Exogenous supplementation of Activin A enhances germ cell differentiation of human embryonic stem cells

Current Progress in Stem Cell Therapy for Male Infertility

Infertility has become one of the most common issues worldwide, which has negatively affected society and infertile couples. Meanwhile, male infertility is responsible for about 50% of infertility. Accordingly, a great number of researchers have focused on its treatment during the last few years; however, current therapies such as assisted reproductive technology (ART) are not effective enough in treating male infertility. Because of their self-renewal and differentiation capabilities and unlimited sources, stem cells have recently raised great hope in the treatment of reproductive system disorders. Stem cells are undifferentiated cells that can induce different numbers of specific cells, such as male and female gametes, demonstrating their potential application in the treatment of infertility. The present review aimed at identifying the causes and potential factors that influence male fertility. Besides, we highlighted the recent studies that investigated the efficiency of stem cells such as spermatogonial stem cells (SSCs), embryonic stem cells (ESCs), very small embryonic-like stem cells (VSELs), induced pluripotent stem cells (iPSCs), and mesenchymal stem cells (MSCs) in the treatment of various types of male infertility.

Plasma activin A concentrations during late gestation in Thoroughbred mares with abnormal pregnancies

Late-term fetal loss in horses is a major problem in the horse-breeding industry globally. Abnormal pregnancies should be diagnosed as early as possible to prevent abortions and other gestational problems. According to our previous longitudinal study in healthy pregnant mares, the plasma activin A concentration increases as pregnancy progresses. The aim of the present study was to compare plasma activin A concentrations in healthy pregnant Thoroughbred mares (n=40) with those in pregnant mares that suffered fetal loss or showed abnormal symptoms (n=30) during late gestation. This field study found that plasma activin A concentrations were higher in the abnormal group (pregnancy loss, red bag delivery, premature udder development, and vaginal discharge) than the normal group (P < 0.001; cutoff value: ≥ 138.2 pg/mL; sensitivity, 74.4%; specificity, 77.5%). More specifically, plasma activin A concentrations in the "symptom" and "abnormal delivery" subgroups were higher than those in gestational-age-matched normal groups (P < 0.001). Nevertheless, the plasma activin A concentration in the "normal delivery" subgroup was not different from that in the "abnormal delivery" subgroup in samples collected within 10 days before delivery. In conclusion, this study is the first to demonstrate a significantly earlier increase in plasma activin A concentration in abnormal pregnancies of Thoroughbred mares during late gestation.

Bend family proteins mark chromatin boundaries and synergistically promote early germ cell differentiation

Understanding the regulatory networks for germ cell fate specification is necessary to developing strategies for improving the efficiency of germ cell production in vitro. In this study, we developed a coupled screening strategy that took advantage of an arrayed bi-molecular fluorescence complementation (BiFC) platform for protein-protein interaction screens and epiblast-like cell (EpiLC)-induction assays using reporter mouse embryonic stem cells (mESCs). Investigation of candidate interaction partners of core human pluripotent factors OCT4, NANOG, KLF4 and SOX2 in EpiLC differentiation assays identified novel primordial germ cell (PGC)-inducing factors including BEN-domain (BEND/Bend) family members. Through RNA-seq, ChIP-seq, and ATAC-seq analyses, we showed that Bend5 worked together with Bend4 and helped mark chromatin boundaries to promote EpiLC induction in vitro. Our findings suggest that BEND/Bend proteins represent a new family of transcriptional modulators and chromatin boundary factors that participate in gene expression regulation during early germline development.

Improving In Vitro Culture of Human Male Fetal Germ Cells

Male human fetal germ cells (hFGCs) give rise to spermatogonial stem cells (SSCs), which are the adult precursors of the male gametes. Human SSCs are a promising (autologous) source of cells for male fertility preservation; however, in contrast to mouse SSCs, we are still unable to culture them in the long term. Here, we investigated the effect of two different culture media and four substrates (laminin, gelatin, vitronectin and matrigel) in the culture of dissociated second trimester testes, enriched for hFGCs. After 6 days in culture, we quantified the presence of POU5F1 and DDX4 expressing hFGCs. We observed a pronounced difference in hFGC number in different substrates. The combination of gelatin-coated substrate and medium containing GDNF, LIF, FGF2 and EGF resulted in the highest percentage of hFGCs (10% of the total gonadal cells) after 6 days of culture. However, the vitronectin-coated substrate resulted in a comparable percentage of hFGCs regardless of the media used (3.3% of total cells in Zhou-medium and 4.8% of total cells in Shinohara-medium). We provide evidence that not only the choices of culture medium but also choices of the adequate substrate are crucial for optimizing culture protocols for male hFGCs. Optimizing culture conditions in order to improve the expansion of hFGCs will benefit the development of gametogenesis assays in vitro.

Activin A-derived human embryonic stem cells show increased competence to differentiate into primordial germ cell-like cells

Protocols for specifying human primordial germ cell-like cells (hPGCLCs) from human embryonic stem cells (hESCs) remain hindered by differences between hESC lines, their derivation methods, and maintenance culture conditions. This poses significant challenges for establishing reproducible in vitro models of human gametogenesis. Here, we investigated the influence of activin A (ActA) during derivation and maintenance on the propensity of hESCs to differentiate into PGCLCs. We show that continuous ActA supplementation during hESC derivation (from blastocyst until the formation of the post-inner cell mass intermediate [PICMI]) and supplementation (from the first passage of the PICMI onwards) is beneficial to differentiate hESCs to PGCLCs subsequently. Moreover, comparing isogenic primed and naïve states prior to differentiation, we showed that conversion of hESCs to the 4i-state improves differentiation to (TNAP [tissue nonspecific alkaline phosphatase]+/PDPN [podoplanin]+) PGCLCs. Those PGCLCs expressed several germ cell markers, including TFAP2C (transcription factor AP-2 gamma), SOX17 (SRY-box transcription factor 17), and NANOS3 (nanos C2HC-type zinc finger 3), and markers associated with germ cell migration, CXCR4 (C-X-C motif chemokine receptor 4), LAMA4 (laminin subunit alpha 4), ITGA6 (integrin subunit alpha 6), and CDH4 (cadherin 4), suggesting that the large numbers of PGCLCs obtained may be suitable to differentiate further into more mature germ cells. Finally, hESCs derived in the presence of ActA showed higher competence to differentiate to hPGCLC, in particular if transiently converted to the 4i-state. Our work provides insights into the differences in differentiation propensity of hESCs and delivers an optimized protocol to support efficient human germ cell derivation.

Cell-free DNA in spent culture medium effectively reflects the chromosomal status of embryos following culturing beyond implantation compared to trophectoderm biopsy

This prospective study evaluated the accuracy of non-invasive preimplantation genetic testing for aneuploidy (niPGT-A) using cell-free DNA in spent culture medium, as well as that of preimplantation genetic testing for aneuploidy (PGT-A) using trophectoderm (TE) biopsy after culturing beyond implantation. Twenty frozen blastocysts donated by 12 patients who underwent IVF at our institution were investigated. Of these, 10 were frozen on day 5 and 10 on day 6. Spent culture medium and TE cells were collected from each blastocyst after thawing, and the embryos were cultured in vitro for up to 10 days. The outgrowths after culturing beyond implantation were sampled and subjected to chromosome analysis using next-generation sequencing. Chromosomal concordance rate, sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), false-positive rate (FPR), and false-negative rate (FNR) of niPGT-A and PGT-A against each outgrowth were analyzed. The concordance rate between the niPGT-A and outgrowth samples was 9/16 (56.3%), and the concordance rate between the PGT-A and outgrowth samples was 7/16 (43.8%). NiPGT-A exhibited 100% sensitivity, 87.5% specificity, 88.9% PPV, 100% NPV, 12.5% FPR, and 0% FNR. PGT-A exhibited 87.5% sensitivity, 77.8% specificity, 87.5% PPV, 75% NPV, 14.3% FPR, and 22.2% FNR. NiPGT-A may be more accurate than PGT-A in terms of ploidy diagnostic accuracy in outgrowths.

Studying human reproductive biology through single-cell analysis and in vitro differentiation of stem cells into germ cell-like cells

BACKGROUND

Understanding the molecular and cellular mechanisms of human reproductive development has been limited by the scarcity of human samples and ethical constraints. Recently, in vitro differentiation of human pluripotent stem cells into germ cells and single-cell analyses have opened new avenues to directly study human germ cells and identify unique mechanisms in human reproductive development.

OBJECTIVE AND RATIONALE

The goal of this review is to collate novel findings and insightful discoveries with these new methodologies, aiming at introducing researchers and clinicians to the use of these tools to study human reproductive biology and develop treatments for infertility.

SEARCH METHODS

PubMed was used to search articles and reviews with the following main keywords: in vitro differentiation, human stem cells, single-cell analysis, spermatogenesis, oogenesis, germ cells and other key terms related to these subjects. The search period included all publications from 2000 until now.

OUTCOMES

Single-cell analyses of human gonads have identified many important gene markers at different developmental stages and in subpopulations of cells. To validate the functional roles of these gene markers, researchers have used the in vitro differentiation of human pluripotent cells into germ cells and confirmed that some genetic requirements are unique in human germ cells and are not conserved in mouse models. Moreover, transcriptional regulatory networks and the interaction of germ and somatic cells in gonads were elucidated in these studies.

WIDER IMPLICATIONS

Single-cell analyses allow researchers to identify gene markers and potential regulatory networks using limited clinical samples. On the other hand, in vitro differentiation methods provide clinical researchers with tools to examine these newly identify gene markers and study the causative effects of mutations previously associated with infertility. Combining these two methodologies, researchers can identify gene markers and networks which are essential and unique in human reproductive development, thereby producing more accurate diagnostic tools for assessing reproductive disorders and developing treatments for infertility.

Human blastocyst outgrowths recapitulate primordial germ cell specification events

Our current knowledge of the mechanisms leading to human primordial germ cell (PGC) specification stems solely from differentiation experiments starting from human pluripotent stem cells. However, information regarding the origin of PGCs in vivo remains obscure. Here we apply an improved system for extended in vitro culture of human embryos to investigate the presence of PGC-like cells (PGCLCs) 12 days post fertilization (dpf). Good quality blastocysts (n = 141) were plated at 6 dpf and maintained in hypoxia, in medium supplemented with Activin A until 12 dpf. We primarily reveal that 12 dpf outgrowths recapitulate human peri-implantation events and demonstrate that blastocyst quality significantly impacts both embryo viability at 12 dpf, as well as the presence of POU5F1⁺ cells within viable outgrowths. Moreover, detailed examination of 12 dpf blastocyst outgrowths revealed a population of POU5F1⁺, SOX2^– and SOX17⁺ cells that may correspond to PGCLCs, alongside POU5F1⁺ epiblast-like cells and GATA6⁺ endoderm-like cells. Our findings suggest that, in human, PGC precursors may become specified within the epiblast and migrate either transiently to the extra-embryonic mesoderm or directly to the dorsal part of the yolk sac endoderm around 12 dpf. This is a descriptive analysis and as such the conclusion that POU5F1⁺ and SOX17⁺ cells represent bona fide PGCs can only be considered as preliminary. In the future, other PGC markers may be used to further validate the observed cell populations. Overall, our findings provide insights into the origin of the human germline and may serve as a foundation to further unravel the molecular mechanisms governing PGC specification in human.

Human Pluripotent Stem Cells in Reproductive Science-a Comparison of Protocols Used to Generate and Define Male Germ Cells from Pluripotent Stem Cells

Globally, fertility-related issues affect around 15% of couples. In 20%–30% of cases men are solely responsible, and they contribute in around 50% of all cases. Hence, understanding of in vivo germ-cell specification and exploring different angles of fertility preservation and infertility intervention are considered hot topics nowadays, with special focus on the use of human pluripotent stem cells (hPSCs) as a source of in vitro germ-cell generation. However, the generation of male germ cells from hPSCs can currently be considered challenging, making a judgment on the real perspective of these innovative approaches difficult. Ever since the first spontaneous germ-cell differentiation studies, using human embryonic stem cells, various strategies, including specific co-cultures, gene over-expression, and addition of growth factors, have been applied for human germ-cell derivation. In line with the variety of differentiation methods, the outcomes have ranged from early and migratory primordial germ cells up to post-meiotic spermatids. This variety of culture approaches and cell lines makes comparisons between protocols difficult. Considering the diverse strategies and outcomes, we aim in this mini-review to summarize the literature regarding in vitro derivation of human male germ cells from hPSCs, while keeping a particular focus on the culture methods, growth factors, and cell lines used.

Robust protocol for feeder-free adaptation of cryopreserved human pluripotent stem cells

Human pluripotent stem cells (hPSCs) are conventionally maintained on mouse embryonic fibroblast (MEF) feeder layers. However, downstream applications, such as directed differentiation protocols, are primarily optimized for feeder-free cultures. Therefore, hPSCs must often be adapted to feeder-free conditions. Here we propose a novel feeder-free adaptation protocol using StemFlex medium, which can be directly applied to thawed hPSC lines.The direct feeder-free adaptation protocol using StemFlex culture medium on Geltrex coating led to robust hPSC cultures in approximately 2 weeks. This approach was tested with three human embryonic stem cell (hESC) lines. All lines were confirmed to be pluripotent, expressing POU5F1, SOX2, and NANOG. No chromosomal imbalances were induced by the feeder-free adaptation.StemFlex medium enabled the efficient adaptation of hPSCs to feeder-free conditions directly after thawing. This protocol is easy to implement in laboratories that perform feeder-free cultures, allowing more convenient adaptation and more robust expansion of cryopreserved hPSCs, even in cases when sample quality is low or unknown.

Germ cell differentiation of mouse embryonic stem cells can be influenced by the culture medium

Although embryonic stem (ES) cells can differentiate into germ cells, little is known about the influence of culture media on this process. We investigated the effect of two culture media on the capacity of ES cells to differentiate into germ cells using embryoid body (EB) and monolayer culture protocols. Germ cell differentiation was induced in mouse ES cells under four experimental conditions: EB/Dulbecco's modified Eagle's medium (EB/DMEM), EB/knockout Dulbecco's modified Eagle's medium (EB/KO-DMEM), monolayer/Dulbecco's modified Eagle's medium (monolayer/DMEM), and monolayer/knockout Dulbecco's modified Eagle's medium (monolayer/KO-DMEM). After incubation for 6 days, quantitative real-time polymerase chain reaction (qRT-PCR) was used to assess expression of the germ cell markers, Mvh, Oct4, Rec8, Scp1, Scp3 and Stra8. Also, Oct4 and Mvh expressions at the protein level were assessed using immunocytochemistry; we evaluated alkaline phosphatase activity in addition to cell number and viability. Germ cell-specific marker expression was increased significantly in cells differentiated in KO-DMEM for both EB and monolayer protocols; the highest level was in cultures using the EB protocol. The highest cell proliferation rate was observed using the monolayer/KO-DMEM protocol and the lowest using the EB/DMEM protocol. Generally, KO-DMEM exhibited the greatest impact on germ cell differentiation and cell proliferation. Optimization of germ cell differentiation of ES cells requires careful selection of culture medium.

Perspectives of small molecule inhibitors of activin receptor‑like kinase in anti‑tumor treatment and stem cell differentiation (Review)

Activin receptor‑like kinases (ALKs), members of the type I activin receptor family, belong to the serine/threonine kinase receptors of the transforming growth factor‑β (TGF‑β) superfamily. ALKs mediate the roles of activin/TGF‑β in a wide variety of physiological and pathological processes, ranging from cell differentiation and proliferation to apoptosis. For example, the activities of ALKs are associated with an advanced tumor stage in prostate cancer and the chondrogenic differentiation of mesenchymal stem cells. Therefore, potent and selective small molecule inhibitors of ALKs would not only aid in investigating the function of activin/TGF‑β, but also in developing treatments for these diseases via the disruption of activin/TGF‑β. In recent studies, several ALK inhibitors, including LY‑2157299, SB‑431542 and A‑83‑01, have been identified and have been confirmed to affect stem cell differentiation and tumor progression in animal models. This review discusses the therapeutic perspective of small molecule inhibitors of ALKs as drug targets in tumor and stem cells.

Dppa3 in pluripotency maintenance of ES cells and early embryogenesis

Embryonic development is precisely regulated by a network of signal pathways and specific genes. Dppa3 (also known as Pgc7 or Stella) plays an important role in early embryonic development during the cleavage stage as a maternal effect gene. Dppa3 expresses in many species, and its homologous gene in human and rat genomes is located at the same chromosomal regions and have the same exon-intron structure. However, unlike mouse embryonic stem (ES) cells, in which the Dppa3 promoter maintains hypomethylation that allows a high transcription level, the DPPA3 promoter region in human ES cells is methylated, much like that of mouse epiblast stem cell. Dppa3 is essential for early embryogenesis and pluripotency maintenance; however, the precise mechanism and downstream passage remains unknown. In this review, we will summarize some important functions of Dppa3 in early embryogenesis and pluripotency maintenance of mouse ES cells.

Let-7a regulates expression of β1-adrenoceptors and forms a negative feedback circuit with the β1-adrenoceptor signaling pathway in chronic ischemic heart failure

BACKGROUND

The aim of the present study was to investigate the role of microRNA (miRNA) let-7a in down-regulation of β1-adrenoceptors (β1-AR) and elucidate the underlying mechanism of chronic ischemia heart failure (CIHF) in rats.

METHODS AND RESULTS

CIHF model was established by occlusion of coronary artery for 4 weeks. β1-AR level was obviously down-regulated and let-7a up-regulated in the failing heart 4 weeks after myocardial infarction. Overexpression of let-7a inhibited β1-AR expression in neonatal rat ventricular cells (NRVCs), which was abolished by anti-let-7a antisense inhibitor. The lentivirus vector containing precursor let-7a (len-pre-let-7a) further down-regulated the reduced β1-AR level by CIHF and the effect was reversed by len-AMO-let-7a. Len-negative control did not produce any significant influence on β1-AR expression. Importantly, there exists a negative feedback loop associated with β1-AR regulation through β1-AR/cAMP/PKA/GATA4/let-7a/β1-AR signaling pathway in CIHF. As demonstrated, GATA4 was activated by β1-AR up-regulation through cAMP-PKA signaling pathway in early phase of ischemia, then GATA4 positively regulated let-7a expression which in turn suppressed β1-AR expression.

CONCLUSIONS

Let-7a regulates β1-AR expression and forms a negative feedback loop with β1-AR signaling pathway in ischemic heart failure. This study provides a new insight into the differential expression of β1-AR in early and later phase of myocardial ischemia.

Activin A induces apoptosis of mouse myeloma cells via the mitochondrial pathway

Activin A is a pleiotropic cytokine belonging to the transforming growth factor β superfamily. Abnormal expression of activin A is associated with tumorigenesis. Multiple myeloma is characterized by the development of osteolytic disease, which ultimately leads to cachexia. However, the involvement of activin A in myeloma cell viability and apoptosis remains to be fully elucidated. For this purpose, mouse myeloma NS-1 cells were treated with activin A, and subsequently subjected to 5-bromo-2'-deoxyuridine analysis, Hoechst 33342 staining, flow cytometry and western blot analysis. The results revealed that activin A significantly suppressed NS-1 cell viability, and induced NS-1 cell apoptosis. In addition, activin A-induced promotion of NS-1 cell apoptosis was accompanied by upregulated expression of BCL2 associated X, apoptosis regulator (Bax), but downregulated expression of B cell lymphoma-2 (Bcl-2), resulting in an increase of the Bax/Bcl-2 ratio. Furthermore, cytochrome c and caspase-3 protein expression also increased following treatment with activin A. These data suggest that activin A induces apoptosis in mouse myeloma NS-1 cells via the mitochondrial pathway, providing a novel insight into multiple myeloma treatment.

Follistatin Rescues Blastocyst Development of Poor Quality Porcine Cumulus-Oocyte Complexes by Delaying Meiotic Resumption With Decreased cGMP

Mammalian oocytes resume maturation when removed from follicles and cultured in vitro. During folliculogenesis, oocytes are bathed in follicular fluid (FF), which provides an important and specialized microenvironment for oocyte competence. Follistatin (FST) is one component of FF that may play a role in oocyte maturation and embryo development. This study was conducted to examine possible effects of FST on porcine oocyte competence and embryo development. Exogenous FST in oocyte maturation medium for 22 or 44 hours did not improve nuclear maturation and had no effect on good quality cumulus-oocyte complexes (COCs). However, FST improved blastocyst rates in embryos derived from oocytes with less than 2 layers of cumulus. Follistatin treatment of the poor quality COC group increased transcript levels for genes indicative of oocyte quality. Transcript levels were also altered for cumulus expansion-related genes in response to FST when measured during the germinal vesicle breakdown stage. Interestingly, high-quality oocytes remained at germinal vesicle stage much longer than low-quality oocytes, FST treatment induced temporary blockage of spontaneous meiotic resumption when added during culture of both good and poor quality COCs, and levels of cyclic guanosine monophosphate (cGMP) were higher in FST-treated versus untreated groups for both good and poor quality oocytes. In conclusion, FST treatment of porcine oocytes during in vitro maturation can rescue competency of poor quality oocytes to develop to blastocyst stage following in vitro fertilization. Beneficial effects of addition of FST to culture medium may be mediated by inhibiting degradation of cGMP and temporarily delaying nuclear maturation.

The crucial role of Activin A on the formation of primordial germ cell-like cells from skin-derived stem cells in vitro

Primordial germ cells (PGCs) are founder cells of the germ cell lineage, and can be differentiated from stem cells in an induced system in vitro. However, the induction conditions need to be optimized in order to improve the differentiation efficiency. Activin A (ActA) is a member of the TGF-β super family and plays an important role in oogenesis and folliculogenesis. In the present study, we found that ActA promoted PGC-like cells (PGCLCs) formation from mouse skin-derived stem cells (SDSCs) in both embryoid body-like structure (EBLS) differentiation and the co-culture stage in a dose dependent manner. ActA treatment (100 ng/ml) during EBLS differentiation stage and further co-cultured for 6 days without ActA significantly increased PGCLCs from 53.2% to 82.8%, and as well as EBLS differentiation without ActA followed by co-cultured with 100 ng/ml ActA for 4 to 12 days with the percentage of PGCLCs increasing markedly in vitro. Moreover, mice treated with ActA at 100 ng/kg body weight from embryonic day (E) 5.5-12.5 led to more PGCs formation. However, the stimulating effects of ActA were interrupted by Smad3 RNAi, and in an in vitro cultured Smad3(-/-) mouse skin cells scenario. SMAD3 is thus likely a key effecter molecule in the ActA signaling pathway. In addition, we found that the expression of some epiblast cell markers, Fgf5, Dnmt3a, Dnmt3b and Wnt3, was increased in EBLSs cultured for 4 days or PGCLCs co-cultured for 12 days with ActA treatment. Interestingly, at 16 days of differentiation, the percentage of PGCLCs was decreased in the presence of ActA, but the expression of meiosis-relative genes, such as Stra8, Dmc1, Sycp3 and Sycp1, was increased. In conclusion, our data here demonstrated that ActA can promote PGCLC formation from SDSCs in vitro, at early stages of differentiation, and affect meiotic initiation of PGCLCs in later stages.

Parthenogenesis and Human Assisted Reproduction

Parthenogenetic activation of human oocytes obtained from infertility treatments has gained new interest in recent years as an alternative approach to create embryos with no reproductive purpose for research in areas such as assisted reproduction technologies itself, somatic cell, and nuclear transfer experiments and for derivation of clinical grade pluripotent embryonic stem cells for regenerative medicine. Different activating methods have been tested on human and nonhuman oocytes, with varying degrees of success in terms of parthenote generation rates, embryo development stem cell derivation rates. Success in achieving a standardized artificial activation methodology for human oocytes and the subsequent potential therapeutic gain obtained from these embryos depends mainly on the availability of gametes donated from infertility treatments. This review will focus on the creation of parthenotes from clinically unusable oocytes for derivation and establishment of human parthenogenetic stem cell lines and their potential applications in regenerative medicine.

Retracted article: In vitro derivation of mammalian germ cells from stem cells and their potential therapeutic application

Pluripotent stem cells (PSCs) are a unique type of cells because they exhibit the characteristics of self-renewal and pluripotency. PSCs may be induced to differentiate into any cell type, even male and female germ cells, suggesting their potential as novel cell-based therapeutic treatment for infertility problems. Spermatogenesis is an intricate biological process that starts from self-renewal of spermatogonial stem cells (SSCs) and leads to differentiated haploid spermatozoa. Errors at any stage in spermatogenesis may result in male infertility. During the past decade, much progress has been made in the derivation of male germ cells from various types of progenitor stem cells. Currently, there are two main approaches for the derivation of functional germ cells from PSCs, either the induction of in vitro differentiation to produce haploid cell products, or combination of in vitro differentiation and in vivo transplantation. The production of mature and fertile spermatozoa from stem cells might provide an unlimited source of autologous gametes for treatment of male infertility. Here, we discuss the current state of the art regarding the differentiation potential of SSCs, embryonic stem cells, and induced pluripotent stem cells to produce functional male germ cells. We also discuss the possible use of livestock-derived PSCs as a novel option for animal reproduction and infertility treatment.

The post-inner cell mass intermediate: implications for stem cell biology and assisted reproductive technology

BACKGROUND

Until recently, the temporal events that precede the generation of pluripotent embryonic stem cells (ESCs) and their equivalence with specific developmental stages in vivo was poorly understood. Our group has discovered the existence of a transient epiblast-like structure, coined the post-inner cell mass (ICM) intermediate or PICMI, that emerges before human ESC (hESCs) are established, which supports their primed nature (i.e. already showing some predispositions towards certain cell types) of pluripotency.

METHODS

The PICMI results from the progressive epithelialization of the ICM and it expresses a mixture of early and late epiblast markers, as well as some primordial germ cell markers. The PICMI is a closer progenitor of hESCs than the ICM and it can be seen as the first proof of why all existing hESCs, until recently, display a primed state of pluripotency.

RESULTS

Even though the pluripotent characteristics of ESCs differ from mouse (naïve) to human (primed), it has recently been shown in mice that a similar process of self-organization at the transition from ICM to (naïve) mouse ESCs (mESCs) transforms the amorphous ICM into a rosette of polarized epiblast cells, a mouse PICMI. The transient PICMI stage is therefore at the origin of both mESCs and hESCs. In addition, several groups have now reported the conversion from primed to the naïve (mESCs-like) hESCs, broadening the pluripotency spectrum and opening new opportunities for the use of pluripotent stem cells.

CONCLUSIONS

In this review, we discuss the recent discoveries of mouse and human transient states from ICM to ESCs and their relation towards the state of pluripotency in the eventual stem cells, being naïve or primed. We will now further investigate how these intermediate and/or different pluripotent stages may impact the use of human stem cells in regenerative medicine and assisted reproductive technology.

Engineering Strategies for the Formation of Embryoid Bodies from Human Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) are powerful tools for regenerative therapy and studying human developmental biology, attributing to their ability to differentiate into many functional cell types in the body. The main challenge in realizing hPSC potential is to guide their differentiation in a well-controlled manner. One way to control the cell differentiation process is to recapitulate during in vitro culture the key events in embryogenesis to obtain the three developmental germ layers from which all cell types arise. To achieve this goal, many techniques have been tested to obtain a cellular cluster, an embryoid body (EB), from both mouse and hPSCs. Generation of EBs that are homogeneous in size and shape would allow directed hPSC differentiation into desired cell types in a more synchronous manner and define the roles of cell-cell interaction and spatial organization in lineage specification in a setting similar to in vivo embryonic development. However, previous success in uniform EB formation from mouse PSCs cannot be extrapolated to hPSCs possibly due to the destabilization of adherens junctions on cell surfaces during the dissociation into single cells, making hPSCs extremely vulnerable to cell death. Recently, new advances have emerged to form uniform human embryoid bodies (hEBs) from dissociated single cells of hPSCs. In this review, the existing methods for hEB production from hPSCs and the results on the downstream differentiation of the hEBs are described with emphases on the efficiency, homogeneity, scalability, and reproducibility of the hEB formation process and the yield in terminal differentiation. New trends in hEB production and directed differentiation are discussed.