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From the * Department of Animal Science and the
Department of Population Health &
Reproduction, School of Veterinary Medicine, University of California, Davis,
California.
| Correspondence to: Dr Janet F. Roser, Department of Animal Science, University of California, One Shields Ave, Davis, CA 95616 (e-mail: jfroser{at}ucdavis.edu). |
| Received for publication November 23, 2005; accepted for publication March 3, 2006. |
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Abstract |
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hydroxylase/17-20 lyase respectively.
Testes of aromatase-inhibited boars initially exhibited delayed lumen
formation, lower testicular weight, fewer detergentresistant spermatids, and
fewer Sertoli cells, but by 7 to 8 months, these boars had recovered and had
larger testes, more detergentresistant spermatids per testis, and more Sertoli
cells. Total Leydig cell volume increased in proportion to testis size.
Reducing endogenous estrogen is consistent with a delay in testicular
maturation/puberty that allows for a longer window for the proliferation of
Sertoli cells and maturation of Leydig cells, resulting in larger testes and
higher spermatid production.
Key words: Aromatase inhibition, Sertoli cell, Leydig cell
Although estrogen is now generally accepted as an essential component for normal testicular development and function in several species, how it does so is not clear. Estrogen secreted by the testes can influence gonadotropic feedback. Alternatively, synthesis by Leydig cells can have local, more direct testicular effects. Moreover, species differ widely in their capacity for estrogen synthesis. Testicular estrogen synthesis in boars is much higher than in males of most other species (Velle, 1966; Claus and Hoffman, 1980). The presence of estrogen receptors on germ cells, Leydig cells, and Sertoli cells at different stages of development (Rago et al, 2004; Mutembei et al, 2005) in the boar is consistent with a paracrine/autocrine role of estrogen in testicular development in this species. Estrogen synthesis is catalyzed by the enzyme aromatase, which is expressed in Leydig cells of this as of most species (Conley and Hinshelwood, 2001; Weng et al, 2005). Importantly, previous studies in our laboratory indicate that inhibiting estrogen synthesis by using an aromatase inhibitor does not alter gonadotropin secretion in boars (At-Taras et al, 2006), providing an unusual opportunity to investigate local effects of estrogen on testicular function. Therefore, the objective of this study was to investigate the effects of inhibiting endogenous estrogen synthesis on testicular development, including testis weight, histology, sperm production (detergent-resistant spermatid number), Sertoli cell number, and Leydig cell volume in the boar.
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Materials and Methods |
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Determination of Detergent-Resistant Spermatids
One gram of testicular parenchyma from each boar was macerated in 10 mL of
sterile, aqueous 0.9% NaCl/0.05% Triton X-100 at room temperature for 3
minutes using a Wheaton Potter-Elvehjem tissue grinder with PTFE pestle
(Fisher Scientific, Pittsburgh, Pa) as previously described
(Amann and Almquist, 1961;
Amann and Lambiase, 1969). The
homogenate was poured into a 50-mL sterile tube and the glass tube was rinsed
with 10 mL of 0.9% NaCl/0.05% Triton X-100 (total volume of 20 mL).
Homogenates were stored at 4°C and the number of detergent-resistant
spermatids per gram of testis tissue, a measure of sperm production, was
determined the next day using a hemacytometer (3 counts within 10% for each
sample). Preliminary experiments had evaluated 1, 2, 3, and 4 minutes of
homogenization; 3 minutes gave equivalent values to 4 minutes but higher
values than 1 or 2 minutes, presumably because shorter intervals did not
completely free spermatids from tissue.
Histology
Tissue Fixation and Preparation—
Tissue was placed in 4% paraformaldehyde for 24 hours at 4°C and then
transferred to 0.1 M phosphate-buffered saline (PBS) at 4°C for 24 hours.
Tissue was then dehydrated in a step-wise fashion through ethanol (30%, 50%,
and 70%, respectively, each for 24 hours at 4°C). After 24 hours or more
in 70% ethanol, tissue was processed (dehydration in ethanol and infiltration
through xylene and paraffin) in a VIP machine (Tissue Tek VIP; Miles
Scientific, Naperville, Ill). Tissue was embedded in paraffin using a tissue
embedding consol system (Sakura Finetek USA, Inc, Torrance, Calif) and
sectioned at 5 µm for hematoxylin and eosin staining, at 5 µm for
17- hydroxylase/17–20 lyase cytochrome P450 (P450c17)
immunostaining, and at 25 µm for identifying and enumerating Sertoli cells
by immunocytochemical staining of GATA-4 within Sertoli cell nuclei.
Immunocytochemistry
GATA-4—
Immunolocalization of the transcription factor GATA-4 was performed as
previously described (McCoard et al,
2001) using the Vectastain Elite ABC kit (Vector Laboratories,
Burlingame, Calif) with slight modifications. Thick sections (25 µm) were
deparaffinized, rehydrated, and incubated in a sodium citrate buffer (antigen
unmasking solution, H-3300; Vector Laboratories) for 15 minutes, followed by
blocking of endogenous peroxidase activity in 3% hydrogen peroxide in methanol
for 10 minutes. Sections were washed 3 times (5 minutes each time) in 50 mM
Tris buffer with 1.5% NaCl pH 7.6 (TB) prior to blocking with 1% normal rabbit
serum (Vector Laboratories) for 20 minutes at room temperature (RT). Sections
were incubated in primary antibody (polyclonal goat anti-mouse, GATA-4
sc-1237, 1:200 dilution in TB; Santa Cruz Biotechnologies, Santa Cruz, Calif)
at RT for 2 hours, washed in TB 3 times, and incubated with biotinylated
rabbit anti-goat secondary antibody. Sections were washed again 3 times in TB
prior to incubation with an avidin-biotin-peroxidase complex (ABC; Vector
Laboratories) at RT for an additional 40 minutes. NovaRed (Vector
Laboratories) was used to visualize immunoreactivity prior to washing in tap
water, dehydrating in CitriSolv (Fisher Health Care, Houston, Tex), and
sealing with Coversafe mounting media (American MasterTech Scientific, Inc,
Lodi, Calif).
P450c17— Immunolocalization of P450c17 to identify Leydig cells was performed as previously described (Conley et al, 1995) with slight modifications. Briefly, sections were deparaffinized and rehydrated before quenching endogenous peroxidase activity with 0.3% hydrogen peroxide in methanol. Sections were washed and then incubated in heated (approximately 200°F for 10 minutes) sodium citrate buffer (antigen unmasking solution), cooled and washed again in PBS (50 mM phosphate, 150 mM NaCl, pH 7.6). Sections were incubated in blocking serum for 20 minutes, then in a rabbit polyclonal antibody raised against P450c17 (1:1000, courtesy of Dr Anita Payne, Stanford University, Stanford, Calif) or normal rabbit serum (1:1000, control sections) overnight, after which they were washed for 5 minutes in PBS and incubated in biotinylated secondary antibody (goat anti-rabbit immunoglobulin G) for 30 minutes. Sections were washed again in PBS (5 minutes) before incubating in an ABC (Vector Laboratories) at RT for 30 minutes and washed again. 3-amino-9-ethylcarbazole (AEC; Vector Laboratories) was used as the peroxidase substrate to visualize antibody labeling.
Sertoli Cell Counting
An Olympus BH2 microscope with a computer-controlled stage (Olympus,
Melville, NY), microcater (Heidenhain MT12), digital camera and stereology
software (CAST; Copenhagen, Denmark) were used to visualize GATA-4 stained
sections and count Sertoli cell nuclei. Adjustments were not made for
shrinkage because this was determined to be consistent among tissue sections.
Fields were chosen randomly by the software, and at least 200 cells were
counted for each animal. Number density of Sertoli cells were calculated using
the optical dissector method (Petersen and
Pakkenberg, 2000). Average number of Sertoli cells per counting
frame was divided by the area of the counting frame and the dissector height
of the optical dissector (Howard and Reed,
1998). Final Sertoli cell number per testis was determined by
multiplying the resulting number by testis weight because volume measurements
of testicular tissue indicated a density of 1 g/cm3.
Determination of Leydig Cell Volume
Determination of Leydig cell volume was based on the percentage of area
stained with P450c17 compared with total testicular area. Sections were
projected onto a computer screen from an Olympus BH2 microscope using a QICAM
monochrome camera (QIMAGING 3; Burnaby, Canada). NIH Scion Image software
(version Beta 4.0.2; Scion Corporation, Frederick, Md) was used to make the
necessary measurements. Six to 8 fields were measured for each animal at a
final magnification of 300x. Percentage of the area stained with P450c17
was then multiplied by testis weight (it having been determined that 1
cm3 = 1 g tissue) to get final testicular Leydig cell volume.
Data Analysis
Testicular weight was analyzed using 2-way ANOVA (Proc GLM; SAS Statistical
Software, SAS Institute Inc, Cary, NC) with treatment and age as the main
effects. Significant treatment by age effects were noted using the LSMEANS
procedure (SAS Statistical Software). Detergent-resistant spermatid number,
Sertoli cell number, and Leydig cell volume were analyzed by 1-way ANOVA with
treatment as the main effect, each age separately, to fulfill homogeneity of
variance criteria. Data are presented as least square means ± pooled
SEM.
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Results |
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Letrozole treatment had additional effects on somatic cells in both tubular and interstitial compartments. The number of Sertoli cells ranged from an average of 2.86 x 109 and 3.18 x 109, respectively, for control and treated boars at 2 months to 2.33 x 1010 and 2.78 x 1010 for control and treated animals, respectively, at 8 months (P < .05). Sertoli cell numbers in aromatase-inhibited boars were increased an average of 137% over control numbers at all ages except 4 months, when Sertoli cells in aromatase-inhibited boars were only 78% of Sertoli cells in control littermates (P < .05; Figure 4). Leydig cell volume as a percentage of total testicular volume, although initially lower in treated boars at 2 months (P < .05), was not different between aromatase-inhibited and control animals at 5 or 8 months of age (Figure 5). Therefore, with the increase in testis size in aromatase-inhibited boars, there was a consequent 48% increase in total Leydig cell volume above control values at 8 months (115.6 ± 21.4 cm3 vs 60.6 ± 21.4 cm3 in treated and control boars, respectively; P < .05).
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Discussion |
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In addition to the development of viable and fertile sperm (Franca and Cardoso, 1998), testicular development and maturation is marked by the development and maturation of the somatic cells (Lunstra et al, 1986, 2003; Franca et al, 2000; McCoard et al, 2001). Leydig cells are the major cellular site of steroidogenic enzyme expression (Conley et al, 1996; Kaminski et al, 1997; Weng et al, 2005), producing both testosterone and estradiol (Raeside and Renaud, 1983; Setchell et al, 1983; Saez et al, 1989). Sertoli cells are the "nurse" cells for developing germ cells, providing factors essential for spermatogenesis (Sylvester and Griswold, 1994). They are the major determinants of testis size (Gondos and Berndtson, 1993; Lunstra et al, 2003), and both testis size and Sertoli cell number are correlated with total sperm production (Chubb, 1992; Okwun et al, 1996; Ford et al, 1997). Sertoli cell proliferation in the pig apparently takes place primarily during the late prenatal and early postnatal period (Putra and Blackshaw, 1985; Kosco et al, 1987; McCoard et al, 2001; Lunstra et al, 2003). Hemicastration studies have indicated that compensatory Sertoli cell proliferation in boars is possible only prior to the onset of puberty (Putra and Blackshaw, 1985; Kosco et al, 1989a). Data from the present study show that the increase in testis size was associated with increased Sertoli cell numbers in reduced estrogen boars above controls, an increase that was apparent as early as 2 months of age. The increment in Sertoli cell numbers with letrozole was sustained through all ages with the exception of the 4-month time point. Rather than representing a decrease in Sertoli cell numbers in the treated boars, the reversal of this trend at 4 months likely reflects a peri-pubertal increase in Sertoli cell numbers in control animals that preceded a delayed increase in Sertoli cell proliferation in treated animals by approximately 1 month. Because initiation of tubular lumen development also occurred later in aromatase-inhibited boars (5 months) compared with control littermates (4 months), there may similarly be a delay in testicular (Sertoli cell) maturation resulting in delayed initiation of puberty. Evidence from Piau pigs shows a biphasic pattern of Sertoli cell proliferation with the second phase occurring just prior to puberty (between 3 and 4 months in these pigs; Franca et al, 2000), consistent with the peri-pubertal increase in Sertoli cell numbers observed in our control animals. Both the timing of the increase in Sertoli cell numbers and the age at tubular lumen formation in letrozole-treated boars suggest that testis development and puberty were delayed by the local inhibition of estrogen synthesis.
Whether estrogen affects Sertoli cell proliferation directly or indirectly is unclear at this time and subject to speculation. However, it is noteworthy that some mitogenic factors, including fibroblast growth factor, epidermal growth factor, and somatomedin, induce porcine Sertoli cell proliferation in vitro (Jaillard et al, 1987), and their production may be regulated by estrogen (Artagaveytia et al, 1997; Smith et al, 2002). In the rat, thyroid hormone appears to be a regulator of Sertoli cell proliferation, growth, and differentiation, as evidenced with neonatal hypothyroidism, which causes delayed puberty and an increase in Sertoli cell number (De Franca et al, 1995). Higher neonatal triiodothyronine concentrations were related to fewer Sertoli cells and earlier onset of puberty in Meishan boars (McCoard et al, 2003), and estrogens are believed to cause an increase in thyroid stimulating hormone production and secretion by pituitary cells in culture in at least 1 species (ovine; Miller et al, 1977). Additionally, an allele of the thyroid-binding globulin gene on the porcine X chromosome that alters the bioavailability of thyroxine (Schussler, 2000) is also associated with variation in testis size (Nonneman et al, 2005). However, 2 independent studies have shown that induction of hypothyroidism in pigs has no effect on testis development (Tarn et al, 1998; Klobucar et al, 2003). Thus far no evidence exists for the direct effect of estrogen on Sertoli cell proliferation in the boar, although this has been documented in the dogfish shark (Betka and Callard, 1998), in which estrogen inhibits DNA synthesis in germ cell–Sertoli cell units within the testes. Further investigation of the mechanisms of action of estrogen on Sertoli cell proliferation is warranted.
Interestingly, the relative percentage Leydig cell volume in testes of aromatase-inhibited boars was conserved, matching the relative volume in testes of control littermate boars. Not only were larger testes in previous studies associated with more Sertoli cells and longer tubules (Kosco et al, 1989a; Lunstra et al, 2003), but there was a noticeable increase in the volume and number of Leydig cells (Lunstra et al, 2003) as well. The present study is consistent with a proportional increase in Leydig cell volume with testis size. Compensatory growth of the remaining testis in hemicastrated boars induced a 250% increase in Leydig cell mass relative to body mass as compared with intact boars (Kosco et al, 1989b). An increase in absolute Leydig cell volume in aromatase-inhibited, reduced-estrogen boars is indicative of increased steroidogenic capacity, because studies have shown a direct correlation between Leydig cell volume and steroidogenic capacity in the boar (Lunstra et al, 1986). In the dog, decreasing estrogen synthesis resulted in hypertrophy of Leydig cells (Walters et al, 1988; Junker Walker and Nogues, 1994), as did immunizing against estradiol in rams (Schanbacher et al, 1987); this effect was likely because of increased luteinizing hormone (LH) tropic drive (Schanbacher et al, 1987; Juniewicz et al, 1988). Unlike the dog and the ram, the boar did not have elevated LH following aromatase enzyme inhibition, and hence it is likely that the increase in total Leydig cell volume observed in 8-month-old boars was caused by reduced estrogen synthesis, whereas the initial reduction at 2 months may be attributed to the delay in testicular maturation in aromatase-inhibited boars.
This study was aimed at determining the role of endogenous estrogens on testicular growth and development in the domestic boar. Although the effect of estrogen on testicular development and function has been studied in several species, we are not aware of any previous studies in the boar that have investigated the role of endogenous estrogen on the development of the somatic cells within the testis that ultimately contribute to testicular size and sperm production. Interestingly, reducing endogenous estrogens caused a delay in the apparent onset of puberty but an ultimate increase in testis size, Sertoli cell number, and total spermatid production per animal without affecting relative Leydig cell volume. These findings suggest that estrogen is an important Sertoli cell maturation factor regulating testicular development in the domestic boar.
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Acknowledgments |
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Footnotes |
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References |
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|
TopAbstractMaterials and MethodsResultsDiscussionReferences |
|---|
Amann RP, Lambiase JT Jr. The male rabbit. III. Determination of daily sperm production by means of testicular homogenates. J Anim Sci. 1969;28: 369 –374.[Medline]
Artagaveytia N, Le Penven S, Falette N, Lucero R, Garofalo EG, Saez S. Epidermal growth factor and transforming growth factor alpha mRNA expression in human breast cancer biopsies; analysis in relation to estradiol, progesterone and EGF receptor content. J Steroid Biochem Mol Biol. 1997;60: 221 –228.[CrossRef][Medline]
At-Taras EE, Conley AJ, Berger T, Roser JF. Reducing estrogen
synthesis does not affect gonadotropin secretion in the developing boar.
Biol Reprod. 2006; 74: 58
–66.
Betka M, Callard GV. Negative feedback control of the spermatogenic progression by testicular oestrogen synthesis: insights from the shark testis model. APMIS. 1998; 106: 252 –257; discussion 257–258.[Medline]
Chubb C. Genes regulating testis size. Biol Reprod. 1992;47: 29 –36.[Abstract]
Claus R, Hoffman B. Oestrogens, compared to other steroids of testicular origin, in bloodplasma of boars. Acta Endocrinol. 1980;94: 404 –411.
Conley A, Hinshelwood M. Mammalian aromatases. Reproduction. 2001; 121: 685 –695.[Abstract]
Conley AJ, Corbin CJ, Hinshelwood MM, Liu Z, Simpson ER, Ford JJ, Harada N. Functional aromatase expression in porcine adrenal gland and testis. Biol Reprod. 1996; 54: 497 –505.[Abstract]
Conley AJ, Kaminski MA, Dubowsky SA, Jablonka-Shariff A, Redmer DA, Reynolds LP. Immunohistochemical localization of 3 beta-hydroxysteroid dehydrogenase and P450 17 alpha-hydroxylase during follicular and luteal development in pigs, sheep, and cows. Biol Reprod. 1995; 52: 1081 –1094.[Abstract]
De Franca LR, Hess RA, Cooke PS, Russell LD. Neonatal hypothyroidism causes delayed Sertoli cell maturation in rats treated with propylthiouracil: evidence that the Sertoli cell controls testis growth. Anat Rec. 1995; 242: 57 –69.[CrossRef][Medline]
Ebling FJ, Brooks AN, Cronin AS, Ford H, Kerr JB. Estrogenic
induction of spermatogenesis in the hypogonadal mouse. Endocrinology
. 2000;141: 2861
–2869.
Fisher CR, Graves KH, Parlow A, Simpson ER. Characterization of
mice deficient in aromatase (ArKO) because of targeted disruption of the
cyp 19 gene. Proc Natl Acad Sci U S A. 1998; 95: 6965
–6970.
Ford JJ, Wise TH, Lunstra DD. Negative relationship between blood
concentrations of follicle-stimulating hormone and testicular size in mature
boars. J Anim Sci. 1997; 75: 790
–795.
Franca LR, Cardoso FM. Duration of spermatogenesis and sperm transit time through the epididymis in the Piau boar. Tissue Cell. 1998;30: 573 –582.[CrossRef][Medline]
Franca LR, Silva VA Jr, Chiarini-Garcia H, Garcia SK, Debeljuk L.
Cell proliferation and hormonal changes during postnatal development of the
testis in the pig. Biol Reprod. 2000; 63: 1629
–1636.
Gancarczyk M, Paziewska-Hejmej A, Carreau S, Tabarowski Z, Bilinska B. Dose- and photoperiod-dependent effects of 17beta-estradiol and the anti-estrogen ICI 182,780 on testicular structure, acceleration of spermatogenesis, and aromatase immunoexpression in immature bank voles. Acta Histochem. 2004; 106: 269 –278.[Medline]
Gondos B, Berndtson WE. Postnatal and pubertal development. In: Russell LD, Griswold MD eds. The Sertoli Cell. Clearwater, Fla: Cache River Press; 1993; 116 –153.
Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching: Consortium for Developing a Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching. 1st ed. Champaign, Ill: American Dairy Association; 1988.
Howard CV, Reed MG, eds. Number estimation. In: Unbiased Stereology: Three-Dimensional Measurement in Microscopy. New York, NY: BIOS Scientific Publishers, 1998: 69 –105.
Jaillard C, Chatelain PG, Saez JM. In vitro regulation of pig Sertoli cell growth and function: effects of fibroblast growth factor and somatomedin-C. Biol Reprod. 1987; 37: 665 –674.[Abstract]
Juniewicz PE, Oesterling JE, Walters JR, Steele RE, Niswender GD, Coffey DS, Ewing LL. Aromatase inhibition in the dog. I. Effect on serum LH, serum testosterone concentrations, testicular secretions and spermatogenesis. J Urol. 1988;139: 827 –831.[Medline]
Junker Walker U, Nogues V. Changes induced by treatment with aromatase inhibitors in testicular Leydig cells of rats and dogs. Exp Toxicol Pathol. 1994; 46: 211 –213.[Medline]
Kaminski MA, Ford SP, Conley AJ. Differences in the expression of cytochromes P450 17 alpha-hydroxylase and aromatase in Meishan and Yorkshire conceptuses at days 10.5–14.0 of gestation. J Reprod Fertil. 1997;111: 213 –219.[Abstract]
Klobucar I, Kosec M, Cebulj-Kadunc N, Majdic G. Postnatal hypothyroidism does not affect prepubertal testis development in boars. Reprod Domest Anim. 2003; 38: 193 –198.[CrossRef][Medline]
Kosco MS, Bolt DJ, Wheaton JE, Loseth KJ, Crabo BG. Endocrine responses in relation to compensatory testicular growth after neonatal hemicastration in boars. Biol Reprod. 1987; 36: 1177 –1185.[Abstract]
Kosco MS, Loseth KJ, Crabo BG. Development of the seminiferous tubules after neonatal hemicastration in the boar. J Reprod Fertil. 1989a;87: 1 –11.[Abstract]
Kosco MS, Loseth KJ, Crabo BG. Development of the testicular interstitium after neonatal hemicastration in the boar. J Reprod Fertil. 1989b;87: 13 –21.[Abstract]
Kula K, Walczak-Jedrzejowska R, Slowikowska-Hilczer J, Oszukowska E. Estradiol enhances the stimulatory effect of FSH on testicular maturation and contributes to precocious initiation of spermatogenesis. Mol Cell Endocrinol. 2001;178: 89 –97.[CrossRef][Medline]
Lunstra DD, Ford JJ, Christenson RK, Allrich RD. Changes in Leydig cell ultrastructure and function during pubertal development in the boar. Biol Reprod. 1986; 34: 145 –158.[Abstract]
Lunstra DD, Wise TH, Ford JJ. Sertoli cells in the boar testis:
changes during development and compensatory hypertrophy after hemicastration
at different ages. Biol Reprod. 2003; 68: 140
–150.
McCoard SA, Lunstra DD, Wise TH, Ford JJ. Specific staining of
Sertoli cell nuclei and evaluation of Sertoli cell number and proliferative
activity in Meishan and White Composite boars during the neonatal period.
Biol Reprod. 2001; 64: 689
–695.
McCoard SA, Wise TH, Ford JJ. Endocrine and molecular influences on testicular development in Meishan and White Composite boars. J Endocrinol. 2003;178: 405 –416.[Abstract]
Miller WL, Knight MM, Gorski J. Estrogen action in vitro: regulation of thyroid stimulating and other pituitary hormones in cell cultures. Endocrinology. 1977; 101: 1455 –1460.[Abstract]
Moran FM, Ford JJ, Corbin CJ, Mapes SM, Njar VC, Brodie AM, Conley
AJ. Regulation of microsomal P450, redox partner proteins, and steroidogenesis
in the developing testes of the neonatal pig.
Endocrinology. 2002; 143: 3361
–3369.
Mutembei HM, Pesch S, Schuler G, Hoffmann B. Expression of oestrogen receptors alpha and beta and of aromatase in the testis of immature and mature boars. Reprod Domest Anim. 2005; 40: 228 –236.[CrossRef][Medline]
Nonneman D, Rohrer GA, Wise TH, Lunstra DD, Ford JJ. A variant of
porcine thyroxine-binding globulin has reduced affinity for thyroxine and is
associated with testis size. Biol Reprod. 2005; 72: 214
–220.
Okwun OE, Igboeli G, Ford JJ, Lunstra DD, Johnson L. Number and function of Sertoli cells, number and yield of spermatogonia, and daily sperm production in three breeds of boar. J Reprod Fertil. 1996; 107: 137 –149.[Abstract]
Oliveira CA, Carnes K, Franca LR, Hess RA. Infertility and
testicular atrophy in the antiestrogen-treated adult male rat. Biol
Reprod. 2001;65: 913
–920.
Pentikainen V, Erkkila K, Suomalainen L, Parvinen M, Dunkel L.
Estradiol acts as a germ cell survival factor in the human testis in vitro.
J Clin Endocrinol Metab. 2000; 85: 2057
–2067.
Petersen PM, Pakkenberg B. Stereological quantitation of Leydig and Sertoli cells in the testis from young and old men. Image Anal Stereol. 2000;19: 215 –218.
Putra DK, Blackshaw AW. Quantitative studies of compensatory testicular hypertrophy following unilateral castration in the boar. Aust J Biol Sci. 1985; 38: 429 –434.[Medline]
Raeside JI, Renaud RL. Estrogen and androgen production by purified Leydig cells of mature boars. Biol Reprod. 1983; 28: 727 –733.[Abstract]
Rago V, Maggiolini M, Vivacqua A, Palma A, Carpino A. Differential expression of estrogen receptors (ERalpha/ERbeta) in testis of mature and immature pigs. Anat Rec A Discov Mol Cell Evol Biol. 2004; 281: 1234 –1239.[CrossRef][Medline]
Robertson KM, O'Donnell L, Jones ME, Meachem SJ, Boon WC, Fisher
CR, Graves KH, McLachlan RI, Simpson ER. Impairment of spermatogenesis in mice
lacking a functional aromatase (cyp 19) gene. Proc Natl Acad Sci U
S A. 1999;96: 7986
–7991.
Saez JM, Sanchez P, Berthelon MC, Avallet O. Regulation of pig Leydig cell aromatase activity by gonadotropins and Sertoli cells. Biol Reprod. 1989; 41: 813 –820.[Abstract]
Schanbacher BD, Pelletier J, Hochereau-de Reviers MT. Follicle
stimulating hormone, luteinizing hormone and testicular Leydig cell responses
to estradiol immunization in Ile-de-France rams. J
Androl. 1987;8: 97
–102.
Schussler G. The thyroxine-binding proteins. Thyroid. 2000;10: 131 –149.[Medline]
Setchell BP, Laurie MS, Flint AP, Heap RB. Transport of free and conjugated steroids from the boar testis in lymph, venous blood and rete testis fluid. J Endocrinol. 1983; 96: 127 –136.[Abstract]
Shetty G, Krishnamurthy H, Krishnamurthy HN, Bhatnagar AS, Moudgal NR. Effect of long-term treatment with aromatase inhibitor on testicular function of adult male bonnet monkeys (M. radiata). Steroids. 1998;63: 414 –420.[CrossRef][Medline]
Smith P, Rhodes NP, Ke Y, Foster CS. Upregulation of estrogen and androgen receptors modulate expression of FGF-2 and FGF-7 in human, cultured, prostatic stromal cells exposed to high concentrations of estradiol. Prostate Cancer Prostatic Dis. 2002; 5: 105 –110.[CrossRef][Medline]
Sylvester SR, Griswold MD. The testicular iron shuttle: a
"nurse" function of the Sertoli cells. J
Androl. 1994;15: 381
–385.
Tarn CY, Rosenkrans CFJr, Apple JK, Kirby JD. Effects of 6-N-propyl-2-thiouracil on growth, hormonal profiles, carcass and reproductive traits of boars. Anim Reprod Sci. 1998; 50: 81 –94.[CrossRef][Medline]
Thompson DL Jr, Honey PG. Active immunization of prepubertal colts
against estrogens: hormonal and testicular responses after puberty.
J Anim Sci. 1984; 59: 189
–196.
Velle W. Urinary oestrogens in the male. J Reprod Fertil. 1966;12: 65 –73.[Medline]
Walters JR, Juniewicz PE, Oesterling JE, Mendis-Handagama SM, Zirkin BR, Ewing LL. The effect of inhibition of aromatase enzyme activity on Leydig cell number and ultrastructure in beagles. Endocrinology. 1988; 123: 2223 –2229.[Abstract]
Weng Q, Medan MS, Watanabe G, Tsubota T, Tanioka Y, Taya K. Immunolocalization of steroidogenic enzymes P450scc, 3betaHSD, P450c17, and P450arom in Gottingen miniature pig testes. J Reprod Dev. 2005;51: 299 –304.[CrossRef][Medline]
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.05. **P 
