首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8419篇
  免费   547篇
  国内免费   3篇
  2023年   77篇
  2022年   109篇
  2021年   196篇
  2020年   157篇
  2019年   182篇
  2018年   225篇
  2017年   202篇
  2016年   357篇
  2015年   435篇
  2014年   487篇
  2013年   569篇
  2012年   758篇
  2011年   630篇
  2010年   459篇
  2009年   403篇
  2008年   505篇
  2007年   499篇
  2006年   428篇
  2005年   359篇
  2004年   341篇
  2003年   308篇
  2002年   254篇
  2001年   78篇
  2000年   69篇
  1999年   63篇
  1998年   79篇
  1997年   55篇
  1996年   56篇
  1995年   60篇
  1994年   50篇
  1993年   39篇
  1992年   38篇
  1991年   31篇
  1990年   22篇
  1989年   25篇
  1988年   30篇
  1987年   15篇
  1986年   21篇
  1985年   22篇
  1984年   29篇
  1983年   19篇
  1982年   18篇
  1981年   29篇
  1980年   20篇
  1979年   22篇
  1978年   18篇
  1976年   13篇
  1975年   12篇
  1974年   19篇
  1973年   17篇
排序方式: 共有8969条查询结果,搜索用时 62 毫秒
991.
ABSTRACT: Centrally necrotizing carcinoma is a rare subtype of breast carcinoma, which is characterized by an extensive central necrotic zone accounting for at least 70% of the cross-sectional area of the neoplasm. This central necrotic zone, in turn, is surrounded by a narrow rim of proliferative viable tumor cells. We report an unusual clinical situation in which a patient whose evident breast mass suggested an ipsilateral local recurrence and for which numerous attempts to confirm the histological diagnosis had failed. The patient was treated with a radical mastectomy based on clinical suspicion of breast cancer recurrence after an undesirable delay. In this case, the narrow rim of viable malignant tissue had a thickness of 0.5 to 8 mm, and the centrally necrotizing carcinoma had a central zone with a predominance of fibrosis. The special features of this case led to a misdiagnosis and to an evident clinical local recurrence.  相似文献   
992.
The posterior lateral line system (PLL) of teleost fish comprises a number of mechanosensory organs arranged in defined patterns on the body surface. Embryonic patterns are largely conserved among teleosts, yet adult patterns are highly diverse. Although changes in pattern modify the perceptual abilities of the system, their developmental origin remains unknown. Here we compare the processes that underlie the formation of the juvenile PLL pattern in Thunnus thynnus, the bluefin tuna, to the processes that were elucidated in Danio rerio, the zebrafish. In both cases, the embryonic PLL comprises five neuromasts regularly spaced along the horizontal myoseptum, but the juvenile PLL comprises four roughly parallel anteroposterior lines in zebrafish, whereas it is a simple dorsally arched line in tuna fish. We examined whether this difference involves evolutionary novelties, and show that the same mechanisms mediate the transition from embryonic to juvenile patterns in both species. We conclude that the marked difference in juveniles depends on a single change (dorsal vs. ventral migration of neuromasts) in the first days of larval life.  相似文献   
993.
The polymeric display of proteins is a method that could be used to increase the immunogenicity of antigens and to enhance the interaction strength of binding domains for their target ligands through an avidity effect. However, the coupling of proteins to oligomeric scaffolds is challenging. The chemical conjugation and recombinant fusion techniques have limitations that prevent their general use. In this work we describe a simple and effective method for coupling proteins to the decameric structure of Brucella abortus Lumazine Synthase based on the use of a pair of high affinity heterodimeric coiled coil peptides complementary fused to the scaffold and the target protein. Results obtained with a series of proteins demonstrate the capability of this approach to generate polyvalent particles. Furthermore, we show that the method is able to increase the immunogenicity of antigens and produce polyfunctional particles with promising biomedical and nanotechnological applications.  相似文献   
994.
The role of hedgehog (HH) signaling in reproductive tract development was studied in mice in which a dominant active allele of the signal transducer smoothened (SmoM2) was conditionally expressed in the Müllerian duct and ovary. Mutant females are infertile, primarily because they fail to ovulate. Levels of mRNA for targets of HH signaling, Gli1, Ptch1, and Hhip, were elevated in reproductive tracts of 24-day-old mutant mice, confirming overactivation of HH signaling. The tracts of mutant mice developed abnormally. The uterine luminal epithelium had a simple columnar morphology in control mice, but in mutants contained stratified squamous cells typical of the cervix and vagina. In mutant mice, the number of uterine glands were reduced and the oviducts were not coiled. Expression of genes within the Hox and Wnt families that regulate patterning of the reproductive tract were altered. Hoxa13, which is normally expressed primarily in the vagina and cervix, was expressed at 12-fold higher levels in the uterus of mutant mice compared with controls. Wnt5a, which is required for development of the cervix and vagina and postnatal differentiation of the uterus, was expressed at higher levels in the oviduct and uterus of mutant mice compared with controls. Mating mutant females with fertile or vasectomized males induced a severe inflammatory response in the tract. In summary, overactivation of HH signaling causes aberrant development of the reproductive tract. The phenotype observed could be mediated by ectopic expression of Hoxa13 in the uterus and elevated levels of Wnt5a in the oviducts and uterus.  相似文献   
995.
A combination of analytical and statistical methods is used to improve a tablet coating process guided by quality by design (QbD) principles. A solid dosage form product was found to intermittently exhibit bad taste. A suspected cause was the variability in coating thickness which could lead to the subject tasting the active ingredient in some tablets. A number of samples were analyzed using a laser-induced breakdown spectroscopy (LIBS)-based analytical method, and it was found that the main variability component was the tablet-to-tablet variability within a lot. Hence, it was inferred that the coating process (performed in a perforated rotating pan) required optimization. A set of designed experiments along with response surface modeling and kriging method were used to arrive at an optimal set of operating conditions. Effects of the amount of coating imparted, spray rate, pan rotation speed, and spray temperature were characterized. The results were quantified in terms of the relative standard deviation of tablet-averaged LIBS score and a coating variability index which was the ratio of the standard deviation of the tablet-averaged LIBS score and the weight gain of the tablets. The data-driven models developed based on the designed experiments predicted that the minimum value of this index would be obtained for a 6% weight gain for a pan operating at the highest speed at the maximum fill level while using the lowest spraying rate and temperature from the chosen parametric space. This systematic application of the QbD-based method resulted in an enhanced process understanding and reducing the coating variability by more than half.  相似文献   
996.
The highest levels of Pb in soils and needles were detected in the Lisbon Botanical Garden, reflecting the intense car circulation since the end of the late nineteenth century, and the presence of tetraethyl lead in petrol. Levels of Pb in Aleppo bark were at or above the threshold values for plant tissues as defined by the UN-ECE/EC (>4–30 μg/g). Nickel was easily washed by distilled water from the bark, indicating a predominant adsorption due to atmospheric deposition, while only Pb was water-extracted from cone scales, but no more than 0.88 μg/g (4.3% of the total). If we take into account the dry weight of each cone, we can assess the amount of extractable lead present in a given time (ranges between 3.0 and 15.5 μg), although its eventual transfer to the ground through rainfall will be hampered by the cone architecture. The average levels of total Pb in the cones (12.7–35.4 μg/g) fall within the range defined by the UN-ECE/EC, generally; only the reference value is below 4 μg/g. Due to the high capabilities of allocating Pb inside the structure and through adsorption, Aleppo bark and cones can be very interesting tools in the monitoring of Pb.  相似文献   
997.
Degenerative aortic stenosis is the most common worldwide cause of valve replacement. While it shares certain risk factors with coronary artery disease, it is not delayed or reversed by reducing exposure to risk factors (e.g., therapies that lower lipids). Therefore, it is necessary to better understand its pathophysiology for preventive measures to be taken. In this work, aortic valve samples were collected from 20 patients that underwent aortic valve replacement (55% males, mean age of 74 years) and 20 normal control valves were obtained from necropsies (40% males, mean age of 69 years). The proteome of the samples was analyzed by quantitative differential electrophoresis (2D-DIGE) and mass spectrometry, and 35 protein species were clearly increased in aortic valves, including apolipoprotein AI, alpha-1-antitrypsin, serum albumin, lumican, alfa-1-glycoprotein, vimentin, superoxide dismutase Cu-Zn, serum amyloid P-component, glutathione S-transferase-P, fatty acid-binding protein, transthyretin, and fibrinogen gamma. By contrast, 8 protein species were decreased (transgelin, haptoglobin, glutathione peroxidase 3, HSP27, and calreticulin). All of the proteins identified play a significant role in cardiovascular processes, such as fibrosis, homeostasis, and coagulation. The significant changes observed in the abundance of key cardiovascular proteins strongly suggest that they can be involved in the pathogenesis of degenerative aortic stenosis. Further studies are warranted to better understand this process before we can attempt to modulate it.  相似文献   
998.

Introduction

Healthcare management is oriented toward single diseases, yet multimorbidity is nevertheless the rule and there is a tendency for certain diseases to occur in clusters. This study sought to identify comorbidity patterns in patients with chronic diseases, by reference to number of comorbidities, age and sex, in a population receiving medical care from 129 general practitioners in Spain, in 2007.

Methods

A cross-sectional study was conducted in a health-area setting of the Madrid Autonomous Region (Comunidad Autónoma), covering a population of 198,670 individuals aged over 14 years. Multiple correspondences were analyzed to identify the clustering patterns of the conditions targeted.

Results

Forty-two percent (95% confidence interval [CI]: 41.8–42.2) of the registered population had at least one chronic condition. In all, 24.5% (95% CI: 24.3–24.6) of the population presented with multimorbidity.In the correspondence analysis, 98.3% of the total information was accounted for by three dimensions. The following four, age- and sex-related comorbidity patterns were identified: pattern B, showing a high comorbidity rate; pattern C, showing a low comorbidity rate; and two patterns, A and D, showing intermediate comorbidity rates.

Conclusions

Four comorbidity patterns could be identified which grouped diseases as follows: one showing diseases with a high comorbidity burden; one showing diseases with a low comorbidity burden; and two showing diseases with an intermediate comorbidity burden.  相似文献   
999.
Modern human populations differ in developmental processes and in several phenotypic traits. However, the link between ontogenetic variation and human diversification has not been frequently addressed. Here, we analysed craniofacial ontogenies by means of geometric-morphometrics of Europeans and Southern Africans, according to dental and chronological ages. Results suggest that different adult cranial morphologies between Southern Africans and Europeans arise by a combination of processes that involve traits modified during the prenatal life and others that diverge during early postnatal ontogeny. Main craniofacial changes indicate that Europeans differ from Southern Africans by increasing facial developmental rates and extending the attainment of adult size and shape. Since other studies have suggested that native subsaharan populations attain adulthood earlier than Europeans, it is probable that facial ontogeny is linked with other developmental mechanisms that control the timing of maturation in other variables. Southern Africans appear as retaining young features in adulthood. Facial ontogeny in Europeans produces taller and narrower noses, which seems as an adaptation to colder environments. The lack of these morphological traits in Neanderthals, who lived in cold environments, seems a paradox, but it is probably the consequence of a warm-adapted faces together with precocious maturation. When modern Homo sapiens migrated into Asia and Europe, colder environments might establish pressures that constrained facial growth and development in order to depart from the warm-adapted morphology. Our results provide some answers about how cranial growth and development occur in two human populations and when developmental shifts take place providing a better adaptation to environmental constraints.  相似文献   
1000.
In certain vertebrates such as the zebrafish, most tissues and organs including the heart and central nervous system possess the remarkable ability to regenerate following severe injury. Both spatial and temporal control of cell proliferation and differentiation is essential for the successful repair and re-growth of damaged tissues. Here, using the regenerating adult zebrafish caudal fin as a model, we have demonstrated an involvement of the circadian clock in timing cell proliferation following injury. Using a BrdU incorporation assay with a short labeling period, we reveal high amplitude daily rhythms in S-phase in the epidermal cell layer of the fin under normal conditions. Peak numbers of S-phase cells occur at the end of the light period while lowest levels are observed at the end of the dark period. Remarkably, immediately following amputation the basal level of epidermal cell proliferation increases significantly with kinetics, depending upon the time of day when the amputation is performed. In sharp contrast, we failed to detect circadian rhythms of S-phase in the highly proliferative mesenchymal cells of the blastema. Subsequently, during the entire period of outgrowth of the new fin, elevated, cycling levels of epidermal cell proliferation persist. Thus, our results point to a preferential role for the circadian clock in the timing of epidermal cell proliferation in response to injury.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号