In 2001 the UK's Human Fertilisation and Embryology Authority changed its regulations to permit the use of human embryos in stem cell research (SCR). Policy-related debates highlighted that questions regarding the meaning and status of embryos or the implications of creating cloned humans cannot be resolved solely by technical expertise and therefore imply the involvement of diverse people with diverse (and often competing) perspectives. This situation poses a practical and political challenge for stem cell scientists, potentially challenging their privileged status in guiding science policy or public discourse by undermining their cognitive authority as knowledge producers par excellence. Yet, scientists do not readily relinquish their cognitive authority and the privileges accorded to this position but instead are continually engaged in boundary-work. With such high stakes, it is important to ask: what discursive strategies do stem cell scientists adopt when seeking to defend their cognitive authority in response to competing claims from multiple actors? In seeking to preserve their cognitive authority, stem cell scientists take on multiple roles in relation to multiple boundaries simultaneously in public debates about SCR. Reframing the embryo question as a technical issue rather than a societal one maintains the cognitive authority of (some) stem cell scientists, while expelling others. This strategy of scientisation operates in stark contrast to processes of politicisation involved when distinguishing embryonic SCR from human reproductive cloning. These multiple discursive practices coalesce, constituting a powerful argument supporting embryonic SCR and contribute to the ongoing expansion of embryonic SCR.