Paving the way for application of next generation risk assessment to safety decision-making for cosmetic ingredients.

Next generation risk assessment (NGRA) is an exposure-led, hypothesis-driven approach that has the potential to support animal-free safety decision-making. However, significant effort is needed to develop and test the in vitro and in silico (computational) approaches that underpin NGRA to enable confident application in a regulatory context. A workshop was held in Montreal in 2019 to discuss where effort needs to be focussed and to agree on the steps needed to ensure safety decisions made on cosmetic ingredients are robust and protective. Workshop participants explored whether NGRA for cosmetic ingredients can be protective of human health, and reviewed examples of NGRA for cosmetic ingredients. From the limited examples available, it is clear that NGRA is still in its infancy, and further case studies are needed to determine whether safety decisions are sufficiently protective and not overly conservative. Seven areas were identified to help progress application of NGRA, including further investments in case studies that elaborate on scenarios frequently encountered by industry and regulators, including those where a 'high risk' conclusion would be expected. These will provide confidence that the tools and approaches can reliably discern differing levels of risk. Furthermore, frameworks to guide performance and reporting should be developed.

Toxicity testing in the 21st century: progress in the past decade and future perspectives.

Advances in the biological sciences have led to an ongoing paradigm shift in toxicity testing based on expanded application of high-throughput in vitro screening and in silico methods to assess potential health risks of environmental agents. This review examines progress on the vision for toxicity testing elaborated by the US National Research Council (NRC) during the decade that has passed since the 2007 NRC report on Toxicity Testing in the 21st Century (TT21C). Concomitant advances in exposure assessment, including computational approaches and high-throughput exposomics, are also documented. A vision for the next generation of risk science, incorporating risk assessment methodologies suitable for the analysis of new toxicological and exposure data, resulting in human exposure guidelines is described. Case study prototypes indicating how these new approaches to toxicity testing, exposure measurement, and risk assessment are beginning to be applied in practice are presented. Overall, progress on the 20-year transition plan laid out by the US NRC in 2007 has been substantial. Importantly, government agencies within the United States and internationally are beginning to incorporate the new approach methodologies envisaged in the original TT21C vision into regulatory practice. Future perspectives on the continued evolution of toxicity testing to strengthen regulatory risk assessment are provided.

Male reproductive hormones in patients treated with pretomanid.

BACKGROUND: Pretomanid (Pa) is a nitroimidazole-class drug recently approved by the US Food and Drug Administration and other regulatory authorities as part of a regimen for treating highly drug-resistant pulmonary Mycobacterium tuberculosis infections. Studies in rodents identified the testis as a target organ of concern, which led to monitoring of reproductive hormones in >800 male patients enrolled in four clinical trials of Pa-containing regimens and the HRZE (isoniazid+rifampin+pyrazinamide+ethambutol) control regimen.METHODS: Serum hormone levels relevant to male reproductive health - follicle stimulating hormone (FSH), luteinizing hormone (LH), inhibin B (InhB) and total testosterone (T) - from the four clinical trials were summarized numerically and analyzed by repeated-measures modeling.RESULTS: Hormone levels generally behaved similarly in Pa-containing and HRZE arms. Relative to baseline, serum T and InhB levels generally increased at the end of treatment and follow-up. FSH and LH levels were variable, but were generally at or below baseline levels by follow-up. Before treatment, many patients were borderline hypogonadal, with T levels near the lower limits of the normal range.CONCLUSION: Changes in male hormones in four clinical trials studying patients with TB indicate that Pa-containing treatment was not associated with testicular toxicity but rather led to improvement in the underlying hypogonadism.

Sertoli cell processes have axoplasmic features: an ordered microtubule distribution and an abundant high molecular weight microtubule-associated protein (cytoplasmic dynein).

Microtubules in the cytoplasm of rat Sertoli cell stage VI-VIII testicular seminiferous epithelium were studied morphometrically by electron microscopy. The Sertoli cell microtubules demonstrated axonal features, being largely parallel in orientation and predominantly spaced one to two microtubule diameters apart, suggesting the presence of microtubule-bound spacer molecules. Testis microtubule-associated proteins (MAPs) were isolated by a taxol, salt elution procedure. Testis MAPs promoted microtubule assembly, but to a lesser degree than brain MAPs. High molecular weight MAPs, similar in electrophoretic mobilities to brain MAP-1 and MAP-2, were prominent components of total testis MAPs, though no shared immunoreactivity was detected between testis and brain high molecular weight MAPs using both polyclonal and monoclonal antibodies. Unlike brain high molecular weight MAPs, testis high molecular weight MAPs were not heat stable. Testis MAP composition, studied on postnatal days 5, 10, 15, and 24 and in the adult, changed dramatically during ontogeny. However, the expression of the major testis high molecular weight MAP, called HMW-2, was constitutive and independent of the development of mature germ cells. The Sertoli cell origin of HMW-2 was confirmed by identifying this protein as the major MAP found in an enriched Sertoli cell preparation and in two rat models of testicular injury characterized by germ cell depletion. HMW-2 was selectively released from testis microtubules by ATP and co-purified by sucrose density gradient centrifugation with MAP-1C, a neuronal cytoplasmic dynein. The inhibition of the microtubule-activated ATPase activity of HMW-2 by vanadate and erythro-(2-hydroxy-3-nonyl)adenine and its proteolytic breakdown by vanadate-dependent UV photocleavage confirmed the dynein-like nature of HMW-2. As demonstrated by this study, the neuronal and Sertoli cell cytoskeletons share morphological, structural and functional properties.

High-quality human and rat spermatozoal RNA isolation for functional genomic studies.

Sperm RNA is a sensitive monitoring endpoint for male reproductive toxicants, and a potential biomarker to assess male infertility and sperm quality. However, isolation of sperm RNA is a challenging procedure due to the heterogeneous population of cells present in the ejaculate, the low yield of RNA per spermatozoon, and the absence of 18S and 28S ribosomal RNA subunits. The unique biology of spermatozoa has created some uncertainty in the field about RNA isolation methods, indicating the need for rigorous quality control checks to ensure reproducibility of data generated from sperm RNA. Therefore, we developed a reliable and effective protocol for RNA isolation from rat and human spermatozoa that delivers highly purified and intact RNA, verified using RNA-specific electrophoretic chips and molecular biology approaches such as RT-PCR and Western blot analysis. The sperm RNA isolation technique was optimized using rat spermatozoa and then adapted to human spermatozoa. Three steps in the sperm isolation procedure, epididymal fluid collection, sperm purification, and spermatozoon RNA extraction, were evaluated and assessed. The sperm RNA extraction methodology consists of collection of rat epididymal fluid with repeated needle punctures of the epididymis, somatic cell elimination using detergent-based somatic cell lysis buffer (SCLB) and the use of RNA isolation Kit. Rat sperm heads are more resistant to disruption than human spermatozoa, necessitating the addition of mechanical lysis with microbeads and heat in the rat protocol, whereas the human sperm protocol only required lysis buffer. In conclusion, this methodology results in reliable and consistent isolation of high-quality sperm RNA. Using this technique will aid in translation of data collected from animal models, and reproducibility of clinical assessment of male factor fertility using RNA molecular biomarkers.

Intra- and inter-individual differences in human sperm DNA methylation.

There is growing evidence that sperm DNA methylation is important in maintaining proper sperm health and function. Previous studies have associated sperm DNA methylation levels with sperm quality and function, however, little is known regarding the intra- and inter-individual variability in sperm methylation levels. This study characterizes this variation. Sperm epigenetic differences between successive semen samples from 12 patients were examined to identify the intra- and inter-individual differences globally across the genome, and in specifically defined genomic regions using the Illumina Infinium HumanMethylation450 BeadChips. Methylation analysis identified a bimodal distribution in the methylation levels that were non-uniformly distributed across the different genomic regions. The methylation levels were highly correlated in both the intra- and inter-individual comparisons. The intra-individual methylation levels were more highly correlated than the inter-individual comparison both globally and across the defined genomic regions, demonstrating that sperm DNA methylation levels are relatively stable between semen sample collections.

Analysis of testes and semen from rabbits treated by intravenous injection with a retroviral vector encoding the human factor VIII gene: no evidence of germ line transduction.

In a phase 1 clinical trial, we are evaluating a murine leukemia virus (MuLV)-based retroviral vector encoding the human factor VIII gene [hFVIII(V)], administered intravenously, as a therapy for hemophilia A. Preclinical biolocalization studies in adult rabbits revealed vector-specific PCR signals in testis tissue at low levels. In follow-up animal studies we used PCR to (1) estimate the frequency with which a given cell in testis tissue is transduced, and (2) determine whether a positive PCR signal could be detected in semen samples from animals treated with hFVIII(V). Using the 99% confidence bound, results indicate that the probability that a given cell within the testis was transduced is less than 1/709,000 (97 days after treatment). This probability decreased with time after hFVIII(V) administration. Moreover, the rate of provector sequence detection in semen samples collected weekly throughout two cycles of spermatogenesis was 3/4281 reactions (0.07%), which is lower than the rate of false positives (1/800, 0.125%) observed for control animals. Using PCR assays with single-copy sensitivity, we have shown that the small number of transduced cells present in testis tissue does not give rise to detectable transduced cells in semen.

Microtubules are oriented with their minus-ends directed apically before tight junction formation in rat Sertoli cells.

We have examined the polarity of microtubules in Sertoli cells of the seminiferous epithelium during testicular development to test the hypothesis that microtubules change their polarity during tight junction formation. Microtubules in a number of polarized epithelial cells, including Sertoli cells, are oriented with their minus-ends directed toward the apical surface of the cell. Indirect evidence from cultured epithelial cell models suggests that this orientation may be achieved during the relocation of centrioles, reorganization of microtubules, and repositioning of the Golgi that occurs during tight junction formation. Using the microtubule hook decoration technique, we have determined the polarity of Sertoli cell microtubules at 5 and 15 days postnatally, prior to the establishment of the tight junctional complex of the blood testis barrier at 19 days. Our results indicate that, although centrioles and the Golgi apparatus migrate from an infranuclear location at 5 days to a supranuclear location by 15 days, the minus-ends of microtubules are already directed toward the apical surface of the cell by 5 days of age. These data indicate that the establishment of the apically directed minus-end orientation of microtubules of mature Sertoli cells precedes rearrangement of the centrioles and Golgi and is not temporally related to the formation of the tight junctional complex of the blood testis barrier.

Binding between mammalian spermatid-ectoplasmic specialization complexes and microtubules.

Ectoplasmic specializations (ESs) are submembrane specializations that consist of Sertoli cell plasma membrane linked by an ordered array of actin filaments to a cisterna of endoplasmic recticulum (ESER). They are thought to function in the spermatid-Sertoli cell adhesion junction. Microtubules occur adjacent to the cytoplasmic face of the ESER and are oriented parallel to the long axis of the Sertoli cell, the direction of spermatid translocation during spermatogenesis. Our hypothesis that spermatid orientation and translocation in the seminiferous epithelium is microtubule dependent predicts that microtubules bind to ESs. To test for binding between microtubules and ESs, we have developed an in vitro assay in which spermatid-ES complexes were isolated from the seminiferous epithelium and incubated with bovine brain microtubules that were labeled with [3H]GTP and stabilized with taxol. Binding was determined by scintillation counts from gradient fractions enriched for spermatid-ES complexes and depleted of unbound microtubules by differential centrifugation. Our data indicate that microtubules bind to spermatid-ES complexes in a substrate concentration-dependent manner and can be released with 5 mM GTP or 10 mM MgATP. Binding is competitively reduced with excess unlabeled microtubules and is inhibited by 100 microM vanadate and 2 mM N-ethylmaleimide (NEM). The amount of binding is unchanged by 10 microM vanadate, 2 mM erythro-(2-hydroxy-3-nonyl)adenine (EHNA) or 1 mM 5'-adenylylimidodiphosphate (AMP-PNP). Immunofluorescence and autoradiographic data confirm that labeled microtubules bind to ES locations on spermatid-ES complexes. These data are consistent with the hypothesis that spermatid translocation is a microtubule-based transport event.

Kinesin localizes to the trans-Golgi network regardless of microtubule organization.

Knowledge of the mechanisms of intracellular membrane trafficking is critical for understanding cell function. Here, the microtubule motor kinesin is localized to the Sertoli cell trans-Golgi network with the SUK4 kinesin monoclonal antibody. Using immunoelectron and immunofluorescence microscopy, kinesin was shown to localize to the trans-Golgi network in primary Sertoli cell cultures. Kinesin immunostaining was not observed in brefeldin A-induced trans-Golgi network-derived membrane tubules and remained associated with trans-Golgi network remnants. Despite dramatic differences in microtubule organization between Sertoli cells in vivo and in vitro, kinesin immunostaining was consistently associated with the trans-Golgi network. In cryosections of control testis and testis treated with colchicine to disrupt microtubules, kinesin immunostaining was juxtaposed with immunostaining for a Golgi cisternal protein; however, brefeldin A exposure partitioned the kinesin immunostaining from the Golgi cisternal protein, indicating that kinesin was associated with the trans-Golgi network of Sertoli cells in vivo. These results are discussed in relation to known Golgi-associated microtubule-dependent transport events in other cell types and suggest that kinesin functions locally at the Sertoli cell trans-Golgi network.

Melanocytotoxicity and the mechanism of activation of gamma-L-glutaminyl-4-hydroxybenzene.

gamma-L-Glutaminyl-4-hydroxybenzene is converted by the tyrosinase of the common mushroom, Agaricus bisporus, to the toxic, dormancy-inducing metabolite 2-hydroxy-4-imino-2,5-cyclohexadiene-1-one. Hydroxylation of gamma-L-glutaminyl-4-hydroxybenzene by mammalian tyrosinase was monitored by determining tritium water release from gamma-L-glutaminyl-[3,5-(3)H[4-hydroxybenzene and occurred at only 25% of the rate found with tyrosine. The dihydroxy product of the hydroxylation reaction, gamma-L-glutaminyl-3,4-dihydroxybenzene, was not oxidized by the mammalian enzyme. Therefore, oxidation of gamma-L-glutaminyl-4-hydroxybenzene to sulfhydryl-reactive quinones by mammalian tyrosinase is an unlikely explanation for the hair depigmentation and inhibition of melanocarcinoma growth observed following administration of this compound. Cleavage of gamma-L-glutaminyl-4-hydroxybenzene by gamma-glutamyl transpeptidase releasing p-aminophenol was demonstrated. p-Aminophenol was an active depigmenting and melanocytotoxic compound. N2-Methyl-gamma-L-glutaminyl-4-hydroxybenzene was synthesized, differing from gamma-L-glutaminyl-4-hydroxybenzene only by the presence of a methylated amide linkage. This chemical modification resulted in a compound resistant to cleavage by gamma-glutamyl transpeptidase and lacking in melanocytotoxic activity. gamma-Glutamyl transpeptidase cleavage is proposed as the route for transformation of gamma-L-glutaminyl-4-hydroxybenzene into an active inhibitor of melanocytes.

Multiple cadherin superfamily members with unique expression profiles are produced in rat testis.

Adhesion between germ and Sertoli cells is thought to be crucial for spermatogenesis. Cadherin superfamily proteins, including classic cadherins and protocadherins, are important mediators of cell-cell adhesion. Using a degenerate PCR cloning strategy, we surveyed the expression of cadherin superfamily members in rat testis. Similar to brain, testis expressed a large number of cadherin superfamily members: 7 classic cadherins of both types I and II, 14 protocadherins, 2 protocadherin-related cadherins, and 1 cadherin-related receptorlike protein. All three protocadherin families (alpha, beta, and gamma) were found in testis. Using a semiquantitative RT-PCR assay, messenger RNA expression was determined for each cadherin superfamily member during a postnatal developmental time-course and following ablation of specific testis cell types by ethanedimethanesulfonate, methoxyacetic acid, and 2,5-hexanedione. Diverse expression patterns were observed among the cadherins, suggesting that cadherin expression is cell type-specific in testis. The large number and variety of cadherin superfamily members found in testis supports a critical function for cadherin-mediated cell-cell adhesion in spermatogenesis.

Leuprolide, a gonadotropin-releasing hormone agonist, reestablishes spermatogenesis after 2,5-hexanedione-induced irreversible testicular injury in the rat, resulting in normalized stem cell factor expression.

2,5-Hexanedione (2,5-HD) exposure in the rat produces irreversible testicular atrophy, a model of human male infertility that can be used for mechanistic and therapeutic studies. Following testicular injury by 2,5-HD, stem cell factor (SCF), a Sertoli cell-derived growth factor that binds the c-kit receptor on spermatogonia, is altered in its expression, changing from predominantly membrane SCF to predominantly soluble SCF. The goals of this study were 2-fold: first, evaluate leuprolide, a GnRH agonist, as a therapy for 2,5-HD-induced testicular atrophy, and second, examine changes in SCF expression during testicular injury and following recovery from injury. Rats exposed to 2,5-HD showed a nearly complete testicular atrophy that could be reversed by leuprolide therapy. Using RT-PCR, preferential expression of membrane SCF was associated with spermatogenesis, whereas soluble SCF expression was associated with atrophy. In conclusion, 2,5-HD exposure altered the form of SCF expressed and disrupted spermatogenesis; leuprolide therapy allowed recovery of spermatogenesis, which correlated with a normalization in growth factor expression in an otherwise irreversibly atrophic testis.

The Fas system is a key regulator of germ cell apoptosis in the testis.

Apoptosis occurs in the testis as an important physiological mechanism to limit the number of germ cells in the seminiferous epithelium. Sertoli cells, which tightly regulate germ cell proliferation and differentiation, are implicated in the control of germ cell apoptosis. Fas (APO-1, CD95), a transmembrane receptor protein, transmits an apoptotic signal within cells when bound by Fas ligand (FasL). The Fas system has been implicated in immune regulation, including cytotoxic T cell-mediated cytotoxicity, activation-induced suicide of T cells, and control of immune-privileged sites. Here we propose the Fas system as a key regulator of spermatogenesis. In this model, FasL expressed by Sertoli cells initiates the apoptotic death of germ cells expressing Fas. Using immunohistochemistry, we localized Fas to germ cells and FasL to Sertoli cells. The expression of these genes was dramatically up-regulated after exposure to mono-(2-ethylhexyl) phthalate and 2,5-hexanedione, two widely studied Sertoli cell toxicants known to induce germ cell apoptosis. Mouse germ cells in vitro were susceptible to anti-Fas antibody-induced death, and the survival of rat germ cells was increased after disruption of FasL by antisense oligonucleotide treatment. Unlike its expression in other tissues, testicular expression of Fas in the lpr mouse, a spontaneous mutant of the Fas gene, is similar to that in the normal mouse, arguing for the importance of the Fas system in maintaining testicular homeostasis. These data implicate the Sertoli cell in the paracrine control of germ cell output during spermatogenesis by a Fas-mediated pathway.

Sensitivity of testicular germ cells to toxicant-induced apoptosis in gld mice that express a nonfunctional form of Fas ligand.

Germ cell apoptosis in testis is essential for functional spermatogenesis. Recent evidence suggests that the Fas signaling system is critical for the regulation of testicular germ cell apoptosis. To further evaluate the Fas signaling system in testis, we examined the incidence of germ cell apoptosis in gld mice that lack a functional Fas-signaling pathway. gld mice have a small, but significant, increase in testis weight and numbers of spermatid heads per testis compared with wild-type mice. In addition, gld mice have a small increase in the spontaneous incidence of germ cell apoptosis, as indicated by characteristic DNA fragmentation via the terminal deoxynucleotidyl-transferase-mediated deoxy-UTP nick end labeling assay. To test the role of the Fas system in toxicant-induced germ cell apoptosis, mice were exposed to either a Sertoli cell- or germ cell-specific toxicant [mono-(2-ethylhexyl)phthalate (MEHP; 1 g/kg) or 5 Gy radiation, respectively]. These two exposure paradigms induced extensive increases in germ cell apoptosis in wild-type mice. However, exposure of gld mice to MEHP caused only a minimal increase in germ cell apoptosis, whereas they were as sensitive as wild-type mice to radiation exposure. These data indicate that the Fas signaling pathway is 1) involved in regulating the numbers of germ cells in the testis, 2) crucial for the initiation of germ cell apoptosis after MEHP-induced Sertoli cell injury, and 3) differentially active in the cell-specific regulation of germ cell apoptosis that occurs as a consequence of Sertoli cell vs. germ cell injury.

The Fas system, a regulator of testicular germ cell apoptosis, is differentially up-regulated in Sertoli cell versus germ cell injury of the testis.

Sertoli cells, the supportive cells in the seminiferous epithelium, orchestrate spermatogenesis by providing structural and nutritional support to germ cells. In the rat, physiological apoptosis occurs continuously to limit the size of the germ cell population to numbers that can be adequately supported. This form of germ cell death is exaggerated after testicular insults such as toxicant treatment, radiation, and heat exposure. The Fas system has been proposed as a key regulator of the activation of germ cell apoptosis. According to this model, Fas ligand (FasL) and Fas, expressed by Sertoli cells and germ cells, respectively, respond to environmental conditions and initiate germ cell death. To assess the role of the Fas system in various testicular injury models, a semiquantitative RT-PCR technique was used to evaluate the expression kinetics of both FasL and Fas after induction of massive germ cell death. Radiation exposure, which targets actively dividing germ cells, produced an up-regulation of Fas gene expression, but not FasL gene expression. However, administration of mono-(2-ethylhexyl)phthalate and 2,5-hexanedione, two widely studied Sertoli cell toxicants, resulted in up-regulated expression of both FasL and Fas. These data support the following hypotheses: 1) up-regulation of Fas is a common and critical step for initiating germ cell death in vivo; and 2) if Sertoli cells are injured, Sertoli cells up-regulate FasL to eliminate Fas-positive germ cells, which cannot be supported adequately.

Cloning and characterization of the 5' flanking region of the stem cell factor gene in rat Sertoli cells.

In order to elucidate the molecular basis of stem cell factor (SCF, or steel factor/kit ligand) expression in Sertoli cells of rat testis, 1.5 kb of the 5' flanking region of the SCF gene was isolated and characterized. The transcriptional start point (tsp) was identified by primer extension assay and a rapid amplification of cDNA ends (RACE) assay. A TATA box was found 29 base pairs (bp) upstream from the tsp, and a number of transcription factor consensus sequences, including several AP2 and Spl sites, were identified. The transcriptional activity of the 1.5 kb 5' flanking region was analyzed by deletion constructs using a firefly luciferase-encoding gene (luc) expression vector transiently transfected into primary rat Sertoli cells and other SCF positive and negative cell lines. For all the cells and cell lines examined, a -119 bp to +43 bp fragment including the tsp was sufficient for SCF promoter activity, and the core promoter activity was not significantly changed by inclusion of upstream sequences as far as -1461 bp. These results indicate that additional sites outside of this promoter region are needed to define the cell-specific regulatory elements of SCF expression. The transcriptional activities of all SCF deletion constructs treated with cyclic adenosine 3',5'-monophosphate (cAMP) and forskolin were increased two- to threefold, indicating that SCF transcription in Sertoli cells is regulated by a cAMP-dependent pathway in the proximal promoter region.

Visualization of Golgi complexes and spermatogonial cohorts of viable, intact seminiferous tubules.

Golgi complexes of cells within the intact, viable seminiferous tubule were examined by light microscopy using the vital Golgi stain C6-NBD-ceramide. This staining technique provided a quick and simple method to visualize all Golgi complexes of cells within the seminiferous tubule that are directly accessible to the basal compartment. Thus, peritubular, spermatogonial, and Sertoli cell Golgi complexes were visualized. Peritubular cells contained simple Golgi complexes which did not change with the stage of the seminiferous epithelium. Both solitary spermatogonial Golgi complexes which varied in size and number with the stage of the seminiferous epithelium and cohorts of spermatogonia connected by intercellular bridges were easily visualized. The Golgi complexes of Sertoli cells were located in the basal, perinuclear cytoplasm except in Stages VII-VIII, when they extended towards the lumen. Exposure of isolated seminiferous tubules to the fungal metabolite brefeldin A caused the Sertoli cell Golgi complex staining pattern to become diffuse or to co-localize with heads of elongate spermatids. The Golgi complexes of the peritubular cells and spermatogonia were resistant to brefeldin A. The C6-NBD-ceramide vital staining method should be useful for studying stage-dependent Sertoli cell Golgi complex movement and spermatogonial maturation.

Distribution of Sertoli cell microtubules, microtubule-dependent motors, and the Golgi apparatus before and after tight junction formation in developing rat testis.

Sertoli cells are polarized epithelial cells of the seminiferous epithelium which provide structural and physiological support for differentiating germ cells. They establish different basal and adluminal environments for the selective nurturing of pre- and post-meiotic germ cells within the seminiferous epithelium, segregated by the Sertoli-Sertoli cell tight junctional complex, the blood-testis barrier. Tight junction formation between epithelial cells in vitro is a critical polarizing event associated with changes in polarized targeting of membrane-specific proteins and reorganization of microtubules, centrioles, and the Golgi apparatus. To investigate whether tight junction formation is associated with organelle reorganization in Sertoli cells in vivo, we have characterized distribution patterns of Sertoli cell microtubules, the mechanoenzymes kinesin and cytoplasmic dynein, and the Golgi apparatus during tight junction formation in developing rat testis. Immunocytochemistry on samples taken at 5, 10, 15, 20, and 25 days of age was used to examine the distribution of these proteins during the extensive cellular reorganization that culminates in the formation of the blood-testis barrier at 19 days of age. Our data show that the distribution patterns reflect the extensive intercellular repositioning of tubule cells in developing seminiferous tubules, but that changes in intracellular organization are not temporally associated with formation of the blood-testis barrier.

2,5-hexanedione exposure in the rat results in long-term testicular atrophy despite the presence of residual spermatogonia.

Charles River CD rats (approximate weight, 208 g) were exposed to 1.0% 2,5-hexanedione (2,5-HD) in drinking water for 5 weeks. Rats were killed 27, 60, and 75 weeks after exposure to evaluate the recovery potential following testicular injury. At 27 weeks, normal serum testosterone and significantly elevated serum luteinizing hormone and serum follicle-stimulating hormone levels were found in treated rats. The 2,5-HD-treated rats had low testicular and epididymal weights at all time points (28% and 72% of controls, respectively, at 75 weeks). Microscopically, there was a generalized loss of postspermatogonial germ cells at all time points, with no seminiferous tubules exhibiting normal spermatogenesis at 75 weeks. However, a relatively constant population of 3.1 to 3.7 spermatogonia/100 Sertoli cells was found in atrophic seminiferous tubules at all time points. The presence of a constant residual population of type A spermatogonia without a normal mass of more mature germ cells and the observed hormonal alterations suggest that 2,5-HD intoxication produced a lengthy disruption in local testicular homeostatic mechanisms that control spermatogenesis.