Dimova Tanya, Alexandrova Marina, Vangelov Ivaylo, You Yuan, Mor Gil
Institute of Biology and Immunology of Reproduction "Acad. Kiril Bratanov", Bulgarian Academy of Sciences, Sofia, Bulgaria.
C.S. Mott Center for Human Growth and Development, Wayne State University, Detroit, MI, USA.
Hum Reprod Update. 2025 Mar 1;31(2):133-163. doi: 10.1093/humupd/dmae033.
Successful implantation is a critical step for embryo survival. The major losses in natural and assisted human reproduction appeared to occur during the peri-implantation period. Because of ethical constraints, the fascinating maternal-fetal crosstalk during human implantation is difficult to study and thus, the possibility for clinical intervention is still limited.
This review highlights some features of human implantation as a unique, ineffective and difficult-to-model process and summarizes the pros and cons of the most used in vivo, ex vivo and in vitro models. We point out the variety of cell line-derived models and how these data are corroborated by well-defined primary cells of the same nature. Important aspects related to the handling, standardization, validation, and modus operandi of the advanced 3D in vitro models are widely discussed. Special attention is paid to blastocyst-like models recapitulating the hybrid phenotype and HLA profile of extravillous trophoblasts, which are a unique yet poorly understood population with a major role in the successful implantation and immune mother-embryo recognition. Despite raising new ethical dilemmas, extended embryo cultures and synthetic embryo models are also in the scope of our review.
We searched the electronic database PubMed from inception until March 2024 by using a multi-stage search strategy of MeSH terms and keywords. In addition, we conducted a forward and backward reference search of authors mentioned in selected articles.
Primates and rodents are valuable in vivo models for human implantation research. However, the deep interstitial, glandular, and endovascular invasion accompanied by a range of human-specific factors responsible for the survival of the fetus determines the uniqueness of the human implantation and limits the cross-species extrapolation of the data. The ex vivo models are short-term cultures, not relevant to the period of implantation, and difficult to standardize. Moreover, the access to tissues from elective terminations of pregnancy raises ethical and legal concerns. Easy-to-culture cancer cell lines have many limitations such as being prone to spontaneous transformation and lacking decent tissue characteristics. The replacement of the original human explants, primary cells or cancer cell lines with cultures of immortalized cell lines with preserved stem cell characteristics appears to be superior for in vitro modeling of human implantation and early placentation. Remarkable advances in our understanding of the peri-implantation stages have also been made by advanced three dimensional (3D) models i.e. spheroids, organoids, and assembloids, as placental and endometrial surrogates. Much work remains to be done for the optimization and standardization of these integrated and complex models. The inclusion of immune components in these models would be an asset to delineate mechanisms of immune tolerance. Stem cell-based embryo-like models and surplus IVF embryos for research bring intriguing possibilities and are thought to be the trend for the next decade for in vitro modeling of human implantation and early embryogenesis. Along with this research, new ethical dilemmas such as the moral status of the human embryo and the potential exploitation of women consenting to donate their spare embryos have emerged. The careful appraisal and development of national legal and ethical frameworks are crucial for better regulation of studies using human embryos and embryoids to reach the potential benefits for human reproduction.
We believe that our data provide a systematization of the available information on the modeling of human implantation and early placentation and will facilitate further research in this field. A strict classification of the advanced 3D models with their pros, cons, applicability, and availability would help improve the research quality to provide reliable outputs.
成功着床是胚胎存活的关键步骤。自然受孕和辅助生殖中，主要的胚胎损失似乎发生在着床期。由于伦理限制，人类着床过程中迷人的母胎交互作用难以研究，因此临床干预的可能性仍然有限。
本综述着重介绍人类着床作为一个独特、低效且难以模拟的过程的一些特征，并总结了最常用的体内、体外和离体模型的优缺点。我们指出了多种细胞系衍生模型，以及这些数据如何通过性质相同的明确原代细胞得到证实。广泛讨论了与先进的3D体外模型的处理、标准化、验证和操作方式相关的重要方面。特别关注模拟绒毛外滋养层细胞的混合表型和HLA谱的囊胚样模型，绒毛外滋养层细胞是一个独特但了解不足的群体，在成功着床和母胎免疫识别中起主要作用。尽管引发了新的伦理困境，但延长胚胎培养和合成胚胎模型也在我们的综述范围内。
我们使用MeSH词和关键词的多阶段检索策略，在电子数据库PubMed中从建库至2024年3月进行检索。此外，我们对所选文章中提到的作者进行了前后向参考文献检索。
灵长类动物和啮齿动物是人类着床研究中有价值的体内模型。然而，伴随着一系列影响胎儿存活的人类特异性因素的深度间质、腺性和血管内侵袭，决定了人类着床的独特性，并限制了数据的跨物种外推。离体模型是短期培养，与着床期无关，且难以标准化。此外，从选择性终止妊娠获取组织引发了伦理和法律问题。易于培养的癌细胞系有许多局限性，如易于自发转化且缺乏良好的组织特征。用具有保留干细胞特征的永生化细胞系培养物替代原始人类外植体、原代细胞或癌细胞系，似乎在人类着床和早期胎盘形成的体外建模方面更具优势。先进的三维（3D）模型，即球体、类器官和组装体，作为胎盘和子宫内膜替代物，在我们对着床期的理解上也取得了显著进展。这些综合且复杂的模型在优化和标准化方面仍有许多工作要做。在这些模型中纳入免疫成分将有助于阐明免疫耐受机制。基于干细胞的胚胎样模型和用于研究的多余体外受精胚胎带来了有趣的可能性，被认为是未来十年人类着床和早期胚胎发生体外建模的趋势。随着这项研究的开展，出现了新的伦理困境，如人类胚胎的道德地位以及对同意捐赠其多余胚胎的女性的潜在剥削。仔细评估和制定国家法律和伦理框架对于更好地规范使用人类胚胎和胚状体的研究以实现对人类生殖的潜在益处至关重要。
我们相信，我们的数据对人类着床和早期胎盘形成建模的现有信息进行了系统化，将有助于该领域的进一步研究。对先进3D模型及其优缺点、适用性和可用性进行严格分类，将有助于提高研究质量以提供可靠的结果。
