Japanese Researchers Pioneer Mouse Testis Organoid from Pluripotent Stem Cells

In a landmark achievement announced on February 27, 2026, scientists from Yokohama City University and Osaka University have successfully generated fetal testicular somatic cell-like cells (fTeSLCs) from mouse embryonic stem cells (ESCs). These cells self-organize into testis-like structures capable of supporting spermatogenesis, marking the world's first reconstruction of functional testis tissue components entirely from pluripotent stem cells in vitro. This breakthrough, detailed in Science Advances, opens new avenues for understanding testis development and addressing male infertility.

The research team, led by Lecturer Takuya Sato and Special Professor Takehiko Ogawa at Yokohama City University's Department of Regenerative Medicine, collaborated with Professor Katsuhiko Hayashi and Associate Professor Takashi Yoshino from Osaka University. Building on prior work where the same group created ovarian organoids in 2021, this study shifts focus to male reproductive tissue, demonstrating the versatility of stem cell-derived organoid technology.

Testis organoids represent three-dimensional (3D) miniaturized models that mimic the structure and function of native testicular tissue. Unlike traditional two-dimensional (2D) cultures, these 3D structures replicate cell-cell interactions, extracellular matrix, and vascular-like networks essential for processes like gametogenesis.

Background: Stem Cell Research and Organoids in Japan

Japan has long been at the forefront of stem cell research, with pioneers like Shinya Yamanaka earning the Nobel Prize in 2012 for induced pluripotent stem cells (iPSCs)—adult cells reprogrammed to an embryonic-like state. Institutions such as RIKEN, Yokohama City University, and Osaka University have advanced organoid technology, creating mini-organs for disease modeling and regenerative medicine.

RIKEN's contributions include earlier work on spermatogonial stem cell (SSC) transplantation into testis organoids, enabling spermatogenesis regeneration as reported in 2022. Recent RIKEN studies in early 2026 explored reverse transcriptase inhibitors for in vitro spermatid production from fetal testes, complementing today's announcement. These efforts align with Japan's national push for regenerative therapies amid a fertility crisis, where the total fertility rate hovers around 1.2 children per woman.

Organoids bridge the gap between animal models and human applications, offering ethical alternatives to animal testing while enabling personalized medicine. For reproductive biology, they promise insights into congenital disorders and environmental impacts on fertility.

The Innovative Methods Behind the Breakthrough

The protocol begins with mouse embryonic stem cells (ESCs), specifically XY reporter lines expressing markers like Nr5a1-hCD271 for gonadal cells and Sox9-CGFP for Sertoli cells. Step 1: EpiLC (epiblast-like cell) induction using activin and basic fibroblast growth factor (bFGF). Step 2: Gonadal somatic cell differentiation with bone morphogenetic protein 4 (BMP4), CHIR99021 (Wnt agonist), retinoic acid (RA), sonic hedgehog (Shh), and FGF9.

Crucially, at day 4, IWR1 (Wnt inhibitor) was added to suppress female pathways, promoting masculinization—a key tweak for Sertoli cell specification. After 10-12 days, fTeSLCs were sorted via flow cytometry, revealing populations resembling fetal Sertoli, interstitial progenitors, and endothelial-like cells. Single-cell RNA sequencing (scRNA-seq) confirmed transcriptomic fidelity to embryonic day 12.5 (E12.5) testis cells.

This stepwise process mimics embryonic gonadogenesis, where somatic cells envelop germ cells to form seminiferous tubules.

Key Findings: Structure and Cellular Composition

fTeSLCs formed compartmentalized clusters with Sertoli-like cells (SerLCs) expressing SOX9, AMH, and GDNF, and interstitial cell-like cells (ICLCs) positive for NR5A1 and DLK1. Chromatin accessibility mirrored in vivo profiles, validating epigenetic maturity.

Co-cultured with germ cells, fTeSLCs self-assembled into tubule-like structures, recapitulating the testis niche. scRNA-seq UMAP plots showed clusters matching fetal Sertoli precursors, Leydig progenitors, and peritubular myoid cells.

Unlike ovarian organoids, full vascularization remains a challenge, but the model advances testicular biomimicry significantly.

Functional Validation: Restoring Spermatogenesis and Fertility

To test functionality, ICLCs were added to organ cultures of ΔFLE mice (lacking Leydig cells). They differentiated into steroidogenic Leydig cells (HSD3B1+), producing testosterone and supporting spermatogenesis to elongating spermatids.

Similarly, SerLCs transplanted into Amh-DTR mice (Sertoli-ablated via diphtheria toxin) repopulated tubules, enabling meiosis (SCP3+) and haploid cells (PNA+). Round spermatids injected via ROSI (round spermatid injection) yielded healthy pups, proving fertility competence.

Success rates: ~10-20% tubules supported full gametogenesis, highlighting efficiency gains needed.

Implications for Male Infertility Treatment

Male factors contribute to ~50% of infertility cases globally; in Japan, ~15-20% of couples face issues amid delayed marriages and low sperm counts. This model could screen drugs for Sertoli cell dysfunction or Leydig cell failure, common in azoospermia.

Transitioning to human iPSCs (patient-derived) could enable autologous organoids for transplantation, restoring fertility without donors. Ethical approvals in Japan, post-STAP scandal, ensure rigorous oversight.

Read the full Science Advances paper for protocols applicable to clinical translation.

Japan's Fertility Crisis: Urgency for Reproductive Innovations

Japan's total fertility rate (TFR) fell to 1.20 in 2023, projected ~1.2 in 2026, with births at record lows (~720,000 in 2025). Male infertility rates mirror global trends at 10-15%, exacerbated by lifestyle and pollution.

• Delayed parenthood: Average paternal age 33+, linked to DNA fragmentation.
• Environmental toxins: Phthalates reduce sperm motility.
• Aging population: 29% over 65, straining IVF resources.

Stem cell therapies could alleviate IVF waitlists (over 50,000 cycles/year).

Challenges: From Mouse Model to Human Application

While promising, hurdles include low efficiency (5-20% functional tubules), absence of full vasculature, and ethical concerns over gamete production. Human testes require longer maturation; iPSC epigenetics may introduce risks.

Japan's JSPS and AMED fund such work, but scaling needs international collaboration. RIKEN's organoid platforms could integrate for hybrid models.

Future Outlook: Human Organoids and Beyond

Lead researchers envision human iPS-derived testis organoids within 10 years, aiding azoospermic patients. Combined with Hayashi's ovarian work, artificial gametes for same-sex couples or singles loom.

Global impact: Drug toxicity testing, toxicology, and personalized fertility. Yokohama City University expands regenerative programs; career advice for stem cell researchers.

Yokohama City University announcement.

Stakeholder Perspectives and Broader Regenerative Medicine

Experts praise the Wnt modulation for sex-specific differentiation. Fertility clinics see potential for non-invasive diagnostics. Policymakers eye it for Japan's '300,000 births/year' goal.

In higher education, this boosts Japan's rankings in life sciences. Professor jobs in reproductive biology are rising.

Photo by Jakub Tomasik on Unsplash

Conclusion: A Step Toward Restoring Fertility

This mouse testis organoid reconstruction exemplifies Japan's leadership in pluripotent stem cell applications. With viable offspring from reconstructed niches, the path to human therapies brightens. Researchers, students, and professionals can engage via Rate My Professor, Higher Ed Jobs, Career Advice, University Jobs, or Recruitment on AcademicJobs.com. Stay informed on regenerative frontiers.