当前位置：文档之家› 2009--JME-Exposure of Xenopus laevis tadpoles to finasteride

2009--JME-Exposure of Xenopus laevis tadpoles to finasteride

Page 1 of 31Accepted Preprint first posted on 24 June 2009 as Manuscript JME-09-0058 Exposure of Xenopus laevis tadpoles to finasteride, an inhibitor of 5-alpha reductase activity, 1

impairs spermatogenesis and alters hypophyseal feedback mechanisms

Abbreviated title: 5-α reductase and spermatogenesis

Urbatzka, R.1, 4, Watermann, B.2,Lutz, I. 1 and Kloas, W.1, 3

1IGB, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Department of Inland Fisheries,

Mueggelseedamm 301, 12587 Berlin, Germany

2Limnomar, Bei der Neuen Muenze 11, 22145 Hamburg, Germany

3Department of Endocrinology, Institute of Biology, Humboldt-University, Invalidenstrasse 43, 10115

Berlin, Germany

4present address: Centre of Marine and Environmental Research, Rua dos Bragas 289, 4050-123 Porto,

Portugal

Corresponding author:

Ralph Urbatzka, IGB, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Department of

Inland Fisheries, Mueggelseedamm 301, 12587 Berlin, Germany, and Centre of Marine and

Environmental Research, Rua dos Bragas 289, 4050-123 Porto, Portugal, rurbatzka@ciimar.up.pt,

Phone: +351 223 401 831, Fax: +351 223 390 608

Page 2 of 31 Abstract

Sexual steroids have major regulatory functions in gonadal development, maturation of gametes and

sexual differentiation in vertebrates. Previous studies in amphibians provided evidence that

dihydrotestosterone and activity of 5-α reductases might play a significant role in androgen mediated

reproductive biology. To test involvement of 5-α reductases in maturation of gametes in amphibians,

X. laevis was exposed to finasteride (FIN), a known inhibitor of 5-α reductase enzyme activity. In a

long-term exposure from stage 46 to 66, severe disruption of spermatogenesis was observed in

histological analysis of testes as detected by occurrence of empty spermatocysts, while ovaries

remained unaffected. Real-time PCR analyses of male and female brain revealed an increase of

luteinizing hormone (LHβ) mRNA and a decrease in follicle stimulating hormone mRNA in males

suggesting a signalling on testes that could result in increased steroidogenesis and reduced Sertoli cell

proliferation. Accordingly, the mRNA expression of P450 side chain cleavage enzyme and of 5-α

reductase type 2 was increased in testes, while no effects could be observed on steroidogenic genes in

ovaries. A short-term exposure to testosterone (T), FIN and T + FIN showed that transient effects of

FIN targeted males selectively and in particular interfered with the hypothalamus-pituitary-gonad axis.

Furthermore, a negative feedback of T on LHβwas observed on males and females. This study

provides evidence that exposure of X. laevis to FIN, an inhibitor of 5-αreductases, impaired

spermatogenesis and involved sex-specific hypophyseal feedback mechanisms.

Keywords: Xenopus laevis, finasteride, 5-alpha reductase, spermatogenesis, luteinizing hormone,

follicle stimulating hormone, gonadotropines, hypothalamus-pituitary-gonad axis.

Page 3 of 31

Introduction

Synthesis of sexual steroids is a conserved process in vertebrates starting with cholesterol that is 2

converted by actions of multiple enzymes into testosterone (T). T can either exert androgen action by 3

itself or can be further converted into dihydrotestosterone (DHT) or into 17β-estradiol (Payne and 4

Hales 2004). Leydig cells in testis and theca/granulosa cells in ovaries are the main producing cells of 5

sexual steroids that are subsequently released into the blood stream. Biological action in peripheral 6

tissues often needs the conversion of T into DHT that is accomplished by the enzyme 5-α reductase.

Two isozymes exist for 5-αreductase, named type 1 (Srd5a1) and type 2 (Srd5a2), which are 8

differentially distributed in many tissues. In rats, Srd5a2 is confined to androgen-dependent tissues 9

while Srd5a1 is present in various tissues (Torres et al. 2003). Especially Srd5a2 activity is believed to

play a key role in androgen regulated reproductive biology.

In mammals as in amphibians the more potent androgen is DHT unlike in fish, where 11-

ketotestosterone is the most potent androgen (Norris 1997). DHT promotes in humans and mammals

the development of external genitalia, or androgenic tissues like prostate while T is responsible for the

development of male internal genitalia. Finasteride (FIN), a well-known inhibitor of 5-αreductase

activity with higher potency to Srd5a2 than to Srd5a1 (George et al.1989; Prahalada et al., 1997;

Ellsworth et al.1998), was frequently used to experimentally unravel functions of 5-αreductases

being involved in male pattern hair loss (Kawashima et al. 2004) or benign prostate hyperplasia or

prostate cancer (Luo et al. 2003). If administered in utero, external genitalia were feminized in rats

(Imperato-McGinley et al.1992) or developed abnormalities in rhesus monkeys (Prahalada et al.

1997). Humans with deficiency of Srd5a2 developed feminized genitalia (Imperato-McGinley et al.

1981) and polymorphisms in Srd5a2 gene were associated with differences in sperm concentration and

motility (Elzanaty et al. 2006).

In the Srd5a2 gene promoter region a progesterone and androgen response element (PRE/ARE) was

identified in mouse suggesting that different androgens might be able to regulate gene expression of

Srd5a2 (Matsui et al.2002). T regulated enzyme activity of 5-αreductase in an in vitro system

Page 4 of 31 (Andersson et al. 1989), mRNA expression of Srd5a2 in prostate of intact and castrated rats in vivo

(Torres et al. 2003) and in rat brain in vivo and in vitro (Poletti et al. 1998). However, also DHT is

described to induce 5-αreductase activity in rats (George et al.1991). Effects of T and DHT are

mediated both via the androgen receptor and it is known that some genes are differentially regulated

by T and DHT, respectively (Avila et al.1998). The mechanism for this is still unclear, but it is

proposed that especially in peripheral tissues with low T level, specific action of 5-αreduced

androgens are needed.

In amphibians, some studies provided evidence that DHT is a decisive androgen for sexual

differentiation in Xenopus laevis: DHT but not T at a concentration of 10-8 M induced sex reversal in

an exposure experiment (B?gi et al.2002; Kloas 2002) and sex-specific pattern of Srd5a2 mRNA

expression during ontogeny suggested an important function in male gamete maturation (Urbatzka et

al. 2007). The aim of the present study was to analyse the effect of FIN on the development of gonads

in X. laevis and to gain insights into the physiological function of 5-αreductases in regulating

reproductive biology of amphibians. Therefore tadpoles of X. laevis were exposed to FIN during their

sensitive phase of gonadal development and sexual differentiation with the intention to decrease the

activities of 5-α reductase enzymes in X. laevis. The effects of FIN on gonad histology of X. laevis

were monitored at stage 66, just after completion of metamorphosis. Additionally, the expression of

LH and FSH mRNA was analysed in brain samples and the expression of selected steroidogenic genes

(P450scc, StAR, Srd5a1, Srd5a2 and Aro) in gonad samples. In complement to the analysis of long

term effects of FIN, a short term exposure was conducted to analyse the transient effects of FIN. The

short-term exposure included three treatment groups, T, FIN, and T + FIN to address the question of a

potential regulation of 5-α reductase by T in amphibians, a regulation that was frequently observed in

mammalian model species. In this study, X. laevis was used for the first time as a comparative model

to unravel the potential function of 5-α reductases on spermatogenesis in lower vertebrates.

Page 5 of 31

Material and Methods

Exposure

Xenopus laevis were taken from the breeding stock of the Institute of Freshwater Ecology and Inland 4

Fisheries, Berlin, Germany. Adult X. laevis were injected with human chorionic gonadotropin 5

(SIGMA, Deisenhofen, Germany) into the dorsal lymph sac to induce spawning as described 6

previously (Kloas et al.1999). Fertilized eggs and tadpoles were reared in 50 L tanks containing 7

reconstituted tap water using deionised water supplemented with 2.5 g marine salt (Tropic Marin 8

Meersalz, Tagis, Dreieich, Germany) per 10 L. Tanks were aerated and temperature was adjusted to 22 9

± 1°C. The light:dark cycle was 12:12 h and tadpoles were fed daily with Sera Micron (Sera GmbH,

Heinsberg, Germany). The developmental stages of the tadpoles were determined according to the

Normal Table of X. laevis (Nieuwkoop and Faber 1994).

For a long-term exposure, tadpoles at stage 46 were transferred to 10 L aerated glass aquaria at a

density of 30 tadpoles per tank containing 7 L of reconstituted tap water. Tadpoles were exposed to

FIN (APIN Chemicals LTD, a gift of Dr. John Ashby, Central Toxicology Laboratory, Cheshire,

Syngenta, UK), a known 5-αreductase enzyme inhibitor. FIN was dissolved in dimethylsulfoxide

(DMSO, SIGMA) and the highest soluble concentration was chosen for exposure that did not re-

crystallize if added to the water. The final concentration of FIN was 7 x 10-6M (2.66 mg/L) and

DMSO concentration was 0.0286%. Exposure was conducted in duplicate tanks (total n = 60) and

included a solvent control. Water and chemicals were renewed every Monday, Wednesday and Friday.

Tadpoles were sampled after completion of metamorphosis (stage 66). Sex of tadpoles was determined

by gross morphology of the gonads using a stereo microscope before gonads were fixed in bouin′s

solution for 24 h. Fixed gonads were transferred to 80% EtOH for 24 h and then stored in fresh 80%

EtOH until further processing for histological analyses. Further gonad and brain samples of male and

female tadpoles were dissected, immediately snap-frozen in liquid nitrogen and stored at -80°C until

further analyses.

Page 6 of 31 In the short-term exposure, tadpoles were reared in aerated glass aquaria until they reached stage 58.

At this stage, tadpoles were transferred to the exposure tanks in a density of 18 tadpoles per tank

containing 7 L of reconstituted tap water. Tadpoles were exposed to T at a supra-physiological

concentration of 10-7 M, to FIN at a concentration of 7 x 10-6M and to a combination of both. A

solvent control (DMSO 0.0867%) and a control without solvent were included in the experimental

setup. Exposure lasted 4 days and water and chemicals were changed daily. Brain and gonads of six

male and female tadpoles were dissected at day 5, directly snap-frozen in liquid nitrogen and stored at

-80°C until further analyses.

Histology

From the long-term exposure, 6 males and 6 females of the control and of the FIN treatment were

analysed for histological alterations of the gonads. 3 – 4 μm sections from paraffin blocks were cut

using a microtome. Tissue sections were deparaffinized in xylene for 15 min, washed sequentially in

96%, 90%, 70%, 50%, 30% EtOH (2 min each) and in aqua dest. for 5 min. Subsequently, one set of

sections was stained with hematoxylin-eosin (H-E), while another set was stained by

immunocytochemistry for the proliferating cell nuclear antigen (PCNA) as indication for cell

proliferation. Heat-induced epitope retrieval was performed by transfer of the slides into a jar

containing S?rensen’s citrate buffer (pH 6) on a hot water bath (96°C to 99°C) for 30 min. After

cooling down the slides for 20 min at room temperature, the slides were washed with phosphate

buffered saline (PBS) and aqua dest. Endogenous peroxidase was quenched in 3% hydrogen peroxide

for 5 min with subsequent rinsing in PBS. 100 μL of anti-PCNA were applied to the slide, incubated at

room temperature in a humidity chamber for 30 min and rinsed in PBS. The working concentration of

anti-PCNA was 1:200, diluted in PBS. Next, 100 μL of peroxidase was added to the slide, followed by

incubation of the slide for 30 min in a humidity chamber and rinsing with PBS. The final step

comprised the application of the chromagen solution NovaRED, incubation in a humidity chamber for

15 min, washing in aqua dest. for 5 min and counterstaining of the slides with hematoxylin.

RNA-Isolation and reverse transcription (RT)

Page 7 of 31

Extraction of total RNA of brain samples of tadpoles from the long-term exposure and of brain and 1

gonad samples of tadpoles from the short-term exposure was performed by using RNA extraction kits 2

of Qiagen (RNeasy Micro Kit, RNeasy Mini Kit, Hilden, Germany) according to the instructions of 3

the manufacturer and included on-column DNAse ingestion (Qiagen). Transcription of RNA into 4

cDNA was accomplished by reverse transcriptase reaction (RT) and was performed with 1 μg total 5

RNA for the brain samples and with 200 ng total RNA for the gonad samples. For RT of brain samples 6

7.5 pM poly (dT) primer (Biometra, G?ttingen, Germany), 10 mM dNTP (Qbiogene, Heidelberg,

Germany) and 10 U AMV reverse transcriptase (Finnzymes, Finnland) were applied in a 30 μl 8

reaction while for RT of gonad samples 10 U AffinityScript reverse transcriptase (Stratagene, 9

Amsterdam, The Netherlands) were used in a 20 μL reaction.

Real-time RT-PCR

Amplifications of cDNA specific for luteinizing hormone (LHβ), follicle stimulating hormone (FSHβ),

P450 side chain cleavage enzyme (P450scc), steroidogenic acute regulatory protein (StAR), 5-α

reductase type 1 (Srd5a1), 5-α reductase type 2 (Srd5a2), aromatase (Aro) and elongation factor 1-α

(EF) as housekeeping gene of X. laevis were carried out by using real-time RT-PCR. Amplification

was carried out in a thermal cycler (Stratagene, Mx3000P) using SYBR green. The following thermal

cycling conditions were applied: 7 min 40 sec at 95°C followed by 40 cycles of 17 s at 95°C, 25 s at

62°C, 25 s at 72°C. Melting curve analyses were performed with the following setting, 40 sec at 95°C,

30 sec at 55°C and 30 sec at 95°C. For all PCR reactions, cDNA was diluted 1:5 (brain samples) or

1:10 (gonad samples) and 3 μl of diluted cDNA were mixed in a 25 μl reaction with 10 mM dNTP′s

(Qbiogene), 1:40.000 SYBR (Invitrogen), 1:200 ROX (Invitrogen), 2 mM MgCl2, 100 - 400 pM

forward and reverse primer and 1 U of Platinum Taq-Polymerase (Invitrogen).

Primers were designed according to sequences published in the National Centre for Biotechnology

Information (NCBI). All sequences were derived from X. laevis with the exception of Srd5a1 and

Srd5a2 that were deduced from X. tropicalis. Table 1 lists the sequences of the designed primers. To

test specificity of the PCR products the bands were run on an agarose gel, extracted (QIAquick, Gel

Page 8 of 31 extraction kit, Qiagen, Hilden, Germany) and forwarded to automated sequencing (SeqLab, Sequence

Laboratories, G?ttingen, Germany). Sequence comparisons by BLAST and sequence alignments

(BioEdit) confirmed the specificity of amplified PCR products.

Data analysis and statistics

MxPro software (Stratagene) was used to analyse real-time PCR data using the ??Ct-method

including the PCR efficiencies of the genes (ranging from 91.6 % to 104.5 %). Values of the mRNA

expressions of the target genes were normalized by the values of the house-keeping gene. After testing

normal distribution, data of long-term exposure was analysed with unpaired t-tests. For data of short-

term exposure, one-way ANOVA was used to examine significant differences between the mRNA

expressions of the treatments. Either Tukey′s test or Dunnet-T3 test was applied depending on the

outcome of testing equality of variance by the Levene-test. Histological data were analysed by using

Mann-Whitney U-test. For all statistical analyses, differences were regarded to be significant if p was

< 0.05.

Page 9 of 31

Results

Long-term exposure

Mortality of tadpoles during exposure was very low and was 2% in control tanks (one individual) and 3

0% in finasteride (FIN) treatment tanks. No differences between control and FIN treatment groups 4

were observed regarding the time needed to complete metamorphosis, and their total length at stage 5

66. Tadpoles developed from stage 46 to stage 66 in about six weeks and the mean length was 1.8 ±

0.2 cm. Exposure of X. laevis tadpoles during ontogeny to FIN did not affect the sex ratio of tadpoles.

Sex ratio in the FIN treatment group was not significantly different from the solvent control group 8

with 60% males and 40% females.

Histological analyses of gonads revealed that male X. laevis from the solvent control group had

relatively immature testes containing mainly spermatogonia and spermatocytes in numerous

spermatocysts (Figure 1, panel A). A further maturation to spermatids or spermatozoa was not

observed in these animals. Proliferation activity of spermatogonia and spermatocytes was high as

indicated by PCNA staining (Figure 1, panel B). FIN exposed male X. laevis possessed comparable

maturation stages as in the solvent control group, namely spermatogonia and spermatocytes but not

spermatids or spermatozoa. In 5 of 6 analysed FIN treated males, empty cavities were observed in the

tubules and this likely corresponded to former spermatocysts with secondary spermatogonia (Figure 1,

panel C). Quantitative analyses of male gonadal sections revealed a statistically significant occurrence

of empty spermatocysts (p < 0.01). While there was no significant difference, FIN treated male

animals seemed to have a lower number of tubuli, spermatogonia and spermatocytes (Table 2).

Intensity of proliferation was dramatically reduced in all FIN treated males compared to the solvent

control group indicated by the lower number of visible mitoses and reduced PCNA staining (Figure 1,

panel D).

In female X. laevis from the solvent control group ovaries were in the maturation stage in that ovarian

cavities were formed or expanded. In the cortical region numerous germ cells and primary oocytes

arranged in nests were observed (Figure 1, panel E). Proliferation activity indicated by PCNA staining

was moderate in germ cells and high in primary oocytes (Figure 1, panel F). Stages of maturation and

structure of gonads of FIN treated female X. laevis were similar compared to females from the solvent

Page 10 of 31 control (Figure 1, panel G) and also proliferation activity was about the same (Figure 1, panel H). The

development of an ovarian cavity was not observed in one ovary.

In brain samples of male X. laevis, LH mRNA expression was about 9-fold increased in the FIN

treatment group (p < 0.001), while FSH mRNA expression was decreased in FIN-treated males (p <

0.05) (Figure 2). In testes, P450scc and Srd5a2 mRNA were significantly increased (p < 0.01), while

no effects were observed for StAR, Srd5a1 and Aro mRNA expression (Figure 2).

In brain of female X. laevis, the expression of both gonadotropins was not altered by waterborne

exposure to FIN (Figure 3). In ovaries, no different expression was observed in the FIN treatment

group relative to the solvent control for all analysed steroidogenic genes.

Short-term exposure

In male tadpoles from the four day exposure, LHβ mRNA in the brain was significant decreased by T

as well as by the combined exposure to T and FIN. In contrast, exposure to FIN alone led to a

significant increase of LHβ mRNA expression and was about 8-fold higher than in the control and in

the solvent control (Figure 4). FSH mRNA expression was not changed by any treatment. In the

gonads, P450scc mRNA expression was 2-fold elevated in response to FIN while the other treatments

had no effect. StAR, Srd5a1, Srd5a2, and Aro mRNA expression stayed at the same expression levels

compared to both control groups.

In female tadpoles, LHβmRNA in the brain was significantly decreased by T and the combined

exposure to T and FIN compared to the solvent control corresponding to the results obtained in male

tadpoles (Figure 5). FIN alone did not change LHβmRNA expression in females. No significant

changes of mRNA expression were observed in the other analysed genes (FSHβ, P450scc, StAR,

Srd5a1, Srd5a2, and Aro) relative to control and solvent control.

Page 11 of 31

Discussion

Tadpoles of X. laevis were exposed to FIN, a known inhibitor of 5-α-reductase enzyme activity 2

(George et al. 1989; Ellsworth et al. 1998), during their sensitive phase of gonadal development and 3

sexual differentiation. In a previous study an important function of 5-αreduced metabolites was 4

suggested for spermatogenesis, observing sex-specific mRNA expression pattern of Srd5a2 in X.

laevis tadpoles during their ontogeny at the beginning of gamete maturation (Urbatzka et al. 2007).

The present study demonstrated that FIN affected specifically males and not females leading to a 7

disruption of spermatogenesis and to altered mRNA expression of gonadotropins in the brain and of 8

steroidogenic genes in the gonads.

Histological analyses of gonads of male X. laevis tadpoles exposed to FIN revealed a severe disruption

of spermatogenesis. Emptied cavities were observed in the analysed gonads that corresponded to

former spermatocysts with secondary spermatogonia (Figure 1) and a reduced proliferation activity in

FIN treated males. According to these results, it was shown in male rats during puberty that a peak of

5-αreductases and 5-αreduced metabolites coincided with the first wave of spermatogenesis

suggesting a role for 5-α reductases in the initiation of spermatogenesis (Killian et al. 2003). In studies

during adulthood, no effects of FIN on testicular histomorphology were found in rats (George et al.

1989; Rhoden et al. 2002) and dogs (Juniewicz et al. 1993). In contrast, a study performed in adult rats

showed that administration of FIN suppressed the T induced restoration of spermatogenesis and

demonstrated clearly a role for 5-αreduced androgens in adult spermatogenesis (O′Donnell et al.

1996). In contrast to the specific mode of action of FIN, the antiandrogen flutamide (FLU) inhibits the

action of various androgens at the androgen receptor and exposure experiments demonstrated the

importance of androgens in spermatogenesis in different vertebrates. If rats were exposed in utero to

FLU, the death of testicular germ cells was observed in a concentration-dependent way mediated by

apoptotic gene expression (Bozec et al.2004). Fish exposed to FLUexperienced spermatocyte

degeneration in fathead minnow (Jensen et al. 2004) or reduced numbers of spermatogenic cysts in

male guppies (Kinnberg and Toft 2003).

In contrast to the results obtained in male tadpoles, histological analyses of ovaries demonstrated no

alterations in the FIN treatment group compared to the control group. The single finding of one female

Page 12 of 31 gonad without the forming of an ovarian cavity may be an effect of FIN, but has to be verified in

future experiments. In tadpoles of Bufo bufo and of Rana dalmatina, the inhibition of 5-α reductase

activity accelerated the ovarian differentiation in females suggesting an inhibiting role for 5-α reduced

metabolites regarding the timing of ovarian differentiation and maybe also for inhibiting oocyte

maturation in females (Zaccanti et al. 1994; Petrini and Zaccanti 1998). However, this finding could

not be confirmed in the present study.

The analysis of gonadotropin mRNA expression in male and female X. laevis from the long-term

exposure revealed a sex-specific hypophyseal feedback mechanism. Selectively in males, LH mRNA

was strongly increased in the FIN treatment group while FSH mRNA was decreased (Figure 2). LH is

known to act mainly on steroidogenesis in gonads while FSH is essential for Sertoli cell proliferation

(Weltzien et al.2004). Therefore, this observed regulation pattern could signal an increased

steroidogenesis in testes and a reduced Sertoli cell proliferation that could potentially have caused the

observed impairment of spermatogonia maturation considering the central role of Sertoli cells in germ

cell development. Mitotic activity of Sertoli cells in fish was highest during spermatogonial

proliferation and considered as an important factor determining testis size and sperm production

(Schulz et al. 2005). The modulatory effect of FIN on gonadotropin mRNA expression in male X.

laevis is shown for the first time and demonstrated an involvement of a physiological regulation of the

H-P-G axis. This finding fits to a study in rats, where a high dose of FIN (250 mg/kg/day) increased

the plasma LH level that correlated with Leydig cell hyperplasia (Prahalada et al.1994), while in

another study plasma LH level remained unchanged in a concentration of 20 mg/kg/day (Ribeiro and

Pereira 2005). The contrary regulation of LH and FSH mRNA expression in response to sexual

steroids was confirmed in further experiments from our workgroup, even if the mechanism remains

unclear (Urbatzka et al., 2008). From studies in fish it is known that sexual steroids can regulate

gonadotropins differently, inhibiting one gene and stimulating the other (Sohn et al., 2001; Mateos et

al., 2002).

Page 13 of 31

In gonads of long-term exposed animals, steroidogenic gene expression was changed in males, but not 1

in females (Fig. 2 and 3). FIN treatment increased P450scc and Srd5a2 mRNA expression in testes, 2

while StAR, Srd5a1 and Aro mRNA expression was not affected. It is well known that steroidogenesis 3

is under control of gonadotropins (Weltzien et al., 2004) and therefore it is likely that increased LH 4

mRNA in brain stimulated the transcription of P450scc. Interestingly, StAR mRNA expression was 5

only slightly, but not significant increased. StAR is the enzyme responsible for cholesterol transport 6

into the mitochondria where it is converted by P450scc to pregnenolone (Arukwe, 2008). Studies in 7

fish showed that StAR and P450scc mRNA expression pattern were different under some 8

experimental conditions regarding to exposure time, compounds and their concentration (Arukwe 9

2005, Kortner et al., 2009a, Kortner et al., 2009b). The increase of Srd5a2 mRNA in testis in FIN

treated animals is suggested to be a counter-regulation to the inhibition of 5-αreductase enzyme

activities and indicated the presence of a regulatory feedback loop. Thereby, the counter-regulation

could be driven directly by the assumed lower DHT level or by the observed feedback on LH mRNA

expression. The Srd5a1 mRNA expression was only slightly increased, but not significant, and could

be associated to the fact that FIN inhibits 5-α reductase type 2 enzyme with higher potency than 5-α

reductase type 1 enzyme (Prahalada et al., 1997; Ellsworth et al., 1998). Aro mRNA expression stayed

at the same expression level indicating no interference of FIN with estrogen synthesis.

Male and female tadpoles at stage 58 were exposed to T, FIN and a combination of both in a short-

term exposure to analyse transient effects on gonadotropin mRNA expression in the brain and on

steroidogenic gene expression in the gonads. T was chosen as androgen since its stimulatory function

on 5-α reductase mRNA expression and enzyme activity in peripheral tissues (Andersson et al. 1989;

Poletti et al. 1998; Torres et al. 2003). LHβ mRNA expression was significantly reduced by T and T +

FIN in males and females, but no effects were observed on FSH mRNA expression or on

steroidogenic gene expression in the gonads. The negative feedback of androgens on the H-P-G axis is

known from previous studies in mature amphibians. DHT reduced the plasma LH level in male, adult

Rana pipiens (Tsai et al. 2005) and MDHT decreased the LHβ mRNA expression in male, adult X.

laevis (Urbatzka et al. 2006). Thus, it is suggested that decreased LHβ mRNA levels in T and T + FIN

Page 14 of 31 treatment are due to negative feedback of T. A regulation of Srd5a2 by T like in mammals or in vitro

(Andersson et al. 1989; George et al. 1991; Poletti et al. 1998; Torres et al. 2003) was not observed in

this study for X. laevis. In accordance to the long-term exposure, the effect of FIN was restricted to

males and included an increase of LHβmRNA and of P450scc mRNA indicating an increase in

steroidogenesis mediated by feedback on LH. FIN treatment in rat resulted in an increase of T and

androstenedione, in a decrease of DHT (George 1997), and additionally increased estradiol in humans

(Castro-Magana et al., 1996). If a similar effect of FIN on steroids is assumed in this study, the

increased LHβ mRNA expression in the FIN treatment group is unlikely to be induced by the increase

of T or estrogens since exposure to T was demonstrated in this study to inhibit LHβmRNA

expression, an effect also known for exposure of amphibians to estrogens (Urbatzka et al., 2006).

Furthermore, androstenedione is known to have only a low affinity to the androgen receptor and is

therefore an improbable candidate for steroidal feedback. In contrary, we suggest that the increase of

LHβ in the FIN treatment could be due to the assumed lower DHT levels, a regulation thatwould fit to

another experiment where exposure to FLU, an androgen receptor antagonist, increased LHβ mRNA

expression in males (Urbatzka et al., 2008). A decrease in FSH mRNA and a increase in Srd5a2

mRNA as detected in males from the long-term exposure, could only slightly be observed in the short-

term exposure and was not significant different from the control. Therefore it may indicate that these

effects are time-dependent. Some effects may be unmasked only after a prolonged exposure time.

Furthermore, transient effects could be mediated by altered enzyme levels, while long term effects

involve de novo synthesis of mRNA.

Interestingly, no effect of FIN was observed in females in the short- and long-term exposures either on

gonadotropin expression in the brain or on steroidogenic gene expression in ovaries. The reasons for

these results remain speculative and may be owed to a differential regulation of feedback mechanisms

in the brain.

The presented study provided clear evidence that FIN affected the expression of gonadotropin mRNA

in long-term and short-term exposures. However, the effect of FIN on gonadotropins could be

potentially mediated by inhibition of 5-αreductase activity or by a direct effect. The present data

Page 15 of 31

indicated an increased Srd5a2 mRNA expression in the long-term exposure as counter-regulation to 1

the assumed inhibition of 5-α reductase enzyme activity. This result is in line with the fact that FIN is 2

a specific inhibitor of 5-αreductase type 2 enzyme activity in mammals (Prahalada et al., 1997;

Ellsworth et al., 1998). Accordingly, specific inhibition of 5-α reductase enzyme activity was reported 4

in sea urchins (Wasson and Watts, 1998) suggesting that FIN is acting specifically in vertebrates and 5

non-vertebrates. An assumed lower DHT level in the FIN treatment group could affect the 6

gonadotropin mRNA expression in the pituitary, since it is well known that sexual steroids feedback 7

on the hypothalamus and pituitary. In further experiments of our workgroup, it was demonstrated that 8

LHβmRNA expression in male X. laevis was increased in an exposure to the androgen receptor 9

antagonist flutamide (Urbatzka et al., 2008) providing further indications for this hypothesis.

Nevertheless, both hypotheses are suggested to be possible until a proof of decreased enzymatic levels

of 5-α reductase or DHT levels is shown in response to FIN treatment in amphibians.

Summarizing, it was shown that exposure of X. laevis tadpoles to FIN, an inhibitor of 5-α reductase

activity, during the phase of gonadal development and sexual differentiation affected only the males

and impaired spermatogenesis in testes of metamorphosed frogs. The long-term and short-term

exposures to FIN demonstrated a potential involvement of sex-specific hypophyseal feedback

mechanisms, in particular an elevation of LHβ mRNA in brain and of P450scc mRNA in gonads. Only

the long-term exposure revealed an additional decrease of FSH mRNA in brain and an increase of

Srd5a2 mRNA in gonads. Decreased FSH mRNA expression may have caused a reduced Sertoli cell

proliferation, important cells for germ cell development, while increased LH mRNA expression was

suggested to stimulate steroidogenesis as counter-regulation to FIN.

Declaration of interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the

impartiality of the research reported.

Page 16 of 31

Funding

This research was funded in part by the European Union within the EU-project EASYRING, contract

Nr. QLK4-CT-2002-02286.

Acknowledgment

We thank Dr. John Ashby (Central Toxicology Laboratory, Cheshire, Syngenta, UK) for the gift of

Finasteride.

References

Andersson S, Bishop RW, Russell DW 1989 Expression cloning and regulation of steroid 5 alpha-

reductase, an enzyme essential for male sexual differentiation. Journal of Biological Chemistry264

16249-16255.

Arukwe A 2005 Modulation of brain steroidogenesis by affecting transcriptional changes of

steroidogenic acute regulatory (StAR) protein and cholesterol side chain cleavage (P450scc) in

juvenile Atlantic salmon (Salmo salar) is a novel aspect of nonylphenol toxicity. Environmental

Science and Technology39 9791-9798.

Arukwe A 2008 Steroidogenic acute regulatory (StAR) protein and cholesterol side-chain cleavage

(P450scc)-regulated steroidogenesis as an organ-specific molecular and cellular target for endocrine

disrupting chemicals in fish. Cell Biology and Toxicology24 527-40.

Avila DM, Fuqua SA, George FW, McPhaul MJ 1998 Identification of genes expressed in the rat

prostate that are modulated differently by castration and Finasteride treatment. Journal of

Endocrinology159 403-411.

Page 17 of 31

B?gi C, Levy G, Lutz I, Kloas W 2002 Functional genomics and sexual differentiation in amphibians.

Comparative Biochemistry and Physiology B133 559-570.

Bozec A, Chuzel F, Chater S, Paulin C, Bars R, Benahmed M, Mauduit C 2004 The mitochondrial-3

dependent pathway is chronically affected in testicular germ cell death in adult rats exposed in utero to 4

anti-androgens. Journal of Endocrinology183 79-90.

Castro-Magana M, Angulo M, Fuentes B, Canas A, Sarrantonio M, Arguello R, Vitollo P 1996 Effect 6

of finasteride on human testicular steroidogenesis. Journal of Andrology17 516-521.

Ellsworth KP, Azzolina BA, Cimis G, Bull HG, Harris GS 1998 Cloning, expression and 8

characterization of rhesus macaque types 1 and 2 5alpha-reductase: evidence for mechanism-based 9

inhibition by finasteride. Journal of Steroid Biochemistry and Molecular Biology66 271-279.

Elzanaty S, Giwercman YL, Giwercman A 2006 Significant impact of 5alpha-reductase type 2

polymorphisms on sperm concentration and motility. International Journal of Andrology29 414-420.

George FW, Johnson L, Wilson JD 1989 The effect of a 5 alpha-reductase inhibitor on androgen

physiology in the immature male rat. Endocrinology 125 2434-2438.

George FW, Russell DW, Wilson JD 1991 Feed-forward control of prostate growth:

dihydrotestosterone induces expression of its own biosynthetic enzyme, steroid 5 alpha-reductase.

Proceedings of the National Academy of Sciences USA88 8044-8047.

George FW 1997 Androgen metabolism in the prostate of the finasteride-treated, adult rat: a possible

explanation for the differential action of testosterone and 5 alpha-dihydrotestosterone during

development of the male urogenital tract. Endocrinology138 871-877.

Imperato-McGinley J, Peterson R E, Gautier T 1981 Male pseudohermaphroditism secondary to 5a-

reductase deficiency: A review. In Fetal Endocrinology, pp 359-382. Eds MJ Novy, JA Resko. New

York: Academic Press.

Page 18 of 31 Imperato-McGinley J, Sanchez RS, Spencer JR, Yee B, Vaughan ED 1992 Comparison of the effects

of the 5 alpha-reductase inhibitor finasteride and the antiandrogen flutamide on prostate and genital

differentiation: dose-response studies. Endocrinology131 1149-1156.

Jensen KM, Kahl MD, Makynen EA, Korte JJ, Leino RL, Butterworth BC, Ankley GT 2004

Characterization of responses to the antiandrogen flutamide in a short-term reproduction assay with

the fathead minnow. Aquatic Toxicology70 99-110.

Juniewicz PE, Hoekstra SJ, Lemp BM, Barbolt TA, Devin JA, Gauthier E, Frenette G, Dube JY,

Tremblay RR 1993 Effect of combination treatment with zanoterone (WIN 49596), a steroidal

androgen receptor antagonist, and finasteride (MK-906), a steroidal 5 alpha-reductase inhibitor, on the

prostate and testes of beagle dogs. Endocrinology133 904-913.

Kawashima M, Hayashi N, Igarashi A, Kitahara H, Maeguchi M, Mizuno A, Murata Y, Nogita T,

Toda K, Tsuboi R, Ueki R, Yamada M, Yamazaki M, Matsuda T, Natsumeda Y, Takahashi K, Harada

S 2004 Finasteride in the treatment of Japanese men with male pattern hair loss. European Journal of

Dermatology14 247-254.

Kinnberg K, Toft G 2003 Effects of estrogenic and antiandrogenic compounds on the testis structure

of the adult guppy (Poecilia reticulata). Ecotoxicology and Environmental Safety54 16-24.

Kloas W, Lutz I, Einspanier R 1999 Amphibians as a model to study endocrine disruptors: II.

Estrogenic activity of environmental chemicals in vitro and in vivo. Science of the Total Environment

225 59-68.

Kloas W 2002 Amphibians as a model for the study of endocrine disruptors. International Reviews of

Cytology216 1-57.

Killian J, Pratis K, Clifton RJ, Stanton PG, Robertson DM, O'Donnell L 2003 5alpha-reductase

isoenzymes 1 and 2 in the rat testis during postnatal development. Biology of Reproduction68 1711-

1718.

Page 19 of 31

Kortner TM, Rocha E, Arukwe A 2009a Androgenic modulation of early growth of Atlantic cod 1

(Gadus morhua L.) previtellogenic oocytes and zona radiata-related genes. Journal of Toxicology and 2

Environmental Health A 72 184-195.

Kortner TM, Rocha E, Arukwe A 2009b Previtellogenic oocyte growth and transcriptional changes of 4

steroidogenic enzyme genes in immature female Atlantic cod (Gadus morhua L.) after exposure to the 5

androgens 11-ketotestosterone and testosterone. Comparative Biochemistry and Physiology A152 6

304-313.

Luo J, Dunn TA, Ewing CM, Walsh PC, Isaacs WB 2003 Decreased gene expression of steroid 5 8

alpha-reductase 2 in human prostate cancer: implications for finasteride therapy of prostate carcinoma.

Prostate57 134-139.

Mateos J, Mananos E, Carrillo M, Zanuy S 2002 Regulation of follicle-stimulating hormone (FSH)

and luteinizing hormone (LH) gene expression by gonadotropin-releasing hormone (GnRH) and

sexual steroids in the Mediterranean Sea bass. Comparative Biochemistry and Physiology B132 75-

86.

Matsui D, Sakari M, Sato T, Murayama A, Takada I, Kim M, Takeyama K, Kato S 2002

Transcriptional regulation of the mouse steroid 5alpha-reductase type II gene by progesterone in brain.

Nucleic Acids Research30 1387-1393.

Nieuwkoop PD & Faber J 1994 Normal Table of Xenopus laevis(Daudin). New York: Garland

Publishing Inc.

Norris DO 1997 Vertebrate Endocrinology, Third Edition, San Diego: Academic Press.

O'Donnell L, Stanton PG, Wreford NG, Robertson DM, McLachlan RI 1996 Inhibition of 5 alpha-

reductase activity impairs the testosterone-dependent restoration of spermiogenesis in adult rats.

Endocrinology137 2703-2710.

Page 20 of 31 Payne AH, Hales DB 2004 Overview of steroidogenic enzymes in the pathway from cholesterol to

active steroid hormones. Endocrine Reviews25 947-970.

Petrini S, Zaccanti F 1998 The effects of aromatase and 5 alpha-reductase inhibitors, antiandrogen,

and sex steroids on Bidder's organs development and gonadal differentiation in Bufo bufo tadpoles.

Journal of Experimental Zoology280 245-259.

Poletti A, Negri-Cesi P, Rabuffetti M, Colciago A, Celotti F, Martini L 1998 Transient expression of

the 5alpha-reductase type 2 isozyme in the rat brain in late fetal and early postnatal life.

Endocrinology139 2171-2178.

Prahalada S, Majka JA, Soper KA, Nett TM, Bagdon WJ, Peter CP, Burek JD, MacDonald JS, van

Zwieten MJ 1994 Leydig cell hyperplasia and adenomas in mice treated with finasteride, a 5 alpha-

reductase inhibitor: a possible mechanism. Fundamental and Applied Toxicology22 211-219.

Prahalada S, Tarantal AF, Harris GS, Ellsworth KP, Clarke AP, Skiles GL, MacKenzie KI, Kruk LF,

Ablin DS, Cukierski MA, Peter CP, vanZwieten MJ, Hendrickx AG 1997 Effects of finasteride, a type

2 5-alpha reductase inhibitor, on fetal development in the rhesus monkey (Macaca mulatta).

Teratology55 119-131.

Rhoden EL, Gobbi D, Menti E, Rhoden C, Teloken C 2002 Effects of the chronic use of finasteride on

testicular weight and spermatogenesis in Wistar rats. BJU International89 961-963.

Ribeiro CM, Pereira OC 2005 5alpha-reductase 2 inhibition impairs brain defeminization of male rats:

reproductive aspects. Pharmacology Biochemistry and Behaviour82 228-235.

Schulz RW, Menting S, Bogerd J, Franca LR, Vilela DAR, Gondinho HP 2005 Sertoli cell

proliferation in the adult testis – evidence from two fish species belonging to different orders. Biology

of Reproduction73 891-898.

如何写先进个人事迹

如何写先进个人事迹篇一：如何写先进事迹材料如何写先进事迹材料一般有两种情况：一是先进个人，如先进工作者、优秀党员、劳动模范等；一是先进集体或先进单位，如先进党支部、先进车间或科室，抗洪抢险先进集体等。无论是先进个人还是先进集体，他们的先进事迹，内容各不相同，因此要整理材料，不可能固定一个模式。一般来说，可大体从以下方面进行整理。 (1)要拟定恰当的标题。先进事迹材料的标题，有两部分内容必不可少，一是要写明先进个人姓名和先进集体的名称，使人一眼便看出是哪个人或哪个集体、哪个单位的先进事迹。二是要概括标明先进事迹的主要内容或材料的用途。例如《王鬃同志端正党风的先进事迹》、《关于评选张鬃同志为全国新长征突击手的材料》、《关于评选鬃处党支部为省直机关先进党支部的材料》等。 (2)正文。正文的开头，要写明先进个人的简要情况，包括：姓名、性别、年龄、工作单位、职务、是否党团员等。此外，还要写明有关单位准备授予他(她)什么荣誉称号，或给予哪种形式的奖励。对先进集体、先进单位，要根据其先进事迹的主要内容，寥寥数语即应写明，不须用更多的文字。然后，要写先进人物或先进集体的主要事迹。这部分内容是全篇材料

的主体，要下功夫写好，关键是要写得既具体，又不繁琐；既概括，又不抽象；既生动形象，又很实在。总之，就是要写得很有说服力，让人一看便可得出够得上先进的结论。比如，写一位端正党风先进人物的事迹材料，就应当着重写这位同志在发扬党的优良传统和作风方面都有哪些突出的先进事迹，在同不正之风作斗争中有哪些突出的表现。又如，写一位搞改革的先进人物的事迹材料，就应当着力写这位同志是从哪些方面进行改革的，已经取得了哪些突出的成果，特别是改革前后的．经济效益或社会效益都有了哪些明显的变化。在写这些先进事迹时，无论是先进个人还是先进集体的，都应选取那些具有代表性的具体事实来说明。必要时还可运用一些数字，以增强先进事迹材料的说服力。为了使先进事迹的内容眉目清晰、更加条理化，在文字表述上还可分成若干自然段来写，特别是对那些涉及较多方面的先进事迹材料，采取这种写法尤为必要。如果将各方面内容材料都混在一起，是不易写明的。在分段写时，最好在每段之前根据内容标出小标题，或以明确的观点加以概括，使标题或观点与内容浑然一体。最后，是先进事迹材料的署名。一般说，整理先进个人和先进集体的材料，都是以本级组织或上级组织的名义；是代表组织意见的。因此，材料整理完后，应经有关领导同志审定，以相应一级组织正式署名上报。这类材料不宜以个人名义署名。写作典型经验材料-般包括以下几部分： (1)标题。有多种写法，通常是把典型经验高度集中地概括出来，一

关于时间管理的英语作文 manage time

How to manage time Time treats everyone fairly that we all have 24 hours per day. Some of us are capable to make good use of time while some find it hard to do so. Knowing how to manage them is essential in our life. Take myself as an example. When I was still a senior high student, I was fully occupied with my studies. Therefore, I hardly had spare time to have fun or develop my hobbies. But things were changed after I entered university. I got more free time than ever before. But ironically, I found it difficult to adjust this kind of brand-new school life and there was no such thing called time management on my mind. It was not until the second year that I realized I had wasted my whole year doing nothing. I could have taken up a Spanish course. I could have read ten books about the stories of successful people. I could have applied for a part-time job to earn some working experiences. B ut I didn’t spend my time on any of them. I felt guilty whenever I looked back to the moments that I just sat around doing nothing. It’s said that better late than never. At least I had the consciousness that I should stop wasting my time. Making up my mind is the first step for me to learn to manage my time. Next, I wrote a timetable, setting some targets that I had to finish each day. For instance, on Monday, I must read two pieces of news and review all the lessons that I have learnt on that day. By the way, the daily plan that I made was flexible. If there’s something unexpected that I had to finish first, I would reduce the time for resting or delay my target to the next day. Also, I would try to achieve those targets ahead of time that I planed so that I could reserve some more time to relax or do something out of my plan. At the beginning, it’s kind of difficult to s tick to the plan. But as time went by, having a plan for time in advance became a part of my life. At the same time, I gradually became a well-organized person. Now I’ve grasped the time management skill and I’m able to use my time efficiently.

英语演讲稿：未来的工作

英语演讲稿：未来的工作这篇《英语演讲稿范文：未来的工作》，是特地，希望对大家有所帮助！热门演讲推荐：竞聘演讲稿 | 国旗下演讲稿 | 英语演讲稿 | 师德师风演讲稿 | 年会主持词 | 领导致辞 everybody good afternoon:. first of all thank the teacher gave me a story in my own future ideal job. everyone has a dream job. my dream is to bee a boss, own a pany. in order to achieve my dreams, i need to find a good job, to accumulate some experience and wealth, it is the necessary things of course, in the school good achievement and rich knowledge is also very important. good achievement and rich experience can let me work to make the right choice, have more opportunities and achievements. at the same time, munication is very important, because it determines whether my pany has a good future development. so i need to exercise their municative ability. i need to use all of the free time to learn

关于坚持的英语演讲稿

关于坚持的英语演讲稿 Results are not important, but they can persist for many years as a commemoration of. Many years ago, as a result of habits and overeating formed one of obesity, as well as indicators of overall physical disorders, so that affects my work and life. In friends to encourage and supervise, the participated in the team Now considered to have been more than three years, neither the fine rain, regardless of winter heat, a day out with 5:00 time. The beginning, have been discouraged, suffering, and disappointment, but in the end of the urging of friends, to re-get up, stand on the playground. 成绩并不重要，但可以作为坚持多年晨跑的一个纪念。多年前，由于庸懒习惯和暴饮暴食，形成了一身的肥胖，以及体检指标的全盘失常，以致于影响到了我的工作和生活。在好友的鼓励和督促下，参加了晨跑队伍。现在算来，已经三年多了，无论天晴下雨，不管寒冬酷暑，每天五点准时起来出门晨跑。开始时，也曾气馁过、痛苦过、失望过，但最后都在好友们的催促下，重新爬起来，站到了操场上。 In fact, I did not build big, nor strong muscles, not a sport-born people. Over the past few years to adhere to it, because I have a team behind, the strength of a strongteam here, very grateful to our team, for a long time, we encourage each other, and with sweat, enjoying common health happy. For example, Friends of the several run in order to maintain order and unable to attend the 10，000 meters race, and they are always concerned about the brothers and promptly inform the place and time, gives us confidence and courage. At the same time, also came on their own inner desire and pursuit for a good health, who wrote many of their own log in order to refuel for their own, and inspiring. 其实我没有高大身材，也没健壮肌肉，天生不属于运动型的人。几年来能够坚持下来，因为我的背后有一个团队，有着强大团队的力量，在这里，非常感谢我们的晨跑队，长期以来，我们相互鼓励着，一起流汗，共同享受着健康带来的快

关于管理的英语演讲

1.How to build a business that lasts100years 0:11Imagine that you are a product designer.And you've designed a product,a new type of product,called the human immune system.You're pitching this product to a skeptical,strictly no-nonsense manager.Let's call him Bob.I think we all know at least one Bob,right?How would that go? 0:34Bob,I've got this incredible idea for a completely new type of personal health product.It's called the human immune system.I can see from your face that you're having some problems with this.Don't worry.I know it's very complicated.I don't want to take you through the gory details,I just want to tell you about some of the amazing features of this product.First of all,it cleverly uses redundancy by having millions of copies of each component--leukocytes,white blood cells--before they're actually needed,to create a massive buffer against the unexpected.And it cleverly leverages diversity by having not just leukocytes but B cells,T cells,natural killer cells,antibodies.The components don't really matter.The point is that together,this diversity of different approaches can cope with more or less anything that evolution has been able to throw up.And the design is completely modular.You have the surface barrier of the human skin,you have the very rapidly reacting innate immune system and then you have the highly targeted adaptive immune system.The point is,that if one system fails,another can take over,creating a virtually foolproof system. 1:54I can see I'm losing you,Bob,but stay with me,because here is the really killer feature.The product is completely adaptive.It's able to actually develop targeted antibodies to threats that it's never even met before.It actually also does this with incredible prudence,detecting and reacting to every tiny threat,and furthermore, remembering every previous threat,in case they are ever encountered again.What I'm pitching you today is actually not a stand-alone product.The product is embedded in the larger system of the human body,and it works in complete harmony with that system,to create this unprecedented level of biological protection.So Bob,just tell me honestly,what do you think of my product? 2:47And Bob may say something like,I sincerely appreciate the effort and passion that have gone into your presentation,blah blah blah-- 2:56(Laughter) 2:58But honestly,it's total nonsense.You seem to be saying that the key selling points of your product are that it is inefficient and complex.Didn't they teach you 80-20?And furthermore,you're saying that this product is siloed.It overreacts, makes things up as it goes along and is actually designed for somebody else's benefit. I'm sorry to break it to you,but I don't think this one is a winner.

关于工作的优秀英语演讲稿

关于工作的优秀英语演讲稿 Different people have various ambitions. Some want to be engineers or doctors in the future. Some want to be scientists or businessmen. Still some wish to be teachers or lawers when they grow up in the days to come. Unlike other people, I prefer to be a farmer. However, it is not easy to be a farmer for Iwill be looked upon by others. Anyway,what I am trying to do is to make great contributions to agriculture. It is well known that farming is the basic of the country. Above all, farming is not only a challenge but also a good opportunity for the young. We can also make a big profit by growing vegetables and food in a scientific way. Besides we can apply what we have learned in school to farming. Thus our countryside will become more and more properous. I believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. All the working position can provide him with a good chance to become a talent. 1 ————来源网络整理，仅供供参考

立春节气的特点是什么_二十四节气立春特点

立春节气的特点是什么_二十四节气立春特点二十四节气，每一个节气都代表着季节气候的转变。立春是春天的头一个节气，那么大家知道立春节气会有哪些特征变化吗?下面是小编给大家整理的有关立春节气的特点，欢迎大家来参阅。立春的节气特点每年2月4日前后，太阳到达黄经315度时交立春节气。“立春”是预示季节转换的节气。这个节气，早在战国末期(约公元前239年)就确定了。 “立春”是二十四节气中的第一个节气。在古时，古人认为，春季开始叫“立春”，夏季开始叫“立夏”，秋季开始叫“立秋”，冬季开始叫“立冬”。其农事意义是“春种、夏长、秋收、冬藏”。春夏秋冬，以春为首。春季六节气，“立春”居先，标志着春天的到来、四季的开始，这是古时的说法。立春物候立春第一候是“东风解冻”，东风送暖，化解了大地冰封已久的寒冻。第二候是“蛰虫始振”，“蛰”是隐藏的意思，指潜伏在地下的小虫都自冬眠中苏醒过来。第三候是“鱼陟负冰”，陟是上升的意思，鱼儿囚为水温渐暖，所以竞相浮游到水面，但水中仍有未溶解的碎冰。在岸上观看，就如同鱼儿背负着冰块在水中游动。立春气候

从现代气候学标准看，以“立春”作为春季的开始，不能和当时全国各地的自然物候现象(即春季应有的景象)吻合。如2月初“立春”后，我国华南已经花红柳绿了，然而华北大地仍会大雪纷飞。现在比较科学的划分，是把候(5天为一候)平均气温在10摄氏度以上的始日，作为春季开始。相连的广州起步北上，开始旅程，2月中旬先是到了昆明、下旬则越过云贵高原进人四川盆地，3月中旬到达武汉三镇、下旬越过郑州和济南，4月上旬抵达京津地区、中旬跨过山海关来到塞外沈阳城、下旬经由长春到达哈尔滨，5月20日前后，才能光临我国最北的地方—漠河。另外，由于我国幅员辽阔，地势复杂，不但春临大地的时间有早有迟，春姑娘在各地停留的时间也有长短之别，如西北地区土壤干燥，春季升温迅速，整个春季仅40多天，可谓来去匆匆;而云南昆明等地则四季如春，被誉为“春城”。虽然“立春”节气后，不能说春天已到，但确实预示着春天就要来了，农业生产大忙季节即将来临。这不，一些农谚即是很好的佐证。如:“立春一日，水热三分”、“春到三分暖”……以安徽省为例，“立春”节气以后，虽处于冬季之尾，但有时气温仍较低，还会发生低温冻害。如安徽省极端最低气温的极值，就出现在1969年2月6日的固镇县，为一24.3‘C。因此，仍应加强冬作物，尤其是温室大棚的保温防寒工作。不过，有时冬季偏暖年份，南方湿热空气势力强于北方冷空气，在“立春” 立春时节立春一到，人们明显地感觉到白昼长了，太阳暖了，气温增高了，日照延长了，降雨也开始了。农谚提醒人们“立春雨水到，早起晚睡觉”。立春时竹北方备耕也开始了。 “一年之计在干春”，在中国是流传最为广泛的格言式谚语之一。在《增广贤文》中，它一共是四句:“一年之计在于春，一日之计在于晨，一家之计在于和，一生之计在于勤。”其中前两句，强调的都是时间的紧迫性和重要性。阳春之季，万象更新，对于农事耕作，这是个关键时刻。对于一个人来说，青年时期也是人生的春天。青春的创造力是无穷的，爱惜青春，珍视青春，就能创造出知识，创造出财富;反之，浪费青春，虚度年华，除了惭愧之外，将一无所得。“一年之计在于春”，强调的是要在一年开始时多做并做好工作，为全年的工作打下坚实的基础。节气特征现象

个人先进事迹简介

个人先进事迹简介 01 在思想政治方面,xxxx同学积极向上,热爱祖国、热爱中国共产党,拥护中国共产党的领导.利用课余时间和党课机会认真学习政治理论,积极向党组织靠拢. 在学习上,xxxx同学认为只有把学习成绩确实提高才能为将来的实践打下扎实的基础,成为社会有用人才.学习努力、成绩优良. 在生活中,善于与人沟通,乐观向上,乐于助人.有健全的人格意识和良好的心理素质和从容、坦诚、乐观、快乐的生活态度,乐于帮助身边的同学,受到师生的好评. 02 xxx同学认真学习政治理论,积极上进,在校期间获得原院级三好生,和校级三好生,优秀团员称号,并获得三等奖学金. 在学习上遇到不理解的地方也常常向老师请教,还勇于向老师提出质疑.在完成自己学业的同时,能主动帮助其他同学解决学习上的难题,和其他同学共同探讨,共同进步. 在社会实践方面,xxxx同学参与了中国儿童文学精品“悦”读书系,插画绘制工作,xxxx同学在班中担任宣传委员,工作积极主动,认真负责,有较强的组织能力.能够在老师、班主任的指导下独立完成学院、班级布置的各项工作. 03 xxx同学在政治思想方面积极进取,严格要求自己.在学习方面刻苦努力,不断钻研,学习成绩优异,连续两年荣获国家励志奖学金；作

为一名学生干部,她总是充满激情的迎接并完成各项工作,荣获优秀团干部称号.在社会实践和志愿者活动中起到模范带头作用. 04 xxxx同学在思想方面,积极要求进步,为人诚实,尊敬师长.严格要求自己.在大一期间就积极参加了党课初、高级班的学习,拥护中国共产党的领导,并积极向党组织靠拢. 在工作上,作为班中的学习委员,对待工作兢兢业业、尽职尽责的完成班集体的各项工作任务.并在班级和系里能够起骨干带头作用.热心为同学服务,工作责任心强. 在学习上,学习目的明确、态度端正、刻苦努力,连续两学年在班级的综合测评排名中获得第1.并荣获院级二等奖学金、三好生、优秀班干部、优秀团员等奖项. 在社会实践方面,积极参加学校和班级组织的各项政治活动,并在志愿者活动中起到模范带头作用.积极锻炼身体.能够处理好学习与工作的关系,乐于助人,团结班中每一位同学,谦虚好学,受到师生的好评. 05 在思想方面,xxxx同学积极向上,热爱祖国、热爱中国共产党,拥护中国共产党的领导.作为一名共产党员时刻起到积极的带头作用,利用课余时间和党课机会认真学习政治理论. 在工作上,作为班中的团支部书记,xxxx同学积极策划组织各类团活动,具有良好的组织能力. 在学习上,xxxx同学学习努力、成绩优良、并热心帮助在学习上有困难的同学,连续两年获得二等奖学金. 在生活中,善于与人沟通,乐观向上,乐于助人.有健全的人格意识和良好的心理素质.

自我管理演讲稿英语翻译

尊敬的领导，老师，亲爱的同学们，大家好！我是5班的梁浩东。今天早上我坐车来学校的路上，我仔细观察了路上形形色色的人，有开着小车衣着精致的叔叔阿姨，有市场带着倦容的卖各种早点的阿姨，还有偶尔穿梭于人群中衣衫褴褛的乞丐。于是我问自己，十几年后我会成为怎样的自己，想成为社会成功人士还是碌碌无为的人呢，答案肯定是前者。那么十几年后我怎样才能如愿以偿呢，成为一个受人尊重，有价值的人呢？正如我今天演讲的题目是：自主管理。大家都知道爱玩是我们孩子的天性，学习也是我们的责任和义务。要怎样处理好这些矛盾，提高自主管理呢？首先，我们要有小主人翁思想，自己做自己的主人，要认识到我们学习，生活这一切都是我们自己走自己的人生路，并不是为了报答父母，更不是为了敷衍老师。我认为自主管理又可以理解为自我管理，在学习和生活中无处不在，比如通过老师，小组长来管理约束行为和同学们对自身行为的管理都属于自我管理。比如我们到一个旅游景点，看到一块大石头，有的同学特别兴奋，会想在上面刻上：某某某到此一游话。这时你就需要自我管理，你需要提醒自己，这样做会破坏景点，而且是一种素质低下的表现。你设想一下，如果别人家小孩去你家墙上乱涂乱画，你是何种感受。同样我们把自主管理放到学习上，在我们想偷懒，想逃避，想放弃的时候，我们可以通过自主管理来避免这些，通过他人或者自己的力量来完成。例如我会制定作息时间计划表，里面包括学习，运动，玩耍等内容的完成时间。那些学校学习尖子，他们学习好是智商高于我们吗，其实不然，在我所了解的哪些优秀的学霸传授经验里，就提到要能够自我管理，规范好学习时间的分分秒秒，只有辛勤的付出，才能取得优异成绩。在现实生活中，无数成功人士告诉我们自主管理的重要性。十几年后我想成为一位优秀的，为国家多做贡献的人。亲爱的同学们，你们们？让我们从现在开始重视和执行自主管理，十几年后成为那个你想成为的人。谢谢大家！

关于工作的英语演讲稿

关于工作的英语演讲稿【篇一：关于工作的英语演讲稿】关于工作的英语演讲稿 different people have various ambitions. some want to be engineers or doctors in the future. some want to be scientists or businessmen. still some wish to be teachers or lawers when they grow up in the days to come. unlike other people, i prefer to be a farmer. however, it is not easy to be a farmer for iwill be looked upon by others. anyway,what i am trying to do is to make great contributions to agriculture. it is well known that farming is the basic of the country. above all, farming is not only a challenge but also a good opportunity for the young. we can also make a big profit by growing vegetables and food in a scientific way. besides we can apply what we have learned in school to farming. thus our countryside will become more and more properous. i believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. all the working position can provide him with a good chance to become a talent. 【篇二：关于责任感的英语演讲稿】 im grateful that ive been given this opportunity to stand here as a spokesman. facing all of you on the stage, i have the exciting feeling of participating in this speech competition. the topic today is what we cannot afford to lose. if you ask me this question, i must tell you that i think the answer is a word---- responsibility. in my elementary years, there was a little girl in the class who worked very hard, however she could never do satisfactorily in her lessons. the teacher asked me to help her, and it was obvious that she expected a lot from me. but as a young boy, i was so restless and thoughtless, i always tried to get more time to play and enjoy myself. so she was always slighted over by me. one day before the final exam, she came up to me and said, could you please explain this to me? i can not understand it. i

关于时间管理的英语演讲

Dear teacher and colleagues: my topic is on “spare time”. It is a huge blessing that we can work 996. Jack Ma said at an Ali's internal communication activity, That means we should work at 9am to 9pm, 6 days a week .I question the entire premise of this piece. but I'm always interested in hearing what successful and especially rich people come up with time .So I finally found out Jack Ma also had said :”i f you don’t put out more time and energy than others ,how can you achieve the success you want? If you do not do 996 when you are young ,when will you ?”I quite agree with the idea that young people should fight for success .But there are a lot of survival activities to do in a day ,I want to focus on how much time they take from us and what can we do with the rest of the time. As all we known ,There are 168 hours in a week .We sleep roughly seven-and-a-half and eight hours a day .so around 56 hours a week . maybe it is slightly different for someone . We do our personal things like eating and bathing and maybe looking after kids -about three hours a day .so around 21 hours a week .And if you are working a full time job ,so 40 hours a week , Oh! Maybe it is impossible for us at

关于人英语演讲稿(精选多篇)

关于人英语演讲稿(精选多篇) 关于人的优美句子 1、“黑皮小子”是我对在公交车上偶遇两次的一个男孩的称呼代号。一听这个外号，你也定会知道他极黑了。他的脸总是黑黑的；裸露在短袖外的胳膊也是黑黑的；就连两只有厚厚耳垂的耳朵也那么黑黑的，时不时像黑色的猎犬竖起来倾听着什么；黑黑的扁鼻子时不时地深呼吸着，像是在警觉地嗅着什么异样的味道。 2、我不知道，如何诠释我的母亲，因为母亲淡淡的生活中却常常跳动着不一样的间最无私、最伟大、最崇高的爱，莫过于母爱。无私，因为她的爱只有付出，无需回报；伟大，因为她的爱寓于

普通、平凡和简单之中；崇高，是因为她的爱是用生命化作乳汁，哺育着我，使我的生命得以延续，得以蓬勃，得以灿烂。 3、我的左撇子伙伴是用左手写字的，就像我们的右手一样挥洒自如。在日常生活中，曾见过用左手拿筷子的，也有像超级林丹用左手打羽毛球的，但很少碰见用左手写字的。中国汉字笔画笔顺是左起右收，适合用右手写字。但我的左撇子伙伴写字是右起左收的，像鸡爪一样迈出田字格，左看右看，上看下看，每一个字都很难看。平时考试时间终了，他总是做不完试卷。于是老师就跟家长商量，决定让他左改右写。经过老师引导，家长配合，他自己刻苦练字，考试能够提前完成了。现在他的字像他本人一样阳光、帅气。 4、老师，他们是辛勤的园丁，帮助着那些幼苗茁壮成长。他们不怕辛苦地给我们改厚厚一叠的作业，给我们上课。一步一步一点一点地给我们知识。虽然

他们有时也会批评一些人，但是他们的批评是对我们有帮助的，我们也要理解他们。那些学习差的同学，老师会逐一地耐心教导，使他们的学习突飞猛进。使他们的耐心教导培养出了一批批优秀的人才。他们不怕辛苦、不怕劳累地教育着我们这些幼苗，难道不是美吗？ 5、我有一个表妹，还不到十岁，她那圆圆的小脸蛋儿，粉白中透着粉红，她的头发很浓密，而且好像马鬓毛一样的粗硬，但还是保留着孩子一样的蓬乱的美，卷曲的环绕着她那小小的耳朵。说起她，她可是一个古灵精怪的小女孩。 6、黑皮小子是一个善良的人，他要跟所有见过的人成为最好的朋友！这样人人都是他的好朋友，那么人人都是好友一样坦诚、关爱相交，这样人与人自然会和谐起来，少了许多争执了。 7、有人说，老师是土壤，把知识化作养分，传授给祖国的花朵，让他们茁壮成长。亦有人说，老师是一座知识的桥梁，把我们带进奇妙的科学世界，让

二十四节气解读

二十四节气解读立春：立春，二十四节气[1]之一，“立”是“开始”的意思，立春就是春季的开始，每年2月4日或5日太阳到达黄经315度时为立春。《月令七十二候集解》：“正月节，立，建始也……立夏秋冬同。”古代“四立”，指春、夏、秋、冬四季开始，其农业意义为“春种、夏长、秋收、冬藏”，概括了黄河中下游农业生产与气候关系的全过程。雨水：雨水，表示降水开始，雨量逐步增多。每年2月18日前后，太阳到达黄经330度，为“雨水”节气。雨水，表示两层意思，一是天气回暖，降水量逐渐增多了，二是在降水形式上，雪渐少了，雨渐多了。《月令七十二候集解》中说：“正月中，天一生水。春始属木，然生木者必水也，故立春后继之雨水。且东风既解冻，则散而为雨矣。惊蛰：惊蛰——春雷乍动，惊醒了蛰伏在土壤中冬眠的动物。这时气温回升较快，渐有春雷萌动。每年公历的3月6日左右为惊蛰。二十四节气之一。蛰是藏的意思。“惊蛰”是指钻到泥土里越冬的小动物被雷震苏醒出来活动。“惊蛰”节气日，地球已经达到太阳黄经345度，一般在每年3月4日～7日。春分：春分，古时又称为“日中”、“日夜分”、“仲春之月”，在每年的3月21日前后(20日～22日)交节，农历日期不固定，这时太阳到达黄经0°。据《月令七十二候集解》：“二月中，分者半也，此当九十日之半，故谓之分。”另《春秋繁露·阴阳出入上下篇》说：“春分者，阴阳相半也，故昼夜均而寒暑平。”有《明史·历一》说：“分者，黄赤相交之点，太阳行至此，乃昼夜平分。”所以，春分的意义，一是指一天时间白天黑夜平分，各为12小时；二是古时以立春至立夏为春季，春分正当春季三个月之中，平分了春季。清明：清明的意思是清淡明智。“清明”是夏历二十四节气之一，中国广大地区有在清明之日进行祭祖、扫墓、踏青的习俗，逐渐演变为华人以扫墓、祭拜等形式纪念祖先的一个中国传统节日。另外还有很多以“清明”为题的诗歌，其中最著名的是唐代诗人杜牧的七绝《清明》。谷雨：谷雨是二十四节气之一。谷雨指雨水增多，大大有利于谷类农作物的生长。每年4月19日～21日视太阳到达黄经30°时为谷雨。

文档之家