Vitrification with DAP 213 and Cryotop ofEx SituandIn SituFeline Cumulus-Oocyte Complexes

Microenvironment factors promoting the quality of vitrified cat oocytes

In oocyte cryopreservation programs, vitrification has overthrown conventional slow freezing both in veterinary and human medicine. In animals, its feasibility in field conditions makes it the preferred technique for the safeguard of genetic resources from zoo or wild animals, including threatened felids, for which the domestic cat is an excellent model. However, many cellular injuries, such as cytoskeleton, mitochondria and meiotic spindle alterations, DNA damage, zona pellucida hardening and cumulus cell loss, might occur following vitrification. After warming, although the exact mechanisms are still unclear, degeneration is a frequent outcome for cat vitrified oocytes. For immature (germinal vesicle) gametes, in vitro maturation after warming is a challenge, and cleavage after fertilization barely reaches 15-30%, while for mature (metaphase II) cryopreserved gametes it can get to 30-50%. Anyway, the progression to late embryos stages is often impaired, and improvements are needed. Standard cryopreservation protocol and the use of conventional in vitro culture systems after warming may not be enough for vitrified oocytes to recover and demonstrate their full developmental potential. Physical or chemical factors applied to oocytes undergoing vitrification, as an enrichment to the vitrification step, or to the culture microenvironment, could create more favorable conditions and promote vitrified oocyte survival and development. From the use of three-dimensional culture systems to the regulation of metabolic activities and cellular pathways, this review aims to explore all the possibilities employed so far, including the studies performed by our own lab, and the future perspectives, to present the most effective strategies for cat oocyte vitrification and the best time for their application (i.e., before, during, or after vitrification-warming).

Granulosa cells in three-dimensional culture: A follicle-like structure for domestic cat vitrified oocytes

Follicle-like structures are three-dimensional matrices joint with living cells that allow the in vitro development of female gametes in more physiological conditions. They have been shown to be beneficial to fresh oocytes in different species, and in this study, domestic cat (Felis silvestris catus) granulosa cells were used to create a functional follicle-like structure aimed at supporting the in vitro maturation of conspecific vitrified oocytes, key players of fertility preservation programmes that usually struggle to acquire their full developmental competence after warming. Cat granulosa cells were cultured for up to 6 days in three-dimensional barium alginate microcapsules (i.e. follicle-like structures) or in two-dimensional monolayers, and their steroidogenic ability (estradiol and progesterone secretion) was assessed to confirm their functionality. The same systems were used (on day 2 or 6 of granulosa cells culture) for the in vitro maturation (IVM) of Cryotop^® vitrified immature cat oocytes and compared with microdrops of IVM medium without cells (control). Granulosa cells were able to maintain their functionality in vitro in both the conditions, even if with a different extent of hormonal secretion along culture (p = .02). Vitrified oocytes resumed meiosis at higher rates when cultured with 2 days old granulosa cells (p = .03), but full maturation rates slightly raised when granulosa cells were cultured longer, albeit without differences with the control group. This study paved the road for the creation of enriched culture systems in the domestic cat, but innovations are strongly needed for vitrified oocytes that deserve better chances to develop in vitro.

Blocking connexin channels during vitrification of immature cat oocytes improves maturation capacity after warming

In the domestic cat, nuclear maturation and embryo development after vitrification of immature oocytes have been obtained but developmental competence after warming remains low. It has been reported that during folliculogenesis, the association and communication between the oocyte and the surrounding cumulus cells through connexin-based gap junctions is essential for normal oocyte and follicular development. Gap junctions result from the head-to-head interaction of two hemichannels; however, there is always a population of hemichannels not incorporated into gap junctions. These unopposed hemichannels are normally closed but may open under certain stress conditions, potentially also during vitrification and warming, turning them into toxic pores inducing cell injury and cell death. The aim of our study was to test whether inhibiting connexin 37 (Cx37) and connexin 43 (Cx43) channels with the connexin-targeting peptide Gap26 during vitrification and warming of cat immature cumulus-oocyte-complexes (COCs) could improve oocyte maturation and competence of resultant blastocysts derived by parthenogenetic activation. In the first experiment, our immunostainings confirmed the presence of Cx43 protein in the cytoplasm of immature cat oocytes and in the plasma membranes of cumulus cells. In the second experiment, COCs were randomly divided in three different groups: a control group (control), a group vitrified without Gap26 (vitrified) and a group vitrified with Gap26 (vitrified-peptide). The maturation rate was checked and oocytes from all three different experimental groups were parthenogenetically activated and cultured in vitro until day 8. After vitrification and warming, 49% of the oocytes in the control group matured, while this was 8% and 19% in the vitrified and vitrified-peptide groups, respectively. Compared to the vitrified group, oocytes in the vitrified-peptide group had significantly larger maturation rates. No blastocysts were detected at day 8 in the vitrified group, while 2% and 13% of the oocytes further developed to blastocyst at day 8 in the vitrified-peptide and control non-vitrified group, respectively. We conclude that the use of Gap26 in vitrification and warming media to vitrify immature cat oocytes improves maturation success and allows such oocytes to reach the blastocyst stage (2%) at day 8 after parthenogenetic activation.

Vitrification of domestic cat oocytes--effect on viability and integrity of subcellular structures

Cryopreservation of female gametes is a crucial part of assisted reproduction techniques. The domestic cat is a model for both wild felids and human research. The aim of this study was to evaluate the effect of different vitrification protocols applied to feline oocytes at different maturational stages on the preservation of oocyte viability and integrity of subcellular structures. The vitrification solutions consisted of 20% ethylene glycol, 20% DMSO, 20% FCS, and 1.5 m Trehalose with and without 10% Ficoll PM-70. Markers for cell viability (PI/FDA), cytoskeleton organization (Anti-α-Tubulin-FITC antibody, Phalloidin-TRITC), as well as nuclear configuration (DAPI) were used for evaluation of vitrified-warmed oocytes. Our results show that 52% and 41% of live mature and immature oocytes, respectively and until 32% of microtubules, 28% of nuclear configuration and 36% of microfilaments in the normal pattern can be obtained with protocol described in this paper. According to our data, Ficoll PM-70 essentially improves the oocytes survival upon vitrification.

Cryosurvival of ex situ and in situ feline oocytes

Cryosurvival of feline oocytes preserved as isolated cells (ex situ) or enclosed in ovarian follicles (in situ) has been demonstrated, and significant advances have recently been achieved. However, an ideal protocol for oocyte cryopreservation has not been established to date because of extreme sensitivity of the structural complex to chilling injury. Several factors, such as stage of maturation, membrane permeability and plasticity of the cytoskeleton, affect cryosurvival of the oocyte. Also, intercellular communications between cumulus cells and oocyte are compromised after freezing or vitrification of ex situ or in situ cumulus-oocyte complexes, which has a detrimental effect on oocyte maturational competence. Despite these issues, embryo development, pregnancies and live kittens have been obtained after in vitro fertilization (by ICSI) and transfer of embryos derived from cryopreserved oocytes. It is a general belief that the efficiency of cryopreservation would increase through a better understanding of oocyte responses to cryoprotectants, cooling rates and all the physical events occurring during the exposure of feline oocytes to low temperatures. Cryobanking of feline oocytes would significantly contribute to the preservation of rare genotypes and to the maintenance of a valuable source of genetic material for research applications.