BACKGROUND In recent decades, a broad range of strategies have been applied to model the testicular microenvironment in vitro. These models have been utilized to study testicular physiology and development. However, a system that allows investigations into testicular organogenesis and its impact in the spermatogonial stem-cell (SSC) niche in vitro has not been developed yet. Recently, the creation of tissue-specific organ-like structures called organoids has resurged, helping researchers to answer scientific questions that previous in vitro models could not help to elucidate. So far, a small number of publications have concerned the generation of testicular organoids and their application in the field of reproductive medicine and biology. OBJECTIVE AND RATIONALE Here, we aim to elucidate whether testicular organoids might be useful in answering current scientific questions about the regulation and function of the SSC niche as well as germ cell proliferation and differentiation, and whether or not the existing in vitro models are already sufficient to address them. Moreover, we would like to discuss how an organoid system can be a better solution to address these prominent scientific problems in our field, by the creation of a rationale parallel to those in other areas where organoid systems have been successfully utilized. SEARCH METHODS We comprehensively reviewed publications regarding testicular organoids and the methods that most closely led to the formation of these organ-like structures in vitro by searching for the following terms in both PubMed and the Web of Science database: testicular organoid, seminiferous tubule 3D culture, Sertoli cell 3D culture, testicular cord formation in vitro, testicular morphogenesis in vitro, germ cell 3D culture, in vitro spermatogenesis, testicular de novo morphogenesis, seminiferous tubule de novo morphogenesis, seminiferous tubule-like structures, testicular in vitro model and male germ cell niche in vitro, with no restrictions to any publishing year. The inclusion criteria were based on the relation with the main topic (i.e. testicular organoids, testicular- and seminiferous-like structures as in vitro models), methodology applied (i.e. in vitro culture, culture dimensions (2D, 3D), testicular cell suspension or fragments) and outcome of interest (i.e. organization in vitro). Publications about grafting of testicular tissue, germ-cell transplantation and female germ-cell culture were excluded. OUTCOMES The application of organoid systems is making its first steps in the field of reproductive medicine and biology. A restricted number of publications have reported and characterized testicular organoids and even fewer have denominated such structures by this method. However, we detected that a clear improvement in testicular cell reorganization is recognized when 3D culture conditions are utilized instead of 2D conditions. Depending on the scientific question, testicular organoids might offer a more appropriate in vitro model to investigate testicular development and physiology because of the easy manipulation of cell suspensions (inclusion or exclusion of a specific cell population), the fast reorganization of these structures and the controlled in vitro conditions, to the same extent as with other organoid strategies reported in other fields. WIDER IMPLICATIONS By way of appropriate research questions, we might use testicular organoids to deepen our basic understanding of testicular development and the SSC niche, leading to new methodologies for male infertility treatment.

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睾丸类器官：体外研究睾丸微环境的新模型？

背景近几十年来，已经应用了广泛的策略来对体外睾丸微环境进行建模。这些模型已被用于研究睾丸生理和发育。然而，尚未开发出一种可以在体外研究睾丸器官发生及其对精原干细胞 (SSC) 生态位的影响的系统。最近，称为类器官的组织特异性器官样结构的创建重新兴起，帮助研究人员回答了以前的体外模型无法帮助阐明的科学问题。到目前为止，少数出版物涉及睾丸类器官的产生及其在生殖医学和生物学领域的应用。目标和理由在这里，我们旨在阐明睾丸类器官是否有助于回答当前有关 SSC 生态位的调节和功能以及生殖细胞增殖和分化的科学问题，以及现有的体外模型是否已经足以解决这些问题。此外，我们想讨论一个类器官系统如何成为解决我们领域中这些突出科学问题的更好解决方案，方法是创建一个与已成功利用类器官系统的其他领域类似的基本原理。搜索方法 我们通过在 PubMed 和 Web of Science 数据库中搜索以下术语，全面审查了有关睾丸类器官的出版物以及最密切地导致这些器官样结构在体外形成的方法：睾丸类器官，生精小管 3D 培养，支持细胞 3D 培养，体外睾丸索形成，体外睾丸形态发生，生殖细胞 3D 培养，体外精子发生，睾丸从头形态发生，生精小管从头形态发生，生精小管样结构，睾丸体外体外模型和雄性生殖细胞生态位，不受任何出版年份的限制。纳入标准基于与主要主题的关系（即睾丸类器官、睾丸和生精样结构作为体外模型）、应用的方法（即体外培养、培养尺寸（2D、3D）、睾丸细胞悬液或片段）和感兴趣的结果（即体外组织）。排除了有关睾丸组织移植、生殖细胞移植和女性生殖细胞培养的出版物。结果 类器官系统的应用正在生殖医学和生物学领域迈出第一步。有限数量的出版物已经报道和描述了睾丸类器官，甚至更少的出版物通过这种方法命名了这种结构。然而，我们发现，当使用 3D 培养条件而不是 2D 条件时，睾丸细胞重组明显改善。根据科学问题，睾丸类器官可能提供更合适的体外模型来研究睾丸发育和生理学，因为细胞悬浮液易于操作（包含或排除特定细胞群）、这些结构的快速重组和受控体外条件，与其他领域报道的其他类器官策略的程度相同。更广泛的影响 通过适当的研究问题，我们可能会使用睾丸类器官来加深我们对睾丸发育和 SSC 生态位的基本理解，从而为男性不育症治疗带来新的方法。