Identification of pig primordial germ cells by immunocytochemistry and lectin binding

Sex bias in utero alters ovarian reserve but not uterine capacity in female offspring†

Environmental stressors to which a fetus is exposed affect a range of physiological functions in postnatal offspring. We aimed to determine the in utero effect of steroid hormones on the reproductive potential of female offspring using a porcine model. Reproductive tracts of pigs from female-biased (>65% female, n = 15), non-biased (45-54.9% female, n = 15), and male-biased litters (<35% females, n = 9) were collected at slaughter (95-115 kg). Ovaries and uterine horns were processed for H&E or immunohistochemistry. Variability of data within groups was analyzed with a Levene's test, while data were analyzed using mixed linear models in R. In the ovarian reserve, there was a significant birth weight by sex ratio interaction (P = 0.015), with low birth weight pigs from male-biased litters having higher numbers of primordial follicles with opposite trends seen in pigs from female-biased litters. Sex bias held no effect on endometrial gland development. A lower birth weight decreased the proportion of glands found in the endometrium (P = 0.045) and was more variable in both male-biased and female-biased litters (P = 0.026). The variability of primordial follicles from male-biased litters was greater than non- and female-biased litters (P = 0.014). Similarly, endometrial stromal nuclei had a greater range in male- and female-biased litters than non-biased litters (P = 0.028). A crucial finding was the greater variability in primordial follicles in the ovaries from females derived from male-biased litters and stromal cell count in the endometrium of females from male- and female-biased litters. This could be inflating the variability of reproductive success seen in females from male-biased litters.

Germline stem cells in human

The germline cells are essential for the propagation of human beings, thus essential for the survival of mankind. The germline stem cells, as a unique cell type, generate various states of germ stem cells and then differentiate into specialized cells, spermatozoa and ova, for producing offspring, while self-renew to generate more stem cells. Abnormal development of germline stem cells often causes severe diseases in humans, including infertility and cancer. Primordial germ cells (PGCs) first emerge during early embryonic development, migrate into the gentile ridge, and then join in the formation of gonads. In males, they differentiate into spermatogonial stem cells, which give rise to spermatozoa via meiosis from the onset of puberty, while in females, the female germline stem cells (FGSCs) retain stemness in the ovary and initiate meiosis to generate oocytes. Primordial germ cell-like cells (PGCLCs) can be induced in vitro from embryonic stem cells or induced pluripotent stem cells. In this review, we focus on current advances in these embryonic and adult germline stem cells, and the induced PGCLCs in humans, provide an overview of molecular mechanisms underlying the development and differentiation of the germline stem cells and outline their physiological functions, pathological implications, and clinical applications.

Porcine Primordial Germ Cell-Like Cells Generated from Induced Pluripotent Stem Cells Under Different Culture Conditions

Culture conditions regulate the process of pluripotency acquisition and self-renewal. This study aimed to analyse the influence of the in vitro environment on the induction of porcine induced pluripotent stem cell (piPSCs) differentiation into primordial germ cell-like cells (pPGCLCs). piPSC culture with different supplementation strategies (LIF, bFGF, or LIF plus bFGF) promoted heterogeneous phenotypic profiles. Continuous bFGF supplementation during piPSCs culture was beneficial to support a pluripotent state and the differentiation of piPSCs into pPGCLCs. The pPGCLCs were positive for the gene and protein expression of pluripotent and germinative markers. This study can provide a suitable in vitro model for use in translational studies and to help answer numerous remaining questions about germ cells.

Step by Step about Germ Cells Development in Canine

Simple Summary

The progression of germ cells is a remarkable event that allows biological discovery in the differ-entiation process during in vivo and in vitro development. This is crucial for understanding one toward making oogenesis and spermatogenesis. Companion animals, such as canine, could offer new animal models for experimental and clinical testing for translation to human models. In this review, we describe the latest and more relevant findings on germ cell development. In addition, we showed the methods available for obtaining germ cells in vitro and the characterization of pri-mordial germ cells and spermatogonial stem cells. However, it is necessary to further conduct basic research in canine to clarify the beginning of germ cell development.

Abstract

Primordial germ cells (PGCs) have been described as precursors of gametes and provide a connection within generations, passing on the genome to the next generation. Failures in the formation of gametes/germ cells can compromise the maintenance and conservation of species. Most of the studies with PGCs have been carried out in mice, but this species is not always the best study model when transposing this knowledge to humans. Domestic animals, such as canines (canine), have become a valuable translational research model for stem cells and therapy. Furthermore, the study of canine germ cells opens new avenues for veterinary reproduction. In this review, the objective is to provide a comprehensive overview of the current knowledge on canine germ cells. The aspects of canine development and germ cells have been discussed since the origin, specifications, and development of spermatogonial canine were first discussed. Additionally, we discussed and explored some in vitro aspects of canine reproduction with germ cells, such as embryonic germ cells and spermatogonial stem cells.

Specification and epigenomic resetting of the pig germline exhibit conservation with the human lineage

Investigations of the human germline and programming are challenging because of limited access to embryonic material. However, the pig as a model may provide insights into transcriptional network and epigenetic reprogramming applicable to both species. Here we show that, during the pre- and early migratory stages, pig primordial germ cells (PGCs) initiate large-scale epigenomic reprogramming, including DNA demethylation involving TET-mediated hydroxylation and, potentially, base excision repair (BER). There is also macroH2A1 depletion and increased H3K27me3 as well as X chromosome reactivation (XCR) in females. Concomitantly, there is dampening of glycolytic metabolism genes and re-expression of some pluripotency genes like those in preimplantation embryos. We identified evolutionarily young transposable elements and gene coding regions resistant to DNA demethylation in acutely hypomethylated gonadal PGCs, with potential for transgenerational epigenetic inheritance. Detailed insights into the pig germline will likely contribute significantly to advances in human germline biology, including in vitro gametogenesis.

Establishment of novel monoclonal antibodies for identification of type A spermatogonia in teleosts†

We recently established a germ cell transplantation system in salmonids. Donor germ cells transplanted into the body cavity of recipient embryos migrate toward and are incorporated into the recipient gonad, where they undergo gametogenesis. Among the various types of testicular germ cells, only type A spermatogonia (A-SG) can be incorporated into the recipient gonads. Enriching for A-SG is therefore important for improving the efficiency of germ cell transplantation. To enrich for A-SG, an antibody against a cell surface marker is a convenient and powerful approach used in mammals; however, little is known about cell surface markers for A-SG in fish. To that end, we have produced novel monoclonal antibodies (mAbs) against cell-surface molecules of rainbow trout (Oncorhynchus mykiss) A-SG. We inoculated mice with living A-SG isolated from pvasa-GFP transgenic rainbow trout using GFP-dependent flow cytometry. By fusing lymph node cells of the inoculated mice with myeloma cells, we generated 576 hybridomas. To identify hybridomas that produce mAbs capable of labeling A-SG preferentially and effectively, we screened them using cell ELISA, fluorescence microscopy, and flow cytometry. We thereby identified two mAbs that can label A-SG. By using flow cytometry with these two antibodies, we could enrich for A-SG with transplantability to recipient gonads from amongst total testicular cells. Furthermore, one of these mAbs could also label zebrafish (Danio rerio) spermatogonia. Thus, we expect these monoclonal antibodies to be powerful tools for germ cell biology and biotechnology.

Sexual precocity in male microminipigs evaluated immunohistologically using spermatogonial stem cell markers

Microminipigs are one of the smallest miniature pigs characterized as sexually precocious; the males achieve sexual maturity at around 3-4.5 months of age. However, the physiology of this sexual precocity is still unclear. To understand sexual precocity in male microminipigs, we analyzed their testes at five developmental stages: neonatal (<7 days), 30-day-old, 45-day-old, 80-day-old, and adult (>24 months) stages. We used 4 pigs in each of the stages. To analyze testicular development histologically, the seminiferous tubule diameter (SD) was measured, and the presence or absence of the seminiferous lumen was confirmed. Changes in the expression of pluripotency markers, DBA, UCHL1, ZBTB16, and vimentin, were evaluated immunohistologically. For the analyses, cells positive for DBA, UCHL1, and ZBTB16 per 150 round seminiferous tubules in cross sections from each testis were counted to evaluate the total number of positive cells. The number of positive cells per 100 Sertoli cells (DBA+/Sertoli, UCHL1+/Sertoli, and ZBTB16+/Sertoli) was calculated to compare the five developmental stages. Histologically, SDs became larger with piglet growth, and precocity was confirmed; seminiferous lumens were observed from the 30-day-old stage. Immunohistologically, the number of DBA+/Sertoli, which indicates the number of gonocytes, decreased rapidly to an undetectable level by the 45-day-old stage. In the same period, the number of UCHL1+/Sertoli, which indicates total SSCs, increased significantly, suggesting that the proliferation of SSCs was accelerated before 30 days of age. Consequently, our study clarified that differentiation of SSCs in microminipigs started during the fetal period, the differentiation of gonocytes and proliferation of SSCs was then accelerated before 30 days of age, and the early phase of spermatogenesis was finally completed at around 45 days after birth. Consequently, sexual precocity in male microminipigs was characterized by a shorter duration of the early phase of spermatogenesis.

Germline development in amniotes: A paradigm shift in primordial germ cell specification

In the field of germline development in amniote vertebrates, primordial germ cell (PGC) specification in birds and reptiles remains controversial. Avians are believed to adopt a predetermination or maternal specification mode of PGC formation, contrary to an inductive mode employed by mammals and, supposedly, reptiles. Here, we revisit and review some key aspects of PGC development that channelled the current subdivision, and challenge the position of birds and reptiles as well as the 'binary' evolutionary model of PGC development in vertebrates. We propose an alternative view on PGC specification where germ plasm plays a role in laying the foundation for the formation of PGC precursors (pPGC), but not necessarily of PGCs. Moreover, inductive mechanisms may be necessary for the transition from pPGCs to PGCs. Within this framework, the implementation of data from birds and reptiles could provide new insights on the evolution of PGC specification in amniotes.

Characterization of the Epigenetic Changes During Human Gonadal Primordial Germ Cells Reprogramming

Epigenetic reprogramming is a central process during mammalian germline development. Genome-wide DNA demethylation in primordial germ cells (PGCs) is a prerequisite for the erasure of epigenetic memory, preventing the transmission of epimutations to the next generation. Apart from DNA demethylation, germline reprogramming has been shown to entail reprogramming of histone marks and chromatin remodelling. Contrary to other animal models, there is limited information about the epigenetic dynamics during early germ cell development in humans. Here, we provide further characterization of the epigenetic configuration of the early human gonadal PGCs. We show that early gonadal human PGCs are DNA hypomethylated and their chromatin is characterized by low H3K9me2 and high H3K27me3 marks. Similarly to previous observations in mice, human gonadal PGCs undergo dynamic chromatin changes concomitant with the erasure of genomic imprints. Interestingly, and contrary to mouse early germ cells, expression of BLIMP1/PRDM1 persists in through all gestational stages in human gonadal PGCs and is associated with nuclear lysine-specific demethylase-1. Our work provides important additional information regarding the chromatin changes associated with human PGCs development between 6 and 13 weeks of gestation in male and female gonads. Stem Cells 2016;34:2418-2428.

Germ cell specification and pluripotency in mammals: a perspective from early embryogenesis

Germ cells are unique cell types that generate a totipotent zygote upon fertilization, giving rise to the next generation in mammals and many other multicellular organisms. How germ cells acquire this ability has been of considerable interest. In mammals, primordial germ cells (PGCs), the precursors of sperm and oocytes, are specified around the time of gastrulation. PGCs are induced by signals from the surrounding extra-embryonic tissues to the equipotent epiblast cells that give rise to all cell types. Currently, the mechanism of PGC specification in mammals is best understood from studies in mice. Following implantation, the epiblast cells develop as an egg cylinder while the extra-embryonic ectoderm cells which are the source of important signals for PGC specification are located over the egg cylinder. However, in most cases, including humans, the epiblast cells develop as a planar disc, which alters the organization and the source of the signaling for cell fates. This, in turn, might have an effect on the precise mechanism of PGC specification in vivo as well as in vitro using pluripotent embryonic stem cells. Here, we discuss how the key early embryonic differences between rodents and other mammals may affect the establishment of the pluripotency network in vivo and in vitro, and consequently the basis for PGC specification, particularly from pluripotent embryonic stem cells in vitro.

Primordial germ-cell development and epigenetic reprogramming in mammals

Primordial germ cells (PGCs) are the embryonic precursors of the gametes and represent the founder cells of the germline. Specification of PGCs is a critical divergent point during embryogenesis. Whereas the somatic lineages will ultimately perish, cells of the germline have the potential to form a new individual and hence progress to the next generation. It is therefore critical that the genome emerges intact and carrying the appropriate epigenetic information during its passage through the germline. To ensure this fidelity of transmission, PGC development encompasses extensive epigenetic reprogramming. The low cell numbers and relative inaccessibility of PGCs present a challenge to those seeking mechanistic understanding of the crucial developmental and epigenetic processes in this most fascinating of lineages. Here, we present an overview of PGC development in the mouse and compare this with the limited information available for other mammalian species. We believe that a comparative approach will be increasingly important to uncover the extent to which mechanisms are conserved and reveal the critical steps during PGC development in humans.

Lentiviral modification of enriched populations of bovine male gonocytes

Undifferentiated germ cells have the capacity to develop into sperm capable of fertilizing oocytes and contributing genetic material to subsequent generations. The most primitive prepubertal undifferentiated germ cells include gonocytes and undifferentiated spermatogonia, including spermatogonial stem cells (SSC). Gonocytes, present in the testis at birth, differentiate into SSC, which maintain spermatogenesis for the remainder of the male's life. Because of their capacity to contribute to lifelong spermatogenesis, undifferentiated germ cells are attractive targets for genetic modification to produce transgenic animals, including cattle. To maximize the efficiency of genetic modification of bovine gonocytes and SSC, effective enrichment techniques need to be developed. Selection of bovine gonocytes using differential plating was improved 8-fold (P < 0.001) when using a combination of extracellular matrix proteins, including laminin, fibronectin, collagen type IV, and gelatin, compared to using laminin and gelatin alone. Selected cells labeled with PKH26 formed colonies of donor-derived germ cells after transplantation into recipient mouse testes, indicating putative stem cell function. Significantly more colonies (P < 0.001) per 1 × 10(5) viable transplanted cells were formed from isolated nonadherent cells (203 ± 23.2) compared to adherent (20 ± 2.7) or Percoll (45.5 ± 4.5) selected cells. After selection, some gonocytes were transduced using a lentiviral vector containing the transgene for the enhanced green fluorescent protein. Transduction efficiency was 17%. Collectively, these data demonstrate effective methods for the selection and genetic modification of bovine undifferentiated germ cells.

Derivation of putative porcine embryonic germ cells and analysis of their multi-lineage differentiation potential

Embryonic germ (EG) cells are cultured pluripotent stem cells derived from the primordial germ cells (PGCs) that migrate from the dorsal mesentery of the hindgut to the developing genital ridge. In this study, the morphology of the porcine genital ridge was assessed in embryos harvested on days 22-30 of pregnancy. PGCs from embryos at these stages were cultured to obtain porcine EG cell lines, and EG-like cells were derived from PGCs from embryos harvested on days 24-28 of pregnancy. The EG-like cells expressed Oct4, Sox2, Nanog, SSEA-3, SSEA-4 and alkaline phosphatase (AP). These cells were able to form embryoid bodies (EBs) in suspension culture and differentiate into cells representative of the three germ layers as verified by a-fetoprotein (AFP), α-smooth muscle actin (α-SMA), and Nestin expression. Spontaneous differentiation from the porcine EG-like cells of delayed passage in vitro showed that they could differentiate into epithelial-like cells, mesenchymal-like cells and neuron-like cells. In vitro directed differentiation generated osteocytes, adipocytes and a variety of neural lineage cells, as demonstrated by alizarin red staining, oil red O staining, and immunofluorescence for neuronal class Ⅲ β-tubulin (Tuj1), glial fibrillary protein (GFAP) and galactosylceramidase (GALC), respectively. These results indicate that porcine EG-like cells have the potential for multi-lineage differentiation and are useful for basic porcine stem cell research.

Embryonic stem cell markers

Embryonic stem cell (ESC) markers are molecules specifically expressed in ES cells. Understanding of the functions of these markers is critical for characterization and elucidation for the mechanism of ESC pluripotent maintenance and self-renewal, therefore helping to accelerate the clinical application of ES cells. Unfortunately, different cell types can share single or sometimes multiple markers; thus the main obstacle in the clinical application of ESC is to purify ES cells from other types of cells, especially tumor cells. Currently, the marker-based flow cytometry (FCM) technique and magnetic cell sorting (MACS) are the most effective cell isolating methods, and a detailed maker list will help to initially identify, as well as isolate ESCs using these methods. In the current review, we discuss a wide range of cell surface and generic molecular markers that are indicative of the undifferentiated ESCs. Other types of molecules, such as lectins and peptides, which bind to ESC via affinity and specificity, are also summarized. In addition, we review several markers that overlap with tumor stem cells (TSCs), which suggest that uncertainty still exists regarding the benefits of using these markers alone or in various combinations when identifying and isolating cells.

The Sda/GM2-glycan is a carbohydrate marker of porcine primordial germ cells and of a subpopulation of spermatogonia in cattle, pigs, horses and llama

Spermatogonia are a potential source of adult pluripotent stem cells and can be used for testis germ cell transplantation. Markers for the isolation of these cells are of great importance for biomedical applications. Primordial germ cells and prepubertal spermatogonia in many species can be identified by their binding of Dolichos biflorus agglutinin (DBA). This lectin binds to two different types of glycans, which are α-linked N-acetylgalactosamine (GalNac) and β-linked GalNac, if this is part of the Sda or GM2 glycotopes. We used the MAB CT1, which is specific for the trisaccharides motif NeuAcα2-3(GalNAcβ1-4)Galβ1-, which is common to both Sda and GM2 glycotopes, to further define the glycosylation of DBA binding germ cells. In porcine embryos, CT1 bound to migratory germ cells and gonocytes. CT1/DBA double staining showed that the mesonephros was CT1 negative but contained DBA-positive cells. Gonocytes in the female gonad became CT1 negative, while male gonocytes remained CT1 positive. In immunohistological double staining of cattle, pig, horse and llama testis, DBA and CT1 staining was generally colocalised in a subpopulation of spermatogonia. These spermatogonia were mainly single, sometimes paired or formed chains of up to four cells. Our data show that the Sda/GM2 glycotope is present in developing germ cells and spermatogonia in several species. Owing to the narrower specificity of the CT1 antibody, compared with DBA, the former is likely to be a useful tool for labelling and isolation of these cells.

A vasa gene from green mud crab Scylla paramamosain and its expression during gonadal development and gametogenesis

VASA is one of the important regulatory factors that determine the development of the reproductive system. However, no information on vasa gene from Pleocyemata Brachyura is available. By using Race, we obtained a full-length cDNA of Sp-vasa of the green mud crab Scylla paramamosain. The full-length (2,851 bp) cDNA of vasa encodes a peptide of 631 amino acids. Real-time PCR results indicated that the expression level of Sp-vasa in the growth stage of ovary was higher than in the maturation stage, and in stage I and II of testis, the expression level of Sp-vasa were higher than in stage III. By using in situ hybridization, Sp-vasa RNAs were detected in the large part of oocyte plasm in stage I, nucleus zone in stage III and perinuclear zone in stage V. As the size of oocytes increases during oogenesis, the signals change from strong to weak. In addition, in stage I and II of testis, the expression levels of Sp-vasa were higher than in stage III, and the hybridization intensity of Sp-vasa gene gradually increased during spermatogenesis from spermatogonia to spermatids. However, no hybridization signal was detected in spermatozoon. Real-time PCR and in situ hybridization were consistent. These findings suggest that Sp-vasa is likely to serve as a useful and specific marker for germ cell development of S. paramamosain.

Spermatogonial stem cells in the testis of an endangered bovid: Indian black buck (Antilope cervicapra L.)

Numerous wild bovids are facing threat of extinction owing to the loss of habitat and various other reasons. Spermatogonial stem cells (SSCs) represent the only germline stem cells in adult body that are capable of self-renewal and that can undergo differentiation to produce haploid germ cells. SSCs can, therefore, serve as a useful resource for preservation of germplasm of threatened and endangered mammals. The Indian black buck (Antilope cervicapra L.) is a small Indian antelope that is listed as endangered by the Indian Wildlife Protection Act, 1972. Immunohistochemical analysis of testes tissues of black buck revealed the presence of spermatogonia that were specifically stained by lectin-Dolichos biflorus agglutinin (DBA). The expression of pluripotent cell-specific markers, NANOG and stage-specific embryonic antigen-1 (SSEA-1), was detected in spermatogonia. Interestingly, the expression of POU5F1 (OCT3/4) was absent from spermatogonia, however, it was detected in differentiating cells such as spermatocytes and round spermatids but not in elongated spermatids. The expression of NANOG protein was also present in spermatocytes but absent in round and elongated spermatids. Using the testis transplantation assay, stem cell potential of black buck spermatogonia was confirmed as indicated by the presence of colonized DBA-stained cells in the basal membrane of seminiferous tubules of xenotransplanted mice testis. The findings from this study suggest the presence of SSCs in the testis of an endangered bovid for the first time and open new possibility to explore the use of SSCs in conservation.

Epigenetic reprogramming in the porcine germ line

Background

Epigenetic reprogramming is critical for genome regulation during germ line development. Genome-wide demethylation in mouse primordial germ cells (PGC) is a unique reprogramming event essential for erasing epigenetic memory and preventing the transmission of epimutations to the next generation. In addition to DNA demethylation, PGC are subject to a major reprogramming of histone marks, and many of these changes are concurrent with a cell cycle arrest in the G2 phase. There is limited information on how well conserved these events are in mammals. Here we report on the dynamic reprogramming of DNA methylation at CpGs of imprinted loci and DNA repeats, and the global changes in H3K27me3 and H3K9me2 in the developing germ line of the domestic pig.

Results

Our results show loss of DNA methylation in PGC colonizing the genital ridges. Analysis of IGF2-H19 regulatory region showed a gradual demethylation between E22-E42. In contrast, DMR2 of IGF2R was already demethylated in male PGC by E22. In females, IGF2R demethylation was delayed until E29-31, and was de novo methylated by E42. DNA repeats were gradually demethylated from E25 to E29-31, and became de novo methylated by E42. Analysis of histone marks showed strong H3K27me3 staining in migratory PGC between E15 and E21. In contrast, H3K9me2 signal was low in PGC by E15 and completely erased by E21. Cell cycle analysis of gonadal PGC (E22-31) showed a typical pattern of cycling cells, however, migrating PGC (E17) showed an increased proportion of cells in G2.

Conclusions

Our study demonstrates that epigenetic reprogramming occurs in pig migratory and gonadal PGC, and establishes the window of time for the occurrence of these events. Reprogramming of histone H3K9me2 and H3K27me3 detected between E15-E21 precedes the dynamic DNA demethylation at imprinted loci and DNA repeats between E22-E42. Our findings demonstrate that major epigenetic reprogramming in the pig germ line follows the overall dynamics shown in mice, suggesting that epigenetic reprogramming of germ cells is conserved in mammals. A better understanding of the sequential reprogramming of PGC in the pig will facilitate the derivation of embryonic germ cells in this species.

Efficient purification of neonatal porcine gonocytes with Nycodenz and differential plating

Gonocytes are the only type of germ cells present in the postnatal testis and give rise to spermatogonial stem cells. Purification of gonocytes has important implications for the study and manipulation of these cells and may provide insights for the ongoing investigation of the male germline stem cells. To obtain a pure population of gonocytes from piglet testis cells, a wide range of Nycodenz concentrations were investigated for density gradient centrifugation. We also examined differential plating of testis cells for various culture durations with different extracellular matrix (ECM) components (fibronectin, poly-d-lysine, poly-l-lysine, laminin and collagen Types I and IV). Gonocytes were highly enriched in pellets of testis cells after using 17% Nycodenz centrifugation to a purity of 81 ± 9%. After culturing testis cells on plates precoated with different ECM components for 120 min, the proportion of gonocytes increased among non-adherent cells (suspended in the medium), with fibronectin or poly-d-lysine resulting in the greatest (up to 85%) and laminin in the lowest (54%) gonocyte proportion. Combining the most promising ECM coatings (fibronectin and poly-d-lysine) and further extension of their culture duration to 240 min did not improve final gonocyte purity. However, centrifugation with 17% Nycodenz followed by differential plating with fibronectin and poly-d-lysine coating further purified gonocytes among the collected cells to >90%. These results provide a simple, quick and efficient approach for obtaining highly enriched populations of piglet gonocytes for use in the study and manipulation of these germline stem cells.

Effects of 6-bromoindirubin-3′-oxime on the maintenance of pluripotency of porcine embryonic germ cells in combination with stem cell factor, leukemia inhibitory factor and fibroblast growth factor

6-Bromoindirubin-3′-oxime (BIO), which is one of the glycogen synthase kinase 3 inhibitors and a key regulator of numerous signaling pathways, was reported to be capable of maintaining the pluripotency of human and mouse embryonic stem cells. Presently, it is unknown whether BIO can influence the derivation of porcine embryonic germ (EG) cells. In this study, porcine primordial germ cells (PGCs) were isolated from gonads of 24- and 28-day embryos, and were then treated with BIO either individually or in combination with other cytokines (stem cell factor (SCF), leukemia inhibitory factor (LIF), and fibroblast growth factor (FGF); abbreviated as ‘3F’), and the effects of the treatment on the proliferation ability of porcine PGCs at early stage were examined using 5-bromo-2-deoxyuridine (Brdu) immunostaining assay. After continuous culture, the effects on the efficiency of porcine undifferentiated EG cells in the third passage and differentiated EG cells from embryoid bodies were examined as well. The results obtained through the observation of the Brdu-labeled PGCs indicated that BIO in combination with 3F resulted in a significant increase in the mitosis index, and also indicated that the BIO in combination with 3F had a higher efficiency in promoting the formation of porcine EG colony derived from porcine day 24 PGCs than BIO used either individually or in combination with LIF. In addition, BIO in combination with 3F exhibited the apparent anti-differentiation activity by reversing the differentiated EG cells to the undifferentiated status. Our results demonstrate that BIO in combination with SCF, LIF, and FGF could significantly contribute to the establishment of a porcine EG cell colony and maintain the undifferentiated status.

The effects of growth factors on in vitro-cultured porcine testicular cells

Cell lines from neonate porcine testis were cultured and characterized and the effect of growth factors were investigated, in order to determine the requirements for the establishment of porcine male germ cell lines. In primary cultures, three different colony types with distinctive morphologies could be recognized. From colonies resembling mouse spermatogonial stem cells (SSCs), two cell lines were derived and maintained for nine passages after which proliferation stopped. Growth of these cell lines depended on the growth factors leukemia inhibitory factor (LIF), epidermal growth factor (EGF), glial derived neurotrophic factor (GDNF), and fibroblast growth factor (FGF). In both cell lines NANOG, promyelocytic leukemia zinc-finger (PLZF), and EPCAM, were expressed at higher levels and GFRA1, ITGA6, and THY1 at lower levels than in neonate porcine testis. Primary cultures of neonate pig testis were subjected to a factorial design of the growth factors LIF, GDNF, EGF, and FGF. EGF and FGF had a positive effect on the number and size of the SSC-like colonies. Addition of EGF and FGF to primary cell cultures of neonate pig testis affected the expression of NANOG, PLZF, POU5F1, and GATA4, whereas effects of LIF or GDNF could not be detected. FGF decreased the expression levels of NANOG, a marker for pluripotency also expressed in neonatal porcine male germ cells. FGF decreased expression of PLZF and enhanced the expression of pluripotency-related gene POU5F1 and Sertoli cell marker GATA4. EGF had a positive effect on PLZF expression levels and counteracted the positive effect of FGF on GATA4 expression. These results suggest that FGF can impede successful derivation of porcine SSCs from neonate pig testis.

Derivation of induced pluripotent stem cells from pig somatic cells

For reasons that are unclear the production of embryonic stem cells from ungulates has proved elusive. Here, we describe induced pluripotent stem cells (iPSC) derived from porcine fetal fibroblasts by lentiviral transduction of 4 human (h) genes, hOCT4, hSOX2, hKLF4, and hc-MYC, the combination commonly used to create iPSC in mouse and human. Cells were cultured on irradiated mouse embryonic fibroblasts (MEF) and in medium supplemented with knockout serum replacement and FGF2. Compact colonies of alkaline phosphatase-positive cells emerged after approximately 22 days, providing an overall reprogramming efficiency of approximately 0.1%. The cells expressed porcine OCT4, NANOG, and SOX2 and had high telomerase activity, but also continued to express the 4 human transgenes. Unlike human ESC, the porcine iPSC (piPSC) were positive for SSEA-1, but negative for SSEA-3 and -4. Transcriptional profiling on Affymetrix (porcine) microarrays and real time RT-PCR supported the conclusion that reprogramming to pluripotency was complete. One cell line, ID6, had a normal karyotype, a cell doubling time of approximately 17 h, and has been maintained through >220 doublings. The ID6 line formed embryoid bodies, expressing genes representing all 3 germ layers when cultured under differentiating conditions, and teratomas containing tissues of ectoderm, mesoderm, and endoderm origin in nude mice. We conclude that porcine somatic cells can be reprogrammed to form piPSC. Such cell lines derived from individual animals could provide a means for testing the safety and efficacy of stem cell-derived tissue grafts when returned to the same pigs at a later age.

Methylation changes in porcine primordial germ cells

Epigenetic re-programming is an important event in the development of primordial germ cells (PGC) into functional gametes, characterized by genome-wide erasure of DNA methylation and re-establishment of epigenetic marks, a process essential for restoration of the potential for totipotency. In this study changes in the methylation status of centromeric repeats and two IGF2-H19 differentially methylated domain (DMD) sequences were examined in porcine PGC between Days 24 and 31 of pregnancy. The methylation levels of centromeric repeats and IGF2-H19 DMD sequences decreased rapidly from Days 24 to 28 in both male and female PGC. At Days 30 and 31 of pregnancy centromeric repeats and IGF2-H19 DMD sequences acquired new methylation in male PGC, while in female PGC these sequences were completely demethylated by Day 30 and remained hypomethylated at Day 31. To characterize methylation changes that PGC undergo in culture, the methylation status of embryonic germ cells (EGCs) derived from PGC at Day 26 of pregnancy was examined. Centromeric repeats and IGF2-H19 DMD sequences were similarly methylated in both male and female EGC and hypermethylated in female EGC compared with female PGC at the same embryonic age. Our results show that, similar to murine PGC, porcine PGC undergo genome-wide DNA demethylation shortly after arrival in the genital ridges. When placed in culture porcine PGC terminate their demethylation program and may acquire new DNA methylation marks. To our knowledge, this is the first report regarding epigenetic re-programming of genital ridge PGC in the pig.

Expression of NANOG, but not POU5F1, points to the stem cell potential of primitive germ cells in neonatal pig testis

Gonocytes are primitive germ cells that are present in the neonatal testis and are committed to male germline development. Gonocytes differentiate to spermatogonia, which establish and maintain spermatogenesis in the postnatal testis. However, it is unknown whether large animal species have pluripotency-specific proteins in the testis. Nanog and Pou5f1 (Oct3/4) have been identified as transcription factors essential for maintaining pluripotency of embryonic stem cells in mice. Here, we show that NANOG protein was expressed in the germ cells of neonatal pig testes, but was progressively lost with age. NANOG was expressed in most of the lectin Dolichos biflorus agglutinin- and ZBTB16-positive gonocytes, which are known gonocyte-specific markers in pigs. NANOG was also expressed in Sertoli and interstitial cells of neonatal testes. Interestingly, POU5F1 expression was not detected at either the transcript or the protein level in neonatal pig testis. In the prepubertal testis, NANOG and POU5F1 proteins were primarily detected in differentiated germ cells, such as spermatocytes and spermatids, and rarely in undifferentiated spermatogonia. By using a testis transplantation assay, we found that germ cells from 2- to 4-day-old pigs could colonize and proliferate in the testes of the recipient mice, suggesting that primitive germ cells from neonatal pig testes have stem cell potential.

Identification, Isolation, and In Vitro Culture of Porcine Gonocytes1

Gonocytes are primitive germ cells that reside in the seminiferous tubules of neonatal testes and give rise to spermatogonia, thereby initiating spermatogenesis. Due to a lack of specific markers, the isolation and culture of these cells has proven to be difficult in the pig. In the present study, we show that a lectin, Dolichos biflorus agglutinin (DBA), which has specific affinity for primordial germ cells (PCGs) in the genital ridge, binds specifically to gonocytes in neonatal pig testes. The specific affinity of DBA for germ cells was progressively lost with age. This suggests that DBA binds strongly to primitive germ cells, such as gonocytes, weakly to primitive spermatogonia, and not at all to spermatogonia. The presence of alkaline phosphatase (AP) activity in the germ cells of neonatal pig testis confirmed the existence of primitive germ cells. Gonocytes from neonatal pig testis were purified, and a cell population that consisted of approximately 70% gonocytes was obtained, as indicated by the DBA binding assay. Purified gonocytes were cultured in DMEM/F12 supplemented with 10% FBS in the absence of any specific growth factors for 7 days. The cells remained viable and proliferated actively in culture. Initially, the gonocytes grew as focal colonies that transformed to three-dimensional colonies by 7 days of culture. Cultured germ cells expressed SSEA-1, a marker for embryonic stem (ES) cells, and were negative for the expression of somatic cell markers. These results should help to establish a male germ cell line that could be used for studying spermatogenesis in vitro and for genetic modification of pigs.

Cell surface markers in human embryonic stem cells

The pluripotent nature of human embryonic stem cells (hESCs) is based on their potential to form every cell type in the body. Prior to use in directed differentiation strategies, these cells need to be thoroughly characterized. The large number of glycoproteins and carbohydrates that exist on the cell surface provide an excellent opportunity for characterizing hESCs and a means to delineate pluripotent and differentiated cell types. A panel of 14 lectins, based on their specificity for a variety of carbohydrates and carbohydrate linkages, along with stage-specific embryonic antigen-4 (SSEA-4), have been chosen to examine hESCs for other potential pluripotent markers. These studies have been achieved by binding quantitation by flow cytometry and binding localization in adherent colonies by immunocytochemistry. We have shown that certain lectins may be used as markers that are associated with the pluripotent state of hESCs because binding percentages and binding localization of these lectins are similar to those of SSEA-4. This presents options for systematic classification of pluripotent hESCs and for distinguishing differentiated hESC types based on glycan presentation that accompanies differentiation.

Identification of mannose moieties in N- and O-linked oligosaccharides of the primordial germ cells of Xenopus embryos

The presence of mannose (Man) in the glycoconjugates of primordial germ cells (PGCs) of Xenopus embryos was elucidated by lectin histochemistry with Concanavalin A (Con A) and snowdrop (Galanthus nivalis) bulb lectin (GNA), in combination with deglycosylative pretreatments: beta-elimination, which removes O-linked oligosaccharides, and incubation with Peptide N glycosidase F (PNGase F), which removes N-linked glycan chains. In addition, histochemistry with Con A, which binds to Man and glucose (Glc), was also performed after glucose-oxidase incubation, which converts Glc into gluconic acid, and GNA was carried out after acid hydrolysis, which removes terminal sialic acid (NeuAc) moieties. PGCs were analyzed during their migration over the mesentery until the genital ridge, and after colonization of this gonad anlage. The results showed that for both lectins: (1) the PGCs and other surrounding tissue showed a similar binding pattern, and (2) the staining in the PGCs was similar in the developmental stages studied. Labeling with Con A was due to Man, and not to Glc, as shown after incubation with glucose-oxidase, and it was assumed that Man was in N-linked oligosaccharides. However, GNA labeling was mainly due to O-linked oligosaccharides, because the pretreatment of beta-elimination turned cells negative. Moreover, acid hydrolysis pretreatment gave rise to a stronger GNA-staining, suggesting that either Man was also in subterminal position to NeuAc or some Man-containing glycans were unmasked after removal of NeuAc from other oligosaccharide chains.

Characterization of pig vasa homolog gene and specific expression in germ cell lineage

The vasa gene is known to be an important factor for germ cell development in both invertebrates and vertebrates. In the present study, we cloned the porcine vasa homolog (Pvh, 2,172 bps) and investigated its expression at mRNA and protein levels. The isolated cDNA had deduced 724 amino acid residues with significant homology to mouse (85%) and human (91%) vasa. In adult tissues, Pvh transcript was restricted to the ovary and testis, and was undetectable in somatic tissues. During preimplantation embryo development, Pvh was transcribed in oocytes and fertilized 2-cell embryos, but not in other preimplantation embryos. In fetal stage, the transcript of Pvh gene was expressed in all fetal stage, except in day 17-18. Immunohistochemical analysis of fetal and adult gonad revealed that the Pvh protein was localized in oocytes and spermatocytes, consistent with mRNA expression. Interestingly, Pvh protein was also observed in proliferating primordial germ cells (PGCs) and freshly isolated PGCs, but not in embryonic germ cells. Our results suggest that Pvh gene can be a useful marker for germ cell development in pigs.

Pluripotent Stem Cells from Germ Cells

To date, stem cells have been derived from three sources of germ cells. These include embryonic germ cells (EGCs), embryonal carcinoma cells (ECCs), and multipotent germ line stem cells (GSCs). EGCs are derived from primordial germ cells that arise in the late embryonic and early fetal period of development. ECCs are derived from adult testicular tumors whereas GSCs have been derived by culturing spermatogonial stem cells from mouse neonates and adults. For each of these lines, their pluripotency has been demonstrated by their ability to differentiate into cell types derived from the three germ layers in vitro and in vivo and in chimeric animals, including germ line transmission. These germ line-derived stem cells have been generated from many species including human, mice, porcine, and chicken albeit with only slight modifications. This chapter describes general considerations regarding critical aspects of their derivation compared with their counterpart, embryonic stem cells (ESCs). Detailed protocols for EGC derivation and maintenance from human and mouse primordial germ cells (PGCs) will be presented.

Confinement and clearance of OCT4 in the porcine embryo at stereomicroscopically defined stages around gastrulation

In the areas of developmental biology and embryonic stem cell research, reliable molecular markers of pluripotency and early lineage commitment are sparse in large animal species. In this study, we present morphological and immunohistochemical findings on the porcine embryo in the period around gastrulation, days 8-17 postinsemination, introducing a stereomicroscopical staging system in this species. In embryos at the expanding hatched blastocyst stage, OCT4 is confined to the inner cell mass. Following detachment of the hypoblast, and formation of the embryonic disk, this marker of pluripotency was selectively observed in the epiblast. A prominent crescent-shaped thickening at the posterior region of the embryonic disk marked the first polarization within this structure reflecting incipient cell ingression. Following differentiation of the epiblast, clearance of OCT4 from the three germ layers was observed at defined stages, suggesting correlations to lineage specification. In the endoderm, clearance of OCT4 was apparent from early during its formation at the primitive streak stage. The endoderm harbored progenitors of the "fourth germ layer," the primordial germ cells (PGCs), the only cells maintaining expression of OCT4 at the end of gastrulation. In the ectodermal and mesodermal cell lineages, OCT4 became undetectable at the neural groove and somite stage, respectively. As in the mouse, PGCs showed onset of c-kit expression when located in extraembryonal compartments. They appeared to follow the endoderm during extraembryonal allocation and the mesoderm on return to the genital ridge.

Lectin binding profiles of SSEA-4 enriched, pluripotent human embryonic stem cell surfaces

Background

Pluripotent human embryonic stem cells (hESCs) have the potential to form every cell type in the body. These cells must be appropriately characterized prior to differentiation studies or when defining characteristics of the pluripotent state. Some developmentally regulated cell surface antigens identified by monoclonal antibodies in a variety of species and stem cell types have proven to be side chains of membrane glycolipids and glycoproteins. Therefore, to examine hESC surfaces for other potential pluripotent markers, we used a panel of 14 lectins, which were chosen based on their specificity for a variety of carbohydrates and carbohydrate linkages, along with stage specific embryonic antigen-4 (SSEA-4), to determine binding quantitation by flow cytometry and binding localization in adherent colonies by immunocytochemistry.

Results

Enriching cells for SSEA-4 expression increased the percentage of SSEA-4 positive cells to 98–99%. Using enriched high SSEA-4-expressing hESCs, we then analyzed the binding percentages of selected lectins and found a large variation in binding percentages ranging from 4% to 99% binding. Lycopersicon (tomato)esculetum lectin (TL), Ricinus communis agglutinin (RCA), and Concanavalin A (Con A) bound to SSEA-4 positive regions of hESCs and with similar binding percentages as SSEA-4. In contrast, we found Dolichos biflorus agglutinin (DBA) and Lotus tetragonolobus lectin (LTL) did not bind to hESCs while Phaseolus vulgaris leuco-agglutinin (PHA-L), Vicia villosa agglutinin (VVA), Ulex europaeus agglutinin (UEA), Phaseolus vulgaris erythro-agglutinin (PHA-E), and Maackia amurensis agglutinin (MAA) bound partially to hESCs. These binding percentages correlated well with immunocytochemistry results.

Conclusion

Our results provide information about types of carbohydrates and carbohydrate linkages found on pluripotent hESC surfaces. We propose that TL, RCA and Con A may be used as markers that are associated with the pluripotent state of hESCs because binding percentages and binding localization of these lectins are similar to those of SSEA-4. Non-binding lectins, DBA and LTL, may identify differentiated cell types; however, we did not find these lectins to bind to pluripotent SSEA-4 positive hESCs. This work represents a fundamental base to systematically classify pluripotent hESCs, and in future studies these lectins may be used to distinguish differentiated hESC types based on glycan presentation that accompanies differentiation.

Development of Novel Markers for the Characterization of Chicken Primordial Germ Cells

This study was undertaken to develop novel markers for chicken primordial germ cells (PGCs), which are of potentially enormous value in transgenic research. Gonadal cells collected from 5.5-day-old chicken embryos were cultured in a Dulbecco's minimal essential medium and the PGC colonies formed during the primary culture period were subcultured three times. Characterization of the PGCs with the candidate marker reagents was performed on the mixed cell population 2 hours after seeding, after the primary culture period (day 10), and after the third passage (day 40). Mouse embryonic stem (ES) cells were used as controls. The cytochemical reagents investigated included periodic acid-Schiff (PAS) stain, antibodies to stage-specific embryonic antigens (SSEA-1, SSEA-3, and SSEA-4), antibody to epithelial membrane antigen (EMA)-1, antibodies to integrins alpha6 and beta1, several lectins (Solanum tuberosum agglutinin [STA], Dolichos biflorus agglutinin [DBA], concanavalin A agglutinin [ConA], and wheat germ agglutinin [WGA]), and double staining with antibodies to SSEA-1, SSEA-3, SSEA-4, integrin alpha6, or integrin beta1 and then with the lectin STA. Densitometric quantification was used to identify PGC-specific markers. The results showed that chicken PGCs were stained selectively by PAS and by antibodies to SSEA-1, SSEA-3, SSEA-4, EMA-1, integrin alpha6, and integrin beta1. The control mouse ES cells reacted with PAS, anti-SSEA-1, and anti-EMA-1 antibodies, as well as with antibodies to integrins alpha6 and beta1, but not with antibodies to SSEA-3 and SSEA-4. Chicken PGCs reacted with the lectins STA and DBA, but mouse ES cells reacted with STA and WGA. The results of double staining of PGC colonies subcultured three times showed that the intensity of staining was not altered by concomitant use of the marker reagents. This study demonstrated that, in addition to PAS and antibodies to SSEA-1 and EMA-1, new specific markers of chicken PGCs are recognized by the lectins STA and DBA and by antibodies to SSEA-3 and SSEA-4 and integrins alpha6 and beta1. Double staining using these newly developed markers might be the method of choice for rapid characterization of chicken PGCs.

Normal embryonic and germ cell development in mice lacking alpha 1,3-fucosyltransferase IX (Fut9) which show disappearance of stage-specific embryonic antigen 1

Stage-specific embryonic antigen 1 (SSEA-1), an antigenic epitope defined as a Lewis x carbohydrate structure, is expressed during the 8-cell to blastocyst stages in mouse embryos and in primordial germ cells, undifferentiated embryonic stem cells, and embryonic carcinoma cells. For many years, SSEA-1 has been implicated in the development of mouse embryos as a functional carbohydrate epitope in cell-to-cell interaction during morula compaction. In a previous study, alpha 1,3-fucosyltransferase IX (Fut9) exhibited very strong activity for the synthesis of Lewis x compared to other alpha 1,3-fucosyltransferases in an in vitro substrate specificity assay. Fut4 and Fut9 transcripts were expressed in mouse embryos. The Fut9 transcript was detected in embryonic-day-13.5 gonads containing primordial germ cells, but the Fut4 transcript was not. In order to identify the role of SSEA-1 and determine the key enzyme for SSEA-1 synthesis in vivo, we have generated Fut9-deficient (Fut9(-/-)) mice. Fut9(-/-) mice develop normally, with no gross phenotypic abnormalities, and are fertile. Immunohistochemical analysis revealed an absence of SSEA-1 expression in early embryos and primordial germ cells of Fut9(-/-) mice. Therefore, we conclude that expression of the SSEA-1 epitope in the developing mouse embryo is not essential for embryogenesis in vivo.

The culture and establishment of embryonic germ (EG) cell lines from Chinese mini swine

As a part of a basic research project on Xeno-transplantion, we have been engaged in the derivation of embryonic stem cell lines from Chinese mini swine. Here, we reported for the first time the establishment of two porcine EG cell lines (BPEG1 and BPEG2) from primordial germ cells of genital ridges of a 28 and a 27 d embryos respectively. Their pluripotent nature has been identified by colony morphology, marker characterization as well as by in vitro and in vivo differentiation. These porcine EG cells are potentially useful for further basic studies.

Germ cell development in Meishan and White Composite gilts

This study compared dynamics of the germ cell population in two swine breeds that differ in prolifacy, White Composite (WC) and Meishan (MS), during fetal and neonatal life and in mature sows. Germ cell populations developed in a similar pattern in these two diverse breeds during fetal life. Maximal germ cell number was observed at 90 days postcoitum (dpc) in both WC and MS gilts, and substantial oogonial apoptosis was evident thereafter with approximately 30% of maximal numbers present at 25 days postpartum (dpp). Neither gilt nor sow germ cell number was correlated with maternal ovulation rate. Postnatal MS gilts had larger pools of primordial follicles and consistently greater proportions and numbers of primary and secondary follicles compared to postnatal WC gilts, indicative of enhanced follicular recruitment and primordial follicle activation. Occasional antral follicles were present in MS ovaries by 25 dpp and numerous surface follicles were observed at 56 dpp in MS but not WC ovaries, indicative of more rapid ovarian maturation and early onset of puberty. Total germ cell number is unlikely to influence or to predict subsequent ovulation rate. These observations highlight important developmental events during late fetal and early postnatal life that prepare the ovarian environment for early onset of puberty and subsequent ovulation in MS gilts.

Lectin histochemistry shows fucosylated glycoconjugates in the primordial germ cells of Xenopus embryos

Previous works have shown that glycoconjugates with terminal fucose (Fuc) are located in the primordial germ cells (PGCs) of some mammals and might play a role in the migration and adhesion processes during development. The aim of this work was to identify the terminal Fuc moieties of Xenopus PGCs by means of three Fuc-binding lectins: from asparagus pea (LTA), gorse seed (UEA-I), and orange peel fungus (AAA). The histochemical procedures were also carried out after deglycosylation pretreatments: beta-elimination with NaOH to remove O-linked oligosaccharides; incubation with PNGase F to remove N-linked carbohydrate chains; and incubation with alpha(1,2)- and alpha(1,6)-fucosidase. The PGCs were always negative for LTA and UEA-I, two lectins that have the highest affinity for Fuc alpha(1,2)-linked. However, the PGCs were strongly labeled with AAA, which preferentially binds to Fuc with alpha(1,3) or alpha(1,4) linkages and to Fuc alpha(1,6)-linked to the proximal N-acetylglucosamine. There was fainter labeling with AAA when the sections were preincubated with alpha(1,6)-fucosidase, but the labeling remained strong when the sections were pretreated with alpha(1,2)fucosidase. When the beta-elimination procedure was carried out, the PGC labeling with AAA was slight. If the PNGase F incubation was performed, the PGCs remained moderately positive for AAA. These data suggest that the Xenopus PGCs have Fuc moieties in O- and N-linked oligosaccharides, including Fuc alpha(1,6) linked to the innermost GlcNAc, and that the Fuc was not in alpha(1,2)-linkage.

Pluripotent stem cells--model of embryonic development, tool for gene targeting, and basis of cell therapy

Embryonic stem (ES) cells are pluripotent cell lines with the capacity of self-renewal and a broad differentiation plasticity. They are derived from pre-implantation embryos and can be propagated as a homogeneous, uncommitted cell population for an almost unlimited period of time without losing their pluripotency and their stable karyotype. Murine ES cells are able to reintegrate fully into embryogenesis when returned into an early embryo, even after extensive genetic manipulation. In the resulting chimeric offspring produced by blastocyst injection or morula aggregation, ES cell descendants are represented among all cell types, including functional gametes. Therefore, mouse ES cells represent an important tool for genetic engineering, in particular via homologous recombination, to introduce gene knock-outs and other precise genomic modifications into the mouse germ line. Because of these properties ES cell technology is of high interest for other model organisms and for livestock species like cattle and pigs. However, in spite of tremendous research activities, no proven ES cells colonizing the germ line have yet been established for vertebrate species other than the mouse (Evans and Kaufman, 1981; Martin, 1981) and chicken (Pain et al., 1996). The in vitro differentiation capacity of ES cells provides unique opportunities for experimental analysis of gene regulation and function during cell commitment and differentiation in early embryogenesis. Recently, pluripotent stem cells were established from human embryos (Thomson et al., 1998) and early fetuses (Shamblott et al., 1998), opening new scenarios both for research in human developmental biology and for medical applications, i.e. cell replacement strategies. At about the same time, research activities focused on characteristics and differentiation potential of somatic stem cells, unravelling an unexpected plasticity of these cell types. Somatic stem cells are found in differentiated tissues and can renew themselves in addition to generating the specialized cell types of the tissue from which they originate. Additional to discoveries of somatic stem cells in tissues that were previously not thought to contain these kinds of cells, they also appear to be capable of developing into cell types of other tissues, but have a reduced differentiation potential as compared to embryo-derived stem cells. Therefore, somatic stem cells are referred to as multipotent rather than pluripotent. This review summarizes characteristics of pluripotent stem cells in the mouse and in selected livestock species, explains their use for genetic engineering and basic research on embryonic development, and evaluates their potential for cell therapy as compared to somatic stem cells.

Temporal and spatial localization patterns of Gata4 during porcine gonadogenesis

The zinc finger transcription factor Gata4, is associated with gonadal development in many species. The present study characterizes temporal and spatial localization of Gata4 throughout gonadogenesis in porcine embryos. Immunohistochemical studies illustrated that Gata4 protein is present in the coelomic epithelium prior to histological differentiation of the nascent bipotential gonad, marking the future site of both XX and XY porcine gonads. Many somatic cells of both XX and XY bipotential gonads continue to retain Gata4 immunoreactivity throughout sexual differentiation and subsequent gonadal development. Testicular cords were evident by 26 days postcoitum. Gata4 was present in Sertoli cells, identified by virtue of coexpression with Müllerian inhibiting substance and also interstitial cells including Leydig cells throughout fetal and postnatal life. Many somatic cells of the differentiating ovary including follicular cells also contained Gata4 protein throughout fetal and postnatal life. Gata4 was not present in germ cells, endothelial cells, or other undifferentiated mesenchymal cells of both XX and XY gonads. A population of Gata4-positive cells in the dorsal mesentery was continuous with the coelomic epithelium of the gonad. This localization pattern led to the hypothesis that a subpopulation of somatic cells in the dorsal mesentery moves toward the gonad. An in vitro cell migration assay demonstrated that Gata4-positive cells preferentially migrate toward explanted gonadal tissue, and morphological features of the developing gonad supported this hypothesis. This study illustrates that Gata4 is a very early marker for gonad formation, highlights species differences in temporal and spatial localization patterns, and suggests a potential role for Gata4 in the development of both XX and XY porcine gonads. Further, we suggest that mesenchymal cells of the dorsal mesentery may provide a source of somatic cells that migrate and incorporate into the gonad and contribute to various somatic cell lineages. Overall, the spatial and temporal localization patterns of Gata4 during porcine gonadogenesis implies a much earlier and wider role for Gata4 than previously reported in other species.

Effects of protease inhibitors and antioxidants on In vitro survival of porcine primordial germ cells

One of the problems associated with in vitro culture of primordial germ cells (PGCs) is the large loss of cells during the initial period of culture. This study characterized the initial loss and determined the effectiveness of two classes of apoptosis inhibitors, protease inhibitors, and antioxidants on the ability of porcine PGCs to survive in culture. Results from electron microscopic analysis and in situ DNA fragmentation assay indicated that porcine PGCs rapidly undergo apoptosis when placed in culture. Additionally, alpha(2)-macroglobulin, a protease inhibitor and cytokine carrier, and N:-acetylcysteine, an antioxidant, increased the survival of PGCs in vitro. While other protease inhibitors tested did not affect survival of PGCs, all antioxidants tested improved survival of PGCs (P: < 0.05). Further results indicated that the beneficial effect of the antioxidants was critical only during the initial period of culture. Finally, it was determined that in short-term culture, in the absence of feeder layers, antioxidants could partially replace the effect(s) of growth factors and reduce apoptosis. Collectively, these results indicate that the addition of alpha(2)-macroglobulin and antioxidants can increase the number of PGCs in vitro by suppressing apoptosis.

Long-term culture and characterization of goat primordial germ cells

While the culture and identification of primordial germ cells (PGCs) in mice is established, only limited investigations on PGCs in livestock have been reported. This study was performed to characterize goat PGCs after culture and cryopreservation. Goat PGCs were isolated from Day 32 fetuses and cultured on a continuous cell line of murine embryonal fibroblasts (STO) as feeder-cells in the presence of leukemia inhibitory factor (LIF). The PGCs proliferated slowly and showed colony formation in early passages. Frozen-thawed PGCs continued to proliferate when stem cell factor (SCF) was added to the culture medium. However, differentiation into epithelial-like polygonal cells or neuronal cells was observed after 1 or 2 passages. The PGCs of 1 female and 1 male cell line were characterized by immunocytochemistry. The PGCs showed positive staining for anti stage-specific embryonic antigen-1 (SSEA-1) and FMA-1 (monoclonal antibody produced against a glycoprotein cell surface antigen of the embryonal carcinoma Nulli SCC1), whereas the reactivity to alkaline phosphatase (AP), an established marker for PGCs in mice, was inconsistent. After differentiation, PGCs lost their positive reaction to SSEA-1, EMA-1 and AP. In conclusion, SSEA-1 and EMA-1 can be used as reliable markers for identifying goat PGCs in addition to morphological criteria. The results indicate that goat PGCs can be kept in long-term culture without losing their morphological characteristics and their positive reaction to SSEA-1 and EMA-1, thus providing a promising source of donor-karyoplasts for nuclear transfer procedures.

Establishment of pluripotent cell lines from vertebrate species--present status and future prospects

Pluripotent embryonic stem (ES) cells are undifferentiated cell lines derived from early embryos and are capable of unlimited undifferentiated proliferation in vitro. They retain the ability to differentiate into all cell types including germ cells in chimeric animals in vivo, and can be induced to form derivatives of all three germ layers in vitro. Mouse ES cells represent one of the most important tools in genetic research. Major applications include the targeted mutation of specific genes by homologous recombination and the discovery of new genes by gene trap strategies. These applications would be of high interest for other model organisms and also for livestock species. However, in spite of tremendous research activities, no proven ES cells colonizing the germ line have been established for vertebrate species other than mouse and chicken thus far. This review summarizes the current status of deriving pluripotent embryonic stem cell lines from vertebrates and recent developments in nuclear transfer technology, which may provide an alternative tool for genetic modification of livestock animals.

Chimeric pigs following blastocyst injection of transgenic porcine primordial germ cells

Porcine primordial germ cell (PGC) derived cell lines of WAPhGH-transgenic pigs have been established that were able to contribute to chimeras. PGCs were isolated from day 25 to 28 genital ridges of more than 30 individual transgenic fetuses in order to have an easy to follow marker gene. To support undifferentiated growth, cell lines were derived and stable maintained on STO no. 8 feeder cells, a murine embryonic fibroblast cell line expressing recombinant, membrane-bound porcine stem cell factor (SCF). Fifteen lines proliferated in an undifferentiated state up to passage 13; two lines were maintained for more than 23 passages. Cell staining experiments for differentiation markers in several cell lines, indicated the presence of pluripotent cells in prolonged cultures. Further characterization using karyotyping revealed a normal, euploid set of chromosomes in cells of passages 15 and higher. Pluripotency of freshly isolated, short-term (up to 24 hr before injection) and long-term cultured, frozen/thawed cells was tested by injection into day 6 recipient blastocysts to give rise to chimeric piglets. The injected embryos (n = 209) were endoscopically transferred into the uterine horns of 11 recipient gilts. Tissue analysis from 49 fetuses and eighteen liveborn piglets for PGC contribution in chimeras was carried out using PCR analysis for the presence of the marker transgene. Thirty-two fetuses showed detectable chimerism in up to five out of 12 tissues analyzed. Skin samples from eight piglets were positive for the transgene, four of them displayed coat colour chimerism.

Short communication: lack of stage-specific embryonic antigen-1 expression by bovine embryos and primordial germ cells

The objective of this study was to determine whether stage-specific embryonic antigen-1, a cellular marker commonly used to identify murine undifferentiated embryonic cells, is also a useful marker for bovine pluripotent cells. Expression of stage-specific embryonic antigen-1 was examined by indirect immunohistochemistry on bovine preimplantation embryos and on primordial germ cells contained in the genital ridge. Expression of stage-specific embryonic antigen-1 was not observed in any of the cleavage-stage bovine embryos examined, including one-cell, two-cell, four-cell, eight-cell, morula, and blastocyst stages, nor in tissue sections of bovine genital ridges collected from embryos on d 34, 37, and 40 of gestation. As expected, expression of stage-specific embryonic antigen-1 was detected on murine preimplantation embryos and on murine teratocarcinoma cells. Results of this study indicate that, unlike in the mouse, stage-specific embryonic antigen-1 is not a useful cellular marker for pluripotent bovine embryonic cells or bovine primordial germ cells.