Importance of Cumulus Cells in Oocyte Maturation and Conservation Strategies to Improve Vitrification Results

Vitrification is not a recent technique, it was firstly described in 1934, but was not successful until years later because the effects of the cryoprotectants (CPAs) used were unknown. Later, in 1985, Rall and Fahy [1] resumed the application of ultrafast cryopreservation by reducing the toxicity of the media and decreasing exposure times, modifying temperatures in order to improve their results [2]. In this way, the cryopreservation basis has been broadened with the aim of optimising conditions for the development of this process. Some of the main concerns are, the formation of intracellular and extracellular ice crystals, the CPAs toxicity and the osmotic changes during vitrification [3].

The large amount of lipids in porcine oocyte cytoplasm and the presence of the surrounding cumulus cells (CC), which are necessary for the meiotic resumption through gap communicating junctions, have been linked to less CPAs cell permeability and protection leading to cell damage [4]. However, previous studies indicate that the presence of the CC is essential to protect oocytes against damage induced by the vitrification procedure compared to the vitrification of denuded oocytes [5]. Also, it was reported that cumulus-oocyte complexes (COCs) vitrification reduces the viability of the CC while preserving oocyte viability [6]. This is important to be considered since these cells establish communication with the oocyte for the exchange of molecules such as: cAMP, ions, pyruvate, among others, which are required for the acquisition of its mitochondrial function, maturation and fertilizaton capacity. Therefore, maintaining the viability of the CC is a key factor for the vitrification of immature oocytes. Consequently, vitrification of immature porcine oocytes has been more difficult to achieve than other species such as sheep, cattle, hamsters, mice, human, among others [7-10] and this is why it still remains a challenge for cryobiologists.