Spermatogenesis in testis xenografts grafted from pre-pubertal Holstein bulls is re-established by stem cell or early spermatogonia

Xenografting of testis explants into recipient mice has resulted in successful restoration of spermatogenesis in several species. Most studies have utilized neonatal donor tissue, although a few have used prepubertal testes. In Holstein bulls, prepubertal development of the testis occurs between 16 and 32 weeks of age. The purpose of the present study was to determine the optimal age during prepubertal development of Holstein bulls for testis grafting. Explants of testis tissue from Holstein bulls between 12 and 32 weeks of age (2 bulls/age; 6 ages) were subcutaneously grafted into castrated or intact immunocompromised mice (n=8/age), then recovered after 75 and 173 days (n=4 mice/grafting period) and evaluated histologically for spermatogenic progression. Seminiferous tubules were assigned a score based on the most advanced type of germ cell present within the tubule and the average for all tubules scored (n=25) within an explant was calculated. Scores for all explants per mouse (n=6) were averaged to give a single spermatogenic progression score per mouse. No difference in spermatogenic progression of grafts between intact and castrated recipients was observed. Spermatocytes were observed in testis grafts from bulls of all ages 75 days post-grafting. At 173 days, the spermatogenic progression score for explants derived from 20 weeks bulls was greater than all ages except 12 weeks donors (p<0.05), with 8% of tubules containing spermatids. Donor material from bulls older than 20 weeks had lesser spermatogenic progression scores largely attributed to the greater number of atrophic tubules in grafts from older donors. Grafts from 28 and 32 weeks donors showed signs of degeneration by 75 days post-grafting, with 30 and 55% atrophic tubules, respectively, and lesser spermatogenic efficiency scores. By 173 days post-grafting, 72% of tubules in explants from 32 weeks donors were atrophic. The results of the present study suggest that the early stages of prepubertal development are optimal for testis grafting while advanced spermatogenesis in the donor tissue prior to grafting had a negative effect on graft development. Spermatogenesis within the grafts apparently needs to be re-established by spermatogonial stem cells or early spermatogonia.     

Spermatogenesis in varicoceles after correction of the blood supply to the testis

By means of light microscopy methods in experiments performed in 60 white rats with modelled venous congestion in the left testis and in 113 men suffering from varicocele of the 2d-3d degree complicated with certain disorders of fertility, the effect of blood correction has been studied in the gonads by switching off the caudal (inferior) epigastric vein. The experimental correction of the blood stream in the testes, according to the data of quantitative estimations, contributes to spermatogenesis. Corresponding positive results, while studying spermograms, are obtained in patients suffering from varicocele complicated with infertility. Application of this operation is expedient when conservative therapy as varicocele is uneffective.     

The isolated, buffer-perfused ferret heart: a new model for the study of cardiac physiology and metabolism.

The isolated, buffer-perfused ferret heart is a new model for the study of cardiac physiology and metabolism. Compared to the more commonly used isolated heart preparation, the rat heart, the ferret has a lower rate-pressure product due to lower heart rate, a remarkably low coronary flow and almost complete oxygen extraction. The ferret heart remains in stable haemodynamic and metabolic conditions for a longer period of time than the rat heart. ATP contents of the two species are similar, but creatine phosphate content is higher in the ferret while NAD content is much lower.     

Evaluation of a black-footed ferret resource utilization function model

Resource utilization function (RUF) models permit evaluation of potential habitat for endangered species; ideally such models should be evaluated before use in management decision-making. We evaluated the predictive capabilities of a previously developed black-footed ferret (Mustela nigripes) RUF. Using the population-level RUF, generated from ferret observations at an adjacent yet distinct colony, we predicted the distribution of ferrets within a black-tailed prairie dog (Cynomys ludovicianus) colony in the Conata Basin, South Dakota, USA. We evaluated model performance, using data collected during post-breeding spotlight surveys (2007–2008) by assessing model agreement via weighted compositional analysis and count-metrics. Compositional analysis of home range use and colony-level availability, and core area use and home range availability, demonstrated ferret selection of the predicted Very high and High occurrence categories in 2007 and 2008. Simple count-metrics corroborated these findings and suggested selection of the Very high category in 2007 and the Very high and High categories in 2008. Collectively, these results suggested that the RUF was useful in predicting occurrence and intensity of space use of ferrets at our study site, the 2 objectives of the RUF. Application of this validated RUF would increase the resolution of habitat evaluations, permitting prediction of the distribution of ferrets within distinct colonies. Additional model evaluation at other sites, on other black-tailed prairie dog colonies of varying resource configuration and size, would increase understanding of influences upon model performance and the general utility of the RUF. © 2011 The Wildlife Society.     

The ferret as a model for inner ear research

Viral infections have long been suspected to be causative agents in a number of inner ear dysfunctions. With few exceptions, the virus has not been demonstrated as the direct agent leading to hearing loss and/or vertigo. Selective inner ear changes have been observed recently in sensory and nonsensory epithelial cells in the ferret model for Reye's syndrome after intranasal inoculation with influenza B combined with aspirin administration and the creation of an arginine deficiency. Such findings suggest that these agents act synergistically on the inner ear, particularly on cells that are metabolically active, and that the ferret may now be a useful model to examine the role of certain upper respiratory tract viruses implicated in inner ear disorders, singly and in combination with other agents that may cause metabolic alterations.     

Spermatogenesis and testis development are normal in mice lacking testicular orphan nuclear receptor 2

Early in vitro cell culture studies suggested that testicular orphan nuclear receptor 2 (TR2), a member of the nuclear receptor superfamily, may play important roles in the control of several pathways including retinoic acids, vitamin D, thyroid hormones, and ciliary neurotrophic factor. Here we report the surprising results showing that mice lacking TR2 are viable and have no serious developmental defects. Male mice lacking TR2 have functional testes, including normal sperm number and motility, and both male and female mice lacking TR2 are fertile. In heterozygous TR2(+/-) male mice we found that beta-galactosidase, the indicator of TR2 protein expression, was first detected at the age of 3 weeks and its expression pattern was restricted mainly in the spermatocytes and round spermatids. These protein expression patterns were further confirmed with Northern blot analysis of TR2 mRNA expression. Together, results from TR2-knockout mice suggest that TR2 may not play essential roles in spermatogenesis and normal testis development, function, and maintenance. Alternatively, the roles of TR2 may be redundant and could be played by other close members of the nuclear receptor superfamily such as testicular orphan receptor 4 (TR4) or unidentified orphan receptors that share many similar functions with TR2. Further studies with double knockouts of both orphan nuclear receptors, TR2 and TR4, may reveal their real physiological roles.     

Effect of vascular endothelial growth factor and testis tissue culture on spermatogenesis in bovine ectopic testis tissue xenografts

Bovine ectopic testis tissue grafting is a technique that can be used to study bovine spermatogenesis and for the production of germ cells for a variety of applications. Approximately 10% of seminiferous tubule cross sections in testis grafts contain spermatids, providing a unique tool to investigate what regulates germ cell differentiation. We hypothesized that manipulation of testis tissue grafts would increase the percentage of seminiferous tubule cross sections undergoing complete germ cell differentiation. To test this hypothesis, bovine testis tissue was treated with vascular endothelial growth factor (VEGF) at the time of grafting or explant cultured for 1 wk prior to grafting. For the VEGF experiment, 8-wk donor tissue and graft sites were treated with 1 microg of VEGF in order to increase angiogenesis at the graft site. For the testis tissue culture experiment, 4-wk-old donor testis was cultured for 1 wk prior to grafting to stimulate spermatogonial stem cell proliferation. Testis tissue grafts were removed from the mice 24 wk after grafting. VEGF treatment increased graft weight and the percentage of seminiferous tubule cross sections with elongating spermatids at the time of graft removal. Cultured testis tissue grafts were smaller and had fewer seminiferous tubules per graft. However, there was no difference in the percentage of seminiferous tubule cross sections that contained any germ cell type between groups. These data indicate for the first time that bovine testis tissue can be manipulated to better support germ cell differentiation in grafted tissue.     

Spermatogenesis from epiblast and primordial germ cells following transplantation into postnatal mouse testis

Primordial germ cells (PGCs) are derived from a population of pluripotent epiblast cells in mice. However, little is known about when and how PGCs acquire the capacity to differentiate into functional germ cells, while keeping the potential to derive pluripotent embryonic germ cells and teratocarcinomas. In this investigation, we show that epiblast cells and PGCs can establish colonies of spermatogenesis after transfer into postnatal seminiferous tubules of surrogate infertile mice. Furthermore, we obtained normal fertile offspring by microinsemination using spermatozoa or spermatids derived from PGCs harvested from fetuses as early as 8.5 days post coitum. Thus, fetal male germ cell development is remarkably flexible, and the maturation process, from epiblast cells through PGCs to postnatal spermatogonia, can occur in the postnatal testicular environment. Primordial germ cell transplantation techniques will also provide a novel tool to assess the developmental potential of PGCs, such as those manipulated in vitro or recovered from embryos harboring lethal mutations.     

The ferret: an animal model to study influenza virus

There has been much critical influenza research conducted in a little-known laboratory animal--the ferret. The authors review some of these findings, discuss the reasons the ferret often becomes a model for influenza infection, and compare the ferret with other animal models.     

Spermatogenesis in some Antarctic teleosts from the Ross Sea: histological organisation of the testis and localisation of bFGF

Testis structure and spermatogenetic activity were studied in two Antarctic teleostean species, Chionodraco hamatus and Trematomus bernacchii, captured during the austral summer in the Ross Sea. The specimens of C. hamatus showed full reproductive activity but, the spermatogenetic cycle being over, only spermatogonia and Sertoli cells were present in the seminiferous tubules whereas the lumina were full of sperm. By contrast, the specimens of T. bernacchii were in the stage of spermatogenetical recrudescence, having not yet entered the reproductive period. In this species, the seminiferous tubules were devoid of lumen and full of spermatogonial cysts, showing some mitoses. Many tubules contained cysts of meiotic spermatocytes I and, in one case only, small cysts of spermatocytes II. The final stages of spermatogenesis were lacking, presumably occurring later, in autumn/winter. The immunocytochemical tests aimed at identifying bFGF and FGFR1 revealed a positive reaction both in Sertoli cells and spermatogonia in the C. hamatus specimens, indicating that this species was ready to start a new spermatogenetic cycle. The weak reaction in the specimens of T. bernacchii suggests that, in this species, the stage of cell division was over and that of meiosis and differentiation was starting. These data indicate that Antarctic fish have an opportunistic spermatogenetic cycle. Accepted: 24 October 1999     

Spermatogenesis in Atlantic cod (Gadus morhua): a novel model of cystic germ cell development

Precocious male puberty significantly compromises sustainability aspects of aquaculture in a number of finfish species. As part of a program aiming to understand and eventually control testis maturation in farmed Atlantic cod, we studied the first reproductive cycle. The gonadosomatic index shows a 41-fold increase from immature (August) to mature (March) stages, reaching almost 10% of the total body weight. The paired cod testes are composed of several lobes arranged around a central collecting duct. In each individual lobe, spermatogenesis occurs in a marked gradient of development, with undifferentiated spermatogonia in the periphery of the lobe and the most advanced germ cells in the vicinity of the collecting duct, suggesting a tight spatiotemporal organization of spermatogenesis in the testis lobes of this species. Spermatogonial proliferation starts in August and continues for about 6 mo. Meiosis and spermiogenesis are first observed in October and are completed in all cysts by February, when a 2-mo-long spawning season starts. Spermatogonia go through 11 mitotic divisions before differentiating to primary spermatocytes. Apoptosis is rare, but when observed it occurs mainly during the last spermatogonial generations. Our observations suggest a model in which a maturational wave progresses through each growing lobe that is first driven by appositional growth from the lobe's periphery, reflecting spermatogonial proliferation and cyst formation which, when ceasing, is terminated by completing spermiogenesis and spermiation that progress toward the lobe's periphery.     

Spermatogenesis in a nematode Nippostrongylus brasiliensis

The structure and development of the spermatozoon of Nippostrongylus brasiliensis was studied with the electron microscope using thinsectioned material and tissue prepared by the freeze-fracture technique.The developing germ cells are connected via a complex anucleate rachis which begins as fine threads of cytoplasm joining the spermatogonia. It rapidly enlarges to a broad, central core which not only anchors and joins the spermatocytes, but also appears to be an important site for protein synthesis. Formation of membranous organelies (MOs) from RER-associated Golgi bodies dominates the activities of the growing spermatocytes. As the latter approach meiosis, the rachis declines in importance and finally becomes the site of breakdown of the residual cytoplasm. The spermatid chromatin condenses into a long cylinder during spermatogensis. A pair of centrioles in an indentation at the anterior end are believed to organize long microtubules which are responsible for moving the nucleus through then out of the sperm cytoplasm to form a tail-like structure. Thus the cytoplasmic region of mature sperm contains only mitochondria and MOs; a small part of the anterior is amoeboid.     

discs large in the Drosophila testis: An old player on a new task

Gamete development requires a coordinated soma-germ line interaction that ensures renewal and differentiation of germline and somatic stem cells. The physical contact between the germline and somatic cell populations is crucial because it allows the exchange of diffusible signals among them. The tumor suppressor gene discs large (dlg) encodes a septate junction protein with functions in epithelial cell polarity, asymmetric neuroblast division and formation of neuromuscular junctions. Our recent work reveals a new role of dlg in the Drosophila testis, as mutations in dlg lead to testis defects and cell death. Dlg is required throughout spermatogenesis in the somatic lineage and its localization changes from a uniform distribution along the plasma membrane of somatic cells in the testis apex, to a restricted localization on the distally located somatic cell in growing cysts. The extensive defects in dlg testis underline the importance of the somatic cells in the establishment and maintenance of the male stem cell niche and somatic cell differentiation. Here, we discuss our latest findings on the role of dlg in the Drosophila testis, supporting the view that junction proteins are dynamic structures, which can provide guiding cues to recruit scaffold proteins or other signaling molecules.Key words: dlg, Drosophila melanogaster, germ cell differentiation, septate junctions, somatic stem cells, somatic cyst cellsThe discovery of mutations causing neoplasia during Drosophila development¹ followed by the molecular characterization of these genes has shown that cell polarity is critically affected in the tumor cells. Three of these genes, lethal (2) giant larvae (lgl), discs large-1 (dlg) and scribble (scrib), encode scaffolding proteins, associated with either the cytoskeleton matrix or septate junctions.^2–9 Analysis over the last decades revealed that these proteins act as more than just static barriers limiting the diffusion of other components along the cortical cell domains. In particular, they function as dynamic organizing centers, targeting site-specific proteins in discrete domains and provide guiding cues for signaling molecules and insertion of membrane components.¹⁰ Nowadays, several studies place Dlg as a key player in numerous tissues at different time points throughout development, contributing to epithelial polarity establishment, polarized membrane insertion, asymmetric neuroblast division, formation of neuromuscular junctions (NMJ) and planar cell polarity in vertebrates.^{4,7,9,11–15} Interestingly, the four mammalian homologs of the Drosophila dlg are also involved in cell polarity and become downregulated in a series of human cancers. Moreover, a mammalian dlg-1 transgene can substitute a defective dlg gene in Drosophila and rescue the development of dlg mutant animals.^8,9,16 Therefore, it is perfectly plausible to envisage that Drosophila functions uncovered in other tissues may be similarly conserved in other species.Similar to vertebrates, the Drosophila testis consists of germ cells and somatic cells. The somatic cells of the hub form the organizing center at the apex of the testis and recruit germline stem cells (GSCs) (Fig. 1A), giving rise to the male stem cell niche. Each GSC attached to the hub is surrounded by two somatic stem cells (SSCs). Upon asymmetric stem cell division, each GSC produces a new GSC attached to the hub and a distally located gonialblast, whereas each SSC pair divides to generate two SSCs and two somatic cyst cells (SCCs).^17–19 The gonialblast divides mitotically four more times to give rise to 16 interconnected spermatogonial cells, forming a cyst surrounded by the two SCCs.²¹ Then, the spermatogonial cyst grows markedly in size and differentiates to primary spermatocytes that enter the pre-meiotic phase (Fig. 1A, B and D–F).¹⁹ We have recently investigated a new role of dlg in the Drosophila testis.²⁰ In contrast to the overgrowth phenotypes observed in imaginal discs and brain hemispheres,^4,6,21 dlg inactivation leads to testis degeneration during early larval development. The dlg testes are extremely small and contain a reduced number of GSCs loosely attached to the hub (Fig. 1C).²⁰ In addition, the few spermatogonial cysts, which become formed, fully degenerate during the second and third larval instars.Open in a separate windowFigure 1(A) Diagram depicting early spermatogenesis in Drosophila. The red line indicates the Dlg distribution in the hub, SSCs, early and late SCCs. GSCs, germline stem cells; SCCs, somatic cyst cells; SSCs, somatic stem cells. (B) Apex of wild-type 3^rd instar larval testis and (C) dlg^m52 3^rd instar larval testis displaying a reduced number of GSCs, spermatogonial and spermatocyte cysts. In mutant dlg^m52 testes, SSCs and early SCCS positively stained for Traffic-jam are still present. However, late SCCs identified by staining for Eye absent remain undetectable.²⁰ Vasa (red), Traffic-jam (green) and Arm + α-Spectrin (blue). (D–F) Pattern of Dlg distribution in 3^rd instar larval testis. (E and F) are enlargements at different optical sections of the testis shown in (D), displaying Dlg staining in the hub region and growing spermatocyte cysts, respectively. Testis hub is oriented towards the left.Recent advances in Drosophila spermatogenesis and the male stem cel niche have clearly shown that the intrinsic signals of the germ cells are important but not sufficient to support stem cell homeostasis. Signals emanating from SSCs and SCCs are also required for testis development. Physical contacts among the cell populations in the Drosophila testis allow the exchange of signals, which promote tissue survival and set the balance between stem cell identity and differentiation.¹⁸ Interestingly, the Dlg protein is present in all somatic cells including the hub, SSCs and SCCs (Fig. 1D–F) and a specific requirement of dlg in these cells is further supported by the finding that the mutant phenotype could be reverted by expressing dlg in somatic cells but not in germ cells.²⁰ Further analysis points out that the mutant GSCs are significantly larger than in wild-type, lower in number and loosely attached to the hub.²⁰ Preliminary results indicate a defective orientation of the daughter centrosome and absence of mitotic spindle in dividing GSCs, which together with the increased GSC size, allows us to speculate that GSCs may grow but fail to undergo mitosis. Similar phenotypes are observed in mutations affecting the insulin pathway,²² further stressing the importance of cell communication between germ cells and somatic cells. However, a functional connection between dlg and the insulin pathway remains yet to be experimentally determined. The defects detected in the dlg mutant testis place dlg as a key regulator in the early development of spermatogonial cysts.During testis differentiation, the Dlg protein displays a dynamic change in its intracellular localization. First, Dlg is uniformly associated with the plasma membrane on all somatic cells in the male stem cell niche and early spermatogonial cysts, and then becomes restricted to the most proximal SCC in late spermatogonial cysts and growing spermatocyte cysts (Fig. 1D–F). The transition from a uniform to a restricted distribution is achieved between the 8- to 16-cell cyst stages, when one of the two SCCs caps the distal side of the growing cyst. Interestingly, the capping corresponds to the axis of cyst growth and points out the direction of cyst expansion. A restoration of nearly normal testis morphology can be obtained by expressing a dlg transgene in SSCs and early SCCs. In contrast, expression of a dlg transgene in later SCCs can still restore the development of already formed cysts, some of which may reach an advanced post-meiotic stage, but the testis is generally depleted in early cysts.²⁰ These data indicate that dlg is required for the differentiation of the somatic cell lineage and, therefore, the early differentiation of the germline into spermatogonial cells. Results of RNAi experiments provide also evidence that dlg silencing in late SSCs results in a fragmentation of the cysts in advanced stages.²⁰ The specific recruitment of Dlg on the membrane of distal SCCs remains an open question, although it is possible to envisage that phosphorylation of Dlg by the PAR-1 kinase may play a role, as it has been shown in the case of postsynaptic targeting of Dlg in NMJs.²³Therefore, Dlg may exert different functions in the somatic cells that are required for (1) GSC attachment to the hub and proper asymmetric GSC division, (2) the architecture and early differentiation of the spermatogonial cysts and (3) the expansion and growth of the spermatocyte cysts. Presumably, dlg is required for establishing and maintaining a tight connection between GSCs and SSCs around the hub. The connection between gonialblast and SCC is also maintained during the mitotic divisions. In SSCs and early SCCs, dlg acts critically to establish a normal cyst structure, whereas in further spermatogonial and spermatocyte stages dlg is critical for the survival, growth and expansion of the cyst. Our rescue experiments further suggest that if proper cyst architecture is not established when the two stem cell populations move away from the hub, it cannot be re-established at later stages. Moreover, the restricted Dlg localization in the distal SCC suggests that dlg may be necessary for the polarized growth of spermatocyte cysts and thus act as a critical factor for planar cell polarity. In the second phase, dlg is involved in spermatogonial and spermatocyte cyst growth, viability and differentiation. Further RNA silencing experiments using GAL4-drivers that target dlg in SCCs during late spermatocyte growth, meiosis and post-meiotic stages may further provide insights into dlg requirement during the whole spermatogenesis. Preliminary results indicate that Dlg is similarly produced and localized on the distal SCC in spermatocyte and spermatid cysts of adult testes, suggesting that dlg may be required from the early stages, from the establishment of male stem cell niche and SCC survival, up to the later stages of sperm formation.An unexpected finding of our analysis deals with the formation of wavy and ruffled plasma membrane in dlg overexpressing cells capping the spermatocyte cysts. One way to interpret this result would be to consider that Dlg regulates the intensity of germ cell encapsulation through the Egfr pathway, which is the major signaling pathway active at the microenvironment of the spermatogonial cysts.^24,25 Membrane ruffling, detected in somatic cells upon dlg overexpression, is highly reminiscent of the formation of lammellipodia-like structures formed upon upregulation of Rac1 in SCCs.²⁶ Rac1 is a downstream component of the Egfr pathway and acts antagonistically to Rho to regulate germ cell encapsulation. As the Dlg protein plays a central role in the organization of epithelial junctions and in signal transduction at sites of cell-cell contact, it is possible to envisage that the C-terminal tail of Egfr interacts with one of the PDZ domains of Dlg.⁹ In this way, dlg inactivation would result in a disruption of the Egfr protein complexes, block the Egfr pathway and impair Rac1 function. Based on these data, we hypothesize that Dlg may act on the cytoskeleton of the somatic cells to mediate cell-shape changes leading to either cellular extensions over the spermatogonial and spermatocyte cysts or reinforcing cell-to-cell contact with the growing germ cells.A second possibility would imply a general role for Dlg in membrane proliferation and expansion of the SCCs. It has already been shown that Dlg regulates membrane proliferation in a subset of NMJ in a dose-dependent fashion.²⁷ Recent focus on membrane growth during cellularization indicates again that Dlg is an important player in the process of polarized membrane insertion.^11,28–30 Up to now, there is no mechanism describing how SCCs in Drosophila testis expand, elongate and envelop germ cell cysts, and how the SCCs direct sperm differentiation and individualization. Membrane proliferation during tissue spreading and cell surface extensions is frequently associated with the formation of membrane ruffles.^31,32 The finding that dlg overexpression in the distal SCC leads to membrane ruffling indicates that Dlg may mediate membrane growth and membrane extension over the cysts but not necessarily at the expense of the proximal SCC devoid of Dlg. Therefore, there should be a physical limitation in the expansion of the dlg-expressing cell, independent of the amount of synthesized Dlg. Further analyses of components at the junctions between the distal and proximal SCCs or components exhibiting a complementary distribution to Dlg may provide ways to identify further regulators of testis morphogenesis.If Dlg defines sites of membrane addition it may provide a link between membrane trafficking and insertion of polarized membrane components. In NMJs, the postsynaptic distribution of the t-SNARE protein Gtaxin depends on its direct interaction to the Dlg GUK domain,¹² whereas in early embryogenesis Dlg genetically interacts with Exo84.³³ Moreover, the Dlg-Strabismus complex recruits membrane associated proteins and lipids from internal membranes to sites of new plasma membrane formation.¹¹ The occurrence of similar proteins in testis was reported in humans where the SNARE-associated component Snapin binds Pumilio2 and Nanos1 proteins in the male germ cells.³⁴ It would be interesting to know whether Dlg plays a similar role in Drosophila testis, in guiding t-SNARE proteins and components of the exocyst complex into intracellular membranes, either directly or indirectly by regulating the distribution of their direct binding partners. Although Dlg may bind to different proteins in epithelial cells, neuroblasts and NMJ according to the protein availability in these tissues, the function of the Dlg protein may be still conserved in a broader sense. Through its PDZ domains Dlg may bind to numerous transmembrane proteins and receptors, and may link them to the cytoskeleton or signaling pathways. The knowledge gained on the role of Dlg in these systems will allow us to study how Dlg mediates membrane proliferation in the early germ cells in male gonads.Recent work has showed that Zero population growth (Zpg), the Drosophila gap junction Innexin 4, is localized to the spermatogonia surface, primarily on the sides adjacent to SCCs³⁵ and is required for the survival and differentiation of early germ cells in both sexes.^35–37 In zpg testes, the spermatogonia are unable to differentiate and are progressively lost, leading to the formation of tiny testes containing a small number of GSCs and germline clusters devoid of branching fusome,³⁵ resembling the dlg phenotype. In contrast, the SCCs that die through apoptosis in dlg testes are present in zpg, indicating that Dlg acts primarily on SCCs and Zpg on the germ cells.^20,35 Moreover, zpg testes display often a considerably enlarged hub. However, a direct comparison of the effect of the two proteins on the hub cannot be made because the null dlg^m52 allele produces a truncated non-functional Dlg protein that could still be detected in the hub.²⁰ Apparently this protein, which contains the PDZ1 and PDZ2 domains, could be recognized by a monoclonal antibody against the PDZ2 domain (data not shown).²⁰ This observation raises the possibility that the truncated Dlg protein may maintain some of its binding properties, which prevents the hub structure from falling apart. Further studies will be performed to determine the requirement of dlg in hub formation and structure.Our results, complementary to current researches conducted in this field, point out the importance of the somatic cell contribution in the organization of the Drosophila testis and the differentiation of the male germline. In mammals, spermatogenesis depends also on interactions between somatic Sertoli cells and germ cells. Sertoli cells act as supportive somatic cells and contain junction proteins with a high degree of similarity to Dlg. These proteins play a critical role in mammalian spermatogenesis.^38,39 Furthermore, the identification of mammalian genes with known function in Drosophila spermatogenesis and the evolutionary conservation among the Dlg proteins suggests that the pathways regulating the balance between stem cell renewal and differentiation might be similarly conserved. Interestingly, recent observations in mammals indicate that Dlg homologs play a role in the formation of mouse gonads and interact with gap junction proteins.^13,40 In addition, Dlg is required for smooth muscle orientation in the mouse ureter¹³ and interacts with the gap protein Connexin 32,⁴¹ whereas ZO-1, a MAGUK protein bearing similarity to Dlg and associated with tight junctions in mammalian Sertoli cells,³⁹ binds also to gap junction proteins, among them connexin 43, which is the predominant gap junction protein in the testis.^38,39,42 All these observations point out to functional similarities between Drosophila and vertebrate Dlg and provide strong indications that our findings in Drosophila may be extended to higher organisms.     

The ferret as a model organism to study influenza A virus infection

Influenza is a human pathogen that continues to pose a public health threat. The use of small mammalian models has become indispensable for understanding the virulence of influenza viruses. Among numerous species used in the laboratory setting, only the ferret model is equally well suited for studying both the pathogenicity and transmissibility of human and avian influenza viruses. Here, we compare the advantages and limitations of the mouse, ferret and guinea pig models for research with influenza A viruses, emphasizing the multifarious uses of the ferret in the assessment of influenza viruses with pandemic potential. Research performed in the ferret model has provided information, support and guidance for the public health response to influenza viruses in humans. We highlight the recent and emerging uses of this species in influenza virus research that are advancing our understanding of virus-host interactions.     

The ferret, Mustela putorius furo, as a new species in toxicology.

Comprehensive studies on the haematology, urine chemistry, serum chemistry and gross pathology have been carried out and reported. Seasonal weight changes were confirmed and were initiated, together with sexual activity, when ferrets were brought in to a constant temperature (18-19 degrees C) and 14-hour daylight cycle during the winter months. Levels of haemoglobin, erythrocytes and haematocrit were higher than in most common laboratory species. Similarly, serum electrolytes and glucose levels were higher than in Wistar rats or beagle dogs. Ovaries, uteri and testes displayed a seasonal maturation and atrophy. All other parameters were similar to common laboratory species.