Age-related changes in the cytoplasmic ultrastructure of feline oocytes

Abstract

This study elucidates the impact of aging on the cellular architecture of feline oocytes, with a particular emphasis on organelles essential for fertilization and embryo development. Using transmission electron microscopy (TEM), the research compares oocytes from prepubescent, cycling adult, and aged cats, revealing notable differences in the arrangement of key structures, particularly mitochondria, lipid droplets, and vacuoles. Oocytes from adult donors are at their metabolic peak, demonstrating a higher concentration of mitochondria near lipid droplets, supporting efficient energy metabolism. In contrast, younger and older oocytes exhibit larger lipid droplets and reduced mitochondrial density, indicative of diminished metabolic activity. These findings not only underscore the necessity of selecting an optimal donor age for in vitro fertilization but also suggest potential biomarkers for oocyte quality assessment. This novel insight offers promising strategies to enhance reproductive success, improve assisted reproduction outcomes, and support feline conservation efforts.

Keywords: Aging; Cats; Electron microscopy; Fertility; Oocytes; Reproductive biology.

Fig. 1

Comparison of the suboolemmal region of oocytes from adult (A, B), prepubertal (C, D) and old queens (E, F). Relatively small magnifications are on the left, larger magnifications are on the right. A, B: Adult oocytes usually show an abundance of cytoplasmic vacuoles (CV) and several small and medium-sized lipid droplets (LD). Mitochondria form clusters that aggregate around cytoplasmic vacuoles and larger LDs. The perivitelline space (PVS) is clear, narrow and distinct from the zona pelucida (ZP). C, D: CVs of oocytes collected from prepubertal queens tend to be less abundant. Fewer mitochondria are usually visible and do not form visible large clusters. Oocytes from old individuals (E, F) show an abundance of severely damaged CVs, the ooplasm is less distinct. Although mitochondrial clusters are present, they show no visible pattern in their distribution relative to other organelles. Microvilli are scarce, perivitelline space is uneven and relatively thick. Abbreviations: CV - cytoplasmic vacuole, CG - cortical granules LD - lipid droplet, Mit - mitochondria, GA - golgi apparatus, Nu - cell nucleus, PVS - periviteline space, ZP - zona pelucida.

Fig. 2

Comparison of the central part of oocytes from adult (A, B), prepubertal (C, D) and old queens (E, F), focusing on the regions rich in lipid droplets. A, B: Mitochondria tend to cluster closely around LDs, possibly as peri-droplet mitochondria (PDM). In some images, a clear association between PDM and LDs is visible (see frames). C, D: Although less abundant, PDM are also visible in prepubertal oocytes. E, F: Many mitochondria are clustered in seemingly random regions of the ooplasm, although some tend to be closely associated with LDs as PDM. Abbreviations: CV - cytoplasmic vacuole, LD - lipid droplet, Mit - mitochondria, Nu - cell nucleus, PDM - peri-droplets mitochondrium.

Fig. 3

Comparison of zona pelucida and corona radiata of oocytes from adult (A, B), prepubertal (C, D) and old queens (E, F). Left, low magnification; right, higher magnification focused on individual cells. The zona pelucida (ZP) tends to be widest in oocytes from adult cats (A), although the cumulus cells tend to be less densely attached than in the other two age groups. In each group, transzonal projections (TZP) originating from cumulus cells penetrate the zona pelucida toward the oolemma. Cumulus cells show a similar morphology in adult and prepubertal cats (B, D); in old cats, the endoplasmic reticulum (ER) tends to be visibly dilated.

Fig. 4

Presents bar plots comparing the number of cytoplasmic vacuoles per square micrometer (A), the percentage of oocyte area occupied by lipid droplets (B), the mean number of peri-droplet mitochondria (or contact sites) per lipid circumference micrometer (C), and the number of cortical granules per micrometer of the oolemma (D). Every trait was calculated in hot-spots. In A, the results approach the statistical significance threshold, whereas in C, they are statistically significant, indicating a more dense distribution of peri-droplet mitochondria than in other groups. * - p-value < 0.05.

Fig. 5

The morphometric characteristics of mitochondria. Mean diameter of mitochondria (A), fraction of damaged mitochondria (B) and mean number of visible cristae per mitochondrium (C). Mitochondria in oocytes from prepubescent queens were significantly smaller and less often damaged. All mitochondria were evaluated in their cross-sections. * - p-value < 0.05.