Ultrastructure and intracellular calcium response during activation in vitrified and slow-frozen human oocytes

Closed vitrification based on DMSO and EG preserves the ultrastructural features and the ability to respond to the Ca²⁺ ionophore A23187 significantly better than does slow freezing with 0.3 M sucrose. Damage to organelles involved in the Ca²⁺(i) modulation might reduce the developmental competence of cryopreserved oocytes.