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   317篇
  免费   22篇
  国内免费   2篇
  2024年   1篇
  2023年   3篇
  2022年   6篇
  2021年   3篇
  2020年   15篇
  2019年   6篇
  2018年   5篇
  2017年   4篇
  2016年   10篇
  2015年   10篇
  2014年   29篇
  2013年   17篇
  2012年   23篇
  2011年   21篇
  2010年   19篇
  2009年   31篇
  2008年   17篇
  2007年   10篇
  2006年   5篇
  2005年   4篇
  2004年   4篇
  2003年   5篇
  2002年   6篇
  2001年   2篇
  2000年   2篇
  1999年   5篇
  1998年   6篇
  1997年   3篇
  1996年   2篇
  1995年   2篇
  1994年   5篇
  1993年   4篇
  1992年   4篇
  1991年   4篇
  1990年   3篇
  1989年   7篇
  1988年   3篇
  1987年   2篇
  1986年   3篇
  1985年   8篇
  1984年   4篇
  1983年   6篇
  1982年   5篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1975年   1篇
排序方式: 共有341条查询结果,搜索用时 15 毫秒
81.
82.
目的:观察白细胞介素17(Interleukin-17,IL-17)在乙肝孕妇和正常孕妇胎盘中表达差异来反映胎盘内的细胞免疫应答的情 况,为外源性细胞因子在增强机体免疫、清除病毒及阻断和治疗HBV宫内感染提供新的科学依据。方法:选择择期剖宫产的足月 妊娠妇女46 例,其中血清HBsAg(+)30 例,血清HBsAg(-)的对照组16 例,采用免疫组化方法检测胎盘组织IL-17 的表达。结 果:IL-17 主要表达在胎盘绒毛的合体滋养层细胞胞浆、毛细血管内皮细胞胞浆以及基质中。血清HBsAb(+)孕妇与HBsAb(-)孕 妇比较,无显著差异(P>0.05),HBsAg(+)乙肝孕妇与对照组的表达比较差异有统计学意义(P<0.05),HBeAg(+)孕妇与HBeAg (-)孕妇的表达比较差异有统计学意义(P<0.05)。结论:IL-17 在乙肝孕妇胎盘组织中的表达高于正常孕妇,IL-17 的表达不但加 强了机体的防御功能,且能促进炎性反应。  相似文献   
83.
Huang Y  Li D  Zhou Y  Zhou Q  Li R  Wang C  Huang Z  Hull V  Zhang H 《Zoo biology》2012,31(5):561-573
Artificial insemination (AI) is an important component of captive breeding programs for endangered species, such as the giant panda. The panda has been the subject of increasingly successful captive breeding programs involving a compilation of assisted breeding techniques, including AI using cryopreserved spermatozoa. AI implementation is currently hampered by a lack of understanding of the factors that may cause failure. We investigated factors influencing the probability of success of AI for 14 giant panda females housed at the China Center for Research and Conservation of the Giant Panda (CCRCGP) inseminated in a total of 20 instances using cryopreserved spermatozoa from 11 males currently residing in 6 different captive breeding institutions. One of the pandas was the oldest giant panda female to ever successfully conceive from AI (20.5 years old). The success of AI was significantly affected by the timing of AI in relationship to both timing of peak urinary estrogen of the female and percent decline in urinary estrogen between the peak level and the first AI attempt. Our results suggest that the window for successful AI in giant pandas may be narrower than previously suspected, although individual differences in rates of decline in urinary estrogen may reflect some degree of variation in this crucial window across females. Our results are consistent with recent research on pandas and other species that demonstrates the efficacy of cryopreserved spermatozoa for AI and highlights the need for more in‐depth analysis of factors related to female physiology that may influence its success. Zoo Biol 31:561‐573, 2012. © 2011 Wiley Periodicals, Inc.  相似文献   
84.
Artificial insemination in deer and non-domestic bovids   总被引:1,自引:0,他引:1  
Artificial insemination technology has revolutionized the domestic cattle breeding industry and allowed for the dissemination of valuable genetics worldwide. This technology has been adapted for use in many other taxa for the conservation of threatened and endangered species, but its use for the genetic management of small populations of deer, antelope and other non-domestic bovids has met numerous challenges and limited success. In practice, adaptation of domestic bovine AI protocols to other artiodactylids for genetic management has been limited by: (1) a lack of understanding of species-specific reproductive characteristics; (2) the inability to minimize handling stress; (3) pregnancy losses; and (4) regulatory challenges in semen importation. To date, AI protocols have been developed for seven species of cervid and 14 species of non-domestic bovids; recent developments in this technology has allowed greater use of AI for dissemination of genetics in farmed deer species. However, despite decades of research in the use of assisted reproduction for the conservation of antelope and other non-domestic bovids, even this simplest technique has not been used repeatedly for genetic management.  相似文献   
85.
Two experiments were designed to evaluate strategies to increase fertility of Bos indicus postpubertal heifers and nonlactating cows submitted to a fixed-time artificial insemination (TAI) protocol consisting of an intravaginal device containing 1.9 g of progesterone (CIDR) insertion + estradiol benzoate on Day 0, CIDR withdrawal + estradiol cypionate on Day 9, and TAI on Day 11. In Experiment 1, heifers (n = 1153) received a new or an 18-d previously used CIDR and, on Day 9, prostaglandin F (PGF) + 0, 200, or 300 IU equine chorionic gonadotropin (eCG). Heifers treated with a new CIDR had greater (least squares means ± SEM) serum concentration of progesterone on Day 9 (3.06 ± 0.09 ng/mL vs. 2.53 ± 0.09 ng/mL; P < 0.05) and a smaller follicle at TAI (11.61 ± 0.11 mm vs. 12.05 ± 0.12 mm; P < 0.05). Heifers with smaller follicles at TAI had lesser serum progesterone concentrations on Day 18 and reduced rates of ovulation, conception, and pregnancy (P < 0.05). Treatment with eCG improved (P < 0.05) follicle diameter at TAI (11.50 ± 0.10 mm, 11.90 ± 0.11 mm, and 12.00 ± 0.10 mm for 0, 100, and 200 IU, respectively), serum progesterone concentration on Day 18 (2.77 ± 0.11 ng/mL, 3.81 ± 0.11 ng/mL, and 4.87 ± 0.11 ng/mL), and rates of ovulation (83.8%, 88.5%, and 94.3%) and pregnancy (41.3%, 47.0%, and 46.7%). In Experiment 2, nonlactating Nelore cows (n = 702) received PGF treatment on Days 7 or 9 and, on Day 9, 0 or 300 IU eCG. Cows receiving PGF on Day 7 had lesser serum progesterone concentrations on Day 9 (3.05 ± 0.21 ng/mL vs. 4.58 ± 0.21 ng/mL; P < 0.05), a larger follicle at TAI (11.54 ± 0.21 mm vs. 10.84 ± 0.21 mm; P < 0.05), and improved (P < 0.05) rates of ovulation (85.4% vs. 77.0%), conception (60.9% vs. 47.2%), and pregnancy (52.0% vs. 36.4%). Treatment with eCG improved (P < 0.05) serum progesterone concentration on Day 18 (3.24 ± 0.14 ng/mL vs. 4.55 ± 0.14 ng/mL) and the rates of ovulation (72.4% vs. 90.0%) and pregnancy (37.5% vs. 50.8%). In conclusion, giving PGF earlier in the protocol in nonlactating cows and eCG treatment in postpubertal heifers and nonlactating cows improved fertility in response to a TAI (progesterone + estradiol) protocol.  相似文献   
86.
Since the establishment of production of viable cloned dogs by somatic cell nucleus transfer, great concern has been given to the reproductive abilities of these animals (Canis familiaris). Therefore, we investigated reproductive activity of cloned dogs by (1) performing sperm analysis using computer-assisted sperm analysis and early embryonic development, (2) assessing reproductive cycling by measuring serum progesterone (P4) levels and performing vaginal cytology, and (3) breeding cloned dogs using artificial insemination. Results showed that most parameters of sperm motility in a cloned male dog were within the reference range, and in vivo–matured oocytes from a noncloned female were successfully fertilized by spermatozoa from a cloned male dog and develop normally to the 8-cell stage. Three cloned female dogs displayed normal patterns of P4 levels and morphologic changes of the vaginal epithelium. Two cloned female dogs became pregnant using semen from a cloned male dog and successfully delivered 10 puppies by natural labor. In conclusion, these data demonstrated that both cloned male and female dogs are fertile, and their puppies are currently alive and healthy with normal growth patterns.  相似文献   
87.
This is the very first report that suggests high pregnancy rates can be obtained with use of the Doublesynch protocol in anestrous dairy cows. Recently, a new synchronization method has been developed (Doublesynch) that resulted in synchronized ovulations both after the first and second gonadotropin-releasing hormone (GnRH) treatments. It was suggested that this protocol has the potential to increase the pregnancy rates in primiparous dairy cows. The aim of the current study was to confirm the success of the Doublesynch protocol and further to investigate the effect of this method on pregnancy rates in anestrous cows. Lactating primiparous Holstein (Bos taurus) cows (n = 165) between 60 and 172 d postpartum were monitored twice with 10-d intervals (on Days -10 and 0) by ultrasonography, and blood samples were collected. Cows were classified as anestrous if both blood samples had progesterone (P4) concentration <1 ng/mL and as cyclic if at least one of the two samples had P4 concentration ≥1 ng/mL. Cyclic cows were classified again as cyclic-high P4 (having an active corpus luteum) if the second blood samples had P4 concentrations ≥1 ng/mL and as cyclic-low P4 if P4 concentrations were <1 ng/mL on Day 0. Then, the cows classified as anestrous (n = 51), cyclic-high P4 (n = 63), or cyclic-low P4 (n = 51) were put into two treatment groups (Ovsynch or Doublesynch) randomly to establish six groups. Cows in the Ovsynch group were administered a GnRH (lecirelin 50 μg, im) on Day 0, PGF (Prostaglandin F2 alpha, D-cloprostenol 0.150 mg, im) on Day 7, and a second dose of GnRH 48 h later. Cows in the Doublesynch group were administered a PGF on Day 0, GnRH on Day 2, a second PGF on Day 9, and a second GnRH on Day 11. Timed artificial insemination (TAI) was performed 16 to 20 h after the second GnRH in both treatment groups. Pregnancy diagnosis was conducted (by ultrasonography) 45 ± 5 d after TAI. In anestrous cows and those with high and low progesterone concentration at treatment onset, Doublesynch treatment led to markedly increased pregnancy rates with respect to Ovsynch treatment (P < 0.05). On the overall analysis of data, it was revealed that the Doublesynch method increased pregnancy rates by 43 percentage units (29.8% vs. 72.8%, P < 0.0001) in relation to Ovsynch. Pregnancy rates of cows having small, medium, or large follicles at the day of second GnRH administration were similar in the Doublesynch group (70.4%, 85.2%, and 63.0%, respectively; P > 0.05), whereas pregnancy rates reduced dramatically as follicle size increased in the Ovsynch group, particularly in cows with follicles greater than 16 mm (45.5%, 28.1%, and 5.3%, respectively; P < 0.05). Our results confirm and support observations that the Doublesynch protocol increases the pregnancy rates in postpartum primiparous cows as reported previously. Our data also demonstrate that the Doublesynch method increases the pregnancy rates in anestrous cows. Thus, these data suggest that the Doublesynch protocol can be used to obtain satisfactory pregnancy rates after TAI in both anestrous and cycling primiparous dairy cows regardless of stage of estrous cycle.  相似文献   
88.
The main objective of the current study was to evaluate the effects of extender type and centrifugation/washing prior to cryopreservation on the postthaw sperm parameters, lipid peroxidation, and superoxide dismutase activity of Angora buck (Capra hircus ancryrensis) sperm. Ejaculates collected from three Angora bucks were used in this study. Two consecutive ejaculates from each buck were pooled and split into equal parts in four Falcon tubes. Two tubes were diluted at 37 °C and then centrifuged to remove semen plasma. After centrifugation, two sediment parts were diluted with a Tris-based extender and commercial Bioxcell extender, respectively. The remaining two parts, which were not centrifuged/washed, were diluted with the above-mentioned extenders, respectively. Diluted samples were cooled to 5 °C and frozen in 0.25-mL French straws to be stored in liquid nitrogen. Frozen straws were thawed individually at 37 °C for 20 sec in a water bath for evaluation. The semen part with centrifugation/washing in the Bioxcell extender (BC) demonstrated a higher rate of subjective motility (58.1 ± 3.0%) compared with that of groups with (TC) or without (T) centrifugation/washing in the Tris-based extender (P < 0.01). Angora buck sperm frozen with (BC) or without (B) centrifugation/washing in the Bioxcell extender demonstrated higher percentages of motility (60.6 ± 2.7% and 54.3 ± 4.8%, respectively) compared with that of groups T and TC. The postthaw progressive motility rate (22.3 ± 2.7%) was significantly greater for semen parts diluted in B compared with that of other groups. BC gave rise to a lower value of average path velocity (90.0 ± 5.2 μm/sec) compared with that of other groups (P < 0.01). For straight linear velocity and linearity index, the highest values (103.2 ± 4.7 μm/sec, 47.5 ± 1.6% and 94.8 ± 3.0 μm/sec, 44.8 ± 1.1%, respectively) were obtained from B and TC (P < 0.001). For sperm acrosome and total abnormalities, TC gave the highest values (11.2 ± 0.6% and 26.6 ± 1.5%, respectively, P < 0.01). In the group frozen in BC, the percentage of membrane integrity assessed by hypo-osmotic swelling test was higher (61.2 ± 2.2%) than that of the other groups (P < 0.001). With respect to fertility results based on 35-d pregnancy rates, BC gave a higher rate (76.5%) than that of TC (27.8%, P < 0.05). Malondialdehyde formation was found to be lower (1.64 ± 0.26 nmol/L) in BC than in the other groups after the freeze-thawing process (P < 0.001). In the semen part frozen in BC, superoxide dismutase activity was higher (0.18 ± 0.02 U/mg protein) compared with that of the other groups (P < 0.05). Further studies are required to obtain more precise results for the characterization of oxidative stress parameters and fertilizing ability in cryopreserved buck spermatozoa.  相似文献   
89.
Cytogenetic analysis of 58 boars at an artificial insemination (AI) centre revealed the presence of a reciprocal chromosome translocation, rcp(1;11)(q−;p+), in two Duroc boars. Pedigree analysis of these two boars suggested familial transmission of the chromosome rearrangement. The reproductive consequences of this translocation were determined in a herd of sows that had received semen doses from these and other boars. All sows underwent multiple AI, with different groups established retrospectively depending on the percentage of semen doses provided by the carrier boars ([number of carrier boar doses/total number doses provided] x 100): 0%, 25%, 50%, 75%, 100%. The fertility rates (percentage of successful multiple AIs/total multiple AIs) recorded for multiple AI including semen doses from the carrier boars were not significantly different from those recorded when all semen doses were supplied by normal-karyotype boars. A reduction in litter size of 29.38% was observed, however, in litters sired by one of the carrier boars when its participation in multiple AI was 100%. The number of live-born piglets per litter gradually decreased (P < 0.05) as the percentage participation in multiple AI (25, 50, or 75%) of the carrier boar increased. In addition, both carrier boars sired some piglets with signs of cleft palate and complex malformations of the front legs; these died soon after birth. In conclusion, the boars carrying the translocation rcp(1;11)(q−;p+) showed reduced reproductive performance.  相似文献   
90.
Theory considering sex ratio optima under ‘strict local mate competition with offspring groups produced by a single foundress’ makes a suite of predictions, one of which is a mean female bias. Treating individual offspring as discrete units, theory further predicts sex ratios to have low variance (precise sex ratio) and to equal the reciprocal of clutch size (one male per clutch). The maternal decision may be complicated by imperfect control of sex allocation, limited insemination capacity of sons and offspring developmental mortality: each can lead to virgin daughters (with zero fitness) and consequently select for less biased sex ratios. When clutches are small and/or developmental mortality is common, appreciable proportions of virgins are expected, even when control of sex allocation is perfect and the mating capacity of males is unlimited. This suite of predictions has been only partially tested. We provide further tests by examining sex ratios and developmental mortalities within and across species of locally mating parasitoids. We find a wide range of mean developmental mortalities (6–67%), but mortality distributions are consistendy overdispersed (have greater than binomial variance) and sexually differential mortality appears to be absent. Sex ratios are female biased and have low variance, but are not perfectly precise and variance is increased by mortality within species and (equivocally) across species. Sex ratios less biased than the reciprocal of clutch size are observed; probably due to a maternal response to developmental mortality in one species, and to limited insemination capacity in others. Cross species comparisons indicate that mean proportions of mortality and virginity are positively correlated. Virginity is more prevalent than predicted among species with higher mortalities but not among lower mortality species. Predicted relationships between virginity and clutch size are supported in species with lower mortalities but only partially supported when mortality rates are higher.  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