Insights into Cytoplasmic Maturation in Human Oocytes: Organelle Dynamics, Ionic Homeostasis, and Translational Regulation

Oocyte maturation is a fundamental biological process required for fertilization and the acquisition of developmental competence. Although nuclear events such as meiotic resumption and chromosomal segregation have historically attracted more scientific interest, cytoplasmic remodeling constitutes an equally essential determinant of oocyte quality. This review synthesizes recent evidence on the dynamic reorganization of organelles, regulation of protein synthesis and post-translational modifications, ionic homeostasis, and the distinct cytoplasmic signatures that characterize in vivo and in vitro maturation. During oocyte maturation, the endoplasmic reticulum (ER), mitochondria, and cortical granules undergo stage-specific structural and spatial reorganization. Changes in maternal protein abundance and intracellular zinc (Zn2+) and calcium (Ca2+) signaling also accompany meiotic progression and acquisition of developmental competence. In the context of assisted reproduction, a subset of oocytes retrieved after controlled ovarian stimulation remains immature, typically at the germinal vesicle (GV) or metaphase I (MI) stage, prompting the use of rescue in vitro maturation (r-IVM) protocols to complete their development. Evidence suggests that such oocytes matured in vitro may display aberrant cytoplasmic architecture, impaired organelle distribution, and reduced functional competence, features that may account for the suboptimal outcomes frequently reported after r-IVM. Advancing our understanding of these cytoplasmic processes is likely to facilitate the refinement of in vitro maturation systems, improve oocyte utilization, and ultimately enhance the efficacy of clinical interventions in human reproduction.