MOL classification: a novel meiotic spindle scoring system for predicting human embryo development and ploidy.

PURPOSE: This study aimed to develop a comprehensive scoring system for assessing the human oocyte meiotic spindle (MS) to predict embryological outcomes from both fresh and cryopreserved oocytes. METHODS: Human oocytes were evaluated using polarized light microscopy before and after cryopreservation. Each oocyte was scored using the MOL system, which assigns a three-digit code reflecting MS Morphology, Orientation, and Localization. Morphology categories included barrel-shaped, altered, enlarged, poor visualization, or absence of MS. Orientation was assessed in relation to the first polar body (PB1), and localization was measured by angular displacement from the PB1. Changes in MS characteristics due to cryopreservation were recorded, and their associations with fertilization and euploid blastocyst development were analyzed. RESULTS: Cryopreservation led to observable alterations in MS morphology, orientation, and localization, resulting in a redistribution of variants across the MOL classification. A strong correlation was found between specific MOL scores and the likelihood of successful fertilization and chromosomally normal embryo development. Each MOL category variant showed distinct predictive value for embryological outcomes in both fresh and cryopreserved oocytes. CONCLUSIONS: The MOL scoring system provides a reliable, structured approach for predicting oocyte fertilization capacity and embryo developmental potential. Its applicability to both fresh and cryopreserved oocytes, along with its potential for automation, suggests significant clinical value in assisted reproductive technologies. Furthermore, the structured and quantifiable data generated by the MOL scoring system offer a valuable foundation for training machine learning models aimed at enhancing predictive accuracy and supporting decision-making in embryo selection.