Sertoli cell tight junction dynamics: their regulation during spermatogenesis

The Sertoli cell: one hundred fifty years of beauty and plasticity

It has been one and a half centuries since Enrico Sertoli published the seminal discovery of the testicular 'nurse cell', not only a key cell in the testis, but indeed one of the most amazing cells in the vertebrate body. In this review, we begin by examining the three phases of morphological research that have occurred in the study of Sertoli cells, because microscopic anatomy was essentially the only scientific discipline available for about the first 75 years after the discovery. Biochemistry and molecular biology then changed all of biological sciences, including our understanding of the functions of Sertoli cells. Immunology and stem cell biology were not even topics of science in 1865, but they have now become major issues in our appreciation of Sertoli cell's role in spermatogenesis. We end with the universal importance and plasticity of function by comparing Sertoli cells in fish, amphibians, and mammals. In these various classes of vertebrates, Sertoli cells have quite different modes of proliferation and epithelial maintenance, cystic vs. tubular formation, yet accomplish essentially the same function but in strikingly different ways.

Identification of the Functions of Liver X Receptor-β in Sertoli Cells Using a Targeted Expression-Rescue Model

Liver X receptors (LXRs) are key regulators of lipid homeostasis and are involved in multiple testicular functions. The Lxrα(-/-);Lxrβ(-/-) mice have illuminated the roles of both isoforms in maintenance of the epithelium in the seminiferous tubules, spermatogenesis, and T production. The requirement for LXRβ in Sertoli cells have been emphasized by early abnormal cholesteryl ester accumulation in the Lxrβ(-/-) and Lxrα(-/-);Lxrβ(-/-) mice. Other phenotypes, such as germ cell loss and hypogonadism, occur later in life in the Lxrα(-/-);Lxrβ(-/-) mice. Thus, LXRβ expression in Sertoli cells seems to be essential for normal testicular physiology. To decipher the roles of LXRβ within the Sertoli cells, we generated Lxrα(-/-);Lxrβ(-/-):AMH-Lxrβ transgenic mice, which reexpress Lxrβ in Sertoli cells in the context of Lxrα(-/-);Lxrβ(-/-) mice. In addition to lipid homeostasis, LXRβ is necessary for maintaining the blood-testis barrier and the integrity of the germ cell epithelium. LXRβ is also implicated in the paracrine action of Sertoli cells on Leydig cells to modulate T synthesis. The Lxrα(-/-);Lxrβ(-/-) and Lxrα(-/-);Lxrβ(-/-):AMH-Lxrβ mice exhibit lipid accumulation in germ cells after the Abcg8 down-regulation, suggesting an intricate LXRβ-dependent cooperation between the Sertoli cells and germ cells to ensure spermiogenesis. Further analysis revealed also peritubular smooth muscle defects (abnormal lipid accumulation and disorganized smooth muscle actin) and spermatozoa stagnation in the seminiferous tubules. Together the present work elucidates specific roles of LXRβ in Sertoli cell physiology in vivo beyond lipid homeostasis.

Proteomics analysis of Xenopus laevis gonad tissue following chronic exposure to atrazine

Atrazine is the most commonly detected pesticide contaminant in ground and surface water. Previous studies have shown that atrazine is an endocrine disruptor owing to its adverse effects on the male reproductive system in several vertebrates, but very few molecular mechanisms for these effects have been revealed. In the present study, Xenopus laevis were exposed to 100 ppb of atrazine for 120 d, and then the isobaric tags for relative and absolute quantitation (iTRAQ) technique was used to detect global changes in protein profiles of the testes and ovaries. The results showed that 100 ppb of atrazine exposure adversely affected the growth of X. laevis and did not induce hermaphroditism but delayed or prevented the development of male seminiferous tubules. Proteomic analysis showed that atrazine altered expression of 143 and 121 proteins in the testes and ovaries, respectively, and most of them are involved in cellular and metabolic processes and biological regulation based on their biological processes. In addition, apoptosis, tight junctions, and metabolic pathways were significantly altered in the atrazine-treated gonads. Based on the above results, it is postulated that the reproductive toxicity of atrazine may be the result of disruption of tight junctions and metabolic signaling pathways and/or induction of apoptosis in germ cells.

Physiological roles of connexins and pannexins in reproductive organs

Reproductive organs are complex and well-structured tissues essential to perpetuate the species. In mammals, the male and female reproductive organs vary on their organization, morphology and function. Connectivity between cells in such tissues plays pivotal roles in organogenesis and tissue functions through the regulation of cellular proliferation, migration, differentiation and apoptosis. Connexins and pannexins can be seen as major regulators of these physiological processes. In the present review, we assembled several lines of evidence demonstrating that these two families of proteins are essential for male and female reproduction.

Specific deletion of Cdh2 in Sertoli cells leads to altered meiotic progression and subfertility of mice

CDH2 (cadherin 2, Neural-cadherin, or N-cadherin) is the predominant protein of testicular basal ectoplasmic specializations (basal ES; a testis-specific type of adhesion junction), one of the major cell junctions composing the blood-testis barrier (BTB). The BTB is found between adjacent Sertoli cells in seminiferous tubules, which divides the tubules into basal and adluminal compartments and prevents the deleterious exchange of macromolecules between blood and seminiferous tubules. However, the exact roles of basal ES protein CDH2 in BTB function and spermatogenesis is still unknown. We thus generated mice with Cdh2 specifically knocked out in Sertoli cells by crossing Cdh2 loxP mice with Amh-Cre mice. Cdh2 deletion in Sertoli cells did not affect Sertoli cell counts, but led to compromised BTB function, delayed meiotic progression from prophase to metaphase I in testes, increased germ cell apoptosis, sloughing of meiotic cells, and, subsequently, reduced sperm counts in epididymides and subfertility of mice. However, the testes with Cdh2-specific deletion in germ cells did not show any difference from the normal control testes, and phenotypes observed in Sertoli cell and germ cell Cdh2 double-knockout mice were indistinguishable from those in mice with Cdh2 specifically knocked out only in Sertoli cells. Taken together, our data demonstrate that the adhesion junction component, Cdh2, functions just in Sertoli cells, but not in germ cells during spermatogenesis, and is essential for the integrity of BTB function, its deletion in Sertoli cells would lead to the BTB damage and subsequently meiosis and spermatogenesis failure.

A somatic permeability barrier around the germline is essential for Drosophila spermatogenesis

Interactions between the soma and germline are essential for gametogenesis. In the Drosophila testis, differentiating germ cells are encapsulated by two somatic cells that surround the germline throughout spermatogenesis. chickadee (chic), the fly ortholog of Profilin, mediates soma-germline interactions. Knockdown of Chic in the soma results in sterility and severely disrupted spermatogenesis due to defective encapsulation. To study this defect further, we developed a permeability assay to analyze whether the germline is isolated from the surrounding environment by the soma. We find that germline encapsulation by the soma is, by itself, insufficient for the formation of a permeability barrier, but that such a barrier gradually develops during early spermatogenesis. Thus, germline stem cells, gonialblasts and early spermatogonia are not isolated from the outside environment. By late spermatocyte stages, however, a permeability barrier is formed by the soma. Furthermore, we find that, concomitant with formation of the permeability barrier, septate junction markers are expressed in the soma and localize to junctional sites connecting the two somatic cells that surround the germline. Importantly, knockdown of septate junction components also disrupts the permeability barrier. Finally, we show that germline differentiation is delayed when the permeability barrier is compromised. We propose that the permeability barrier around the germline serves an important regulatory function during spermatogenesis by shaping the signaling events that take place between the soma and the germline.

Androgen-dependent sertoli cell tight junction remodeling is mediated by multiple tight junction components

Sertoli cell tight junctions (SCTJs) of the seminiferous epithelium create a specialized microenvironment in the testis to aid differentiation of spermatocytes and spermatids from spermatogonial stem cells. SCTJs must be chronically broken and rebuilt with high fidelity to allow the transmigration of preleptotene spermatocytes from the basal to adluminal epithelial compartment. Impairment of androgen signaling in Sertoli cells perturbs SCTJ remodeling. Claudin (CLDN) 3, a tight junction component under androgen regulation, localizes to newly forming SCTJs and is absent in Sertoli cell androgen receptor knockout (SCARKO) mice. We show here that Cldn3-null mice do not phenocopy SCARKO mice: Cldn3(-/-) mice are fertile, show uninterrupted spermatogenesis, and exhibit fully functional SCTJs based on imaging and small molecule tracer analyses, suggesting that other androgen-regulated genes must contribute to the SCARKO phenotype. To further investigate the SCTJ phenotype observed in SCARKO mutants, we generated a new SCARKO model and extensively analyzed the expression of other tight junction components. In addition to Cldn3, we identified altered expression of several other SCTJ molecules, including down-regulation of Cldn13 and a noncanonical tight junction protein 2 isoform (Tjp2iso3). Chromatin immunoprecipitation was used to demonstrate direct androgen receptor binding to regions of these target genes. Furthermore, we demonstrated that CLDN13 is a constituent of SCTJs and that TJP2iso3 colocalizes with tricellulin, a constituent of tricellular junctions, underscoring the importance of androgen signaling in the regulation of both bicellular and tricellular Sertoli cell tight junctions.

Gold nanoparticles elevate plasma testosterone levels in male mice without affecting fertility

ω-Methoxy and ω-aminoethyl poly(ethylene glycol)-modified 14-nm gold nanoparticles can accumulate in mouse testes, pass through the blood-testis barrier, and enter germ cells. Furthermore, PEG-NH2 @AuNP accumulate more easier in the testes and increase plasma T levels. However, these two gold nanoparticle types have no effect on male fertility, fetal survival, or fetal development.

Neonatal exposure to zearalenone induces long term modulation of ABC transporter expression in testis

Mycotoxin zearalenone (ZEN) is a cereal contaminant produced by various species of Fusarium fungi. When interacting with estrogen receptors, ZEN leads to animal fertility disturbances and other reproductive pathologies. Few data are available on the effects of perinatal exposure to ZEN, particularly in the blood-testis barrier. The aim of this study was to assess the impact of ZEN in adult rats exposed neonatally. We focused on the expression and cellular localization of major ABC transporters expressed in adult rat testis, comparing ZEN effects with those of Estradiol Benzoate (EB) neonatal exposure. Dose-dependent and long term modulations of mRNA and protein levels of Abcb1, Abcc1, Abcg2, Abcc4 and Abcc5 were observed, along with Abcc4 protein cellular delocalization. ZEN exposure of SerW3 Sertoli cells showed modulation of Abcb1, Abcc4 and Abcc5. Comparison with EB exposure showed similar modulation profiles for Abcg2 but differential modulations for Abcb1, Abcc1, Abcc4 and Abcc5 in vivo, and a similar profile for Abcb1 modulation by ZEN and EB, but differential modulation for Abcc4 and Abcc5 in vitro. ZEN and EB effects were inhibited by in vitro addition of the pure anti-estrogen ICI 182.780, suggesting the at least partial implication of ZEN estrogenic activity in these modulations. These results suggested that ZEN neonatal exposure could affect the exposure of testis to ABC transporter substrates, and negatively influence spermatogenesis and male fertility.

Diabetes, insulin-mediated glucose metabolism and Sertoli/blood-testis barrier function

Blood testis barrier (BTB) is one of the tightest blood-barriers controlling the entry of substances into the intratubular fluid. Diabetes Mellitus (DM) is an epidemic metabolic disease concurrent with falling fertility rates, which provokes severe detrimental BTB alterations. It induces testicular alterations, disrupting the metabolic cooperation between the cellular constituents of BTB, with dramatic consequences on sperm quality and fertility. As Sertoli cells are involved in the regulation of spermatogenesis, providing nutritional support for germ cells, any metabolic alteration in these cells derived from DM may be responsible for spermatogenesis disruption, playing a crucial role in fertility/subfertility associated with this pathology. These cells have a glucose sensing machinery that reacts to hormonal fluctuations and several mechanisms to counteract hyper/hypoglycemic events. The role of DM on Sertoli/BTB glucose metabolism dynamics and the metabolic molecular mechanisms through which DM and insulin deregulation alter its functioning, affecting male reproductive potential will be discussed.

A novel subcellular machine contributes to basal junction remodeling in the seminiferous epithelium

Tubulobulbar complexes are cytoskeleton-related membrane structures that develop at sites of intercellular attachment in mammalian seminiferous epithelium. At apical junctions between Sertoli cells and spermatids, the structures internalize adhesion junctions and are a component of the sperm release mechanism. Here we explore the possibility that tubulobulbar complexes that form at the blood-testis barrier are subcellular machines that internalize basal junction complexes. Using electron microscopy, we confirmed that morphologically identifiable tight and gap junctions are present in basal tubulobulbar complexes in rats. In addition, immunological probes for claudin-11 (CLDN11), connexin-43 (GJA1), and nectin-2 (PVRL2) react with linear structures at the light level that we interpret as tubulobulbar complexes, and probes for early endosome antigen 1 (EEA1) and Rab5 (RAB5A) react in similar locations. Significantly, fluorescence patterns for actin and claudin-11 indicate that the amount of junction present is dramatically reduced over the time period that tubulobulbar complexes are known to be most prevalent during spermatogenesis. We also demonstrated, using electron microscopy and fluorescence microscopy, that tubulobulbar complexes develop at basal junctions in primary cultures of Sertoli cells and that like their in vivo counterparts, the structures contain junction proteins. We use this culture system together with transfection techniques to show that junction proteins from one transfected cell occur in structures that project into adjacent nontransfected cells as predicted by the junction internalization hypothesis. On the basis of our findings, we present a new model for basal junction remodeling as it relates to spermatocyte translocation in the seminiferous epithelium.

The tyrosine phosphatase SHP2 regulates Sertoli cell junction complexes

The blood-testis barrier (BTB) is a large junctional complex composed of tight junctions, adherens junctions, and gap junctions between adjacent Sertoli cells in the seminiferous tubules of the testis. Maintenance of the BTB as well as the controlled disruption and reformation of the barrier is essential for spermatogenesis and male fertility. Tyrosine phosphorylation of BTB proteins is known to regulate the integrity of adherens and tight junctions found at the BTB. SHP2 is a nonreceptor protein tyrosine phosphatase (PTP) and a key regulator of growth factor-mediated tyrosine kinase signaling pathways. We found that SHP2 is localized to Sertoli-Sertoli cell junctions in rat testis. The overexpression of a constitutive active SHP2 mutant, SHP2 Q79R, up-regulated the BTB disruptor ERK1/2 via Src kinase in primary rat Sertoli cells in culture. Furthermore, focal adhesion kinase (FAK), which also supports BTB integrity, was found to interact with SHP2 and constitutive activation of SHP2 decreased FAK tyrosine phosphorylation. Expression of the SHP2 Q79R mutant in primary cultured Sertoli cells also resulted in the loss of tight junction and adherens junction integrity that corresponded with the disruption of the actin cytoskeleton and mislocalization of adherens junction and tight junction proteins N-cadherin, β-catenin, and ZO-1 away from the plasma membrane. These results suggest that SHP2 is a key regulator of BTB integrity and Sertoli cell support of spermatogenesis and fertility.

Distribution of Zonula Occludens-1 and Occludin and alterations of testicular morphology after in utero radiation and postnatal hyperthermia in rats

In utero irradiation (IR) and postnatal hyperthermia (HT) exposure cause infertility by decreasing spermatogenic colony growth and the number of sperm in rats. Four groups were used: (i) Control group, (ii) HT group (rats exposed to hyperthermia on the 10th postnatal day), (iii) IR group (rats exposed to IR on the 17th gestational day) and (iv) IR + HT group. Three and six months after the procedures testes were examined by light and electron microscopy. Some degenerated tubules in the HT group, many vacuoles in spermatogenic cells and degenerated tight junctions in the IR group, atrophic tubules and severe degeneration of tight junctions in the IR + HT group were observed. ZO-1 and occludin immunoreactivity were decreased and disorganized in the HT and IR groups and absent in the IR + HT group. The increase in the number of apoptotic cells was accompanied by a time-dependent decrease in haploid, diploid and tetraploid cells in all groups. Degenerative findings were severe after 6 months in all groups. The double-hit model may represent a Sertoli cell only model of infertility due to a decrease in spermatogenic cell and alterated blood-testis barrier proteins in rat.

New insights into roles of tubulobulbar complexes in sperm release and turnover of blood-testis barrier

Tubulobulbar complexes are actin-filament-related structures that form at intercellular junctions in the seminiferous epithelium of mammalian testis. The structures occur both at adhesion junctions between Sertoli cells and the maturing spermatids in apical regions of the epithelium, and at junction complexes between neighboring Sertoli cells near the base of the epithelium. Here, we review the general morphology and molecular composition of tubulobulbar complexes, and also include a description of tubulobulbar complex structure in the human seminiferous epithelium. Although tubulobulbar complexes are unique to the seminiferous epithelium, they have the molecular signature of clathrin-based endocytosis machinery present generally in cells. We review the evidence that tubulobulbar complexes internalize intact intercellular junctions and are significant components of the sperm-release mechanism and the process by which spermatocytes translocate from basal to adluminal compartments of the epithelium.

Cyclical and patch-like GDNF distribution along the basal surface of Sertoli cells in mouse and hamster testes

Background and Aims

In mammalian spermatogenesis, glial cell line-derived neurotrophic factor (GDNF) is one of the major Sertoli cell-derived factors which regulates the maintenance of undifferentiated spermatogonia including spermatogonial stem cells (SSCs) through GDNF family receptor α1 (GFRα1). It remains unclear as to when, where and how GDNF molecules are produced and exposed to the GFRα1-positive spermatogonia in vivo.

Methodology and Principal Findings

Here we show the cyclical and patch-like distribution of immunoreactive GDNF-positive signals and their close co-localization with a subpopulation of GFRα1-positive spermatogonia along the basal surface of Sertoli cells in mice and hamsters. Anti-GDNF section immunostaining revealed that GDNF-positive signals are mainly cytoplasmic and observed specifically in the Sertoli cells in a species-specific as well as a seminiferous cycle- and spermatogenic activity-dependent manner. In contrast to the ubiquitous GDNF signals in mouse testes, high levels of its signals were cyclically observed in hamster testes prior to spermiation. Whole-mount anti-GDNF staining of the seminiferous tubules successfully visualized the cyclical and patch-like extracellular distribution of GDNF-positive granular deposits along the basal surface of Sertoli cells in both species. Double-staining of GDNF and GFRα1 demonstrated the close co-localization of GDNF deposits and a subpopulation of GFRα1-positive spermatogonia. In both species, GFRα1-positive cells showed a slender bipolar shape as well as a tendency for increased cell numbers in the GDNF-enriched area, as compared with those in the GDNF-low/negative area of the seminiferous tubules.

Conclusion/Significance

Our data provide direct evidence of regionally defined patch-like GDNF-positive signal site in which GFRα1-positive spermatogonia possibly interact with GDNF in the basal compartment of the seminiferous tubules.

Lack of protein 4.1G causes altered expression and localization of the cell adhesion molecule nectin-like 4 in testis and can cause male infertility

Protein 4.1G is a member of the protein 4.1 family, which in general serves as adaptors linking transmembrane proteins to the cytoskeleton. 4.1G is thought to be widely expressed in many cells and tissues, but its function remains largely unknown. To explore the function of 4.1G in vivo, we generated 4.1G(-/-) mice and bred the mice in two backgrounds: C57BL/6 (B6) and 129/Sv (129) hybrids (B6-129) and inbred B6. Although the B6 4.1G(-/-) mice showed no obvious abnormalities, deficiency of 4.1G in B6-129 hybrids was associated with male infertility. Histological examinations of these 4.1G(-/-) mice revealed atrophy, impaired cell-cell contact and sloughing off of spermatogenic cells in seminiferous epithelium, and lack of mature spermatids in the epididymis. Ultrastructural examination revealed enlarged intercellular spaces between spermatogenic and Sertoli cells as well as the spermatid deformities. At the molecular level, 4.1G is associated with the nectin-like 4 (NECL4) adhesion molecule. Importantly, the expression of NECL4 was decreased, and the localization of NECL4 was altered in 4.1G(-/-) testis. Thus, our findings imply that 4.1G plays a role in spermatogenesis by mediating cell-cell adhesion between spermatogenic and Sertoli cells through its interaction with NECL4 on Sertoli cells. Additionally, the finding that infertility is present in B6-129 but not on the B6 background suggests the presence of a major modifier gene(s) that influences 4.1G function and is associated with male infertility.

Adenosine regulates Sertoli cell function by activating AMPK

This work evaluates adenosine effects on Sertoli cell functions, which are different to those resulting from occupancy of purinergic receptors. The effects of adenosine and N(6)-cyclohexyladenosine (CHA) - an A(1) receptor agonist resistant to cellular uptake - on Sertoli cell physiology were compared. Adenosine but not CHA increased lactate production, glucose uptake, GLUT1, LDHA and MCT4 mRNA levels, and stabilized ZO-1 protein at the cell membrane. These differential effects suggested a mechanism of action of adenosine that cannot be solely explained by occupancy of type A(1) purinergic receptors. Activation by adenosine but not by CHA of AMPK was observed. AMPK participation in lactate production and ZO-1 stabilization was confirmed by utilizing specific inhibitors. Altogether, these results suggest that activation of AMPK by adenosine promotes lactate offer to germ cells and cooperates in the maintenance of junctional complex integrity, thus contributing to the preservation of an optimum microenvironment for a successful spermatogenesis.

Prenatal testosterone excess alters Sertoli and germ cell number and testicular FSH receptor expression in rams

Exposure to excess testosterone (T) during fetal life has a profound impact on the metabolic and reproductive functions in the female's postnatal life. However, less is known about the effects of excess testosterone in males. The aim of the present study was to evaluate the impact (consequences) of an excess of T during fetal development on mature male testis. The testicular evaluation was by histological analysis and by determination of mRNA expression of the FSH receptor (FSH-R), transforming growth factor-β type I receptor (TβR-I), and two members of the TGF-β superfamily, transforming growth factor-β3 (TGFβ3) and anti-Müllerian hormone (AMH) in males born to mothers receiving an excess of T during pregnancy. At 42 wk of age, postpubertal males born to mothers treated with 30 mg of T propionate twice weekly from day 30 to 90, followed by 40 mg of T propionate from day 90 to 120 of pregnancy (T males), showed higher concentrations of FSH in response to a GnRH analog, a higher number of Sertoli cells/seminiferous tubule cross-section, and a lower number of germ cells/tubules (P < 0.05) than control males (C males) born to mothers treated with the vehicle. The mRNA expression of FSH-R and of TβR-I was higher in T males compared with C males (P < 0.05). Moreover, in T males, AMH expression level correlated negatively with the expression level of TGFβ3. In C males, this latter correlation was not observed. These results suggest that prenatal exposure to an excess of T can negatively modify some histological and molecular characteristics of the mature testis.

Characterization and functionality of proliferative human Sertoli cells

It has long been thought that mammalian Sertoli cells are terminally differentiated and nondividing postpuberty. For most previous in vitro studies immature rodent testes have been the source of Sertoli cells and these have shown little proliferative ability when cultured. We have isolated and characterized Sertoli cells from human cadaveric testes from seven donors ranging from 12 to 36 years of age. The cells proliferated readily in vitro under the optimized conditions used with a doubling time of approximately 4 days. Nuclear 5-ethynyl-2'-deoxyuridine (EdU) incorporation confirmed that dividing cells represented the majority of the population. Classical Sertoli cell ultrastructural features, lipid droplet accumulation, and immunoexpression of GATA-4, Sox9, and the FSH receptor (FSHr) were observed by electron and fluorescence microscopy, respectively. Flow cytometry revealed the expression of GATA-4 and Sox9 by more than 99% of the cells, and abundant expression of a number of markers indicative of multipotent mesenchymal cells. Low detection of endogenous alkaline phosphatase activity after passaging showed that few peritubular myoid cells were present. GATA-4 and SOX9 expression were confirmed by reverse transcription polymerase chain reaction (RT-PCR), along with expression of stem cell factor (SCF), glial cell line-derived neurotrophic factor (GDNF), and bone morphogenic protein 4 (BMP4). Tight junctions were formed by Sertoli cells plated on transwell inserts coated with fibronectin as revealed by increased transepithelial electrical resistance (TER) and polarized secretion of the immunoregulatory protein, galectin-1. These primary Sertoli cell populations could be expanded dramatically in vitro and could be cryopreserved. The results show that functional human Sertoli cells can be propagated in vitro from testicular cells isolated from adult testis. The proliferative human Sertoli cells should have important applications in studying infertility, reproductive toxicology, testicular cancer, and spermatogenesis, and due to their unique biological properties potentially could be useful in cell therapy.

Non-classical actions of testosterone and spermatogenesis

Testosterone is essential to maintain spermatogenesis and male fertility. In the absence of testosterone stimulation, spermatogenesis does not proceed beyond the meiosis stage. After withdrawal of testosterone, germ cells that have progressed beyond meiosis detach from supporting Sertoli cells and die, whereas mature sperm cannot be released from Sertoli cells resulting in infertility. The classical mechanism of testosterone action in which testosterone activates gene transcription by causing the androgen receptor to translocate to and bind specific DNA regulatory elements does not appear to fully explain testosterone regulation of spermatogenesis. This review discusses two non-classical testosterone signalling pathways in Sertoli cells and their potential effects on spermatogenesis. Specifically, testosterone-mediated activation of phospholipase C and calcium influx into Sertoli cells is described. Also, testosterone activation of Src, EGF receptor and ERK kinases as well as the activation of the CREB transcription factor and CREB-mediated transcription is reviewed. Regulation of germ cell adhesion to Sertoli cells and release of mature sperm from Sertoli cells by kinases regulated by the non-classical testosterone pathway is discussed. The evidence accumulated suggests that classical and non-classical testosterone signalling contribute to the maintenance of spermatogenesis and male fertility.

Organic anion transporter 6 (Slc22a20) specificity and Sertoli cell-specific expression provide new insight on potential endogenous roles

Organic anion transporter 6 (Oat6; Slc22a20), a member of the OAT family, was demonstrated previously to mediate the transport of organic anions (Am J Physiol Renal Physiol 291:F314-F321, 2006). In the present study, we sought to further delineate the function of murine Oat6 (mOat6) by analyzing the effect of select organic anions on mOat6-mediated transport by using a Chinese hamster ovary (CHO) cell line stably expressing mOat6 (CHO-mOat6). When examined, kinetic analysis demonstrated that the mechanism of inhibition of mOat6 and mOat3 was competitive. Homovanillic acid, 5-hydroxyindole acetic acid, 2,4-dihydroxyphenylacetate, hippurate, and dehydroepiandrosterone sulfate (DHEAS) each significantly reduced mOat6 activity with inhibitory constant (K(i)) values of 3.0 +/- 0.5, 48.9 +/- 10.3, 61.4 +/- 7.1, 59.9 +/- 4.9, and 38.8 +/- 3.1 microM, respectively. Comparison to K(i) values determined for mOat3 (67.8 +/- 7.2, 134.5 +/- 27.0, 346.7 +/- 97.9, 79.3 +/- 4.0, and 3.8 +/- 1.1 microM, respectively) revealed that there are significant differences in compound affinity between each transporter. Fluoroquinolone antimicrobials and reduced folates were without effect on mOat6-mediated uptake. Investigation of testicular cell type-specific expression of mOat6 by laser capture microdissection and quantitative polymerase chain reaction revealed significant mRNA expression in Sertoli cells, but not in Leydig cells or spermatids. Overall, these data should aid further refinements to the interpretation and modeling of the in vivo disposition of OAT substrates. Specifically, expression in Sertoli cells suggests Oat6 may be an important determinant of blood-testis barrier function, with Oat6-mediated transport of estrone sulfate and DHEAS possibly representing a critical step in the maintenance of testicular steroidogenesis.

EPAS1 Is Required for Spermatogenesis in the Postnatal Mouse Testis

Spermatogenesis, a process involving the differentiation of spermatogonial stem cells into mature spermatozoa, takes place throughout masculine life. A complex system in the testis, including endocrine signaling, physical interactions between germ and somatic cells, spermatocyte meiosis, and timely release of spermatozoa, controls this cycle. We demonstrate herein that decreased O(2) levels and Epas1 activation are critical components of spermatogenesis. Postnatal Epas1 ablation leads to male infertility, with reduced testis size and weight. While immature spermatogonia and spermatocytes are present in Epas1(Delta/Delta) testes, spermatid and spermatozoan numbers are dramatically reduced. This is not due to germ cell-intrinsic defects. Rather, Epas(Delta/Delta) Sertoli cells exhibit decreased ability to form tight junctions, thereby disrupting the blood-testis barrier necessary for proper spermatogenesis. Reduced numbers of tight junction complexes are due to decreased expression of multiple genes encoding tight junction proteins, including TJP1 (ZO1), TJP2 (ZO2), and occludin. Furthermore, Epas1(Delta/Delta) testes exhibit disrupted basement membranes surrounding the seminiferous tubules, causing the premature release of incompletely differentiated germ cells. We conclude that low O(2) levels in the male gonad regulate germ cell homeostasis in this organ via EPAS1.

Diabetic rat testes: morphological and functional alterations

Reproductive dysfunction is a consequence of diabetes, but the underlying mechanisms are poorly understood. This study investigated the histological and molecular alterations in the testes of rats injected with streptozotocin at prepuperal (SPI rats) and adult age (SAI rats) to understand whether diabetes affects testicular tissue with different severity depending on the age in which this pathological condition starts. The testes of diabetic animals showed frequent abnormal histology, and seminiferous epithelium cytoarchitecture appeared altered as well as the occludin distribution pattern. The early occurrence of diabetes increased the percentage of animals with high number of damaged tubules. The interstitial compartment of the testes was clearly hypertrophic in several portions of the organs both in SPI and SAI rats. Interestingly, fully developed Leydig cells were present in all the treated animals although abnormally distributed. Besides the above-described damages, we found a similar decrease in plasma testosterone levels both in SPI and SAI rats. Oxidative stress (OS) is involved in the pathogenesis of various diabetic complications, and in our experimental models we found that manganese superoxide dismutase was reduced in diabetic animals. We conclude that in STZ-induced diabetes, the altered spermatogenesis, more severe in SPI animals, is possibly due to the effect of OS on Leydig cell function which could cause the testosterone decrease responsible for the alterations found in the seminiferous epithelium of diabetic animals.

Immature rat seminiferous tubules reconstructed in vitro express markers of Sertoli cell maturation after xenografting into nude mouse hosts

Sertoli cells undergo a maturation process during post-natal testicular development that leads to the adult-type Sertoli cell, which is required for spermatogenesis. Understanding Sertoli cell maturation is therefore necessary to gain insight into the underlying causes of impaired spermatogenesis and male infertility. The present study characterized the cellular and molecular differentiation of Sertoli cells in a xenograft model of mammalian testicular development. Immature rat Sertoli cells were cultured in a three-dimensional culture system to allow the formation of cord-like structures. The in vitro Sertoli cell cultures were then grafted into nude mice. Sertoli cell proliferation, morphological differentiation and mRNA expression of Sertoli cell maturation markers were evaluated in xenografts. Sertoli cell proliferation significantly decreased between 1 and 4 weeks (6.7 +/- 0.9 versus 1.2+/- 0.1%, P < 0.001), and was maintained at low levels thereafter. Sertoli cell cord-like structures significantly decreased between 1 and 4 weeks (59.6 versus 21%, P < 0.05), whereas Sertoli cell tubules were more frequently observed after 4 weeks (13.3 versus 73.1%, P < 0.05). Furthermore, expression of androgen binding protein, transferrin and follicle stimulating hormone receptor, markers for mature Sertoli cells, was detected after 1 week of grafting and increased significantly thereafter. We conclude from these results that rat Sertoli cells continue maturation after xenografting to the physiological environment of a host. This model of in vitro tubule formation will be helpful in future investigations addressing testicular maturation in the mammalian testis.

Modulatory role of lipoic acid on lipopolysaccharide-induced oxidative stress in adult rat Sertoli cells in vitro

Inflammatory reactions to microbial infections may cause male infertility. The mechanisms of inhibition of spermatogenesis can be studied in vitro using rat Sertoli cells. Bacterial lipopolysaccharides (LPS) induce acute inflammations. So LPS treated Sertoli cells can be used to test for new therapeutic compounds. The present study aimed to investigate the protective efficacy of dl-alpha-lipoic acid (LA) on lipopolysaccharide (LPS)-induced oxidative stress in adult rat Sertoli cells. Sertoli cells were divided into 4 groups. Group I served as a control incubated with water (vehicle). Groups II and IV were incubated with 100 microM LA for 24h before incubating Groups III and IV with 50 microg/ml lipopolysaccharide (LPS) for 12h. In Group III cells (LPS-treated, no LA) the lactate concentration was decreased whereas hydrogen peroxide production and lipid peroxidation were significantly increased. Moreover, the activities of antioxidant enzymes such as superoxide dismutase, glutathione peroxidase, catalase, glutathione-S-transferase, glutathione reductase were reduced. The concentrations of antioxidant molecules such as reduced glutathione and vitamin C were significantly decreased. The activities of enzymes normally elevated in Sertoli cells, gamma-glutamyl transpeptidase and beta-glucuronidase, were significantly decreased. Treatment with LA (100 microM) for 24h before LPS-treatment (Group IV), prevented these changes in enzyme activities and metabolite concentrations. Therefore, LA may have a cyto-protective role during LPS-induced inflammation in adult rat Sertoli cells.

Zona occludens-2 is critical for blood-testis barrier integrity and male fertility

Tight junction integral membrane proteins such as claudins and occludin are tethered to the actin cytoskeleton by adaptor proteins, notably the closely related zonula occludens (ZO) proteins ZO-1, ZO-2, and ZO-3. All three ZO proteins have recently been inactivated in mice. Although ZO-3 knockout mice lack an obvious phenotype, animals deficient in ZO-1 or ZO-2 show early embryonic lethality. Here, we rescue the embryonic lethality of ZO-2 knockout mice by injecting ZO-2(-/-) embryonic stem (ES) cells into wild-type blastocysts to generate viable ZO-2 chimera. ZO-2(-/-) ES cells contribute extensively to different tissues of the chimera, consistent with an extraembryonic requirement for ZO-2 rather than a critical role in epiblast development. Adult chimera present a set of phenotypes in different organs. In particular, male ZO-2 chimeras show reduced fertility and pathological changes in the testis. Lanthanum tracer experiments show a compromised blood-testis barrier. Expression levels of ZO-1, ZO-3, claudin-11, and occludin are not apparently affected. ZO-1 and occludin still localize to the blood-testis barrier region, but claudin-11 is less well restricted and the localization of connexin-43 is perturbed. The critical role of ZO-2 for male fertility and blood-testis barrier integrity thus provides a first example for a nonredundant role of an individual ZO protein in adult mice.

Growth differentiation factor 9 is a germ cell regulator of Sertoli cell function

Oocyte-secreted growth differentiation factor (GDF) 9 and bone morphogenetic protein (BMP) 15 are critical regulatory factors in female reproduction. Together, they promote granulosa cell proliferation and stimulate the maturation of preovulatory follicles. Despite their importance in female fertility, GDF9 and BMP15 expression patterns and function during spermatogenesis have not been investigated. In this study we show that the expression and stage-specific localization of both factors are limited to the germ cells of the rat seminiferous epithelium, with GDF9 being principally localized in round spermatids and BMP15 in gonocytes and pachytene spermatocytes. To identify potential cellular targets for GDF9 actions, cells of the seminiferous tubule were isolated and screened for the expression of signaling receptors [activin-like kinase (ALK) 5, ALK6, and BMP receptor, type II)]. Individual receptor types were expressed throughout the seminiferous epithelium, but coexpression of ALK5 and BMP receptor, type II was limited to Sertoli cells and round spermatids. Based on the reproductive actions of related TGFbeta ligands in the ovary and testis, GDF9 was assessed for its ability to regulate tight junction function and inhibin B production in rat Sertoli cell cultures. When recombinant mouse GDF9 was added to immature Sertoli cell cultures, it inhibited membrane localization of the junctional proteins claudin-11, occludin, and zonula occludens-1, thereby disrupting tight junction integrity. Concomitantly, GDF9 up-regulated inhibin subunit expression and significantly stimulated dimeric inhibin B protein production. Together, these results demonstrate that GDF9 and BMP15 are germ cell-specific factors in the rat testis, and that GDF9 can modulate key Sertoli cell functions.

Characterization of nucleobase transport by mouse Sertoli cell line TM4

In the spermatogenesis, many nucleosides and nucleobases are needed for the salvage nucleotide biosynthesis. One of the roles of Sertoli cells is to provide such nutrients to spermatogenic cells located within the blood-testis barrier (BTB). We have already shown that nucleoside transporters are expressed and are functional in primary-cultured rat Sertoli cells and TM4 cells derived from mouse testis. Here, we examined the uptakes of purine ([3H]guanine) and pyrimidine ([3H]uracil) nucleobases using TM4 cells. Uptakes of both nucleobases were time- and concentration-dependent, and kinetic analysis indicated the involvement of high-affinity transport systems. Uptake of uracil was significantly reduced in the absence of Na+, although guanine uptake was mainly mediated by a sodium-independent transport system in TM4 cells. Guanine uptake was inhibited by other purine nucleobases, but not by pyrimidine nucleobases. Only pyrimidine nucleobases reduced uracil uptake. In addition, mycophenolic acid, an inosine monophosphate dehydrogenase inhibitor, up-regulated guanine uptake. These results suggested that there are distinct transport systems for purine and pyrimidine nucleobases in cells of mouse Sertoli cell line TM4.

Mitogen-activated protein kinases in male reproductive function

Recent studies have shown that male reproductive function is modulated via the mitogen-activated protein kinase (MAPK) cascade. The MAPK cascade is involved in numerous male reproductive processes, including spermatogenesis, sperm maturation and activation, capacitation and acrosome reaction, before fertilization of the oocyte. In this review, we discuss the latest findings in this rapidly developing field regarding the role of MAPK in male reproduction in animal models and in human spermatozoa in vitro. This research will facilitate the design of future studies in humans, although much work is needed before this information can be used to manage male infertility and environmental toxicant-induced testicular injury in men, such as blood-testis-barrier disruption.

Immune response to bacteria in seminiferous epithelium

The luminal part of the seminiferous epithelium, a tissue compartment protected by the blood-testis barrier, has been considered a site of immune privilege. However, there are reports describing the production of anti-microbial peptides and the expression of Toll-like receptors in cells present in the seminiferous epithelium, evoking the possibility that this tissue compartment is immunologically active at least with regard to the innate immune response. To test this, we injected Escherichia coli into seminiferous tubules of live mice and examined the fate of bacteria, the production of chemokines and inflammatory cytokines, and the infiltration of neutrophils. The bacteria actively propagated and reached a maximal level in a day, but started to decrease after 5 days and completely disappeared in 2 months. The expression of macrophage inflammatory protein-2 and tumor necrosis factor-alpha became evident in macrophages present in the interstitial compartment of testes as early as 1-3 h after the inoculation of bacteria. Neutrophils first accumulated in the interstitial space at 9-12 h and entered the tubules after a day. On the other hand, impairment of spermatogenesis was observed a day after bacteria injection and seemed unrecoverable even after the bacteria were eliminated. By contrast, bacteria injected into the interstitial compartment were more rapidly cleared with no damage in the seminiferous epithelium. These results suggest the existence of immunity against invading microbes in the seminiferous epithelium although its effectiveness in maintaining tissue homeostasis remains equivocal.

Regulation of testicular tight junctions by gonadotrophins in the adult Djungarian hamster in vivo

This study aimed to assess the effect of gonadotrophin suppression and FSH replacement on testicular tight junction dynamics and blood-testis barrier (BTB) organisation in vivo, utilising the seasonal breeding Djungarian hamster. Confocal immunohistology was used to assess the cellular organisation of tight junction proteins and real-time PCR to quantify tight junction mRNA. The effect of tight junction protein organisation on the BTB permeability was also investigated using a biotin-linked tracer. Tight junction protein (claudin-3, junctional adhesion molecule (JAM)-A and occludin) localisation was present but disorganised after gonadotrophin suppression, while mRNA levels (claudin-11, claudin-3 and occludin) were significantly (two- to threefold) increased. By contrast, both protein localisation and mRNA levels for the adaptor protein zona occludens-1 decreased after gonadotrophin suppression. FSH replacement induced a rapid reorganisation of tight junction protein localisation. The functionality of the BTB (as inferred by biotin tracer permeation) was found to be strongly associated with the organisation and localisation of claudin-11. Surprisingly, JAM-A was also recognised on spermatogonia, suggesting an additional novel role for this protein in trans-epithelial migration of germ cells across the BTB. It is concluded that gonadotrophin regulation of tight junction proteins forming the BTB occurs primarily at the level of protein organisation and not gene transcription in this species, and that immunolocalisation of the organised tight junction protein claudin-11 correlates with BTB functionality.

Essential roles of COUP-TFII in Leydig cell differentiation and male fertility

Chicken Ovalbumin Upstream Promoter-Transcription Factor II (COUP-TFII; also known as NR2F2), is an orphan nuclear receptor of the steroid/thyroid hormone receptor superfamily. COUP-TFII-null mice die during the early embryonic development due to angiogenesis and cardiovascular defects. To circumvent the early embryonic lethality and investigate the physiological function of COUP-TFII, we knocked out COUP-TFII gene in a time-specific manner by using a tamoxifen inducible Cre recombinase. The ablation of COUP-TFII during pre-pubertal stages of male development results in infertility, hypogonadism and spermatogenetic arrest. Homozygous adult male mutants are defective in testosterone synthesis, and administration of testosterone could largely rescue the mutant defects. Notably, the rescued results also provide the evidence that the major function of adult Leydig cell is to synthesize testosterone. Further phenotypic analysis reveals that Leydig cell differentiation is arrested at the progenitor cell stage in the testes of null mice. The failure of testosterone to resumption of Leydig cell maturation in the null mice indicates that COUP-TFII itself is essential for this process. In addition, we identify that COUP-TFII plays roles in progenitor Leydig cell formation and early testis organogenesis, as demonstrated by the ablation of COUP-TFII at E18.5. On the other hand, when COUP-TFII is deleted in the adult stage after Leydig cells are well differentiated, there are no obvious defects in reproduction and Leydig cell function. Taken together, these results indicate that COUP-TFII plays a major role in differentiation, but not the maintenance of Leydig cells.

Expression of occludin in testis and epididymis of wild rabbits, Lepus sinensis coreanus

Tight junctions (TJs) in inter-Sertoli junctional areas and epididymal epithelia build up the blood-testis barrier (BTB) and the blood-epididymal barrier (BEB), respectively. In this study, the expression of occludin, an integral member of the TJs, was examined in testis and different regions of epididymis of Lepus sinensis coreanus, an Korean wild rabbit species. In testis, intense occludin immunoreactivity was found in the basally located inter-Sertoli junctional area together with diffused immunoreactivity of occludin in the cytoplasm of Sertoli cells. It can be suggested that occludin is one of the robust elements of BTB in seminiferous tubules of rabbit testis. In proximal and distal caput epididymis, occludin immunoreactivity was found in the lateral as well as apical contacts of epithelial cells. In corpus epididymis, intense occludin immunoreactivity was found in the basolateral as well as apical contacts of epithelial cells together with cytoplasmic signal. In cauda epididymis, occludin immunoreactivity in luminal epithelia was relatively strong but largely found in the cytoplasm. This suggests that intriguing regulatory mechanisms differentially recruit occludin to the TJ in the different regions of epididymal epithelia. The differences in the subcellular localization as well as expression levels of occludin among the epididymal segments may reflect differential paracellular permeability of epithelia along the epididymal tubules and be correlated with sperm maturation in rabbit. In Western blot, a major form of occludin was MW 62 kDa together with small fragments of MW 34-39 kDa in testis and epididymis, suggesting the peptide cleavage of occludin. This is the first report on the molecular nature of TJs in a wild rabbit testis and epididymis.

Continual maintenance of the blood-testis barrier during spermatogenesis: the intermediate compartment theory revisited

Tight junctions occur between the lateral processes of neighboring Sertoli cells that divide the seminiferous epithelium into two compartments: basal and adluminal compartments. These tight junctions constitute the blood-testis barrier (BTB). The established theory that the BTB must open when spermatocytes translocate from the basal compartment to the adluminal compartment is marked by one contradiction, that is, normal spermatogenesis occurs in the testis because the BTB is expected to constantly seclude the adluminal compartment from the basal compartment in order to protect haploid germ cells from the autoimmune system. Subsequently, another concept was proposed in which two BTBs divide the seminiferous epithelium into three compartments: basal, intermediate and adluminal compartments. It has been suggested that the transition from the basal region to the adluminal region without the BTB open occurs through the agency of a short-lived intermediate compartment embodying some primary spermatocytes. In contrast, the results of recent findings in the molecular architecture of the BTB suggest that the BTB in the seminiferous epithelium must "open". In this paper, I re-examine the BTBs of boar and experimental cryptorchid mouse testes by transmission electron microscope (TEM). TEM analysis showed that an atypical basal compartment existed in the thin seminiferous epithelium of 14-day post-cryptorchid mice testes. In developmental boar testes, ectoplasmic specialization (ES) of the seminiferous epithelium showed dynamic behavior. The intermediate compartment was clearly observed between the basal and adluminal compartments of the mature boar seminiferous epithelium. ESs were observed between Sertoli cells and spermatids at all developmental stages, including early, late and mature. Furthermore, ESs were situated on the apical surface of the seminiferous epithelium. From these results, I propose that the BTB is continually maintained during spermatogenesis and suggest a model of ES circulation in the seminiferous epithelium.

On the necessity of different statistical treatment for Illumina BeadChip and Affymetrix GeneChip data and its significance for biological interpretation

Background

The original spotted array technology with competitive hybridization of two experimental samples and measuring relative expression levels is increasingly displaced by more accurate platforms that allow determining absolute expression values for a single sample (for example, Affymetrix GeneChip and Illumina BeadChip). Unfortunately, cross-platform comparisons show a disappointingly low concordance between lists of regulated genes between the latter two platforms.

Results

Whereas expression values determined with a single Affymetrix GeneChip represent single measurements, the expression results obtained with Illumina BeadChip are essentially statistical means from several dozens of identical probes. In the case of multiple technical replicates, the data require, therefore, different stistical treatment depending on the platform. The key is the computation of the squared standard deviation within replicates in the case of the Illumina data as weighted mean of the square of the standard deviations of the individual experiments. With an Illumina spike experiment, we demonstrate dramatically improved significance of spiked genes over all relevant concentration ranges. The re-evaluation of two published Illumina datasets (membrane type-1 matrix metalloproteinase expression in mammary epithelial cells by Golubkov et al. Cancer Research (2006) 66, 10460; spermatogenesis in normal and teratozoospermic men, Platts et al. Human Molecular Genetics (2007) 16, 763) significantly identified more biologically relevant genes as transcriptionally regulated targets and, thus, additional biological pathways involved.

Conclusion

The results in this work show that it is important to process Illumina BeadChip data in a modified statistical procedure and to compute the standard deviation in experiments with technical replicates from the standard errors of individual BeadChips. This change leads also to an improved concordance with Affymetrix GeneChip results as the spermatogenesis dataset re-evaluation demonstrates.

Reviewers

This article was reviewed by I. King Jordan, Mark J. Dunning and Shamil Sunyaev.

Effect of electromagnetic pulse exposure on permeability of blood-testicle barrier in mice

OBJECTIVE

To study the effect of electromagnetic pulse (EMP) exposure on the permeability of blood-testicle barrier (BTB) in mice.

METHODS

Adult male BALB/c mice were exposed to EMP at 200 kV/m for 200 pulses with 2 seconds interval. The mice were injected with 2% Evans Blue solution through caudal vein at different time points after exposure, and the permeability of BTB was monitored using a fluorescence microscope. The testis sample for the transmission electron microscopy was prepared at 2 h after EMP exposure. The permeability of BTB in mice was observed by using Evans Blue tracer and lanthanum nitrate tracer.

RESULTS

After exposure, cloudy Evans Blue was found in the testicle convoluted seminiferous tubule of mice. Lanthanum nitrate was observed not only between testicle spermatogonia near seminiferous tubule wall and sertoli cells, but also between sertoli cells and primary spermatocyte or secondary spermatocyte. In contrast, lanthanum nitrate in control group was only found in the testicle sertoli cells between seminiferous tubule and near seminiferous tubule wall.

CONCLUSION

EMP exposure could increase the permeability of BTB in the mice.

Junction restructuring and spermatogenesis: the biology, regulation, and implication in male contraceptive development

Spermatogenesis that occurs in the seminiferous epithelium of adult mammalian testes is associated with extensive junction restructuring at the Sertoli-Sertoli cell, Sertoli-germ cell, and Sertoli-basement membrane interface. While this morphological phenomenon is known and has been described in great details for decades, the biochemical and molecular changes as well as the mechanisms/signaling pathways that define changes at the cell-cell and cell-matrix interface remain largely unknown until recently. In this chapter, we summarize and discuss findings in the field regarding the coordinated efforts of the anchoring [e.g., adherens junction (AJ), such as basal ectoplasmic specialization (basal ES)] and tight junctions (TJs) that are present in the same microenvironment, such as at the blood-testis barrier (BTB), or at distinctly opposite ends of the Sertoli cell epithelium, such as between apical ectoplasmic specialization (apical ES) in the apical compartment, and the BTB adjacent to the basal compartment of the epithelium. These efforts, in turn, regulate and coordinate different cellular events that occur during the seminiferous epithelial cycle. For instance, the events of spermiation and of preleptotene spermatocyte migration across the BTB both take place concurrently at stage VIII of the epithelial cycle of spermatogenesis. Recent findings suggest that these events are coordinated by protein complexes found at the apical and basal ES and TJ, which are located at different ends of the Sertoli cell epithelium. Besides, we highlight important areas of research that can now be undertaken, and functional studies that can be designed to tackle different issues pertinent to junction restructuring during spermatogenesis.