192 PREMATURATION OF BOVINE CUMULUS-OOCYTE COMPLEXES WITH CYCLIC ADENOSINE MONOPHOSPHATE MODULATORS AFFECTS BOTH OOCYTE AND BLASTOCYST ULTRASTRUCTURE

Abstract

Oocytes resume meiosis spontaneously when subjected to in vitro maturation (IVM). Cyclic adenosine monophosphate (cAMP) elevating agents have been used for artificial blocking of meiotic resumption (pre-IVM) to allow the oocyte to prepare for maturation, potentially increasing its developmental competence. However, the ultrastructural effects of this pharmacological approach on oocytes and embryos remain to be addressed. We assessed the effects of pre-IVM with cAMP modulators in oocytes (10 for each group) at the end of IVM and in blastocyst (10 for each group) after 7 days of culture. Cumulus-oocyte complexes (COC) were subjected to pre-IVM for 2 h with forskolin (Sigma, St. Louis, MO, USA; 100 μM) and 3-isobutyl-1-methylxanthine) (IBMX, Sigma, 500 μM) followed by 24 h of IVM with FSH-enriched media (IVF Vet Solutions, Adelaide, Australia). Simultaneously, another group of COC was subjected to conventional IVM (con-IVM) for 24 h (EmbryoTransBiotech, Copenhagen, Denmark) with BSA (4 mg mL^–1, Sigma), gentamycin (50 mg mL^–1), and FSH (0.1 IU mL^–1). Matured oocytes were collected for qualitative ultrastructural analysis or followed to IVF. The morphology was carefully evaluated on serially sectioned oocytes and embryos, where each serial section (~60.2-μm section per oocyte/embryo) was analysed under light microscopy. Subsequently, the equatorial section from each oocyte and the section giving the optimal representation of the inner cell mass in each blastocyst was re-embedded and sectioned for electron microcopy as previously described (Hyttel and Madsen 1987 Acta Anat. 129, 12–14). Blastocyst rates did not differ between groups. Ultrastructural analyses revealed subtle ultrastructural differences between pre-IVM and con-IVM conditions. In both groups, oocytes had matured to metaphase II. The perivitelline space of pre-IVM oocytes was significantly narrower than con-IVM. The cytoplasmic vesicles were more abundant and globally distributed in pre-IVM oocytes, whereas at con-IVM a vesicle-free periphery of the ooplasm was frequent, except for cortical granules and clusters of mitochondria associated with smooth endoplasmic reticulum (SER). We observed typical hooded mitochondria and cortical granules either clustered in the periphery or solitarily distributed in the cortical ooplasmic region for both groups. In the blastocysts, differences were noted with respect to especially distribution of ribosomes. In pre-IVM blastocysts, ribosomes were mostly organised in free clusters (polysomes) and peripherally located in cells of the inner cell mass. Con-IVM blastocysts showed ribosomes preferentially associated with the rough ER and often associated with mitochondria. Lipid droplets and rounded mitochondria were observed in both groups as well as apically located tight junctions and desmosomes between adjacent trophectoderm (TE) cells. Pleomorphic and elongated mitochondria were abundant in the TE of pre-IVM blastocysts, whereas the mitochondrial population was more homogenous at con-IVM. These findings suggest that pre-IVM for 2 h affects oocyte and blastocyst ultrastructure.

Research was supported by grants 12/50533-2 and 12/23409-9 from FAPESP.