# Comparative Assessment of Functional and Morphological Markers in Guinea Pig (Cavia porcellus) Oocytes Collected at Different Estrous Cycle Phases

## TL;DR

This study compares guinea pig oocyte quality at different reproductive cycle stages to determine the best time for collection to improve in vitro maturation success.

## Contribution

The study identifies stage-specific oocyte traits in guinea pigs, offering a framework for optimizing oocyte selection based on morphological and functional markers.

## Key findings

- Diestrus oocytes showed larger size and better lipid distribution, indicating higher developmental competence.
- Periovulatory oocytes had higher metabolic activity and lower early apoptosis, suggesting better maturation potential.
- Oocyte quality varied with estrous cycle stage and cumulus–oocyte complex classification.

## Abstract

Reproductive technologies offer valuable tools to enhance both animal production and biomedical research. However, their success is critically dependent on the selection of high-quality oocytes. In guinea pigs, the application of these technologies has yielded limited efficiency, potentially due to the intrinsic oocyte characteristics. This study aimed to assess oocyte quality in female guinea pigs at two distinct stages of the reproductive cycle in order to identify the optimal timing for oocyte retrieval. Morphological and cytological parameters were assessed, including oocyte diameter, structural integrity, cytoplasmic composition, and cellular viability. Oocytes obtained during the diestrus stage exhibited greater diameter and improved cytoplasmic organization, suggestive of enhanced developmental competence. Conversely, oocytes retrieved during the periovulatory stage—closer to the time of natural ovulation—showed increased nuclear maturity and fewer indicators of early cellular degeneration. These findings indicate that the optimal stage for oocyte collection may vary depending on the specific attributes prioritized in in vitro applications. A deeper understanding of these stage-dependent differences may assist researchers and clinicians in selecting the most appropriated collection window, thereby improving the efficiency of embryo production protocols and contributing to advances in both animal husbandry and medical research.

The efficiency of in vitro maturation (IVM) in Cavia porcellus remains suboptimal compared to other species. This study aimed to characterize the morphological and functional quality of oocytes based on the stage of the estrous cycle and the classification of the cumulus–oocyte complexes (COCs) from which they were derived. A total of 744 oocytes were recovered postmortem from females in the diestrus and periovulatory phases. Oocytes were evaluated for metabolic activity, lipid distribution, apoptosis, nuclear maturation, and diameter. Oocytes collected during diestrus exhibited larger diameters and more homogeneous lipid distribution, particularly in oocytes from Type A COCs. In contrast, a higher proportion of BCB+ oocytes and reduced rates of early apoptosis were observed during the periovulatory stage, suggesting enhanced metabolic competence. Nuclear maturation rates varied with both cycle stage and COC classification, with oocytes from Type A COCs showing higher maturation rates in diestrus. These findings indicate that both intrinsic (e.g., morphological quality) and extrinsic (e.g., estrous cycle stage) factors modulated oocyte competence. Selecting oocytes based on integrated morphological and physiological criteria may improve the efficiency of IVM protocols in this species.