Vol. 11, Issue 1, Part G (2026)

Round Spermatids (RS) represent the earliest haploid stage of male germ cells, possessing the essential chromosomal imprint required for fertilization despite lacking the structural attributes of mature spermatozoa. This review explores the biological potential and clinical application of Round Spermatid Injection (ROSI) as a therapeutic strategy for severe male infertility caused by spermiogenic arrest. Experimental evidence from animal models, including mice and rabbits, has demonstrated that RS nuclei can support full-term embryonic development, a finding subsequently translated to successful human pregnancies. Beyond reproduction, RS are also highlighted as effective vectors for transgenesis due to their accessible chromatin structure prior to protamine condensation. However, the clinical efficacy of ROSI is challenged by the difficulty in distinguishing viable spermatids from somatic contaminants like lymphocytes under standard optics. Consequently, this review critically examines isolation methodologies, emphasizing the superior purity achieved through Fluorescence-Activated Cell Sorting (FACS) compared to traditional sedimentation. Furthermore, it details molecular validation techniques using specific markers such as PRM2 and SP-10, alongside non-invasive mitochondrial selection protocols, which are pivotal for ensuring the accuracy and safety of using immature germ cells in assisted reproductive technologies.