soluble yscF activity, a mutant KEX2 gene lacking 600 bp coding for the C-terminal 200 amino acids was constructed. Expression of the truncated KEX2 gene in yeast led to the secretion of an active soluble yscF protein (yscFs). The soluble yscF protein is able to efficiently cleave heterologous protein precursors in-vitro, as demonstrated for -factor leader-hIGF1 and -factor leader-hirudin fusion proteins.
Offprint requests to: P. G. Seeboth 相似文献
9.
East African material of the genus Hypoxis L. has preliminarily been divided into the heterogenous, probably apomictic H. obtusa Burch- complex (2n = 40–50, ca. 75, 76, ca. 85, >86, ca. 92, ca. 98, ca. 108, 130–135, 160–200) and 5 rather homogenous species: H. angustifolia Lam. (2n = 14, 28), H. goetzei Harms (2n = ca. 62), H. kilimanjarica Bak., H. malosana Bak. (2n = 14) and H. macrocarpa Holt & Staubo sp. nov. H. kilimanjarica is divided into ssp. kilimanjarica and ssp. prostrata Holt & Staubo ssp. nov. 相似文献