Advocating distinct regulatory paths for embryos and embryo-like structures.

Human embryo-like structures (ELSs) are novel entities emulating aspects of embryogenesis to advance understanding of early human life and enable future clinical applications. ELSs frequently fall into a regulatory gap: the laws that govern embryo research do not commonly apply, but nor are there bespoke regulatory schemes. There is international consensus that the gap must be addressed, but disagreement as to when and how this should be achieved. To date ELSs model embryos, mimicking aspects of embryonic development. In 2024 a UK Nuffield Council on Bioethics report recommended that these `stem cell-based embryo models' should be regulated separately to embryos. Building on this report, this paper considers a subset of ELSs that may in future lose their model status because they replicate rather than model embryos. Distinguishing between models and replicas it considers what circumstances, in the UK and internationally, would require regulation as an embryo, the circumstances in which replicas might justifiably be regulated separately to embryos and why maintaining distinct regulatory paths for embryos and ELSs is beneficial.