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Biology of Reproduction 60, 635-641 (1999)
©Copyright 1999 Society for the Study of Reproduction, Inc.

Effects of Spinal Cord Injury on Spermatogenesis and the Expression of Messenger Ribonucleic Acid for Sertoli Cell Proteins in Rat Sertoli Cell-Enriched Testes1

Hosea F.S. Huang2,a,b, Ming-tang Lib, Robert Anesettia, William Giglioa, John E. Ottenwellera,c, and Leonard M. Pogacha

a Veterans Affairs Medical Center, East Orange, New Jersey 07019 b Department of Surgery Section of Urology and c Department of Neurosciences, UMD-New Jersey Medical School, Newark, New Jersey 07103


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The study was an examination of the effects of spinal cord injury (SCI) on spermatogenesis and Sertoli cell functions in adult rats with Sertoli cell-enriched (SCE) testes. The effects of SCI on the seminiferous epithelium were characterized by abnormalities in the remaining spermatogenic cells during the first month after SCI. Three days after SCI, serum testosterone levels were 80% lower, while serum FSH and LH levels were 25% and 50% higher, respectively, than those of sham control SCE rats. At this time, the levels of mRNA for androgen receptor (AR), FSH receptor (FSH-R), and androgen-binding protein (ABP) were normal whereas those for transferrin (Trf) had decreased by 40%. Thereafter, serum testosterone levels increased, but they remained lower than those of the sham control rats 28 days after SCI; and serum FSH and LH levels returned to normal. The levels of mRNA for AR, ABP, and Trf exhibited a biphasic increase 7 days after SCI and remained elevated 28 days after SCI. FSH-R mRNA levels were also elevated 90 days after SCI. Unexpectedly, active spermatogenesis, including qualitatively complete spermatogenesis, persisted in > 40% of the tubules 90 days after SCI. These results suggest that the stem cells and/or undifferentiated spermatogonia in SCE testes are less susceptible to the deleterious effects of SCI than the normal testes and that they were able to proliferate and differentiate after SCI. The presence of elevated levels of mRNA for Sertoli cell FSH-R and AR, as well as of that for the Sertoli cell proteins, in the SCE testes during the chronic stage of SCI suggests a modification of Sertoli cell physiology. Such changes in Sertoli cell functions may provide a beneficial environment for the proliferation of the stem cells and differentiation of postmeiotic cells, thus resulting in the persistence of spermatogenesis in these testes.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In the rat, impairment in spermatogenesis, including a decrease in the number of types A1 and B spermatogonia (the first and the last generation of proliferating spermatogonia, respectively) and preleptotene spermatocytes, occurs shortly after surgically induced spinal cord injury (SCI) [1, 2]. These changes occur while serum FSH and LH and testicular testosterone concentrations are severely suppressed [1], suggesting that Sertoli cell dysfunction due to hormone deprivation may contribute to the early effects of SCI on spermatogenesis. The seminiferous epithelium of the SCI rats continues to regress even after normal function of the pituitary-testis hormone axis has recovered, and it is characterized by the disappearance of proliferating spermatogonia and preleptotene spermatocytes [2, 3]. However, the pre-A1 undifferentiated spermatogonia persist during the chronic stage of the injury [3]. These results suggest that undifferentiated spermatogonia are unable to undergo normal proliferation after SCI, thus resulting in the gradual disappearance of proliferating spermatogonia and eventual regression of the seminiferous epithelium during the chronic stage of SCI.

Spermatogenesis is supported and regulated by Sertoli cells, which provide both physical and biochemical/molecular signals for specific stages of spermatogenic differentiation [4–6]. Our earlier studies revealed a transient suppression of transferrin (Trf) mRNA without a concomitant change in androgen-binding protein (ABP) mRNA during the acute phase of SCI [1]. In addition, the mRNAs for various Sertoli cell proteins in the testes of SCI rats were up-regulated by exogenous testosterone and/or FSH while spermatogenesis was regressing [7, 8]. Moreover, Sertoli cells exhibit unusual morphology during the chronic phase of SCI [3]. Taken together, these results suggest that SCI may affect several aspects of Sertoli cell function.

In order to develop remedies capable of preserving normal spermatogenesis after SCI, understanding the effects of SCI on Sertoli cell functions and the mechanisms responsible for such effects is essential. However, because the functions of Sertoli cells, including protein expression, can be modulated by the status of the seminiferous epithelium [9, 10], changes in Sertoli cell functional markers after SCI while spermatogenic cells are degenerating may not reflect direct effects of SCI on Sertoli cells. The testes of rats born to females subjected to x-irradiation on the 20th day of gestation are Sertoli cell enriched due to the killing of spermatogenic stem cells [11]. The residual stem cells in these testes are able to proliferate and differentiate, resulting in the presence of spermatogenesis in some area of the seminiferous tubules. These animals provide a unique model for the study of Sertoli cell function under various experimental conditions with reduced interference from spermatogenic cells, as well as of proliferation and differentiation of the stem cells and/or undifferentiated spermatogonia after testicular injuries. In the current study we examined the effects of SCI on spermatogenic function and the expression of mRNA for Sertoli cell proteins in rats with Sertoli cell-enriched testes (SCE rats).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals

Dated-pregnant female Sprague-Dawley rats (Taconic Farm, Taconi, NY) were delivered to the Animal Research Facility of UMD-New Jersey Medical School 1 wk before the expected delivery date. Animals were individually caged in an air-conditioned, light-controlled animal room and were fed Purina (Ralston-Purina, St. Louis, MO) rat chow and water ad libitum. On the 20th day of gestation, each female rat was exposed to 100 R whole-body x-irradiation using a Mark 1 Irradiator (J.L. Shepherd, San Francisco, CA). All females delivered within 1–2 days.

Male pups born to the irradiated female rats were weaned at 20 days of age and were fed Purina rat chow and water ad libitum. When these rats reached 65–70 days of age, their testes were palpated to monitor the status of spermatogenesis. Animals with testis length > 0.5 cm were considered to have normal spermatogenesis and were discarded. The remaining animals were considered SCE and were transported to the Animal Research Facility of VA Medical Center, East Orange, NJ. Animals were caged individually in an air-conditioned, light-controlled animal room used solely for housing SCI rats and were maintained on Purina rat chow and water ad libitum.

Experiment

After 2 wk of acclimation, SCI was induced by surgical transection of the spinal cord at the level of the 9th thoracic vertebra as described previously [1–3]. Control animals received sham operation without laminectomy. The postoperative care included creding three times daily and cleaning of the perineum of the SCI animals as has been described previously [2]. The SCI (n = 8–10) and sham control animals (n = 6) were killed by decapitation 3, 7, 14, 28, and 90 days after the surgery. Trunk blood was collected, and serum was isolated and stored at -80ÅC for subsequent hormone measurement. One half of one testis from each rat was fixed in Bouin's solution and processed for histology [1–3]. The remaining testicular tissue was frozen in heptane cooled by the mixture of methanol and dry ice and stored at -80ÅC for subsequent RNA isolation and Northern blot analysis. The procedures used in the treatment of the animals, including irradiation of pregnant rats and spinal cord transection, were approved by Institutional Animal Care and Use Committee of both the East Orange VA Medical Center and UMD-New Jersey Medical School.

Sertoli Cell Enrichment of the Testes

Enrichment of Sertoli cells in the testes was determined by quantifying the number of tubules containing only Sertoli cells and of those containing active spermatogenesis. In each animal, a total of 300–400 tubular cross sections in at least 2 different areas were evaluated. The normalcy of the remaining spermatogenesis in SCI-SCE rats was evaluated by the criteria described previously [1–3].

Northern Blot cDNA Hybridization

Details of procedures for the isolation of total RNA by the single-step method [12], isolation of poly(A)+ RNA by oligo(dT) cellulose chromatography [13], and agarose electrophoresis and Northern blotting of RNA [14] have been reported previously [1]. The cDNA probes for FSH receptor (FSH-R) [15], androgen receptor (AR) [16], ABP [17], and Trf [18] were isolated by agarose electrophoresis after appropriate endonuclease digestion. Each of these probes was radiolabeled with [32P]dCTP using a random priming kit (Boehringer-Mannheim, Indianapolis, IN). The blots were prehybridized for 2–4 h in a solution containing 6-strength SSC (150 mM NaCl, 15 mM sodium citrate, pH 7.0), 0.5% SDS, 50% formamide, and 100 mg/ml denatured sperm DNA at 42ÅC. Subsequently, the blots were hybridized overnight in the same buffer containing approximately 106 cpm/ml of radiolabeled cDNA probe. The membranes were washed in single-strength SSC solution containing 0.1% SDS at 68ÅC for 20–30 min, followed by several changes in 0.2-strength SSC and 0.1% SDS at 68ÅC for 2–4 h. The autoradiographs were developed after 2–10 days exposure with an intensifying screen. The membranes were subsequently stripped and rehybridized with 32P-labeled cDNA for 18S rRNA. The relative abundance of mRNA transcripts (FSH-R: 2.6 kilobases [kb]; AR: 9.4 kb; ABP: 2.3 kb; Trf: 2.7 kb; and 18S rRNA) was subsequently estimated by densitometry. The steady-state level of each mRNA transcript was normalized against that of the 18S rRNA in each sample. The average ratio between the mRNA and the 18S rRNA of the control animals in each blot was considered 100%, and the results for individual control and experimental samples in each blot were expressed as a percentage of this average.

Statistics

All data were analyzed to determine that they were normally distributed. Subsequently, the organ weights and serum hormone concentrations were analyzed by time points x treatment groups ANOVA. When the treatment effects were significant (p < 0.05), Dunn's tests were used to determine the significance of differences among treatment groups. For Sertoli cell protein mRNA levels, there were no significant differences among the control animals at the five time points. Thus, the results for these animals were pooled in a single control group. Then the five SCI groups were each compared with the control group simultaneously using the Statistical Analysis Systems General Linear Models procedure (SAS Institute, Cary, NC).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Testis Weight

The testis weights in sham control SCE rats were all below 800 mg and were maintained throughout the experiment. The testis weights of SCI-SCE rats were persistently lower than those of the sham control SCE rats at each time point (p < 0.05; except Day 14) and remained relatively unchanged (or gradually rose) during the course of the experiment (Fig. 1).



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  		FIG. 1. The effect of SCI on testis weight of SCE rats. Results are expressed as mean ± SEM of 8–10 (SCI) or 6 (sham control) rats; *p < 0.05. The insert shows changes in testis weight in normal rats subjected to SCI, expressed as percentage of sham control values, at various times after SCI. Data presented in the inset was previously published in [1] and [3].

Spermatogenesis

Active spermatogenesis was observed in 33 ± 3% and 31 ± 3% of the seminiferous tubules of the sham control SCE and SCI-SCE rats 3 days after the surgery (Fig. 2, A and B). These results demonstrate that the testes of these animals were indeed SCE. By 2 wk after SCI, abnormalities in spermatogenesis, including delayed spermiation (Fig. 2C) and vacuolization of spermatid nuclei (Fig. 2D), were seen in the remaining spermatogenic cells. By the end of the fourth week, abnormalities in spermatogenesis similar to those that we have reported previously [1, 3] were common in SCI-SCE rats.



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  		FIG. 2. Photomicrographs of testicular histology. A) A control SCE testis 3 days after sham operation, showing the presence of active spermatogenesis in some tubules and absence of spermatogenesis in the others. B) An SCE testis 3 days after SCI surgery, showing the presence of active spermatogenesis in some tubules and absence of spermatogenesis in the others. C) Portion of a tubule from an SCE testis 7 days after SCI, showing the presence of mature spermatids (arrowheads) in stage XI epithelium. This phenomenon denotes a failure in spermiation. D) A stage VII or VIII tubule from an SCE testis 14 days after SCI. The absence of mature spermatids and vacuolization of spermatid nuclei demonstrate the effects of SCI on spermatogenesis. A, B: x60; C, D: x150. Reproduced at 86%.

Three months after the surgery, active spermatogenesis was observed in 46 ± 4% of the tubules in sham control SCE rats (Fig. 3A). Unexpectedly, while the seminiferous epithelium was regressing and abnormalities including sloughing of germ cells and failure of normal spermiation were apparent (Fig. 3, B and C), active spermatogenesis including qualitatively complete spermatogenesis persisted in 41 ± 8% of the tubules of the SCI-SCE rats (Fig. 3C). Overall, the extent of seminiferous epithelial regression was far less severe than that in regular SCI rats, in which total regression of the seminiferous epithelium was seen in > 95% of the tubules at the same stage after the injury (Fig. 3D and [3]).



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  		FIG. 3. Photomicrographs of testicular histology. A) Sham control SCE testis 3 mo after the surgery, showing the persistence of spermatogenesis in some tubules and absence of spermatogenesis in the others. B) An SCE testis 3 mo after SCI surgery. While regression of the seminiferous epithelium is apparent in some tubules, active spermatogenesis persisted in the others. C) Enlargement of portion of Figure 2B. The presence of mature spermatids in the luminal edge of stage VIII epithelium and persistence of mature spermatids (arrowheads) in an adjacent stage IX–X tubule denote a failure of normal spermiation. D) Testicular histology of a normal adult rat, killed 3 mo after SCI from another experiment conducted 3 mo earlier, showing complete regression of the seminiferous epithelium in most of the tubules. A, B: x60; C: x150; D: x25. Reproduced at 86%.

Sertoli Cell Protein mRNAs

Because of the normalization to the average for the sham control rats at each time point, there could be no difference among the results for control animals at different times. For this reason, results from all control animals were pooled, and the results from the SCI rats at each time point were compared to the mean of this pool.

Three days after SCI, the levels of mRNA transcripts for AR, FSH-R, and ABP were not different from those of the controls (p > 0.1), whereas Trf mRNA was reduced by 40% (p = 0.06; Fig. 4). Thereafter, the level of AR mRNA became significantly higher than in the sham control rats, at 7, 28 (p < 0.05), and 90 days (p < 0.01) after the SCI surgery. The level of FSH-R mRNA remained normal during the first 4 wk after SCI but also became elevated (p < 0.05) 3 mo after SCI (Fig. 4). Figure 4 shows that ABP and Trf mRNAs in the testes of SCI-SCE rats were also higher than those for sham-operated SCI rats 7 days after SCI and that these values remained elevated after Day 28.



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  		FIG. 4. Northern blot analysis of mRNA for Sertoli cell proteins in SCE testes at various times after SCI. Upper) Representative radiographs of 3, 28, and 90 days after SCI. Each lane contained 10 µg poly(A)+ RNA, and the radiographs were developed 1–2 days (ABP, Trf, and 18s) or 6–8 days (AR and FSH-R) after exposure. Lower) Quantitative analysis of the effects of SCI on mRNA for Sertoli cell proteins at various time points. Each bar represents the mean ± SEM of 6 sham control rats and 8–10 SCI rats. * p < 0.05; ** p < 0.01.

Serum Hormone Concentrations

Three days after SCI, serum FSH and LH concentrations of the SCE rats were significantly higher than those of the sham control SCE rats (p < 0.01, Fig. 5). At this time, serum testosterone levels in the sham control SCE rats were in the low normal range for adult male rats, probably because of surgery-related stress [1]. Serum testosterone concentrations of the SCI-SCE rats were 80% lower than those of the sham control rats despite the fact that the difference was only marginal (p < 0.06). Thereafter, serum FSH and LH concentrations of the SCI-SCE rats decreased and were not different from those of the sham control SCE rats by Day 28. On the other hand, serum testosterone concentrations of the sham control SCE rats increased toward the normal level in adult male rats. While serum testosterone of the SCI-SCE rats also increased, this value remained significantly lower (p < 0.05) than that for the sham control rats 28 days after the surgery. Serum hormone levels of the rats killed at 7 and 90 days after the surgery were not available owing to the loss of samples.



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  		FIG. 5. The effects of SCI on serum FSH, LH, and testosterone concentration in SCE rats. Results are expressed as mean ± SEM ng/ml of 6 sham control rats and 8–10 SCI rats. * p < 0.05; ** p < 0.01.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In normal adult rats, spermatogenesis becomes impaired within 1 wk after surgically induced SCI, and the seminiferous epithelium regresses rapidly during the following 1–2 mo [1–3]. These changes are associated with a decrease and eventual disappearance of proliferating spermatogonia, attributable to the failure of the undifferentiated spermatogonia to undergo normal proliferation and differentiation [3]. Sertoli cells and undifferentiated spermatogonia are all that remain in the seminiferous epithelium 3 mo after SCI [3]. Although the effects of SCI on SCE testes were manifested in the presence of abnormalities in the remaining spermatogenic cells at different times, active spermatogenesis persisted unexpectedly in > 40% of the tubules in SCE testes 3 mo after SCI, consistent with the lack of a significant decrease in testis weight. These results suggest that the seminiferous epithelium in the SCE testes is more resistant to the deleterious effects of SCI. Because degeneration of spermatogenic cells and regression of the seminiferous epithelium were apparent in all SCI-SCE rats, the spermatogenic cells persisting during the chronic stage of SCI are likely to have derived from spermatogonia that survived the early effects of SCI. These results suggest that the stem cells and/or undifferentiated spermatogonia in the SCE testes were able to undergo proliferation and differentiation continuously after SCI. Previously, it has been reported that pretreatment of rats with exogenous testosterone and/or GnRH antagonist prior to the administration of chemotherapeutic agents facilitates the survival of stem cells and subsequent recovery of spermatogenesis [19–21]. It is postulated that modulation of Sertoli cell paracrine functions, as the result of the suppression of testicular testosterone, may facilitate survival and regrowth of the stem cells after testicular injury [22]. Although persistence of spermatogenesis in SCE testes might be related to the decrease in testicular testosterone shortly after SCI, regression of the seminiferous epithelium in non-SCE testes in which testosterone concentrations were also severely suppressed shortly after SCI [1] argues against this possibility. It has been hypothesized that disappearance of more advanced spermatogenic cells after x-irradiation may trigger the proliferation of stem cells and facilitate repopulation of the seminiferous epithelium [23]. Since over 60% of the tubules in SCE testes were devoid of spermatogenic cells at the time of SCI surgery, the paracrine environment in these testes might be beneficial for the proliferation and subsequent differentiation of the remaining stem cells and/or undifferentiated spermatogonia. This may thus contribute to the persistence of spermatogenesis during the chronic phase of SCI.

Endocrine regulation of the expression of Trf and ABP has been studied extensively [24–26], and these proteins have been used as markers for the functional status of Sertoli cells under various experimental conditions. A transient decrease in Trf mRNA in the SCE testes without a concomitant change in ABP mRNA 3 days after SCI is consistent with our previous findings [1]. The decrease in Trf mRNA can be attributed to the decrease in testosterone production at this time, because Trf mRNA decreases after hypophysectomy [10], and decreased Trf mRNA levels in SCI rats given exogenous testosterone also correlated with the decrease in testicular testosterone [8]. While the subsequent increase in ABP and Trf mRNA levels is consistent with recovering testosterone production, a significant increase in ABP and Trf mRNAs 4 wk after SCI cannot be correlated with the hormone status of the animals. In hypophysectomized rats, daily injections of FSH for 3 mo failed to induce a significant increase in testicular ABP but enhanced the effects of 2- or 5-cm testosterone capsule implants on testicular ABP production [27]. These results are consistent with the lack of FSH effects on Sertoli cells isolated from adult animals [24] and emphasize the importance of the interaction between FSH and testosterone in the adult testis [28]. In the testes of normal rats subjected to SCI, the levels of ABP and Trf mRNAs were stimulated by FSH, and this effect can be modulated by testosterone [8]. These results suggest that after SCI, Sertoli cells might become more sensitive to FSH and/or testosterone. The current observation of elevated levels of ABP and Trf mRNAs in the SCI-SCE testes without a concomitant increase in serum FSH and testosterone levels is consistent with this notion. Previous findings including modulation of Sertoli cell functions by ß-endorphin [29–31] and the adrenergic agonist, isoproterenol [32, 33], suggest possible neural-hormonal interactions in the control of Sertoli cell function. Thus, the increase in Sertoli cell ABP and Trf mRNA levels during the chronic stage of SCI may reflect an increase in Sertoli cell activities due to an increase in cellular responses to tropic hormones or secondary to the neurogenic effect of SCI.

The level of FSH-R mRNA in Sertoli cells cultured in vitro is down-regulated by FSH and exhibits minimal, if any, response to testosterone [15]. In contrast, AR protein and mRNA level in cultured Sertoli cells can be stimulated by both androgen and FSH [34–36]. In short term-hypophysectomized rats, testicular AR and FSH-R mRNAs are generally increased, an increase attributable to the absence of respective ligands [15, 36]. Recently, we observed higher testicular AR mRNA levels in untreated SCI rats that had normal testicular testosterone concentrations, and normal AR mRNA levels in those SCI rats that had received exogenous testosterone and had < 5% of normal testicular testosterone concentrations [8]. The presence of normal levels of AR mRNA in the SCE testes 3 days after SCI, while testosterone production was severely depressed, and higher AR mRNA at later times, while testosterone production was recovering, is consistent with an altered feedback regulation of testicular AR mRNA by its own ligand. A similar phenomenon has also been observed in the prostate of SCI rats [37]. A recent study demonstrated that AR expression in the motoneurons in the spinal nucleus of the bulbocavernosus can be modulated by brain-derived neurotrophic factors and axotomy [38], suggesting a neuronal-target interaction in the control of AR expression. Accordingly, the increased AR mRNA seen in the SCI-SCE testes could be related to the absence of a normal neuronal input to the testes. The concomitant increase in the levels of AR mRNA and mRNA for ABP and Trf at various times suggests a possible link between the status of AR and expression of Sertoli cell proteins.

A significant increase in FSH-R mRNA 90 days after SCI is consistent with the increase in mRNA for other Sertoli cell proteins. Recently, we found that FSH-R mRNA in the testes of normal rats subjected to SCI rats was elevated after 2 wk of FSH or testosterone treatment [8]. These results are different from those in the testes of normal or hypophysectomized rat testes in which FSH-R mRNA levels are negatively regulated by FSH and testosterone [8, 14], thus suggesting alteration of the regulation of FSH-R gene after SCI. While the cause for such changes cannot be delineated by current results, the concurrence of a higher abundance of FSH-R and AR mRNAs may result in more receptor proteins that could facilitate the responses of Sertoli cells to FSH and testosterone. This may provide some mechanistic explanation for the increased expression of ABP and Trf mRNAs, and perhaps facilitate other aspects of Sertoli cell activities, at this time. The latter could then contribute to the maintenance of spermatogenesis in the SCI-SCE rats.

In summary, current results demonstrate the persistence of spermatogenesis in SCE testes during the chronic phase of SCI. These effects are associated with elevated levels of mRNA for Sertoli cell FSH-R, AR, ABP, and Trf. It is postulated that absence of spermatogenic cells in most of the seminiferous tubules, and/or Sertoli cell enrichment, in the SCE testes may render the stem cells and/or undifferentiated spermatogonia less susceptible to the deleterious effects of SCI on the testes. Alternatively, disruption of normal neuronal input to the testes after SCI may modulate the responses of Sertoli cells to FSH and/or testosterone, leading to an overall increase in Sertoli cell activities. The changes in Sertoli cell physiology may render the intratesticular paracrine environment more favorable for the survival and proliferation of the remaining stem cells, as well as differentiation of their daughter cells, thus resulting in the persistence of spermatogenesis during the chronic phase of SCI.


   ACKNOWLEDGMENTS
 
We thank the following investigators for their generous gifts of cDNA probes used in the present experiment: Drs. David Joseph (ABP), Michael Griswold (transferrin and FSH-R), and Shutsung Liao (AR). The RIA kits for FSH and LH were the gift of the NIH Pituitary Program and Dr. Parlow, and are gratefully acknowledged.


   FOOTNOTES
 
1 Supported by the Department Veterans Affairs Rehabilitation Research and Development Services (B885-RA). Back

2 Correspondence. FAX: 973 972 6803; huanghf{at}umdnj.edu Back

Accepted: October 29, 1998.

Received: August 21, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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This article has been cited by other articles:


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J. E. Ottenweller, M.-T. Li, W. Giglio, R. Anesetti, L. M. Pogach, and H. F.S. Huang
Alteration of Follicle-Stimulating Hormone and Testosterone Regulation of Messenger Ribonucleic Acid for Sertoli Cell Proteins in the Rat During the Acute Phase of Spinal Cord Injury
Biol Reprod, September 1, 2000; 63(3): 730 - 735.
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