From in vitro culture to in vivo models to study testis development and spermatogenesis

Transplantation of spermatogonial stem cells in stallions

Spermatogonial stem cells originate from gonocytes and undergo self-renewal and differentiation to generate mature spermatozoa via spermatogenesis in the seminiferous tubules of the testis in male mammals. Owing to the unique capacity of these cells, the spermatogonial stem cell transplantation technique, which enables the restoration of male fertility by transfer of germlines between donor and recipient males, has been developed. Thus, spermatogonial stem cell transplantation can be used as an important next-generation reproductive and breeding tool in livestock production. However, in large animals, this approach is associated with many technical limitations and inefficiency. Furthermore, research regrading spermatogonial stem cell transplantation in stallions is limited. Therefore, this review article describes the history and current knowledge regarding spermatogonial stem cell transplantation in animals and challenges in establishing an experimental protocol for successful spermatogonial stem cell transplantation in stallions, which have been presented under the following heads: spermatogonial stem cell isolation, recipient preparation, and spermatogonial stem cell transplantation. Additionally, we suggest that further investigation based on previous unequivocal evidence regarding donor-derived spermatogenesis in large animals must be conducted. A detailed and better understanding of the physical and physiological aspects is required to discuss the current status of this technique field and develop future directions for the establishment of spermatogonial stem cell transplantation in stallions.

KIF3A regulates the Wnt/β-catenin pathway via transporting β-catenin during spermatogenesis in Eriocheir sinensis

The Wnt/β-catenin pathway participates in many important physiological events such as cell proliferation and differentiation in the male reproductive system. We found that Kinesin-2 motor KIF3A is highly expressed during spermatogenesis in Eriocheir sinensis; it may potentially promote the intracellular transport of cargoes in this process. However, only a few studies have focused on the relationship between KIF3A and the Wnt/β-catenin pathway in the male reproductive system of decapod crustaceans. In this study, we cloned and characterized the CDS of β-catenin in E. sinensis for the first time. Fluorescence in situ hybridization and immunofluorescence results showed the colocalization of Es-KIF3A and Es-β-catenin at the mRNA and the protein level respectively. To further explore the regulatory function of Es-KIF3A to the Wnt/β-catenin pathway, the es-kif3a was knocked down by double-stranded RNA (dsRNA) in vivo and in primary cultured cells in testes of E. sinensis. Results showed that the expression of es-β-catenin and es-dvl were decreased in the es-kif3a knockdown group. The protein expression level of Es-β-catenin was also reduced and the location of Es-β-catenin was changed from nucleus to cytoplasm in the late stage of spermatogenesis when es-kif3a was knocked down. Besides, the co-IP result demonstrated that Es-KIF3A could bind with Es-β-catenin. In summary, this study indicates that Es-KIF3A can positively regulate the Wnt/β-catenin pathway during spermatogenesis and Es-KIF3A can bind with Es-β-catenin to facilitate the nuclear translocation of Es-β-catenin.

A Testis-Derived Hydrogel as an Efficient Feeder-Free Culture Platform to Promote Mouse Spermatogonial Stem Cell Proliferation and Differentiation

Fertility preservation and assisted reproductive medicine require effective culture systems for the successful proliferation and differentiation of spermatogonial stem cells (SSCs). Many SSC culture systems require the addition of feeder cells at each subculture, which is tedious and inefficient. Here, we prepared decellularized testicular matrix (DTM) from testicular tissue, which preserved essential structural proteins of testis. The DTM was then solubilized and induced to form a porous hydrogel scaffold with randomly oriented fibrillar structures that exhibited good cytocompatibility. The viability of SSCs inoculated onto DTM hydrogel scaffolds was significantly higher than those inoculated on Matrigel or laminin, and intracellular gene expression and DNA imprinting patterns were similar to that of native SSCs. Additionally, DTM promoted SSC differentiation into round spermatids. More importantly, the DTM hydrogel supported SSC proliferation and differentiation without requiring additional somatic cells. The DTM hydrogel scaffold culture system provided an alternative and simple method for culturing SSCs that eliminates potential variability and contamination caused by feeder cells. It might be a valuable tool for reproductive medicine.

A testis-derived macroporous 3D scaffold as a platform for the generation of mouse testicular organoids

Extracellular matrix-derived scaffolds provide an efficient platform for the generation of organ-like structures.

Development of Spermatogenesis In Vitro in Three-Dimensional Culture from Spermatogonial Cells of Busulfan-Treated Immature Mice

Aggressive chemotherapy may lead to permanent male infertility. Prepubertal males do not generate sperm, but their testes do contain spermatogonial cells (SPGCs) that could be used for fertility preservation. In the present study, we examined the effect of busulfan (BU) on the SPGCs of immature mice, and the possible induction of the survivor SPGCs to develop spermatogenesis in 3D in-vitro culture. Immature mice were injected with BU, and after 0.5⁻12 weeks, their testes were weighed and evaluated histologically compared to the control mice. The spermatogonial cells [Sal-like protein 4 (SALL4) and VASA (a member of the DEAD box protein family) in the testicular tissue were counted/seminiferous tubule (ST). The cells from the STs were enzymatically isolated and cultured in vitro. Our results showed a significant decrease in the testicular weight of the BU-treated mice compared to the control. This was in parallel to a significant increase in the number of severely damaged STs, and a decrease in the number of SALL4 and VASA/STs compared to the control. The cultures of the isolated cells from the STs of the BU-treated mice showed a development of colonies and meiotic and post-meiotic cells after four weeks of culture. The addition of homogenates from adult GFP mice to those cultures induced the development of sperm-like cells after four weeks of culture. This is the first study demonstrating the presence of biologically active spermatogonial cells in the testicular tissue of BU-treated immature mice, and their capacity to develop sperm-like cells in vitro.

Influence of warming and reanimation conditions on seminiferous tubule morphology, mitochondrial activity, and cell composition of vitrified testicular tissues in the domestic cat model

Understanding critical roles of warming and reanimation is critical to improve the survival of vitrified testicular tissue in domestic cats. The objective was to study structural and functional properties of testicular tissues from prepubertal domestic cats after standard vitrification followed by two warming protocols (directly at 37°C or with a 5-second pre-exposure to 50°C) and three reanimation time points (immediately, 24 h and 5 days post-warming). In Experiment 1, tissues were evaluated for histo-morphology and mitochondrial activity immediately or 24 h after warming protocols. In Experiment 2, cell viability, DNA fragmentation, and germ cell composition were assessed immediately, 24 h, or 5 days after optimal warming. Preservation of seminiferous tubule structure was better using warming at 50°C for five seconds, and survival of somatic as well as germinal cells was higher compared to direct warming at 37°C for one minute. Short term in vitro culture (for reanimation) also proved that cellular composition and functionality were better preserved when warmed for a short time at 50°C. Collective data showed that short warming at 50°C led to better quality of seminiferous tubule structure and cell composition after vitrification and short-term culture. In addition, data suggest clear directions to further understand and optimize testicular tissue survival after fertility preservation procedures.

Exposure to phthalate esters induces an autophagic response in male germ cells

Phthalate esters are plasticizers that impart flexibility to polvinylchloride plastics. As they are not covalently bound, they can leach from a wide range of products, including food containers, medical devices, clothing, and toys, leading to widespread environmental exposure. Phthalate toxicity has been linked to male infertility by disrupting testosterone production and testis development. Phthalates also impair proliferation and viability of spermatogonial stem cells (SSC), the role of which is to support lifelong spermatogenesis. To elucidate cellular mechanisms in spermatogonia affected by long-term phthalate exposure, we grafted primate testis tissue into mice. Grafts treated with di-n-butyl phthalate showed an increase in autophagy compared to controls. Short term in vitro exposure of porcine germ cells to mono(2-ethylhexyl) phthalate, also resulted in an increase in autophagy. Viability was lower in cells exposed to phthalates, but treatment with rapamycin to induce autophagy significantly increased viability. The data suggests autophagy is triggered in spermatogonia as a response to a toxic insult, which may constitute a survival mechanism in spermatogonia.

Spermatogonial stem cell autotransplantation and germline genomic editing: a future cure for spermatogenic failure and prevention of transmission of genomic diseases

BACKGROUND

Subfertility affects approximately 15% of all couples, and a severe male factor is identified in 17% of these couples. While the etiology of a severe male factor remains largely unknown, prior gonadotoxic treatment and genomic aberrations have been associated with this type of subfertility. Couples with a severe male factor can resort to ICSI, with either ejaculated spermatozoa (in case of oligozoospermia) or surgically retrieved testicular spermatozoa (in case of azoospermia) to generate their own biological children. Currently there is no direct treatment for azoospermia or oligozoospermia. Spermatogonial stem cell (SSC) autotransplantation (SSCT) is a promising novel clinical application currently under development to restore fertility in sterile childhood cancer survivors. Meanwhile, recent advances in genomic editing, especially the clustered regulatory interspaced short palindromic repeats-associated protein 9 (CRISPR-Cas9) system, are likely to enable genomic rectification of human SSCs in the near future.

OBJECTIVE AND RATIONALE

The objective of this review is to provide insights into the prospects of the potential clinical application of SSCT with or without genomic editing to cure spermatogenic failure and to prevent transmission of genetic diseases.

SEARCH METHODS

We performed a narrative review using the literature available on PubMed not restricted to any publishing year on topics of subfertility, fertility treatments, (molecular regulation of) spermatogenesis and SSCT, inherited (genetic) disorders, prenatal screening methods, genomic editing and germline editing. For germline editing, we focussed on the novel CRISPR-Cas9 system. We included papers written in English only.

OUTCOMES

Current techniques allow propagation of human SSCs in vitro, which is indispensable to successful transplantation. This technique is currently being developed in a preclinical setting for childhood cancer survivors who have stored a testis biopsy prior to cancer treatment. Similarly, SSCT could be used to restore fertility in sterile adult cancer survivors. In vitro propagation of SSCs might also be employed to enhance spermatogenesis in oligozoospermic men and in azoospermic men who still have functional SSCs albeit in insufficient numbers. The combination of SSCT with genomic editing techniques could potentially rectify defects in spermatogenesis caused by genomic mutations or, more broadly, prevent transmission of genomic diseases to the offspring. In spite of the promising prospects, SSCT and germline genomic editing are not yet clinically applicable and both techniques require optimization at various levels.

WIDER IMPLICATIONS

SSCT with or without genomic editing could potentially be used to restore fertility in cancer survivors to treat couples with a severe male factor and to prevent the paternal transmission of diseases. This will potentially allow these couples to have their own biological children. Technical development is progressing rapidly, and ethical reflection and societal debate on the use of SSCT with or without genomic editing is pressing.

A Review of New Technologies that may Become Useful for in vitro Production of Boar Sperm

Making sperm cells outside the original testicular environment in a culture dish has been considered for a long time as impossible due to the very complicated process of spermatogenesis and sperm maturation, which altogether, encompasses a 2-month period. However, new approaches in complex three-dimensional co-cell cultures, micro-perfusion and micro-fluidics technologies, new knowledge in the functioning, culturing and differentiation of spermatogonial stem cells (SSC) and their precursor cells have revolutionized this field. Furthermore, the use of better molecular markers as well as stimulatory factors has led to successful in vitro culture of stem cells either derived from germ line stem cells, from induced pluripotent stem cells (iPSC) or from embryonic stem cells (ESC). These stem cells when placed into small seminiferous tubule fragments are able to become SSC. The SSC beyond self-renewal can also be induced into haploid sperm-like cells under in vitro conditions. In mouse, this in vitro produced sperm can be injected into a mature oocyte and allow post-fertilization development into an early embryo in vitro. After transferring such obtained embryos into the uterus of a recipient mouse, they can further develop into healthy offspring. Recently, a similar approach has been performed with combining selected cells from testicular cell suspensions followed by a complete in vitro culture of seminiferous cords producing sperm-like cells. However, most of the techniques developed are laborious, time-consuming and have low efficiency, placing questionable that it will become useful used for setting up an efficient in vitro sperm production system for the boar. The benefits and drawbacks as well as the likeliness of in vitro pig sperm production to become applied in assisted technologies for swine reproduction are critically discussed. In this contribution, also the process of sperm production in the testis and sperm maturation is reviewed.

Beyond the mouse monopoly: studying the male germ line in domestic animal models

Spermatogonial stem cells (SSCs) are the foundation of spermatogenesis and essential to maintain the continuous production of spermatozoa after the onset of puberty in the male. The study of the male germ line is important for understanding the process of spermatogenesis, unravelling mechanisms of stemness maintenance, cell differentiation, and cell-to-cell interactions. The transplantation of SSCs can contribute to the preservation of the genome of valuable individuals in assisted reproduction programs. In addition to the importance of SSCs for male fertility, their study has recently stimulated interest in the generation of genetically modified animals because manipulations of the male germ line at the SSC stage will be maintained in the long term and transmitted to the offspring. Studies performed mainly in the mouse model have laid the groundwork for facilitating advancements in the field of male germ line biology, but more progress is needed in nonrodent species in order to translate the technology to the agricultural and biomedical fields. The lack of reliable markers for isolating germ cells from testicular somatic cells and the lack of knowledge of the requirements for germ cell maintenance have precluded their long-term maintenance in domestic animals. Nevertheless, some progress has been made. In this review, we will focus on the state of the art in the isolation, characterization, culture, and manipulation of SSCs and the use of germ cell transplantation in domestic animals.

Spermatogenic Cell-Specific Gene Mutation in Mice via CRISPR-Cas9

Tissue-specific knockout technology enables the analysis of the gene function in specific tissues in adult mammals. However, conventional strategy for producing tissue-specific knockout mice is a time- and labor-consuming process, restricting rapid study of the gene function in vivo. CRISPR-Cas9 system from bacteria is a simple and efficient gene-editing technique, which has enabled rapid generation of gene knockout lines in mouse by direct injection of CRISPR-Cas9 into zygotes. Here, we demonstrate CRISPR-Cas9-mediated spermatogenic cell-specific disruption of Scp3 gene in testes in one step. We first generated transgenic mice by pronuclear injection of a plasmid containing Hspa2 promoter driving Cas9 expression and showed Cas9 specific expression in spermatogenic cells. We then produced transgenic mice carrying Hspa2 promoter driven Cas9 and constitutive expressed sgRNA targeting Scp3 gene. Male founders were infertile due to developmental arrest of spermatogenic cells while female founders could produce progeny normally. Consistently, male progeny from female founders were infertile and females could transmit the transgenes to the next generation. Our study establishes a CRISPR-Cas9-based one-step strategy to analyze the gene function in adult tissues by a temporal-spatial pattern.

Xenografting of testicular tissue pieces: 12 years of an in vivo spermatogenesis system

Spermatogenesis is a dynamic and complex process that involves endocrine and testicular factors. During xenotransplantation of testicular tissue fragments into immunodecifient mice, a functional communication between host brain and donor testis is established. This interaction allows for the progression of spermatogenesis and recovery of fertilisation-competent spermatozoa from a broad range of mammalian species. In the last few years, significant progress has been achieved in testis tissue xenografting that improves our knowledge about the factors determining the success of grafting. The goal of this review is to provide up to date information about the role of factors such as donor age, donor species, testis tissue preservation or type of recipient mouse on the efficiency of this technique. Applications are described and compared with other techniques with similar purposes. Recent work has demonstrated that testicular tissue xenografting is used as a model to study gonadotoxicity of drugs and to obtain sperm from valuable young males.

Primary cilia in the developing pig testis

In vertebrates, a variety of cell types generate a primary cilium. Cilia are implicated in determination and differentiation of a wide variety of organs and during embryonic development. However, there is little information on the presence or function of primary cilia in the mammalian testis. Therefore, the objective of this study was to characterize expression of primary cilia in the developing pig testis. Testicular tissue from pigs at 2-10 weeks of age was analyzed for primary cilia by immunocytochemistry. Expression of primary cilia was also analyzed in testicular tissue formed de novo from a single cell suspension ectopically grafted into a mouse host. Functionality of primary cilia was monitored based on cilia elongation after exposure to lithium. Analysis showed that the primary cilium is present in testis cords as well as in the interstitium of the developing pig testis. Germ cells did not express primary cilia. However, we identified Sertoli cells as one of the somatic cell types that produce a primary cilium within the developing testis. Primary cilium expression was reduced from the second to the third week of pig testis development in situ and during de novo morphogenesis of testis tissue from a single cell suspension after xenotransplantation. In vitro, primary cilia were elongated in response to lithium treatment. These results indicate that primary cilia on Sertoli cells may function during testicular development. De novo morphogenesis of testis tissue from single cell suspensions may provide an accessible platform to study and manipulate expression and function of primary cilia.

The roles of testicular c-kit positive cells in de novo morphogenesis of testis

C-kit positive (c-kit⁺) cells are usual tissue-specific stem cells. However, in postnatal testis, undifferentiated spermatogonial stem cells (SSCs) are c-kit negative (c-kit⁻) and activation of c-kit represents the start of SSC differentiation, leaving an intriguing question whether other c-kit⁺ cells exist and participate in the postnatal development of testis. To this end, a feasible system for testicular reconstitution, in which a specific type of cells can be manipulated, is needed. Here, we first establish de novo morphogenesis of testis by subcutaneous injection of testicular cells from neonatal testes into the backs of nude mice. We observe testicular tissue formation and spermatogenesis from all injected sites. Importantly, functional spermatids can be isolated from these testicular tissues. Using this system, we systemically analyze the roles of c-kit⁺ cells in testicular reconstitution and identify a small population of cells (c-kit⁺:CD140a⁺:F4/80⁺), which express typical markers of macrophages, are critical for de novo morphogenesis of testis. Interestingly, we demonstrate that these cells are gradually replaced by peripheral blood cells of recipient mice during the morphogenesis of testis. Thus, we develop a system, which may mimic the complete developmental process of postnatal testis, for investigating the testicular development and spermatogenesis.

Sertolin mediates blood-testis barrier restructuring

Two important events that occur during mammalian spermatogenesis are the release of elongated spermatids at late stage VIII of the seminiferous epithelial cycle and the restructuring of the blood-testis barrier (BTB) during stages VIII-XI. Still, it is not completely understood how these cellular events are accomplished within the seminiferous epithelium. In the present study, we investigate how sertolin, a protein that was initially identified, cloned, and partially characterized by our laboratory, functions in these critical events. Sertolin was found at the BTB, as well as at the apical ectoplasmic specialization and apical tubulobulbar complex, where it colocalized with epidermal growth factor receptor kinase substrate 8 and actin-related protein 3, two actin-regulatory proteins. Knockdown of sertolin by RNA interference showed Sertoli cell barrier function to be enhanced when assessed by transepithelial electrical resistance measurements and immunolocalization experiments. By contrast, the integrity of the BTB was disrupted when sertolin was overexpressed in vitro and in vivo. Sertolin overexpression also prompted germ cell loss from the seminiferous epithelium. Taken collectively, these results suggest that sertolin may be involved in coordinating spermatid release and BTB restructuring during spermatogenesis in the rat.

Spermatogonial stem cells from domestic animals: progress and prospects

Spermatogenesis, an elaborate and male-specific process in adult testes by which a number of spermatozoa are produced constantly for male fertility, relies on spermatogonial stem cells (SSCs). As a sub-population of undifferentiated spermatogonia, SSCs are capable of both self-renewal (to maintain sufficient quantities) and differentiation into mature spermatozoa. SSCs are able to convert to pluripotent stem cells during in vitro culture, thus they could function as substitutes for human embryonic stem cells without ethical issues. In addition, this process does not require exogenous transcription factors necessary to produce induced-pluripotent stem cells from somatic cells. Moreover, combining genetic engineering with germ cell transplantation would greatly facilitate the generation of transgenic animals. Since germ cell transplantation into infertile recipient testes was first established in 1994, in vivo and in vitro study and manipulation of SSCs in rodent testes have been progressing at a staggering rate. By contrast, their counterparts in domestic animals, despite the failure to reach a comparable level, still burgeoned and showed striking advances. This review outlines the recent progressions of characterization, isolation, in vitro propagation, and transplantation of spermatogonia/SSCs from domestic animals, thereby shedding light on future exploration of these cells with high value, as well as contributing to the development of reproductive technology for large animals.

Separation of spermatogenic cell types using STA-PUT velocity sedimentation

Mammalian spermatogenesis is a complex differentiation process that occurs in several stages in the seminiferous tubules of the testes. Currently, there is no reliable cell culture system allowing for spermatogenic differentiation in vitro, and most biological studies of spermatogenic cells require tissue harvest from animal models like the mouse and rat. Because the testis contains numerous cell types--both non-spermatogenic (Leydig, Sertoli, myeloid, and epithelial cells) and spermatogenic (spermatogonia, spermatocytes, round spermatids, condensing spermatids and spermatozoa)--studies of the biological mechanisms involved in spermatogenesis require the isolation and enrichment of these different cell types. The STA-PUT method allows for the separation of a heterogeneous population of cells--in this case, from the testes--through a linear BSA gradient. Individual cell types sediment with different sedimentation velocity according to cell size, and fractions enriched for different cell types can be collected and utilized in further analyses. While the STA-PUT method does not result in highly pure fractions of cell types, e.g. as can be obtained with certain cell sorting methods, it does provide a much higher yield of total cells in each fraction (~1 x 10(8) cells/spermatogenic cell type from a starting population of 7-8 x 10(8) cells). This high yield method requires only specialized glassware and can be performed in any cold room or large refrigerator, making it an ideal method for labs that have limited access to specialized equipment like a fluorescence activated cell sorter (FACS) or elutriator.

In vitro propagation of male germline stem cells from piglets

PURPOSE

To study the effects of serum and growth factors on propagation of porcine male germline stem cells (MGSCs) in vitro and develop a culture system for these stem cells.

METHODS

Fresh testicular cells from neonatal piglets were obtained by mechanical dissociation and collagenase-trypsin digestion. After differential plating, non-adhering cells were cultured in media supplemented with different concentrations of serum (0, 1 %, 2 %, 5 %, 10 %). After 10 days of primary culture, the cells were maintained in media supplemented with different concentrations of growth factors (basic fibroblast growth factor and epidermal growth factor at 1, 5, 10 ng/ml). The number of MGSC-derived colonies with different sizes was determined in each treatment to assess the effects of serum concentrations and growth factors.

RESULTS

The number of MGSC-derived colonies was significantly higher in the presence of 1 % rather than 10 % fetal bovine serum (FBS). Basic fibroblast growth factor (bFGF) at 1, 5 ng/ml and epidermal growth factor (EGF) at 5, 10 ng/ml significantly promoted colony formation. Immunocytochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and xenotransplantation assays demonstrated the presence of functional stem cells in cultured cell population.

CONCLUSIONS

In vitro propagation of porcine MGSCs could be maintained in the presence of 1 % FBS and supplementation of growth factors for 1 month.

Cell biology solves mysteries of reproduction

Reproduction and fertility have been objects of keen inquiry since the dawn of humanity. Medieval anatomists provided the first accurate depictions of the female reproductive system, and early microscopists were fascinated by the magnified sight of sperm cells. Initial successes were achieved in the in vitro fertilization of frogs and the artificial insemination of dogs. Gamete and embryo research was in the cradle of modern cell biology, providing the first evidence of the multi-cellular composition of living beings and pointing out the importance of chromosomes for heredity. In the 20th century, reproductive research paved the way for the study of the cytoskeleton, cell signaling, and the cell cycle. In the last three decades, the advent of reproductive cell biology has brought us human in vitro fertilization, animal cloning, and human and animal embryonic stem cells. It has contributed to the development of transgenesis, proteomics, genomics, and epigenetics. This Special Issue represents a sample of the various areas of reproductive biology, with emphasis on molecular and cell biological aspects. Advances in spermatology, ovarian function, fertilization, and maternal-fetal interactions are discussed within the framework of fertility and diseases such as endometriosis and diabetes.

Clinical review: Present and future prospects of male fertility preservation for children and adolescents

CONTEXT

Rapid progress in fertility preservation strategies has led to the investigation of ways in which fertile gametes could be generated from cryopreserved immature testicular tissue. Childhood cancer patients remain the major group that can benefit from these techniques. Other potential candidates include patients undergoing gonadectomy and patients with Klinefelter's syndrome and cryptorchid testes. This review aims to present an overview of the current state of knowledge in experimental germ cell transplantation, testicular tissue transplantation, and germ cell culture as fertility preservation methods for males.

METHODOLOGY

We included English articles published in PubMed as well as personal files with the focus on studies including human or nonhuman material.

MAIN FINDINGS

Germ cell and testicular tissue transplantation demonstrate clinical options to mature germ cells from immature primate testicular tissue. The most promising approach involves autologous grafting of immature testicular tissue, whereas germ cell maturation in vitro provides the best strategies to overcome problems of cancer contamination in cryopreserved testicular tissue. Three-dimensional and organ culture systems offer the possibility to differentiate immature male germ cells up to the stage of elongated spermatids. Further characterization of early germ cell development in humans is needed to modify these systems for clinical use.