Abstract
In assisted reproductive technolog, both invasive and non-invasive criteria are widely used to identify oocytes and embryos with high developmental potential, with the ultimate goal of improving live birth rates. Since the earliest applications of in vitro fertilization in the 1970s, non-invasive assessment methods have played a central role in evaluating oocyte and embryo competence. These approaches rely primarily on morphological analysis, including assessment of the mature cumulus-oocyte complex (COC), the presence and morphology of the first polar body, zona pellucida thickness, characteristics of the perivitelline space, and cytoplasmic appearance. Oocyte quality is a fundamental determinant of successful fertilization, subsequent embryo development, implantation, and the establishment of a healthy term pregnancy. The follicular environment also plays a crucial role in regulating oocyte maturation and developmental competence. Within the follicle, the COC functions as a coordinated unit in which the oocyte regulates its growth and development through metabolic interactions with the surrounding cumulus and granulosa cells. During maturation, the oocyte’s energy requirements are largely met through the β-oxidation of fatty acids stored in lipid droplets (LDs). These LDs undergo spatial redistribution during maturation, which may be associated with changes in the oocyte’s viscoelastic properties. Alterations in lipid metabolism within the COC can impair oocyte developmental competence and may help explain the reduced survival of some oocytes after intracytoplasmic sperm injection. This review critically examines the relationship between the follicular environment and oocyte metabolism, with particular emphasis on lipid metabolism and oocyte and embryo quality. It also discusses established non-invasive markers of oocyte quality and emerging approaches based on oocyte biomechanical properties that may be associated with embryo viability, implantation, and term pregnancy.