Meiotic Development of Canine Oocytes from Poly-Ovular and Mono-Ovular Follicles after In Vitro Maturation

SIMPLE SUMMARY: Mammalian ovaries contain many oocytes that are enclosed in different follicles, commonly one oocyte in one follicle. Poly-ovular follicles are those with more than one oocyte within a single follicle. This type of follicle has been observed in many mammals, with an especially high incidence in canines, representing an intriguing condition. The quality of oocytes from poly-ovular follicles is heterogeneous, as different oocyte morphologies have been observed. Oocyte meiotic potential has been a key factor in determining the success rate of the in vitro maturation process in canines because of the mixed population of oocytes collected from follicles at different stages of development and at random phases of the estrus cycle. Several oocytes may be derived from poly-ovular follicles without knowledge of their meiotic capacity in vitro. Therefore, to improve in vitro maturation and reproductive biotechnologies in canines, evaluating the maturity potential of oocytes according to follicle type and estrous cycle is necessary. Our study demonstrated that the efficiency of in vitro maturation of oocytes retrieved from poly-ovular or mono-ovular follicles depends on the follicle type according to the phases of the estrous cycle during collection, which is important to consider when improving in vitro maturation protocols in this species. ABSTRACT: Poly-ovular follicles are defined as those with more than one oocyte present in single follicles. The occurrence frequency of this follicle type is higher in canines than that in other species. This study aimed to evaluate the in vitro meiotic maturation of dog oocytes from this follicle type in comparison to those from mono-ovular follicles of various sizes (small antral, medium antral, and large antral) considering different phases of the estrus cycle (anestrus, proestrus, estrus, and diestrus). Canine oocytes were obtained separately from the poly-ovular and mono-ovular antral follicles from the ovaries of adult females. In each experimental replicate, cumulus-oocyte complexes (COCs) from poly-ovular and mono-ovular follicles were incubated in supplemented TCM-199 at 38.5 °C and 5% CO(2) for 72 h. After culturing, the meiotic development of each oocyte was evaluated using epifluorescence microscopy. Meiotic stages were classified into germinal vesicle (GV), germinal vesicle breakdown (GVBD), first metaphase (MI), and second metaphase (MII). Data were evaluated using an analysis of variance. Oocytes from poly-ovular follicles at all phases exhibited a higher (p < 0.05) percentage of oocytes arrested at the GV stage than those from mono-ovular follicles, showing the highest rate of GV in small antral follicles during anestrus. In contrast, there were no differences in MII rates (p < 0.05) in oocytes from mono-ovular and poly-ovular follicles during the estrus and diestrus phases in all sizes evaluated, with the highest MII rate in estrus. These results suggest that oocytes from poly-ovular follicles can resume meiosis at a slower rate than those from mono-ovular follicles; however, the maturation in vitro of such oocytes is possible. Furthermore, the relationship between the maturation capacity of oocytes from both poly-ovular and mono-ovular follicles depends on the ovarian cycle and follicular development.