全文获取类型
收费全文 | 88篇 |
免费 | 12篇 |
出版年
2017年 | 1篇 |
2015年 | 2篇 |
2014年 | 2篇 |
2013年 | 1篇 |
2011年 | 2篇 |
2010年 | 2篇 |
2009年 | 2篇 |
2007年 | 2篇 |
2006年 | 2篇 |
2004年 | 1篇 |
2003年 | 4篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 7篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1968年 | 10篇 |
1967年 | 3篇 |
1966年 | 3篇 |
1965年 | 2篇 |
1964年 | 1篇 |
1919年 | 1篇 |
排序方式: 共有100条查询结果,搜索用时 187 毫秒
71.
Fabíola C Toledo Juliana E Perobelli Flávia PC Pedrosa Janete A Anselmo-Franci Wilma DG Kempinas 《Reproductive biology and endocrinology : RB&E》2011,9(1):1-9
Background
Hyperglycemia can impair the male reproductive system in experimental animals and in men during reproductive age. Studies have shown that vitamin C has some good effects on male reproductive system, and therefore vitamin C treatment could attenuate the dysfunctions in this system caused by hyperglycemia. Thus, the objective of this work was to evaluate whether vitamin C treatment could attenuate reproductive dysfunctions in hyperglycemic male rats.Methods
Adult male rats were divided into 3 groups: a normoglycemic (n = 10) and two hyperglycemic (that received a single dose of streptozotocin - 40 mg/kg BW). The two last groups (n = 10 per group) were divided into: hyperglycemic control (Hy) and hyperglycemic + 150 mg of vitamin C (HyC), by gavage during 30 consecutive days. The normoglycemic and hyperglycemic control groups received the vehicle (water). The first day after the treatment, the rats were anesthetized and killed to evaluate oxidative stress biomarkers (TBARS, SOD, GSHt and GSH-Px) in the erythrocytes, body and reproductive organ weights, sperm parameters, plasma hormone levels (FSH, LH and testosterone), testicular and epididymal histo-morphometry and histopathology.Results
Compared with the normoglycemic animals, hyperglycemic control rats showed reduced weight of the body and reproductive organ but testis weight was maintained. It was also observed reduction of testosterone and LH levels, seminiferous tubular diameter, sperm motility and sperm counts in the epididymis. In addition, there was an increase in morphological abnormalities on spermatozoa as well as in oxidative stress level. Vitamin C reduced the oxidative stress level, diminished the number of abnormal sperm, and increased testosterone and LH levels and seminiferous tubular diameter but did not show improvement of sperm motility in relation to the hyperglycemic control group. Hyperglycemia caused a rearrangement in the epididymal tissue components (stroma, ephitelium and lumen) as demonstrated by the stereological analysis results. However, this alteration was partially prevented by vitamin C treatment.Conclusions
We conclude that vitamin C partially attenuated some male reproductive system dysfunctions in hyperglycemic rats. 相似文献72.
Molecular divergence and phylogeny: rates and patterns of cytochrome b evolution in cranes 总被引:6,自引:0,他引:6
Analyses of complete cytochrome b sequences from all species of cranes
(Aves: Gruidae) reveal aspects of sequence evolution in the early stages of
divergence. These DNA sequences are > or = 89% identical, but expected
departures from random substitution are evident. Silent, third- position
pyrimidine transitions are the dominant substitution type, with
transversion comprising only a small fraction of sequence differences.
Substitution patterns are not clearly manifested until divergence has
reached a moderate level (> 3%), as expected for a stochastic process.
Variation in the frequency of mismatch types among lineages decreases at
larger divergences, but the level of bias does not decay. Divergence varies
up to fivefold among gene regions but is not correlated with structural
domain. All protein structural domains except extramembrane 4 display <
20% variable residues. Regions corresponding to putative functional domains
show the excepted conservation of amino acids, although the C-terminal
portion of the Q0 reaction center displays several nonconservative
replacements. Phylogenetic analyses incorporating substitution asymmetries
produced mixed results. Distances estimated with multiple parameters
(transition, codon-position, composition, and pyrimidine-transition biases)
yielded identical additive tree topologies with comparable bootstrap
values, all consistent with uncontroversial species relationships. Maximum
likelihood analysis incorporating these biases, as well as equally weighted
parsimony analysis, produced similar results. Static, differential
weighting for parsimony did not improve the phylogenetic signal but
produced unusual trees with low bootstraps. The overall rate of nucleotide
substitution varies slightly but significantly among cranes, and
calibration of distances against fossil dates suggests divergence rates of
0.7%-1.7% per million years.
相似文献
73.
74.
Summary There are two regions of steroidogenic cells in the duck adrenal gland. An outer, subcapsular zone (SCZ), consisting of cells with irregularly shaped nuclei, shows relatively little smooth endoplasmic reticulum and mitochondria with shelf-like cristae. This region surrounds the inner zone (IZ) of the gland which is comprised of smaller cells with rounded nuclei, a more abundant smooth endoplasmic reticulum and mitochondria with tubular cristae. When samples of tissue from these distinct regions of the gland are superfused in vitro with media containing concentrations of 1–24 ACTH ranging from 100 to 1000 ng per ml (0.034 to 0.34 M) the steroidogenic cells in both zones release corticosterone in a dose-dependent manner. The dose-responsiveness of both the SCZ and the IZ cells over this range is a complex quadratic function of the 1–24 ACTH concentration in the medium and the semilogarithmic linear portions of the dose-response curves are restricted to a narrow midrange of ACTH concentrations. Throughout the dose-response range, however, the steroidogenic cells of the IZ are more responsive to corticotropic stimulation than are the cells of the SCZ. The cells of the two zones are further distinguished by their responses to a challenge for a second time with medium containing 1–24 ACTH; the responses of the IZ cells to a second challenge were greater than those of the SCZ cells, and at a high concentration of ACTH the SCZ slices showed no significant second response.This work was supported by a grant from the National Science Foundation (PCM 79-15777) to James Cronshaw and W.N. Holmes 相似文献
75.
The patterns of synonymous codon usage in 91 Drosophila melanogaster genes
have been examined. Codon usage varies strikingly among genes. This
variation is associated with differences in G+C content at silent sites,
but (unlike the situation in mammalian genes) these differences are not
correlated with variation in intron base composition and so are not easily
explicable in terms of mutational biases. Instead, those genes with high
G+C content at silent sites, resulting from a strong "preference" for a
particular subset of the codons that are mostly C- ending, appear to be the
more highly expressed genes. This suggests that G+C content is reduced in
sequences where selective constraints are weaker, as indeed seen in a
pseudogene. These and other data discussed are consistent with the effects
of translational selection among synonymous codons, as seen in unicellular
organisms. The existence of selective constraints on silent substitutions,
which may vary in strength among genes, has implications for the use of
silent molecular clocks.
相似文献
76.
W. N. Holmes S. C. Al-Ghawas James Cronshaw K. E. Rohde 《Cell and tissue research》1991,263(3):557-566
Summary The adrenal steroidogenic tissue of the neonatal mallard duckling is differentiated into an outer subcapsular zone where the cells contain many large lipid droplets, and an inner zone in which the cells appear to contain less lipid. The cells in both zones contain numerous mitochondria and an abundance of smooth endoplasmic reticulum, and their interdigitating plasma membranes possess many filipodia, coated pits and desmosome-like junctions. Islands of chromaffin cells are distributed throughout the steroidogenic tissue. Two types of chromaffin cell are present, one with vesicles containing densely staining material and the other more lightly staining material. Non-myelinated preganglionic fibers synapse with the chromaffin cells and the axonal terminals contain two types of dense-cored vesicles as well as acetylcholine-containing vesicles. The basal rates of corticosterone (B) and aldosterone (Aldo) release from tissue superfused with buffer containing no secretogogue were low and almost equal (B: Aldo=1.25); the corresponding rate of deoxycorticosterone (DOC) release was less than one-fortieth of the rates of B and Aldo release. The addition of 1–24 ACTH to the medium caused the rate of release of each hormone to increase as a semi-logarithmic function of the concentration and the induced increase in B release was always significantly higher than that of Aldo (B: Aldo=4.8). The corticotropin-induced rates of B and Aldo, but not DOC, release reflected do novo hormone synthesis. Norepinephrine, epinephrine and dopamine each suppressed the basal rates of B and Aldo release, but had no effect when the medium contained 1–24 ACTH. Acetylcholine (ACh) similarly suppressed the basal rates of hormone release, and neither suppressed nor enhanced the responses to medium containing 1–24 ACTH. The suppressive effects of the catecholamines and ACh were not dose-related. [Asp1, Val5] angiotensin II induced significant semi-logarithmic dose-dependent increases in Aldo synthesis but had no effect on the release of either B or DOC.This work was supported by grants to J. Cronshaw and W.N. Holmes from the University of California Committee on Research and the National Science Foundation (DIR-8820923), Washington, D.C., USA 相似文献
77.
Lathe WC rd; Burke WD; Eickbush DG; Eickbush TH 《Molecular biology and evolution》1995,12(6):1094-1105
R1 is a non-long terminal repeat (non-LTR) retrotransposable element that
inserts into a specific sequence of insect 28S ribosomal RNA genes. We have
previously shown that this element has been maintained through vertical
transmission in the melanogaster species subgroup of Drosophila. To address
whether R1 elements have been vertically transmitted for longer periods of
evolutionary time, the analysis has been extended to 11 other species from
four species groups of the genus Drosophila (melanogaster, obscura,
testecea, and repleta). All sequenced elements appeared functional on the
basis of the preservation of their open-reading frames and consistently
higher rate of substitution at synonymous sites relative to replacement
sites. The phylogenetic relationships of the R1 elements from all species
analyzed were congruent with the species phylogenies, suggesting that the
R1 elements have been vertically transmitted since the inception of the
Drosophila genus, an estimated 50-70 Mya. The stable maintenance of R1
through the germ line appears to be the major mechanism for the widespread
distribution of these elements in Drosophila. In two species, D.
neotestecea of the testecea group and D. takahashii of the melanogaster
group, a second family of R1 elements was also present that differed in
sequence by 46% and 31%, respectively, from the family that was congruent
with the species phylogeny. These second families may represent occasional
horizontal transfers or, alternatively, they could reflect the ability of
R1 elements to diverge into new families within a species and evolve
independently.
相似文献
78.
79.
Summary In horizontally placedAcer rubrum seedlings the development of tension wood is inhibited by auxin especially when applied along the upper side of the axis. Treatment separately with GA and K does not alter the normal pattern of tension wood formation. The development of tension wood appears to be correlated with a reduced auxin level on the upper side of the stem.Geotropic reorientation of the woody stem ofAcer rubrum seedlings that are placed in the horizontal position is related closely to the amount of tension wood that is formed on the upper side of the axis. Evidence is discussed which indicates that tension wood itself participates actively in the reorientation mechanism.The following abbreviations will be used TIBA
(2,3,5-tri-iodobenzoic acid)
- IAA
(indole-3-acetic acid)
- GA
(gibberellic acid)
- NAA
(naphthaleneacetic acid)
- 2,4-D
(2,4-dichloro-phenoxyacetic acid)
- K
(kinetin)
This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven. 相似文献
80.
Summary The thin-walled aesthetasc pegs on the antennules of a small hermit crab,Pagurus hirsutiusculus, were studied by light and electron microscopy. The lumen of each aesthetasc was found to be filled with the dendrites of 300–500 bipolar neurons whose cell bodies lie beneath the base of the aesthetasc. These dendrites are ciliary in nature, having well developed basal bodies and rootlets.Each basal body gives rise to a cilium which divides to form a cluster of slender branches, each of which contains a microtubule running lengthwise. These structures occupy most of the length of the hair.The cuticle of the aesthetasc wall is thin and tenuous. Except for the pore canals in the basal region, we have found no pores at either light or electron microscope level, but as the hair is extremely permeable, we conclude that the cuticle itself may permit the passage of solutions. This permeability of the cuticle and the large numbers of dendrites within support the hypothesis that the aesthetascs are chemoreceptors. 相似文献