Modelling mammalian implantation in vitro

Embryo implantation is a major reproductive bottleneck, requiring precise spatiotemporal coordination between the embryo and the maternal endometrium. This intricate communication is governed by synchronised molecular signalling, mechanical cues, and endocrine regulation. Although some of these mechanisms are conserved across mammals, marked species differences in the degree of invasiveness of implantation, decidualisation, and placental morphology, as well as in embryonic developmental milestones, limit comparative understanding of early pregnancy across species. Moreover, ethical restrictions on human embryo research, the need to reduce animal use in research, and the oversimplified nature of 2D cultures, which cannot replicate the 3D, multicellular organisation of the implantation site, have further constrained progress. Recent advances in 3D in vitro technologies, including organoids, blastoids, and biomaterial-based cultures, provide more physiologically relevant platforms for studying embryo-endometrium interactions. When integrated with organ-on-a-chip systems, these models enable dynamic control of hormonal gradients, paracrine signalling, and fluid flow, closely mimicking the native implantation microenvironment, unlike static cultures. These systems have yielded key insights into epithelial-stromal crosstalk, immune regulation, and vascular remodelling at the maternal-foetal interface. However, challenges remain regarding reproducibility and rigorous biological validation. This review explores the biological principles of implantation across mammalian species. It evaluates in vivo animal models and 2D and 3D in vitro models for their ability to recapitulate species-specific implantation strategies. Furthermore, we highlight how integrating cellular diversity, biomechanical forces, and species-specific tissue architecture into bioengineered 3D platforms, together with advanced imaging, can bridge the gap between in vitro approaches and complex in vivo physiology. The convergence of these technologies is poised to transform implantation research and ultimately improve reproductive outcomes.