Intrafollicular immature oocyte transfer (IFIOT): technical advances, biological insights and applications in reproductive biotechnologies.

Intrafollicular immature oocyte transfer (IFIOT) represents an intermediate approach between in vitro embryo production (IVP) and conventional in vivo-derived embryo systems, combining key advantages of both. Similar to IVP, it enables repeated use of donor females through ovum pick-up without gonadotropin-based superstimulation, while allowing oocyte maturation, fertilization and early embryonic development to occur under physiological in vivo conditions. The development of IFIOT has progressed from early experimental demonstrations of intrafollicular maturation to ultrasound-guided applications and, more recently, to the production of viable embryos and live offspring, confirming its biological feasibility. However, efficiency remains limited, with embryo recovery rates of 3-17%, reflecting constraints such as oocyte retention, follicular disruption and suboptimal injection conditions. Beyond embryo production, IFIOT provides a valuable model to investigate follicular function and oocyte competence, with emerging applications in oocyte cryopreservation, embryo production from prepubertal donors, and somatic cell nuclear transfer. Therefore, IFIOT should be viewed not only as an embryo production alternative, but as a physiological platform for in vivo oocyte maturation with broad applications in farm animals and wildlife species, while further technical refinement may enable its consolidation as an efficient system for embryo production.