The effect of vitrification on ultrastructure of human in vitro matured germinal vesicle oocytes

ObjectiveTo describe the possible effects of cryotop vitrification on maturation rate and ultrastructural morphology of human in vitro matured germinal vesicle (GV) oocytes.Study designA total of 301surplus immature GV oocytes obtained from infertile patients were allocated into two groups: (i) GV oocytes (n = 150) matured in vitro (fIVM), and (ii) GV oocytes (n = 151) that were first vitrified, then matured in vitro (vIVM). Supernumerary fresh in vivo matured oocytes (n = 10) were used as controls. The maturation media was Ham's F10 supplemented with FSH + LH and human follicular fluid. After 36 h of incubation, the oocytes were investigated for nuclear maturation and ultrastructural changes using transmission electron microscopy (TEM).ResultsOocyte maturation rates were reduced (P < 0.001) in vIVM (45.92%) in comparison with fIVM oocytes (75.33%). The rate of degeneration was also significantly higher in vIVM than in the fIVM group (44.4% vs. 6.0%). Large and numerous mitochondria and minute vesicles of smooth endoplasmic reticulum (SER) complexes (MV complexes) were observed in both fIVM and vIVM groups. In addition, TEM revealed a drastic reduction in amount of cortical granules (CGs) at the cortex of vitrified-warmed GV oocytes, as well as appearance of vacuoles and small mitochondria-SER aggregates in the ooplasm.ConclusionThe vitrification procedure is associated with ultrastructural alterations in specific oocyte microdomains, presumably related to the reduced competence of cryopreserved oocytes for maturation. This information emphasizes the need for further work on advancing the cryotechnology of human oocytes.