全文获取类型
收费全文 | 33968篇 |
免费 | 3228篇 |
国内免费 | 3604篇 |
出版年
2024年 | 74篇 |
2023年 | 453篇 |
2022年 | 1004篇 |
2021年 | 1819篇 |
2020年 | 1307篇 |
2019年 | 1596篇 |
2018年 | 1615篇 |
2017年 | 1124篇 |
2016年 | 1540篇 |
2015年 | 2190篇 |
2014年 | 2621篇 |
2013年 | 2721篇 |
2012年 | 3226篇 |
2011年 | 2916篇 |
2010年 | 1813篇 |
2009年 | 1713篇 |
2008年 | 1865篇 |
2007年 | 1677篇 |
2006年 | 1379篇 |
2005年 | 1164篇 |
2004年 | 1100篇 |
2003年 | 1012篇 |
2002年 | 898篇 |
2001年 | 683篇 |
2000年 | 584篇 |
1999年 | 512篇 |
1998年 | 342篇 |
1997年 | 273篇 |
1996年 | 268篇 |
1995年 | 175篇 |
1994年 | 205篇 |
1993年 | 126篇 |
1992年 | 155篇 |
1991年 | 137篇 |
1990年 | 118篇 |
1989年 | 78篇 |
1988年 | 62篇 |
1987年 | 49篇 |
1986年 | 36篇 |
1985年 | 50篇 |
1984年 | 18篇 |
1983年 | 23篇 |
1982年 | 20篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1978年 | 5篇 |
1973年 | 5篇 |
1968年 | 3篇 |
1965年 | 8篇 |
1950年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 287 毫秒
81.
82.
Earlier observations in our laboratory indicated that i.v. infusion of human/rat corticotropin-releasing hormone (hCRH) suppresses pulsatile luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in ovariectomized rhesus monkeys. Since cortisol secretion increased significantly as well, it was not possible to exclude the possibility that this inhibitory effect of hCRH on gonadotropins was related to the activation of the pituitary/adrenal axis. The purpose of the present study was to determine the role of pituitary/adrenal activation in the effect of hCRH on LH and FSH secretion. We compared the effects of 5-h i.v. infusions of hCRH (100 micrograms/h, n = 7) and of human adrenocorticotropic hormone (ACTH) (1-24) (5 micrograms/h, n = 3; 10 micrograms/h, n = 3, 20 micrograms/h, n = 3) to ovariectomized monkeys on LH, FSH, and cortisol secretion. As expected, during the 5-h ACTH infusions, cortisol levels increased by 176-215% of baseline control, an increase similar to that observed after CRH infusion (184%). However, in contrast to the inhibitory effect observed during the CRH infusion, LH and FSH continued to be released in a pulsatile fashion during the ACTH infusions, and no decreases in gonadotropin secretion were observed. The results indicated that increases in ACTH and cortisol did not affect LH and FSH secretion and allowed us to conclude that the rapid inhibitory effect of CRH on LH and FSH pulsatile release was not mediated by activation of the pituitary/adrenal axis. 相似文献
83.
Transforming growth factor beta regulates the inhibitory actions of epidermal growth factor during granulosa cell differentiation 总被引:1,自引:0,他引:1
The effects of transforming growth factor beta (TGF-beta) on epidermal growth factor (EGF) receptor content and EGF action were studied in cultured granulosa cells from immature diethylstilbestrol-implanted rats. During follicle-stimulating hormone (FSH)-induced differentiation in vitro, EGF receptors increased by 20-fold as measured by the binding of 125I-EGF to the intact cells. Addition of TGF-beta during the 48-h culture period amplified the stimulatory effects of FSH on EGF receptors up to 2-fold, with ED50 and maximal concentrations of 2.5 and 8 pM, respectively. Also TGF-beta alone in amounts from 1.6 to 16 pM increased EGF receptor content 4-fold. The stimulatory effects of TGF-beta were due to increased numbers of EGF receptors/cell, since the growth factor had no effect on the Kd (3-5 X 10(-11) M) of the high-affinity EGF binding site. TGF-beta action was observed within 20 h of granulosa cell culture and was maximal by 48 h of a 96-h culture. The stimulatory actions of TGF-beta in gonadotropin-induced cells were exerted through the cAMP effector system of the granulosa cell, since the growth factor also amplified the induction of EGF receptors by cholera toxin, forskolin, and 8-bromo-cAMP. The augmentation of EGF receptors by TGF-beta resulted in a parallel 2-fold increase in the inhibitory effects of EGF on FSH-induced cAMP production and luteinizing hormone receptor expression during granulosa cell development. TGF-beta did not increase granulosa cell numbers during culture although it elevated [3H]thymidine incorporation into DNA by 2-fold over that of FSH-treated cells. These results indicate that TGF-beta regulates the effects of both FSH and EGF during granulosa cell differentiation and provides evidence that ovarian function may be controlled by the combined actions of gonadotropins and multiple growth factors. 相似文献
84.
M. Vater A. S. Feng M. Betz 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1985,157(5):671-686
Summary The frequency-place map of the horseshoe bat cochlea was studied with the horseradish peroxidase (HRP) technique involving focal injections into various, physiologically defined regions of cochlear nucleus (CN). The locations of labeled spiral ganglion cells and their termination sites on inner hair cells of the organ of Corti from injections into CN-regions responsive to different frequencies were analyzed in three dimensional reconstructions of the cochlea. Horseshoe bats from different geographical populations were investigated. They emit orientation calls with constant frequency (CF) components around 77 kHz (Rhinolophus rouxi from Ceylon) and 84 kHz (Rhinolophus rouxi from India) and their auditory systems are sharply tuned to the respective CF-components.The HRP-map shows that in both populations: (i) the frequency range around the CF-component of the echolocation signal is processed in the second half-turn of the cochlea, where basilar membrane (BM) is not thickened, secondary spiral lamina (LSS) is still present and innervation density is maximal; (ii) frequencies more than 5 kHz above the CF-component are processed in the first halfturn, where the thickened BM is accompanied by LSS and innervation density is low; (iii) frequencies below the spectral content of the orientation call are represented in apical turns showing no morphological specializations. The data demonstrate that the cochlea of horseshoe bats is normalized to the frequency of the individual specific CF-component of the echolocation call.The HRP-map can account for the overrepresentation of neurons sharply tuned to the CF-signal found in the central auditory system. A comparison of the HRP-map with a map derived with the swollen nuclei technique following loud sound exposure (Bruns 1976b) reveals that the latter is shifted towards cochlear base by about 4 mm. This discrepancy warrants a new interpretation of the functional role of specialized morphological structures of the cochlea within the mechanisms giving rise to the exceptionally high frequency selectivity of the auditory system.Abbreviations
AVCN
anteroventral CN
-
BF
best frequency
-
BM
basilar membrane
-
CF
constant frequency
-
CN
cochlear nucleus
-
DCN
dorsal CN
-
FM
frequency modulated
-
HRP
horseradish peroxidase
-
IHC
inner hair cell
-
LSS
secondary spiral lamina
-
OHC
outer hair cell
-
PVCN
posteroventral CN
-
RF
resting frequency
-
RRc
Rhinolophus rouxi from Ceylon
-
RRi
Rhinolophus rouxi from India 相似文献
85.
花椰菜下胚轴外植体在MS+6BA 5 ppm的培养基上能分化出芽,在MS+2,4-D2ppm的培养基上能脱分化而形成愈伤组织。用3种不同的酚类物质(咖啡酸、阿魏酸、愈创木酚及联苯胺)作氢供体发现分化过程中的过氧化物酶活性高于脱分化过程,其中以咖啡酸作氢供体显示的活性最高,阿魏酸及愈创木酚次之,而联苯胺最小。用聚丙烯酰胺凝胶电泳分离阴极向及阳极向过氧化物酶同工酶,在分化及脱分化培养过程中均不断出现新的酶带,前者有13条,后者为11条,两者的差别主要在阴极向酶带,在分化过程中多了两条酶带(C_1和C_3),同时C_2带活性也比脱分化的高。阳极向酶带也有差别,A_2和A_2两条酶带在分化过程中逐渐加强,但是在脱分化过程中却逐渐消失。反映了两个过程生理上的差别。 相似文献
86.
87.
本文讨论了对带瘤小鼠注射香菇多糖、松口蘑多糖和羊脾RNA后,活体内有关组织中cAMP含量与抑瘤率之间的关系。 结果表明,上述活性物质不仅增强了机体免疫活性,同时亦促使机体内脾、瘤等组织的cAMP增高,此种增高,与抑瘤率成正相关。 相似文献
88.
杂交水稻及三系在发育过程中的酯酶同工酶比较研究 总被引:7,自引:1,他引:6
刘文芳;吴文瑜;肖翊华 《武汉植物学研究》1987,5(3):267-274
本文报道了利用聚丙烯酰胺凝胶电泳技术,测定杂交水稻及三系亲本共50个组合的萌动胚、芽,不同发育时期的叶片、根、雄蕊等12个组织或器官的酯酶同工酶的结果。根据所测结果,可把杂交水稻的酯酶同工酶酶谱分为5种类型:互补型、偏父型、偏母型、同型和“杂种”酶谱。强优势组合以互补型酶谱居多,弱优势组合都是同型酶谱,“杂种”酶谱仅见于V优64的幼穗分化期叶片和V优63、汕优63的三叶期叶片中。不同器官的互补酶谱都可作为预测杂种比势,鉴定杂交稻种子的纯度和真实性以及选配新杂交组合的一个手段或依据,但以对萌动胚或幼芽的测定更有实践意义。 相似文献
89.
<正> 红素(Prodigiosin)是某些沙雷氏杆菌(Serratia marcescens)菌株所合成的一种三吡咯衍生物的红色色素,它对某些细菌,真菌和原虫具有抗生作用,因而引起人们的注意。在活菌体内,它由两个中间前体:2-甲基-3-戊基吡咯(MAP)和4-甲氧基-2,2’-双吡咯-5-甲醛(MBC)缩合而成。催化这一反应的酶,我们称之为红素缩合酶。除MAP外,某些人 相似文献
90.
从大鼠自然诱发的肉瘤细胞中,我们建立了一个四倍体细胞系(4n=84),命名为RC(ratcell)。它具有典型的成纤维细胞外形,能在玻璃表面贴壁生长,但不能生长在琼脂半固体培养基中。该细胞在含15%小牛血清的RPMI 1640培养基中生长良好,至今已连续繁殖112世代,细胞群体倍增时间约为15小时。染色体G-带分析表明,RC为整四倍体细胞,它的1条X染色体在第32至34区为均染区。RC细胞核仁组织者(NORs)活性显然比大鼠二倍体细胞NORs活性的加倍还高(P<0.001)。这个具有非常高NORs活性的RC细胞系对于研究细胞18S+28S rRNA基因转录活性的调控、基因表达与基因剂量关系有一定的意义。RC细胞还有异常高的磷酸酯酶活性,而且它的同工酶谱也与大鼠肌肉细胞明显不同。体内接种实验和扫描电镜的观察表明,RC是非致瘤细胞。RC细胞各号染色体的C-带图样与大鼠二倍体细胞无明显的差异。 相似文献