全文获取类型
收费全文 | 1728篇 |
免费 | 169篇 |
出版年
2023年 | 12篇 |
2022年 | 14篇 |
2021年 | 43篇 |
2020年 | 34篇 |
2019年 | 41篇 |
2018年 | 29篇 |
2017年 | 33篇 |
2016年 | 54篇 |
2015年 | 90篇 |
2014年 | 94篇 |
2013年 | 93篇 |
2012年 | 167篇 |
2011年 | 121篇 |
2010年 | 65篇 |
2009年 | 60篇 |
2008年 | 62篇 |
2007年 | 81篇 |
2006年 | 65篇 |
2005年 | 62篇 |
2004年 | 53篇 |
2003年 | 49篇 |
2002年 | 46篇 |
2001年 | 43篇 |
2000年 | 36篇 |
1999年 | 30篇 |
1998年 | 19篇 |
1997年 | 11篇 |
1996年 | 18篇 |
1995年 | 15篇 |
1994年 | 14篇 |
1993年 | 14篇 |
1992年 | 24篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 13篇 |
1986年 | 15篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1979年 | 11篇 |
1977年 | 9篇 |
1974年 | 7篇 |
1973年 | 8篇 |
1972年 | 8篇 |
1971年 | 10篇 |
1885年 | 8篇 |
1883年 | 8篇 |
排序方式: 共有1897条查询结果,搜索用时 15 毫秒
1.
F F Smith J R Mertz I Krebs L L Tres C B Chae Z Zakeri J Engelhardt D Hoover M Tenniswood A L Kierszenbaum 《Molecular reproduction and development》1992,33(4):363-372
We have previously reported that a heterodimeric protein secreted by rat Sertoli cells is antigenically related to a protein associated with outer dense fibers of the sperm tail. Therefore, we have explored the possibility that Sertoli and spermatogenic cells express a similar gene encoding a homologous protein. A Sertoli cell heterodimeric protein cDNA probe recognizes specific mRNA in pachytene and round spermatids fractionated by centrifugal elutriation; however, this specific mRNA was less prominent than in cultured Sertoli cells. In agreement with these observations, in situ hybridization experiments show that Sertoli cells are predominantly engaged in active heterodimeric protein mRNA synthesis, while meiotic prophase spermatocytes and spermatids also show significant but less abundant specific mRNA. Immunoblotting experiments demonstrate that, while Sertoli cells synthesize a heterodimeric protein consisting of two disulfide-linked components with molecular masses of 45 and 35 kD, both primary spermatocytes and round spermatids synthesize single 30 kD monomers not associated by disulfide linkage but recognized by antisera to Sertoli cell heterodimeric protein. Immunoblotting and immunogold electron microscopic studies show that antisera to Sertoli cell heterodimeric protein recognize a protein associated with outer dense fibers. This immunoreactivity was abolished by a 5-min pronase treatment, without affecting the integrity of outer dense fibers. Results of this study and previous studies demonstrate that both Sertoli and spermatogenic cells express a similar gene and that an antigenically related product encoded by this gene becomes associated with outer dense fibers during their assembly at spermiogenesis. 相似文献
2.
Natalia V. Engelhardt Valentina M. Factor Alexander L. Medvinsky Vladimir N. Baranov Maria N. Lazareva Valentina S. Poltoranina 《Differentiation; research in biological diversity》1993,55(1):19-26
Abstract. The A6 antigen - a surface-exposed component shared by mouse oval and biliary epithelial cells - was examined during prenatal development of mouse in order to elucidate its relation to liver progenitor cells. Immunohistochemical demonstration of the antigen was performed at the light and electron microscopy level beginning from the 9.5 day of gestation (26–28 somite pairs).
Up to the 11.5 day of gestation A6 antigen is found only in the visceral endoderm of yolk sac and gut epithelium, while liver diverticulum and liver are A6-negative. In the liver epithelial lineages A6 antigen behaves as a strong and reliable marker of biliary epithelial cells where it is found beginning from their emergence on the 15th day of gestation. It was not revealed in immature hepato-cytes beginning from the 16th day of gestation. However weak expression of the antigen was observed in hepato-blasts on 12–15 days of gestation possibly reflecting their ability to differentiate along either hepatocyte or biliary epithelial cell lineages.
Surprisingly, A6 antigen turned out to be a peculiar marker of the crythroid lineage: in mouse fetuses it distinguished A6 positive liver and spleen erythroblasts from A6 negative early hemopoietic cells of yolk sac origin. Moreover in the liver, A6 antigen probably distinguishes two waves of erythropoiesis: it is found on the erythroblasts from the 11.5 day of gestation onward while first extravascular erythroblasts appear in the liver on the 10th day of gestation. Both fetal and adult erythrocytes are A6-negative.
In the process of organogenesis A6 antigen was revealed in various mouse fetal organs. Usually it was found on plasma membranes of mucosal or ductular epithelial cells. Investigation of A6 antigen's physiological function would probably explain such specific localization. 相似文献
Up to the 11.5 day of gestation A6 antigen is found only in the visceral endoderm of yolk sac and gut epithelium, while liver diverticulum and liver are A6-negative. In the liver epithelial lineages A6 antigen behaves as a strong and reliable marker of biliary epithelial cells where it is found beginning from their emergence on the 15th day of gestation. It was not revealed in immature hepato-cytes beginning from the 16th day of gestation. However weak expression of the antigen was observed in hepato-blasts on 12–15 days of gestation possibly reflecting their ability to differentiate along either hepatocyte or biliary epithelial cell lineages.
Surprisingly, A6 antigen turned out to be a peculiar marker of the crythroid lineage: in mouse fetuses it distinguished A6 positive liver and spleen erythroblasts from A6 negative early hemopoietic cells of yolk sac origin. Moreover in the liver, A6 antigen probably distinguishes two waves of erythropoiesis: it is found on the erythroblasts from the 11.5 day of gestation onward while first extravascular erythroblasts appear in the liver on the 10th day of gestation. Both fetal and adult erythrocytes are A6-negative.
In the process of organogenesis A6 antigen was revealed in various mouse fetal organs. Usually it was found on plasma membranes of mucosal or ductular epithelial cells. Investigation of A6 antigen's physiological function would probably explain such specific localization. 相似文献
3.
4.
5.
C. F. C. Klütsch B. Misof W.-R. Grosse R. F. A. Moritz 《Journal of Biogeography》2007,34(7):1124-1135
Aim Anole lizards (Reptilia: Sauria: Polychrotidae) display remarkable morphological and genetic differentiation between island populations. Morphological differences between islands are probably due to both adaptive (e.g. differential resource exploitation and intra‐ or interspecific competition) and non‐adaptive differentiation in allopatry. Anoles are well known for their extreme diversity and rapid adaptive speciation on islands. The main aim of this study was to use tests of morphological and genetic differentiation to investigate the population structure and colonization history of islands of the Islas de Bahia, off the coast of Honduras. Location Five populations of Norops bicaorum and Norops lemurinus were sampled, four from islands of the Islas de Bahia and one from the mainland of Honduras. Methods Body size and weight differentiation were measured in order to test for significant differences between sexes and populations. In addition, individuals were genotyped using the amplified fragment length polymorphism technique. Bayesian model‐based and assignment/exclusion methods were used to study genetic differentiation between island and mainland populations and to test colonization hypotheses. Results Assignment tests suggested migration from the mainland to the Cayos Cochinos, and from there independently to both Utila and Roatán, whereas migration between Utila and Roatán was lacking. Migration from the mainland to Utila was inferred, but was much less frequent. Morphologically, individuals from Utila appeared to be significantly different in comparison with all other localities. Significant differentiation between males of Roatán and the mainland was found in body size, whereas no significant difference was detected between the mainland and the Cayos Cochinos. Main conclusions Significant genetic and morphological differentiation was found among populations. A stepping‐stone model for colonization, in combination with an independent migration to Utila and Roatán, was suggested by assignment tests and was compatible with the observed morphological differentiation. 相似文献
6.
Identification of endogenous sugar-binding proteins (lectins) in human placenta by histochemical localization and biochemical characterization 总被引:2,自引:0,他引:2
H J Gabius P L Debbage R Engelhardt R Osmers W Lange 《European journal of cell biology》1987,44(2):265-272
Human placentas of different stages of development were histochemically analyzed for expression of endogenous sugar-binding proteins using a panel of biotin-conjugated, chemically glycosylated probes with specificity for beta-galactosides, alpha-galactosides, alpha-mannosides, alpha-fucosides and alpha-glucosides. Temporal differences in the expression of sugar-binding proteins and different patterns of staining of the component cell types of human placenta were discerned, especially pronounced for alpha-fucoside-specific binding in the trophoblast and alpha-glucoside-specific binding in fetal and maternal macrophages. Fractionation of salt and detergent extracts from human placentas by affinity chromatography on columns with immobilized carbohydrates or glycoproteins substantiated the histochemically detectable temporal changes on the basis of alterations in the pattern of individual sugar-binding proteins, as determined by gel electrophoresis under denaturing conditions. Analysis of the trophoblastic layer primarily disclosed the presence of several additional sugar-binding proteins (lectins) in comparison to full-term placenta. The presence and developmental changes of such endogenous sugar receptors may lead to specific carbohydrate-protein interactions of physiological significance with similarly developmentally regulated carbohydrated portions of glyco-conjugates, already detected in human placenta by plant lectins. 相似文献
7.
A note on the hatching and viability ofCeriodaphnia ephippia collected from lake sediment 总被引:3,自引:3,他引:0
Christian Moritz 《Hydrobiologia》1987,145(1):309-314
Ephippia ofCeriodaphnia pulchella Sars were collected at 2 sites from successive sediment layers.
Hatching observed in the laboratory gave information about the duration of their viability. Conclusions about the hatching
situation in the lake were drawn from the ratio of intact to total ephippia at various lake depths. The results are discussed. 相似文献
8.
Common antigens of mouse oval and biliary epithelial cells. Expression on newly formed hepatocytes 总被引:17,自引:0,他引:17
Natalya V. Engelhardt Valentina M. Factor Alla K. Yasova Valentina S. Poltoranina Vladimir N. Baranov Maria N. Lasareva 《Differentiation; research in biological diversity》1990,45(1):29-37
Two antigens - A6 and G7 - shared by mouse biliary epithelial and oval cells were revealed by monoclonal antibodies raised in rat immunized with oval-cell-enriched liver fraction. Oval cells were induced in CBA or F1 (CBA x C57BL6) mice by a combination of a single injection of the alkylating drug Dipin with partial hepatectomy. In normal liver A6 antigen was localized, using light and electron microscopy, in biliary epithelial cells of all ducts including Hering canals. Some bile ductal and Hering cells were A6-negative. Occasionally, A6 antigen was present in single hepatocytes forming the periportal ends of hepatic cords. In preneoplastic and tumorous liver A6 antigen was present in bile ductal and oval cells and in a fraction of newly formed hepatocytes and tumor cells. G7 antigen was revealed in normal, precancerous and tumorous liver in biliary epithelial and oval cells but not in hepatocytes. A6 and G7 antigens were not liver-specific: they were expressed in various normal organs and tissues, especially in epithelia. In studies of mouse liver lineages A6 antigen can be used as a common marker of biliary epithelial and oval cells and hepatocytes at certain stages of differentiation. G7 antigen is a marker of oval and biliary epithelial cells. There was a striking similarity in A6 antigen localization to that of human blood group antigens in normal liver and liver tumors. A6 antigen may thus provide a useful tool for the study of neoexpression of human blood group antigens in liver tumors. 相似文献
9.
Roselyne Rousseaux-Prvost Ren-Pierre Engelhardt Jean Rousseaux Danile Wouters-Tyrou Pierre Sautire 《Molecular reproduction and development》1988,19(3):277-290
The changes in basic nuclear proteins throughout cuttle-fish spermiogenesis were investigated both by immunocytochemical procedures and by isolation of late spermatid nuclei (by virtue of their resistance to sonication). Antibodies were raised in rabbits to a protein, named protein T, isolated from testis chromatin. The anti-protein T immune serum was found to recognize protein T and not histones from the testis. Immunoperoxidase staining of sections or of smears of testis with anti-protein T antibodies showed that protein T appears in the nuclei of round spermatids, is abundant in elongating spermatid nuclei, but cannot be detected in elongated spermatids. Nuclei from these elongated spermatids were isolated by sonication treatment of testis cells. A protein, named protein Sp, with the characteristic mobility of a protamine, was isolated from elongated spermatid nuclei. This protein has the same mobility as the protamine present in mature spermatozoa. Taken together, the results indicate that in cuttle-fish, nuclear protein transitions involve the replacement of histones by a spermatid-specific protein (protein T), which is replaced at the end of elongation of the nucleus by a protamine (protein Sp). Thus, spermiogenesis of the cuttle-fish (and perhaps of other cephalopods), shows two basic nuclear protein transitions, which are similar to the transitions observed in higher vertebrates such as mammals. 相似文献
10.
Flavonol triglycosides containing galactose in tea. 总被引:3,自引:0,他引:3
The isolation and structural elucidation of new quercetin and kaempferol triglycosides from Camellia sinensis is described. Their structures were determined as quercetin and kaempferol 3-glucosyl(1----3) rhamnosyl(1----6)galactosides. The content of quercetin glucosylrhamnosylgalactoside ranged between 0 and 87 mg per 100 g, and that of the kaempferol homologue between 0 and 119 mg per 100 g dry wt. 相似文献