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R. Anderson, K. Schaible, J. Heasman and C. Wylie

During normal embryonic development, mammalian germ cells use both cell migration and aggregation to form the primitive sex cords. Germ cells must be able to interact with their environment and each other to accomplish this; however, the molecular basis of early germ cell adhesion is not well characterized. Differential adhesion is also thought to occur in the adult seminiferous tubules, since germ cells move from the periphery to the lumen as they differentiate. In a screen for additional adhesion molecules expressed by the germ line, expression of the homophilic adhesion molecule, Ep-CAM, was identified in embryonic, neonatal and adult germ cells using immunocytochemistry and flow cytometry with an Ep-CAM-specific monoclonal antibody. At embryonic stages, germ cells were found to express Ep-CAM during migration at embryonic day 10.5 and early gonad assembly at embryonic day 12.5. Expression of Ep-CAM was also found on neonatal male and female germ cells. In the adult testis, Ep-CAM was detected only on spermatogonia, and was absent from more differentiated cells. Finally, embryonic stem cells were shown to express this receptor. It is proposed that Ep-CAM plays a role in the development of the germ line and the behaviour of totipotent cells.

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Heiner Niemann, X Cindy Tian, W Allan King and Rita S F Lee

The birth of ‘Dolly’, the first mammal cloned from an adult donor cell, has sparked a flurry of research activities to improve cloning technology and to understand the underlying mechanism of epigenetic reprogramming of the transferred somatic cell nucleus. Especially in ruminants, somatic cell nuclear transfer (SCNT) is frequently associated with pathological changes in the foetal and placental phenotype and has significant consequences for development both before and after birth. The most critical factor is epigenetic reprogramming of the transferred somatic cell nucleus from its differentiated status into the totipotent state of the early embryo. This involves an erasure of the gene expression program of the respective donor cell and the establishment of the well-orchestrated sequence of expression of an estimated number of 10 000–12 000 genes regulating embryonic and foetal development. The following article reviews the present knowledge on the epigenetic reprogramming of the transferred somatic cell nucleus, with emphasis on DNA methylation, imprinting, X-chromosome inactivation and telomere length restoration in bovine development. Additionally, we briefly discuss other approaches towards epigenetic nuclear reprogramming, including the fusion of somatic and embryonic stem cells and the overexpression of genes crucial in the formation and maintenance of the pluripotent status. Improvements in our understanding of this dramatic epigenetic reprogramming event will be instrumental in realising the great potential of SCNT for basic biological research and for various agricultural and biomedical applications.

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Jingmei Hou, Shi Yang, Hao Yang, Yang Liu, Yun Liu, Yanan Hai, Zheng Chen, Ying Guo, Yuehua Gong, Wei-Qiang Gao, Zheng Li and Zuping He

Infertility is a major and largely incurable disease caused by disruption and loss of germ cells. It affects 10–15% of couples, and male factor accounts for half of the cases. To obtain human male germ cells ‘especially functional spermatids’ is essential for treating male infertility. Currently, much progress has been made on generating male germ cells, including spermatogonia, spermatocytes, and spermatids, from various types of stem cells. These germ cells can also be used in investigation of the pathology of male infertility. In this review, we focused on advances on obtaining male differentiated germ cells from different kinds of stem cells, with an emphasis on the embryonic stem (ES) cells, the induced pluripotent stem (iPS) cells, and spermatogonial stem cells (SSCs). We illustrated the generation of male differentiated germ cells from ES cells, iPS cells and SSCs, and we summarized the phenotype for these stem cells, spermatocytes and spermatids. Moreover, we address the differentiation potentials of ES cells, iPS cells and SSCs. We also highlight the advantages, disadvantages and concerns on derivation of the differentiated male germ cells from several types of stem cells. The ability of generating mature and functional male gametes from stem cells could enable us to understand the precise etiology of male infertility and offer an invaluable source of autologous male gametes for treating male infertility of azoospermia patients.

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Sharmila A Bapat

The isolation and identification of stem-like cells in solid tumors or cancer stem cells (CSCs) have been exciting developments of the last decade, although these rare populations had been earlier identified in leukemia. CSC biology necessitates a detailed delineation of normal stem cell functioning and maintenance of homeostasis within the organ. Ovarian CSC biology has unfortunately not benefited from a pre-established knowledge of stem cell lineage demarcation and functioning in the normal organ. In the absence of such information, some of the classical parameters such as long-term culture-initiating assays to isolate stem cell clones from tumors, screening and evaluation of other epithelial stem cell surface markers, dye efflux, and label retention have been applied toward the putative isolation of CSCs from ovarian tumors. The present review presents an outline of the various approaches developed so far and the various perspectives revealed that are now required to be dealt with toward better disease management.

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Shosei Yoshida

Spermatogenesis in mice and other mammalians is supported by a robust stem cell system. Stem cells maintain themselves and continue to produce progeny that will differentiate into sperm over a long period. The pioneering studies conducted from the 1950s to the 1970s, which were based largely on extensive morphological analyses, have established the fundamentals of mammalian spermatogenesis and its stem cells. The prevailing so-called Asingle (As) model, which was originally established in 1971, proposes that singly isolated As spermatogonia are in fact the stem cells. In 1994, the first functional stem cell assay was established based on the formation of repopulating colonies after transplantation in germ cell-depleted host testes, which substantially accelerated the understanding of spermatogenic stem cells. However, because testicular tissues are dissociated into single-cell suspension before transplantation, it was impossible to evaluate the As and other classical models solely by this technique. From 2007 onwards, functional assessment of stem cells without destroying the tissue architecture has become feasible by means of pulse-labeling and live-imaging strategies. Results obtained from these experiments have been challenging the classical thought of stem cells, in which stem cells are a limited number of specialized cells undergoing asymmetric division to produce one self-renewing and one differentiating daughter cells. In contrast, the emerging data suggest that an extended and heterogeneous population of cells exhibiting different degrees of self-renewing and differentiating probabilities forms a reversible, flexible, and stochastic stem cell system as a population. These features may lead to establishment of a more universal principle on stem cells that is shared by other systems.

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D A Rappolee, S Zhou, E E Puscheck and Y Xie

Development can happen in one of two ways. Cells performing a necessary function can differentiate from stem cells before the need for it arises and stress does not develop. Or need arises before function, stress develops and stress signals are part of the normal stimuli that regulate developmental mechanisms. These mechanisms adjust stem cell differentiation to produce function in a timely and proportional manner. In this review, we will interpret data from studies of null lethal mutants for placental stress genes that suggest the latter possibility. Acknowledged stress pathways participate in stress-induced and -regulated differentiation in two ways. These pathways manage the homeostatic response to maintain stem cells during the stress. Stress pathways also direct stem cell differentiation to increase the first essential lineage and suppress later lineages when stem cell accumulation is diminished. This stress-induced differentiation maintains the conceptus during stress. Pathogenic outcomes arise because population sizes of normal stem cells are first depleted by decreased accumulation. The fraction of stem cells is further decreased by differentiation that is induced to compensate for smaller stem cell populations. Analysis of placental lethal null mutant genes known to mediate stress responses suggests that the labyrinthine placenta develops during, and is regulated by, hypoxic stress.

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Xiaoqing Yang, Meivita Devianti, Yuan H Yang, Yih Rue Ong, Ker Sin Tan, Shanti Gurung, Jean L Tan, Dandan Zhu, Rebecca Lim, Caroline E Gargett and James A Deane

Perivascular mesenchymal stem/stromal cells can be isolated from the human endometrium using the surface marker SUSD2 and are being investigated for use in tissue repair. Mesenchymal stem/stromal cells from other tissues modulate T cell responses via mechanisms including interleukin-10, prostaglandin E2, TGF-β1 and regulatory T cells. Animal studies demonstrate that endometrial mesenchymal stem/stromal cells can also modify immune responses to implanted mesh, but the mechanism/s they employ have not been explored. We examined the immunomodulatory properties of human endometrial mesenchymal stem/stromal cells on lymphocyte proliferation using mouse splenocyte cultures. Endometrial mesenchymal stem/stromal cells inhibited mitogen-induced lymphocyte proliferation in vitro in a dose-dependent manner. Inhibition of lymphocyte proliferation was not affected by blocking the mouse interleukin-10 receptor or inhibiting prostaglandin production. Endometrial mesenchymal stem/stromal cells continued to restrain lymphocyte proliferation in the presence of an inhibitor of TGF-β receptors, despite a reduction in regulatory T cells. Thus, the in vitro inhibition of mitogen-induced lymphocyte proliferation by endometrial mesenchymal stem/stromal cells occurs by a mechanism distinct from the interleukin-10, prostaglandin E2, TGF-β1 and regulatory T cell-mediated mechanisms employed by MSC from other tissues. eMSCs were shown to produce interleukin-17A and Dickkopf-1 which may contribute to their immunomodulatory properties. In contrast to MSC from other sources, systemic administration of endometrial mesenchymal stem/stromal cells did not inhibit swelling in a T cell-mediated model of skin inflammation. We conclude that, while endometrial mesenchymal stem/stromal cells can modify immune responses, their immunomodulatory repertoire may not be sufficient to restrain some T cell-mediated inflammatory events.

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Deepa Bhartiya and Jarnail Singh

Despite extensive research, genetic basis of premature ovarian failure (POF) and ovarian cancer still remains elusive. It is indeed paradoxical that scientists searched for mutations in FSH receptor (FSHR) expressed on granulosa cells, whereas more than 90% of cancers arise in ovary surface epithelium (OSE). Two distinct populations of stem cells including very small embryonic-like stem cells (VSELs) and ovarian stem cells (OSCs) exist in OSE, are responsible for neo-oogenesis and primordial follicle assembly in adult life, and are modulated by FSH via its alternatively spliced receptor variant FSHR3 (growth factor type 1 receptor acting via calcium signaling and the ERK/MAPK pathway). Any defect in FSH–FSHR3–stem cell interaction in OSE may affect folliculogenesis and thus result in POF. Ovarian aging is associated with a compromised microenvironment that does not support stem cell differentiation into oocytes and further folliculogenesis. FSH exerts a mitogenic effect on OSE and elevated FSH levels associated with advanced age may provide a continuous trigger for stem cells to proliferate resulting in cancer, thus supporting gonadotropin theory for ovarian cancer. Present review is an attempt to put adult ovarian biology, POF, aging, and cancer in the perspective of FSH–FSHR3–stem cell network that functions in OSE. This hypothesis is further supported by the recent understanding that: i) cancer is a stem cell disease and OSE is the niche for ovarian cancer stem cells; ii) ovarian OCT4-positive stem cells are regulated by FSH; and iii) OCT4 along with LIN28 and BMP4 are highly expressed in ovarian cancers.

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D. J. Tisdall, A. E. Fidler, P. Smith, L. D. Quirke, V. C. Stent, D. A. Heath and K. P. McNatty

The aim of this study was to investigate stem cell factor and c-kit gene expression and protein localization in the mesonephros and ovary of sheep fetuses at different days of gestation, using RNA in situ hybridization and immunohistochemical procedures. At days 24 and 26 of gestation, stem cell factor mRNA and protein were present in cells throughout the developing gonad and mesonephros. From day 28 to day 40 of gestation, stem cell factor mRNA and protein became increasingly localized to the cortical region of the ovary, where most germ cells were present as actively proliferating oogonia. From day 40 to day 90 of gestation, stem cell factor mRNA and protein localization were confined mainly to the ovarian cortex. Moreover, within the cortical region, stem cell factor mRNA was low or absent where follicles were first forming and highest in the outer ovarian cortex, where germ cells were undergoing mitosis or the early stages of meiosis. In contrast, stem cell factor protein was present in newly forming follicles, as well as in mitotic and meiotic germ cells, which is consistent with the presence of both membrane-bound and soluble forms of this ligand. However, by day 90 of gestation, both stem cell factor mRNA and protein were observed in the granulosa cells of most (> 90%) primordial follicles. C-kit mRNA and protein were observed from day 24 of gestation in both germ cells and somatic cells but, with increasing gestational age, preferentially in germ cells (for example, pre-meiotic germ cells and both isolated oocytes and follicle-enclosed oocytes). C-kit protein, but not mRNA, was also observed in germ cells that were undergoing meiosis. The results indicate that the cells containing stem cell factor mRNA within the ovary up to day 90 of gestation originated from the gonadal blastema and from cells that migrated from the mesonephros before day 28 of gestation. Since stem cell factor mRNA was absent in both mesonephric cells migrating after day 28 of gestation and in regions where follicles were first forming, it is suggested that these later migrating mesonephric cells are the progenitors of the granulosa cells in the first forming follicles. In conclusion, during follicle formation, c-kit mRNA is localized to germ cells whereas c-kit, together with stem cell factor protein, is localized to both germ cells and somatic cells, consistent with the hypothesis that the presence of this receptor–ligand pair is essential to prevent apoptosis.

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DS Johnston, LD Russell and MD Griswold

Spermatogonial stem cell transplantation was first reported by Ralph Brinster's laboratory in 1994. It has proven to be a technological breakthrough in the study of both stem cells and Sertoli cell-germ cell interactions. This technique can be used to transfer testicular stem cells successfully from one animal to another of the same species (referred to as syngeneic transplants) and sometimes to an animal of a different species (xenogeneic transplants). This transfer technique, combined with developments in cryopreservation, long-term culture, and the enrichment of stem cell populations makes more significant breakthroughs likely in the near future. Ultimately, the application of spermatogonial stem cell transfer will allow transplantation of cultured stem cells manipulated genetically in vitro to give rise to functional male gametes with an altered genotype. This achievement will have applications in basic science, human medicine, and domestic and wild animal reproduction. Although progress toward this goal has been swift, potentially significant barriers, such as the stable incorporation of genetic material into stem cells and immunological responses to the introduced germ cells, remain to be overcome. This article is a review of the scientific advances made since the initial report of successful transplantation in 1994.