Role of fibroblast growth factors and their receptors in mouse primordial germ cell growth

In vitro culture of reptile PGCS to preserve endangered species

Primordial germ cells (PGCs), are the source of gametes in vertebrates. There are similarities in the development of PGCs of reptiles with avian and mammalian species PGCs development. PGCs culture has been performed for avian and mammalian species but there is no report for reptilian PGCs culture. In vitro culture of PGCs is needed to produce transgenic animals, preservation of endangered animals and for studies on cell behaviour and research on fertility. Reptiles are traded as exotic pets and a source of food and they are valuable for their skin and they are useful as model for medical research. Transgenic reptile has been suggested to be useful for pet industry and medical research. In this research different aspects of PGCs development was compared in three main classes of vertebrates including mammalian, avian and reptilian species. It is proposed that a discussion on similarities between reptilian PGCs development with avian and mammalian species helps to find clues for studies of reptilian PGCs development details and finding an efficient protocol for in vitro culture of reptilian PG.

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.

Mouse Primordial Germ Cells: In Vitro Culture and Conversion to Pluripotent Stem Cell Lines

Primordial germ cells (PGCs) are the embryonic precursors of the gametes. Despite decades of research, in vitro culture of PGCs remains a major challenge and has previously relied on undefined components such as serum and feeders. Notably, PGCs cultured for extended periods do not maintain their lineage identity but instead undergo conversion to form pluripotent stem cell lines called embryonic germ (EG) cells in response to LIF/STAT3 signaling. Here we report both established and new methodologies to derive EG cells, in a range of different conditions. We show that basic fibroblast growth factor is not required for EG cell conversion. We detail the steps taken in our laboratory to systematically remove complex components and establish a fully defined protocol that allows efficient conversion of isolated PGCs to pluripotent EG cells. In addition, we demonstrate that PGCs can adhere and proliferate in culture without the support of feeder cells or serum. This may well suggest novel approaches to establishing short-term culture of PGCs in defined conditions.

Regulation of Folliculogenesis by Growth Factors in Piglet Ovary Exposed Prenatally to β-Hydroxy-β-Methylbutyrate (HMB)

Β-hydroxy-β-methylbutyrate (HMB) is one of the leucine metabolites with protein anabolic effects which makes it very popular among athletes. Previously, it was shown that HMB administered during the prenatal period reduced the pool of primordial follicles and increased the proportion of developing follicles in newborn piglets. This work is a further step to understand these morphological alterations. Therefore, the aim of this study was to examine the effect of prenatal HMB treatment on the expression of the Kit ligand, BMP-4, bFGF, and the IGF-1/IGF-1R system which are the main growth factors controlling follicular development. Excised ovaries from 12 newborn piglets, originated from the control (n=6) and HMB-treated (n=6) sows were used for immunohistochemical and western-blot analysis. The tested proteins were localized within egg nests and ovarian follicles. Furthermore, the western-blot assay indicated higher BMP-4, Kit ligand, and IGF-1R expression, while the level of bFGF and IGF-1 proteins decreased after HMB dietary treatment. These findings show that HMB included into sow diet can modulate the expression of growth factors and thereby alter ovarian morphology in offspring. Therefore, this study opens a discussion about the benefits and risks of the diet supplemented with HMB and its potential application in medicine and animal husbandry, and further research is necessary in this area.

Mechanisms by which fibroblast growth factor 20 improves motor performance in a mouse model of Parkinson's disease

Genome-wide studies have reported that Parkinson’s disease is associated with abnormal expression of various growth factors. In this study, male C57BL/6 mice aged 10 weeks were used to establish Parkinson’s disease models using an intraperitoneal injection of 60 mg/kg 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. 28 days later, 10 or 100 ng fibroblast growth factor 20 was injected intracerebroventricularly. The electrophysiological changes in the mouse hippocampus were recorded using a full-cell patch clamp. Expression of Kv4.2 in the substantia nigra was analyzed using a western blot assay. Serum malondialdehyde levels were analyzed by enzyme-linked immunosorbent assay. The motor coordination of mice was evaluated using the rotarod test. The results showed that fibroblast growth factor 20 decreased A-type potassium current in neurons of the substantia nigra, increased long-term potentiation amplitude in the hippocampus, and downregulated Kv4.2 expression. A high dose of fibroblast growth factor 20 reduced serum malondialdehyde levels and enhanced the motor coordination of mice. These findings confirm that fibroblast growth factor 20 has a therapeutic effect on the toxicity induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, and its mechanism of action is associated with the inhibition of A-type K⁺ currents and Kv4.2 expression. All animal procedures were approved by the Animal Care and Use Committee of Qilu Hospital of Shandong University, China in 2017 (approval No. KYLL-2017-0012).

Female reproductive system morphology of crab-eating fox (Cerdocyon thous) and cryopreservation of genetic material for animal germplasm bank enrichment

The sprawl of the urbanization and road network process without building ecological corridors contributes to the high mortality rates and a threat to the population decline of wild species such as the crab-eating fox. A strategy for the ex situ conservation is the study of the reproductive biology of the species and cryopreservation of their genetic heritage through the formation of an animal germplasm bank. This research is in accordance with the principles adopted by Brazilian College of Animal Experimentation. Reproductive systems of Cerdocyon thous females (n = 7) were examined macroscopically and microscopically by histological techniques and scanning electron microscopy. Gross features showed the shape of the ovaries was similar to a bean, and the elongated oviducts lengths were between 5 and 8 cm, with body of the uterus (3 cm) with long and narrow uterine horns (9-11 cm). The cervix was as a single annular conformation carrying out communication between the uterus and the vagina. The vagina has lengthened and circular muscle and the vulva with dense anatomical conformation with a quite pronounced clitoris. In addition, with regard to the establishment of a cell line (fibroblasts) for the gene bank enrichment, cells showed a low clonogenic capacity, especially when compared to domestic dogs, which can be explained by "in vitro" environment, age and diet of the animal. However, it was possible to create a bank of limited cell number. This study had morphological and preservationist character and aimed to help at long term in the conservation of wild animal's genetic resources.

Wnt/β-catenin signaling pathway activation is required for proliferation of chicken primordial germ cells in vitro

Here, we investigated the role of the Wnt/β-catenin signaling pathway in chicken primordial germ cells (PGCs) in vitro. We confirmed the expression of Wnt signaling pathway-related genes and the localization of β-catenin in the nucleus, revealing that this pathway is potentially activated in chicken PGCs. Then, using the single-cell pick-up assay, we examined the proliferative capacity of cultured PGCs in response to Wnt ligands, a β-catenin-mediated Wnt signaling activator (6-bromoindirubin-3′-oxime [BIO]) or inhibitor (JW74), in the presence or absence of basic fibroblast growth factor (bFGF). WNT1, WNT3A, and BIO promoted the proliferation of chicken PGCs similarly to bFGF, whereas JW74 inhibited this proliferation. Meanwhile, such treatments in combination with bFGF did not show a synergistic effect. bFGF treatment could not rescue PGC proliferation in the presence of JW74. In addition, we confirmed the translocation of β-catenin into the nucleus by the addition of bFGF after JW74 treatment. These results indicate that there is signaling crosstalk between FGF and Wnt, and that β-catenin acts on PGC proliferation downstream of bFGF. In conclusion, our study suggests that Wnt signaling enhances the proliferation of chicken PGCs via the stabilization of β-catenin and activation of its downstream genes.

Physiologic Course of Female Reproductive Function: A Molecular Look into the Prologue of Life

The genetic, endocrine, and metabolic mechanisms underlying female reproduction are numerous and sophisticated, displaying complex functional evolution throughout a woman's lifetime. This vital course may be systematized in three subsequent stages: prenatal development of ovaries and germ cells up until in utero arrest of follicular growth and the ensuing interim suspension of gonadal function; onset of reproductive maturity through puberty, with reinitiation of both gonadal and adrenal activity; and adult functionality of the ovarian cycle which permits ovulation, a key event in female fertility, and dictates concurrent modifications in the endometrium and other ovarian hormone-sensitive tissues. Indeed, the ultimate goal of this physiologic progression is to achieve ovulation and offer an adequate environment for the installation of gestation, the consummation of female fertility. Strict regulation of these processes is important, as disruptions at any point in this evolution may equate a myriad of endocrine-metabolic disturbances for women and adverse consequences on offspring both during pregnancy and postpartum. This review offers a summary of pivotal aspects concerning the physiologic course of female reproductive function.

Very small embryonic-like stem cells as a novel developmental concept and the hierarchy of the stem cell compartment

Our current understanding of stem cells suffers from a lack of precision, as the stem cell compartment is a broad continuum between early stages of development and adult postnatal tissues, and it is not fully understood how this transition occurs. The definition of stem cell pluripotency is adapted from embryology and excludes the possibility that some early-development stem cells with pluri- and/or multipotential differentiation potential may reside in postnatal tissues in a dormant state in which they are protected from uncontrolled proliferation and thus do not form teratomas or have the ability to complement blastocyst development. We will discuss the concept that a population of very small embryonic-like stem cells (VSELs) could be a link between early-development stages and adult stem cell compartments and reside in a quiescent state in adult tissues. The epigenetic mechanism identified that changes expression of certain genes involved in insulin/insulin-like growth factor signaling (IIS) in VSELs, on the one hand, keeps these cells quiescent in adult tissues and, on the other hand, provides a novel view of the stem cell compartment, IIS, tissue/organ rejuvenation, aging, and cancerogenesis.

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.

Pluripotent and multipotent stem cells in adult tissues

One of the most intriguing questions in stem cell biology is whether pluripotent stem cells exist in adult tissues. Several groups of investigators employing i) various isolation protocols, ii) detection of surface markers, and iii) experimental in vitro and in vivo models, have reported the presence of cells that possess a pluripotent character in adult tissues. Such cells were assigned various operational abbreviations and names in the literature that added confusion to the field and raised the basic question of whether these are truly distinct or overlapping populations of the same primitive stem cells. Unfortunately, these cells were never characterized side-by-side to address this important issue. Nevertheless, taking into consideration their common features described in the literature, it is very likely that various investigators have described overlapping populations of developmentally early stem cells that are closely related. These different populations of stem cells will be reviewed in this paper.

Basic fibroblast growth factor suppresses meiosis and promotes mitosis of ovarian germ cells in embryonic chickens

Basic fibroblast growth factor (bFGF or FGF2) plays diverse roles in regulating cell proliferation, migration and differentiation during embryo development. In this study, the effect of bFGF on ovarian germ cell development was investigated in the embryonic chicken by in vitro and in vivo experiments. Results showed that a remarkable decrease in bFGF expression in the ovarian cortex was manifested during meiosis progression. With ovary organ culture, we revealed that meiosis was initiated after retinoic acid (RA) treatment alone but was decreased after combined bFGF treatment that was detected by real time RT-PCR, fluorescence immunohistochemistry and Giemsa staining. Further, no significant difference in mRNA expression of either RA metabolism-related enzymes (Raldh2 and Cyp26b1) or RA receptors was displayed after bFGF challenge. This result suggests that the suppression of bFGF on meiosis was unlikely through inhibition of RA signaling. In addition, as a mitogen, bFGF administration increased germ cell proliferation (via BrdU incorporation) in cultured organ or cells in vitro and also in developing embryos in vivo. In contrast, blockade of bFGF action by SU5402 (an FGFR1 antagonist) or inhibition of protein kinase C signaling showed inhibited effect of bFGF on mitosis. In conclusion, bFGF suppresses RA-induced entry of germ cells into meiosis to ensure embryonic ovarian germ cells to maintain at undifferentiated status and accelerate germ cell proliferation by binding with FGFR1 involving PKC activation in the chicken.

Reinterpretation of evidence advanced for neo-oogenesis in mammals, in terms of a finite oocyte reserve

The central tenet of ovarian biology, that the oocyte reserve in adult female mammals is finite, has been challenged over recent years by proponents of neo-oogenesis, who claim that germline stem cells exist in the ovarian surface epithelium or the bone marrow. Currently opinion is divided over these claims, and further scrutiny of the evidence advanced in support of the neo-oogenesis hypothesis is warranted - especially in view of the enormous implications for female fertility and health. This article contributes arguments against the hypothesis, providing alternative explanations for key observations, based on published data. Specifically, DNA synthesis in germ cells in the postnatal mouse ovary is attributed to mitochondrial genome replication, and to DNA repair in oocytes lagging in meiotic progression. Lines purported to consist of germline stem cells are identified as ovarian epithelium or as oogonia, from which cultures have been derived previously. Effects of ovotoxic treatments are found to negate claims for the existence of germline stem cells. And arguments are presented for the misidentification of ovarian somatic cells as de novo oocytes. These clarifications, if correct, undermine the concept that germline stem cells supplement the oocyte quota in the postnatal ovary; and instead comply with the theory of a fixed, unregenerated reserve. It is proposed that acceptance of the neo-oogenesis hypothesis is erroneous, and may effectively impede research in areas of ovarian biology. To illustrate, a novel explanation that is consistent with orthodox theory is provided for the observed restoration of fertility in chemotherapy-treated female mice following bone marrow transplantation, otherwise interpreted by proponents of neo-oogenesis as involving stimulation of endogenous germline stem cells. Instead, it is proposed that the chemotherapeutic regimens induce autoimmunity to ovarian antigens, and that the haematopoietic chimaerism produced by bone marrow transplantation circumvents activation of an autoreactive response, thereby rescuing ovarian function. The suggested mechanism draws from animal models of autoimmune ovarian disease, which implicate dysregulation of T cell regulatory function; and from a surmised role for follicular apoptosis in the provision of ovarian autoantigens, to sustain self-tolerance during homeostasis. This interpretation has direct implications for fertility preservation in women undergoing chemotherapy.

Epiblast/germ line hypothesis of cancer development revisited: lesson from the presence of Oct-4+ cells in adult tissues

The morphology of several tumors mimics developmentally early tissues; tumors often express early developmental markers characteristic for the germ line lineage. Recently, our group identified a population of very small stem cells (SCs) in murine bone marrow (BM) and other adult organs that express several markers characteristic for epiblast/germ line-derived SCs. We named these rare cells "Very Small Embryonic/Epiblast-like Stem Cells (VSELs)." We hypothesized that these cells that express both epiblast and germ line markers are deposited during early gastrulation in developing tissues and organs and play an important role in the turnover of tissue-committed (TC) SCs. To support this, we envision that the germ line is not only the origin of SCs, but also remains as a scaffold or back-up for the SC compartment in adult life. Furthermore, we noticed that VSELs are protected from uncontrolled proliferation and teratoma formation by a unique DNA methylation pattern in some developmentally crucial imprinted genes, which show hypomethylation or erasure of imprints in paternally methylated genes and hypermethylation of imprints in the maternally methylated. In pathological situations, however, we hypothesize that VSELs could be involved in the development of several malignancies. Therefore, potential involvement of VSELs in cancerogenesis could support century-old concepts of embryonic rest- or germ line-origin hypotheses of cancer development. However, we are aware that this working hypothesis requires further direct experimental confirmation.

A self-renewal program controls the expansion of genetically unstable cancer stem cells in pluripotent stem cell-derived tumors

Human germ cell tumors are often metastatic, presumably due to distal site tumor growth by cancer stem cells. To determine whether cancer stem cells can be identified in a transplantation model of testicular germ cell tumor, we transplanted murine embryonic germ cells (EGCs) into the testis of adult severe combined immunodeficient mice. Transplantation resulted in a locally invasive solid tumor, with a cellular component that generated secondary tumors upon serial transplantation. The secondary tumors were invariably metastatic, a feature not observed in the primary tumors derived from EGCs. To characterize the differences between EGCs and the tumor-derived stem cells, we performed karyotype and microarray analysis. Our results show that generation of cancer stem cells is associated with the acquisition of nonclonal genomic rearrangements not found in the originating population. Furthermore, pretreatment of EGCs with a potent inhibitor of self-renewal, retinoic acid, prevented tumor formation and the emergence of these genetically unstable cancer stem cells. Microarray analysis revealed that EGCs and first- and second-generation cancer stem cells were highly similar; however, approximately 1,000 differentially expressed transcripts could be identified corresponding to alterations in oncogenes and genes associated with motility and development. Combined, the data suggest that the activation of oncogenic pathways in a cellular background of genetic instability, coupled with an inherent ability to self-renew, is involved in the acquisition of metastatic behavior in the cancer stem cell population of tumors derived from pluripotent cells.

Bone-marrow-derived stem cells--our key to longevity?

Bone marrow (BM) was for many years primarily regarded as the source of hematopoietic stem cells. In this review we discuss current views of the BM stem cell compartment and present data showing that BM contains not only hematopoietic but also heterogeneous non-hematopoietic stem cells. It is likely that similar or overlapping populations of primitive non-hematopoietic stem cells in BM were detected by different investigators using different experimental strategies and hence were assigned different names (e.g., mesenchymal stem cells, multipotent adult progenitor cells, or marrow-isolated adult multilineage inducible cells). However, the search still continues for true pluripotent stem cells in adult BM, which would fulfill the required criteria (e.g. complementation of blastocyst development). Recently our group has identified in BM a population of very small embryonic-like stem cells (VSELs), which express several markers characteristic for pluripotent stem cells and are found during early embryogenesis in the epiblast of the cylinder-stage embryo.

The role of exogenous fibroblast growth factor-2 on the reprogramming of primordial germ cells into pluripotent stem cells

The germ cell lineage is a specified cell population that passes through a series of differentiation steps before giving rise, eventually, to either eggs or sperm. We have investigated the manner in which primordial germ cells (PGCs) are reprogrammed in vitro to form pluripotent stem cells in response to exogenous fibroblast growth factor-2 (FGF-2). The response is dependent on time of exposure and concentration of FGF-2. PGCs isolated in culture show a motile phenotype and lose any expression of a characteristic germ cell marker, mouse vasa homolog. Subsequently, some but not all of the cells show further changes of phenotype, accompanied by changes in expression of endogenous FGF-2 and up-regulation of its receptor, fibroblast growth factor receptor-3, in the nucleus. We propose that it is from this reprogrammed component of the now heterogeneous PGC population that pluripotent stem cells arise.

The roles of FGF signaling in germ cell migration in the mouse

Fibroblast growth factor (FGF) signaling is thought to play a role in germ cell behavior. FGF2 has been reported to be a mitogen for primordial germ cells in vitro, whilst combinations of FGF2, steel factor and LIF cause cultured germ cells to transform into permanent lines of pluripotent cells resembling ES cells. However, the actual function of FGF signaling on the migrating germ cells in vivo is unknown. We show, by RT-PCR analysis of cDNA from purified E10.5 germ cells, that germ cells express two FGF receptors: Fgfr1-IIIc and Fgfr2-IIIb. Second, we show that FGF-mediated activation of the MAP kinase pathway occurs in germ cells during their migration, and thus they are potentially direct targets of FGF signaling. Third, we use cultured embryo slices in simple gain-of-function experiments,using FGF ligands, to show that FGF2, a ligand for FGFR1-IIIc, affects motility, whereas FGF7, a ligand for FGFR2-IIIb, affects germ cell numbers. Loss of function, using a specific inhibitor of FGF signaling, causes increased apoptosis and inhibition of cell shape change in the migrating germ cells. Lastly, we confirm in vivo the effects seen in slice cultures in vitro,by examining germ cell positions and numbers in embryos carrying a loss-of-function allele of FGFR2-IIIb. In FGFR2-IIIb-/- embryos,germ cell migration is unaffected, but the numbers of germ cells are significantly reduced. These data show that a major role of FGF signaling through FGFR2-IIIb is to control germ cell numbers. The data do not discriminate between direct and indirect effects of FGF signaling on germ cells, and both may be involved.

Effects of growth factors and feeder cells on porcine primordial germ cells in vitro

As embryonic stem (ES) cells are not available in swine, embryonic germ (EG) cells derived from primordial germ cells (PGCs) are an alternate source of pluripotent embryonic cells for genetic modification through homologous recombination. Although morphological and biochemical characteristics are similar between ES and EG cells, culture conditions are quite different. To optimize the culture condition for the establishment of porcine EG cells, porcine PGCs were cultured in vitro with various combinations of growth factors (leukemia inhibitory factor [LIF], stem cell factor [SCF], and basic fibroblast growth factor [bFGF]) and on different kinds of feeder cells (STO, TM(4), Sl/Sl(4) m220, porcine embryonic fibroblasts, and COS-7 cells). Optimal results were obtained when all three growth factors (LIF, SCF, and bFGF) were present in the media. Also, feeder cells expressing membrane-bound SCF are required for survival and establishment of porcine EG cells. Therefore, a combination of growth factors and proper feeder cells are critical for the establishment of undifferentiated porcine EG cells.

Analysis of Hypomorphic KitlSl Mutants Suggests Different Requirements for KITL in Proliferation and Migration of Mouse Primordial Germ Cells1

Germ cell development in mice is initiated when a small number of primordial germ cells (PGCs) are set aside from somatic cells during gastrulation. In the subsequent 4 to 5 days, PGCs enter the hindgut, undergo a directed migration away from the hindgut into the developing gonads, and undergo a massive increase in cell number. It is well established that Kit ligand (KITL, also known as stem cell factor and mast cell growth factor) is required for the survival and proliferation of PGCs. However, there is little information on a direct role for KITL in PGC migration. By comparing the effects of multiple Kitl mutations, including two N-ethyl-N-nitrosourea-induced hypomorphic mutations, we were able to distinguish stages of PGC development that are preferentially affected by certain mutations. We provide evidence that the requirements for KITL in proliferation are different in PGCs before and after they start migrating, and different levels of KITL function are required to support PGC proliferation and migration. This study illustrates the usefulness of an allelic series of mutations to dissect developmental processes and suggests that these mutants may be useful for further studies of molecular mechanisms of KITL functions in gametogenesis.

Evaluating human embryonic germ cells: concord and conflict as pluripotent stem cells

The realization of cell replacement therapy derived from human pluripotent stem cells requires full knowledge of the starting cell types as well as their differentiated progeny. Alongside embryonic stem cells, embryonic germ cells (EGCs) are an alternative source of pluripotent stem cell. Since 1998, four groups have described the derivation of human EGCs. This review analyzes the progress on derivation, culture, and differentiation, drawing comparison with other pluripotent stem cell populations.

Establishment of oocyte population in the fetal ovary: primordial germ cell proliferation and oocyte programmed cell death

Strict control of cell proliferation and cell loss is essential for the coordinated functions of different cell populations in complex multicellular organisms. Oogenesis is characterized by a first phase occurring during embryo-fetal life and in common with spermatogenesis, during which mitotic proliferation of the germline stem cells, the primordial germ cells (PGC), prevails over germ cell death. The result is the formation of a relatively high number of germ cells depending on the species, ready to enter sex specific differentiation. In the female, PGC enter into meiosis and become oocytes, thereby ending their stem cell potential. After entering into meiosis in the fetal ovary, oocytes pass through leptotene, zygotene and pachytene stages before arresting in the last stage of meiotic prophase I, the diplotene or dictyate stage at about the time of birth. The most part of oocytes die during the fetal period or shortly after birth. It is widely accepted that in mammals a female is born with a fixed number of oocytes within the ovaries, which over the years progressively decreases without possibility for renewal. Once the oocyte reserve has been exhausted, ovarian senescence, driving what is referred to as the menopause in women, rapidly ensues. The fertile lifespan of a female depends by the size of the oocyte pool at birth and the rapidity of the oocyte pool depletion. Which mechanisms control PGC proliferation? Why do most of the oocytes die during fetal life and what are the mechanisms of such massive degeneration? Is it possible to prolong the lifespan of a female by reducing oocyte lost during the fetal life? This review reports some of the most recent results obtained in an attempt to answer these questions.

Autocrine and paracrine mechanisms regulating primordial germ cell proliferation

Although several mitogens and survival factors have been previously shown to act on primordial germ cells (PGCs) in culture, it is not clear whether they are responsible for controlling proliferation of PGCs in the embryo. We show here that during their migratory phase, PGCs do not express FGF-4, FGF-8, or FGF-17, but these FGFs are expressed by neighboring cells. Thus, any FGF action on migrating PGCs would appear to be through a paracrine mechanism. We found that after entering into the gonads, PGCs start to express FGF-4 and FGF-8. On this basis, we hypothesize that FGF signaling is involved in both a paracrine manner in initiating PGC proliferation during their migration and an autocrine manner in sustaining PGC proliferation after their arrival in the gonads. We then studied the role of soluble stem cell factor (SCF), which acts as a survival factor or a mitogen in culture, to determine whether it interacts with FGFs. We found that SCF has a complex effect on PGC proliferation. On one hand, soluble SCF promoted PGC proliferation synergistically with FGF in the absence of membrane-bound SCF. Conversely, soluble SCF inhibited FGF-stimulated proliferation of PGCs in the presence of membrane-bound SCF. We account for these findings in a model involving regulation of PGC proliferation, in which SCF modulates the response to FGFs.

Turning germ cells into stem cells

Primordial germ cells (PGCs), the embryonic precursors of the gametes of the adult animal, can give rise to two types of pluripotent stem cells. In vivo, PGCs can give rise to embryonal carcinoma cells, the pluripotent stem cells of testicular tumors. Cultured PGCs exposed to a specific cocktail of growth factors give rise to embryonic germ cells, pluripotent stem cells that can contribute to all the lineages of chimeric embryos including the germline. The conversion of PGCs into pluripotent stem cells is a remarkably similar process to nuclear reprogramming in which a somatic nucleus is reprogrammed in the egg cytoplasm. Understanding the genetics of embryonal carcinoma cell formation and the growth factor signaling pathways controlling embryonic germ cell derivation could tell us much about the molecular controls on developmental potency in mammals.

Regulation of the proliferation of cocultured gonocytes and Sertoli cells by retinoids, triiodothyronine, and intracellular signaling factors: differences between fetal and neonatal cells

The regulation of early fetal germ cell growth has not been studied in cell culture, probably due to the poor survival of these cells. However, cell culture is the only system in which the control of cell growth can be studied independently of the influence of secreted testicular factors, which are diluted in the medium. We successfully cultured dispersed testicular cells from 16.5-day-old rat fetuses in defined medium and compared the growth of these cells with that of cells from 3-day-old neonates. In this system, fetal gonocytes displayed low levels of mitotic activity and their numbers remained stable. In contrast, neonatal gonocytes displayed high levels of mitotic activity and increased in number, these characteristics resembling those observed in vivo. We found that retinoic acid had deleterious effects on the number of gonocytes but did not affect Sertoli cell proliferation in fetal and neonatal cell cultures. Moreover, in fetal cell cultures, the decrease in the number of gonocytes resulted from a decrease in mitotic activity, probably due to a direct effect of retinoids on fetal gonocytes. Among the selective agonists for the retinoic acid receptor (RARalpha agonist, RARbeta agonist, and RARgamma agonist) and the retinoic X receptor (pan-RXR agonist) tested, only the RARalpha agonist reproduced the effects of retinoic acid at concentrations lower than its Kd value in both fetal and neonatal cell cultures. As both RARalpha and RXRalpha are present in fetal and neonatal gonocytes, we suggest that retinoic acid exerts its effects on gonocytes via a RARalpha-RXRalpha heterodimer, with RARalpha functioning as an active partner and RXRalpha as a passive partner. In this culture system, we show for the first time that triiodothyronine (T3) inhibits testicular fetal Sertoli cell and germ cell growth. We also tested intracellular signaling factors and found that a cAMP analog increased Sertoli cell proliferation and germ cell survival in both fetal and neonatal cells whereas phorbol esters (PMA) strongly inhibited the proliferation of fetal but not of neonatal gonocytes. None of the tested factors (T3, dbcAMP, and PMA) seemed to interact with the all-trans retinoic acid pathway. Thus, fetal gonocytes and neonatal gonocytes differ in intrinsic properties, and their growth is not regulated in the same manner. Despite their low level of mitotic activity, fetal gonocytes were more sensitive to various factors than neonatal gonocytes.

Conditional loss of PTEN leads to testicular teratoma and enhances embryonic germ cell production

The tumor suppressor gene PTEN, which is frequently mutated in human cancers, encodes a lipid phosphatase for phosphatidylinositol 3,4,5-triphosphate [PtdIns(3,4,5)P3] and antagonizes phosphatidylinositol 3 kinase. Primordial germ cells (PGCs), which are the embryonic precursors of gametes, are the source of testicular teratoma. To elucidate the intracellular signaling mechanisms that underlie germ cell differentiation and proliferation, we have generated mice with a PGC-specific deletion of the Pten gene. Male mice that lacked PTEN exhibited bilateral testicular teratoma, which resulted from impaired mitotic arrest and outgrowth of cells with immature characters. Experiments with PTEN-null PGCs in culture revealed that these cells had greater proliferative capacity and enhanced pluripotent embryonic germ (EG) cell colony formation. PTEN appears to be essential for germ cell differentiation and an important factor in testicular germ cell tumor formation.

Male germ line stem cells: from cell biology to cell therapy

Research using stem cells has several applications in basic biology and clinical medicine. Recent advances in the establishment of male germ line stem cells provided researchers with the ability to identify, isolate, maintain, expand and differentiate the spermatogonia, the primitive male germ cells, as cell lines under in vitro conditions. The ability to culture and manipulate stem cell lines from male germ cells has gradually facilitated research into spermatogenesis and male infertility, to an extent beyond that facilitated by the use of somatic stem cells. After the introduction of exogenous genes, the spermatogonial cells can be transplanted into the seminiferous tubules of recipients, where the transplanted cells can contribute to the offspring. The present review concentrates on the origin, life cycle and establishment of stem cell lines from male germ cells, as well as the current status of transplantation techniques and the application of spermatogonial stem cell lines.

Basic fibroblast growth factor modulates the expression of glycophorin A and c-kit and inhibits erythroid differentiation in K562 cells

Basic fibroblast growth factor (bFGF) is produced by bone marrow stromal cells as well as by normal and leukemic hematopoietic cells. In this study, we examine the direct effects of bFGF on erythroid differentiation in K562 cells in order to determine whether bFGF can promote the expression of a primitive phenotype. Low levels of bFGF inhibited erythroid differentiation as evidenced by decreased expression of glycophorin A and increased expression of c-kit. bFGF also increased both the numbers and the sizes of colonies of K562 cells in soft agar assays. The addition of TGF-beta to these cells induced erythroid differentiation which resulted in an increase in glycophorin A and a decrease in c-kit. The simultaneous addition of bFGF and TGF-beta to K562 cells prevented both the TGF-beta-mediated increase in glycophorin A expression and the decrease in c-kit expression associated with erythroid differentiation. bFGF antagonised the TGF-beta-mediated promotion of erythroid differentiation in K562 cells in a dose dependent manner and these two cytokines counteracted each other on an approximately molar basis. These results indicate that bFGF alone increases expression of c-kit and promotes a primitive phenotype in K562 cells. In addition, bFGF counteracts the effects of differentiation-inducing cytokines, such as TGF-beta, on hematopoietic cells. It is therefore possible that enhanced production of bFGF by leukemic cells could contribute to their neoplastic phenotype by opposing the effects of negative regulators or cytokines that induce differentiation.

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.

Oocyte attrition

During oogenesis, germ cell numbers sharply decrease when meiosis is initiated. There is solid evidence (DNA ladders, in situ detection) that this loss is through apoptosis. Oocyte apoptosis appears to hit mitotic primordial germ cells (PGC), pachytene oocytes and early primordial follicles. The control of oocyte apoptosis is not fully understood, although survival factors (LIF, kit ligand and FGF), as well as death inducing factors (fas ligand, TGFbeta), have been identified. Fas ligand binding on oocytic fas may result in caspase 8 activation. Two pathways inducing oocyte apoptosis may then be operating. In the first one, activated caspase 8 will induce activation of executioner caspases. In the second one, activated caspase 8 will trigger the cleavage of the bcl(2) family member Bid, which will act on mitochondria, resulting in cytochrome c release, caspase 9 activation and finally, activation of all executioner caspases. As a consequence of caspase activation, alterations in the cell nucleus (DNAse activation, PARP fragmentation), in the cell cytoskeleton (lamin) and cell metabolism will occur, producing cell death. During folliculogenesis, germ cell loss, owing to oocyte apoptosis, has been postulated within primordial and preantral follicles. Its regulatory mechanisms may be even more complex than those operating in foetal oocytes since additional control factors include EGF/TGFalpha and bcl(2) (survival) and activin (death inducer). In contrast, oocytes from antral follicles appear to be very unsensitive to death inducing stimuli.

Soluble factors and the emergence of chick primordial germ cells in vitro

Previous observations obtained from a culture of blastodermal cells on a mouse fibroblast feeder layer (STO) suggested that STO cells provide a factor or factors that facilitate development of avian primordial germ cells (PGC) from dispersed embryo cells. The purpose of the current study was to test the hypothesis that soluble factors produced by STO cells are responsible, at least in part, in supporting the development of PGC in culture and to examine the effect of stem cell factor (SCF), ciliary neurotrophic factor (CNTF), and basic fibroblast growth factor (bFGF) in the development of PGC in culture. Blastodermal cells on gelatin-coated plastic or on feeder layers of CV-1 cells yielded a small number of PGC. When blastodermal cells were cultured on STO cells, a marked increase in PGC was observed. The addition of STO cell-conditioned medium (STO-CM) to blastodermal cells cultured on gelatin-coated plastic and on feeder layers of CV-1 cells resulted in a significant increase in the number of PGC, indicating that soluble factors produced by STO cells can enhance the development of chicken PGC in culture. Supplementation of blastodermal cells with SCF (100 ng/mL) or CNTF (2 ng/mL) or with CNTF and SCF together resulted in a significant increase in the number of PGC after 48 h of culture on feeder layers of CV-1 cells. However, addition of bFGF (100 ng/mL) did not increase PGC. We concluded from these observations that the culture of blastodermal cells on feeder layers of STO and CV-1 cells can be used as a useful biological system in examining the regulatory factors that govern the ontogeny of the germ cell lineage in the avian embryo.