全文获取类型
收费全文 | 23711篇 |
免费 | 2032篇 |
国内免费 | 2篇 |
出版年
2021年 | 181篇 |
2020年 | 138篇 |
2019年 | 166篇 |
2018年 | 408篇 |
2017年 | 426篇 |
2016年 | 465篇 |
2015年 | 427篇 |
2014年 | 534篇 |
2013年 | 860篇 |
2012年 | 1510篇 |
2011年 | 1646篇 |
2010年 | 883篇 |
2009年 | 613篇 |
2008年 | 1415篇 |
2007年 | 1456篇 |
2006年 | 1393篇 |
2005年 | 1253篇 |
2004年 | 1219篇 |
2003年 | 1119篇 |
2002年 | 1141篇 |
2001年 | 858篇 |
2000年 | 970篇 |
1999年 | 475篇 |
1998年 | 263篇 |
1997年 | 210篇 |
1996年 | 248篇 |
1995年 | 225篇 |
1994年 | 203篇 |
1993年 | 214篇 |
1992年 | 217篇 |
1991年 | 188篇 |
1990年 | 192篇 |
1989年 | 181篇 |
1988年 | 177篇 |
1987年 | 173篇 |
1986年 | 152篇 |
1985年 | 207篇 |
1984年 | 221篇 |
1983年 | 179篇 |
1982年 | 230篇 |
1981年 | 199篇 |
1980年 | 175篇 |
1979年 | 174篇 |
1978年 | 150篇 |
1977年 | 137篇 |
1976年 | 129篇 |
1975年 | 119篇 |
1974年 | 129篇 |
1973年 | 111篇 |
1972年 | 93篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
81.
Ectopic head and foot formation in Hydra: Diacylglycerol-induced increase in positional value and assistance of the head in foot formation 总被引:1,自引:0,他引:1
Werner A. Müller 《Differentiation; research in biological diversity》1990,42(3):131-143
In wild type Hydra magnipapillata, daily application of the protein kinase C activator diacylglycerol (DAG) evokes sprouting of periodically spaced ectopic heads along the body column and leads to loss of the ability to regenerate proximal structures including the foot. The present transplantation studies show that the appearance of ectopic heads is preceded by an early increase in the 'positional value' (P-value) or 'head activation potential' of the gastric column. Long before ectopic head structures emerge, pieces of DAG-treated tissue transplanted into the corresponding positional level of untreated hosts induce head formation instead of being integrated, whereas pieces implanted from untreated donors into DAG-treated hosts form feet. Foot formation implies a decrease in the P-value. This down-regulation is promoted through long-range assistance by the head. Thus, after termination of the DAG treatment ectopic feet are intercalated midway between the periodically spaced heads; moreover, untreated polyps onto which additional distal heads have been grafted regenerate feet faster than do one-headed polyps and may form supernumerary feet. Multiheaded animals can also be produced using two substances (K-252a and xanthate D609) that interfere with signal transduction, but the mode by which secondary heads arise is different from DAG-induced ectopic head formation. Presumably because the assistance by the parental head is impaired, buds fail to form a foot and detach and instead give rise to stable secondary body axes. It is assumed that the P-value along the body varies according to the number of cellular receptors for factors serving as intercellular signals.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
82.
The 76 kD cell-adhesion factor from crayfish haemocytes promotes encapsulation in vitro 总被引:2,自引:0,他引:2
Summary Semigranular cells from the crayfish, Pacifastacus leniusculus, were separated by Percoll gradient centrifugation and were used to study the encapsulation of foreign particles. The semigranular cells were found strongly to encapsulate glass beads coated with haemocyte lysate in which the prophenoloxidase-activating system had been activated with laminarin or with a low concentration of calcium ions. The granular cells only weakly encapsulated these particles. The encapsulationpromoting factor was purified from haemocyte lysates and found to be a 76 kD protein which was recognized by an antiserum to the previously described 76 kD cell-adhesion factor. After the last step in purification (Con A-Sepharose chromatography), the flowthrough consisted of several proteins, which had some, but less, encapsulation-promoting activity and contained a 30 kD band that was also recognized by the antiserum to the 76 kD cell-adhesion factor. If the haemocyte lysate prepared in low [Ca2+] was incubated with a -1,3-glucan prior to purification, no 76 kD protein could be isolated but only a 30 kD protein. The 30 kD protein thus seems to be a degradation product of the 76 kD cell-adhesion factor. We conclude that the 76 kD protein which is released from degranulating haemocytes, and to a lesser extent its 30 kD fragment, can promote encapsulation. Phenoloxidase did not have any encapsulation-promoting activity. 相似文献
83.
Päivi Heikkilä 《Cell and tissue research》1990,259(3):421-427
Summary The present study was undertaken to define the effects of lipoprotein-derived cholesterol and endogenous, de novo synthesized cholesterol on the ultrastructure and function of undifferentiated rat adrenocortical cells [lipoprotein (HDL3 and LDL) receptor-negative, zona glomerulosa-like adrenocortical cells] in primary culture. For this purpose human plasma high density lipoprotein (HDL3) or low density lipoprotein (LDL) was added to culture medium devoid of cholesterol. Steroid secretion remained at the low basal level even after addition of lipoproteins, and the amount of intracellular lipid droplets did not increase. When mevinolin (0.96 µg/ml), an inhibitor of cholesterol synthesis, was added to the culture medium, a low secretion of corticosterone was measured both in serum-free and serum-containing media. Ultrastructurally, lipid droplets disappeared after treatment with mevinolin in both media used. At this concentration of mevinolin cell proliferation was similar to that in the controls, but at higher concentrations (4.8 or 9.6 µg/ml) proliferation was inhibited to 42% and 26% in serum-free medium, and 20% and 12% in serum-supplemented medium, respectively. This study demonstrates that cell proliferation and synthesis of corticosterone by undifferentiated rat adrenocortical cells is identical in the absence or presence of exogenous lipoprotein cholesterol. Inhibition of de novo cholesterol synthesis by mevinolin over a period of 7 days does not inhibit corticosterone secretion or proliferation of cells but decreases the amount of intracellular lipid droplets, thus suggesting utilization of intracellular cholesterol esters. However, higher concentrations of mevinolin inhibit proliferation of cells both in serum-free and serum-containing media. 相似文献
84.
A native high molecular complex (Mr 850000) containing about 50% of the allphycocyanin of the phycobilisome but lacking allophycocyanin B was separated from isolated phycobilisomes by gel electrophoresis. It was designated APCM since the large linker polypeptide LCM was exclusively localized in this complex. The complex exhibited a ?196°C fluorescence emission maximum at 673 nm (671 nm at 25°C). In addition, a core complex (designated APC, Mr≥1000000) consisting of both APCM and AP 680 was isolated by combined gel filtration and linear gradient centrifugation. At 25°C this complex showed dual emission peaks at 670 and 680 nm demonstrating functional independence of the terminal emitters. A complex similar to APCM can be isolated from phycobilisomes of Anabaena variabilis. This is evidence that APCM is the constitutive center of the tricylindrical core of hemidiscoidal cyanobacterial phycobilisomes. Two models summarizing the structural and functional consequences of the results are presented in the discussion. 相似文献
85.
Peter Bräunig 《Cell and tissue research》1990,260(1):95-108
Summary The nervus corporis cardiaci III (NCC III) of the locust Locust migratoria was investigated with intracellular and extracellular cobalt staining techniques in order to elucidate the morphology of neurons within the suboesophageal ganglion, which send axons into this nerve. Six neurons have many features in common with the dorsal, unpaired, median (DUM) neurons of thoracic and abdominal ganglia. Three other cells have cell bodies contralateral to their axons (contralateral neuron 1–3; CN 1–3). Two of these neurons (CN2 and CN3) appear to degenerate after imaginal ecdysis. CN3 innervates pharyngeal dilator muscles via its anterior axon in the NCC III, and a neck muscle via an additional posterior axon within the intersegmental nerve between the suboesophageal and prothoracic ganglia. A large cell with a ventral posterior cell body is located close to the sagittal plane of the ganglion (ventral, posterior, median neuron; VPMN). Staining of the NCC III towards the periphery reveals that the branching pattern of this nerve is extremely variable. It innervates the retrocerebral glandular complex, the antennal heart and pharyngeal dilator muscles, and has a connection to the frontal ganglion.Abbreviations
AH
antennal heart
-
AN
antennal nerves
-
AO
aorta
-
AV
antennal vessel
-
CA
corpus allatum
-
CC
corpus cardiacum
-
CN1, CN2, CN3
contralateral neuron 1–3
-
DIT
dorsal intermediate tract
-
DMT
dorsal median tract
-
DUM
dorsal, unpaired, median
-
FC
frontal connective
-
FG
frontal ganglion
-
HG
hypocerebral ganglion
-
LDT
lateral dorsal tract
-
LMN, LSN
labral motor and sensory nerves
-
LN+FC
common root of labral nerves and frontal connective
-
LO
lateral ocellus
-
MDT
median dorsal tract
-
MDVR
ventral root of mandibular nerve
-
MVT
median ventral tract
-
NCA I, II
nervus corporis allati I, II
-
NCC I, II, III
nervus corporis cardiaci I, III
-
NR
nervus recurrens
-
NTD
nervus tegumentarius dorsalis
-
N8
nerve 8 of SOG
-
OE
oesophagus
-
OEN
oesophageal nerve
-
PH
pharynx
-
SOG
suboesophageal ganglion
-
T
tentorium
-
TVN
tritocerebral ventral nerve
-
VLT
ventral lateral tract
-
VIT
ventral intermediate tract
-
VMT
ventral median tract
-
VPMN
ventral, posterior, median neuron
-
1–7
peripheral nerves of the SOG
-
36, 37, 40–45
pharyngeal dilator muscles 相似文献
86.
Günter Kämper Hans-Ulrich Kleindienst 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1990,167(2):193-200
1. | Filiform hairs of various lengths on the cerci of adult crickets vibrate in a sound field. These movements were measured with a photodetector for sound frequencies from 10 Hz to 200 Hz in the species Acheta domestica, Gryllus bimaculatus and Phaeophilacris spectrum. |
2. | With low air-particle velocities, the hair shafts were deflected sinusoidally from their resting position, without bending or secondary oscillations (Figs. 2 A, 3 A). At higher velocities (from ca. 80 mm/s peak velocity, depending on the properties of the individual hairs), the shaft struck the cuticular rim of the socket in which the base of the hair is seated (Fig. 2B). This contact was made at an average angular displacement from the resting position of 5.16°±1.0°. |
3. | The best frequencies of the hairs were found to be between 40 Hz and 100 Hz (Fig. 5A). The slope of the amplitude curve for constant peak air-particle velocity at frequencies below the best frequencies was between 0 and 6 dB/octave. Long hairs had smaller slope values than short hairs (Fig. 5C). |
4. | At its best frequency the ratio of maximal tip displacement of a hair to the displacement of the air particles in the sound field was between 0.2 and 2. Only a small number of hairs (2 out of 36) showed tip displacements exceeding twice the air-particle displacement. The values of maximal angular displacement were not correlated to hair length (Fig. 5 B). |
5. | The angular displacement of the hairs was phase shifted with respect to the air-particle velocity by 0° to +45° (phase lead) at sound frequencies around 10 Hz and by -45° to -120° (phase lag) at 200 Hz (Figs. 3C, 4B). At a particular frequency long hairs tended to have larger phase lags than shorter hairs (Fig. 5D). |
87.
Dr. Kauko K. Mäkinen Eva Söderling Salam A. Syed Pirkko-Liisa Mäkinen 《Current microbiology》1990,20(1):5-11
The cell extracts of two human oral strains (FN2 and FN3) ofFusobacterium nucleatum displayed exceptionally high-glutamylpeptidase activity as determined withN--l-glutamyl-2-naphthylamine as substrate. This activity was so dominant that the hydrolysis of otherN-aminoacyl-2-naphthylamines progressed at a rate <10% of the former. Two major enzymes (I and II) were partially purified from FN2. I had a molecular weight of 115,000 and did not hydrolyze-glutamylcysteinylglycine (glutathione). II had a molecular weight of 70,000 and rapidly liberated only glutamic acid from glutathione. Strain FN3 contained several enzymes hydrolyzing-glu-2NA. Direct anion exchange chromatography of FN3 cell extracts separated one enzyme that liberated both glutamic acid and glycine from glutathione, one that was inactive against glutathione (but hydrolyzed-glu-2NA), and one that liberated only glutamic acid. Although-glu-2NA was a good synthetic substrate, glutathione was hydrolyzed at least 500 times faster by an enzyme present in both strains. These results indicate that the presence of-glutamylpeptidase activity is very characteristic of theseF. nucleatum strains. 相似文献
88.
The effects of , , and receptor-agonists on forskolin stimulated cyclic adenosine-3, 5-monophosphate (cAMP) formation were examined in astroglial enriched primary cultures from the cerebral cortex of newborn rats. Intracellular cAMP accumulation was quantified by radioimmunoassay. Morphine was used as a -receptor agonist, D-Ala-D-Leu-Enkephalin (DADLE) as a -receptor agonist and dynorphine 1–13 (Dyn) as a -receptor agonist. Basal cAMP levels were unaffected by either the opiate agonists or the antagonists used. In the presence of the cAMP stimulator forskolin, morphine had no significant effect on the cytoplasmic cAMP levels. DADLE caused a dose related inhibition of the forskolin stimulated cAMP accumulation. The effects of this receptor stimulation was blocked with the selective antagonist ICI 174.864. In the presence of Dyn, the forskolin stimulated cAMP accumulation was inhibited in a dose related manner. This receptor stimulation was blocked with the selective antagonist MR 2266. Co-administration of DADLE and Dyn resulted in a non additive inhibition of the forskolin stimulated accumulation of cAMP. These findings indicate that astroglial enriched cultures from the cerebral cortex of rats express and -receptors co-localized ont he same population of cells, and that these receptors are inhibitory coupled to adenylate cyclase. 相似文献
89.
Summary We raised monoclonal antibodies against a membrane fraction ofXenopus neurulae in order to detect tissue-specific cell-surface markers. Here we describe a monoclonal antibody that recognizes an epithelial membrane-associated antigen (EMA) in immunohistological stainings. The tissue-specific and membrane-associated antigen detected in immunohistological stainings could serve as useful marker in epithelium differentiation and membrane organization of the early embryo. In tadpoles and adults EMA was found in specific epithelial tissues derived from different germ layers such as kidney, skin, gut, pancreas, epiphysis and choroid plexus. In the cleaving embryo this antibody stained newly formed membranes between blastomeres from the two-cell stage onwards. Cytoplasmic staining in large oocytes and early embryos was also observed. The possibility that the cytoplasmic signal represents a maternal store of membrane material is discussed. 相似文献
90.
Michael Kroiher Günter Plickert Werner A. Müller 《Development genes and evolution》1990,199(3):156-163
Summary During embryogenesis and planula development of the colonial hydroidHydractinia echinata cell proliferation decreases in a distinct spatio-temporal pattern. Arrest in S-phase activity appears first in cells localized at the posterior and then subsequently at the anterior pole of the elongating embryo. These areas do not resume S-phase activity, even during the metamorphosis of the planula larva into the primary polyp. Tissue containing the quiescent cells gives rise to the terminal structures of the polyp. The posterior area of the larva becomes the hypostome and tentacles, while the anterior part of the larva develops into the basal plate and stolon tips. In mature planulae only a very few cells continue to proliferate. These cells are found in the middle part of the larva. Labelling experiments indicate that the prospective material of the postmetamorphic tentacles and stolon tips originates from cells which have exited from the cell cycle in embryogenesis or early in planula development. Precursor cells of the nematocytes which appear in the tentacles of the polyp following metamorphosis appear to have ceased cycling before the 38th hour of embryonic development. The vast majority of the cells that constitute the stolon tips of the primary polyp leave the cell cycle not later than 58 h after the beginning of development. We also report the identification of a cell type which differentiates in the polyp without passing through a post-metamorphic S-phase. The cell type appears to be neural in origin, based upon the identification of a neuropeptide of the FMRFamide type. 相似文献