The ultrastructural nature of human oocytes’ cytoplasmatic abnormalities and the role of cytoskeleton dysfunction

Background Egg quality is a limiting factor of female fertility and assisted reproductive technology (ART) success. Oocytes recovered from hyperstimulated ovaries often display morphological anomalies suspected to compromise their fertilization and developmental potential. Knowledge of (ab)normal oocyte’s intracellular organization is vital to establish reliable criteria for morphological evaluation of oocytes intended for in vitro fertilization (IVF). Methods Transmission electron microscopy (TEM) was used to investigate the fine morphology of 22 dysmorphic IVF eggs exhibiting different types of cytoplasmic irregularities, namely ([1][1]) refractile bodies, ([2][2]) centrally-located cytoplasmic granularity (CLCG), ([3][3]) smooth endoplasmic reticulum (SER) disc, and ([4][4]) vacuoles. The cytoskeleton targeting compounds were employed to address the causative mechanism behind the anomalous cytoplasmic architecture observed in abnormal egg samples. A total of 133 immature oocytes were exposed to chemical inhibitors/control conditions, and their morphology was examined by fluorescent and electron microscopy. Results TEM exposed the structural basis of the common oocyte aberrations and revealed that the underlying cause of two of the studied morphotypes was excessive organelle clustering. Inhibition experiments showed that disruption of actin, not microtubules, allows inordinate aggregation of subcellular structures resembling the ultrastructural pattern seen in morphologically abnormal eggs retrieved in IVF cycles. These results imply that actin serves as a regulator of organelle distribution during human oocyte maturation. Conclusions The ultrastructural analogy between dysmorphic eggs and oocytes, in which actin network integrity was perturbed, suggests that malfunction of the actin cytoskeleton might be implicated in generating common cytoplasmic aberrations. Knowledge of human oocytes’ inner workings and the origin of morphological abnormalities is a step forward to more objective egg quality assessment in clinical practice. ### Competing Interest Statement The authors have declared no competing interest. * ### LIST OF ABBBREVIATIONS ART : assisted reproduction technology BFA : - Brefeldin A CLCG : - centrally-located cytoplasmic granularity COS : - controlled ovarian stimulation CytD : - Cytochalasin D DMSO : - dimethyl sulfoxide GV : - germinal vesicle ER : - endoplasmic reticulum MII : - Metaphase II mRNA : – messenger ribonucleic acid ICSI : – intracytoplasmic sperm injection IVF : - in vitro fertilization NC : - Nocodazole PB - polar body PBT : - phosphate-buffered saline supplemented with 0.1% Triton X-100 PLM : – polarized light microscopy PVS : – perivitelline space PX – paclitaxel SER : – smooth endoplasmic reticulum TEM : – transmission electron microscopy ZP : - zona pellucida [1]: #ref-1 [2]: #ref-2 [3]: #ref-3 [4]: #ref-4