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This study aimed to determine the effect of presence of the corpus luteum (CL) and its influence on cumulus–oocyte complexes (COCs) obtained from the ipsilateral or contralateral ovary in bovine on the recovery and capacity of the oocytes to sustain mono-spermic fertilization, undergo preimplantation development, and develop to the blastocyst stage. Ovaries were collected at a local slaughterhouse and kept in pairs corresponding to the same animal. In the first experiment the variables evaluated were compared between cows with (CCL+) and without (CCL-) CL, and for the second experiment, comparisons were made between ovaries with an ipsilateral (CL+), contralateral (CL−), and no (NCL). The recovery rate of COCs was higher in ovaries from CCL− cows, and a higher proportion of grade 1 COCs were recovered from this group. A higher proportion of metaphase I oocytes at 7 h of maturation, and a higher rate of cleavage were observed in the CCL+ group; however, a higher proportion of embryos were obtained from the CCL− group. Besides, COCs from the CL+ group had a lower proportion of grades 1 and 2 morphological qualities, lower rate of metaphase II oocytes at 22 h of maturation, and lower rate of formation of two pronuclei, whereas a higher proportion of unfertilized oocytes after in vitro fertilization. On the other hand, the COCs from the CL− group displayed a lower proportion of oocytes with more than two pronuclei, higher cleavage rate, and higher final blastocyst production were obtained when compared to CL+. Thus, the effects of CL on the competence of bovine COCs are different depending on the anatomical proximity of their location in the animal, negatively affecting the quality of COCs located in the same ovary, but not having negative effects on the competence of COCs in the ovaries contralateral to their location. © The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Incluye referencias bibliográficas.

Background: The brilliant cresyl blue (BCB) staining is a non-invasive test to select the best-suited oocytes for embryonic development. This makes it a useful tool to select best-quality oocytes at the times of the year when there is forage restriction. Objective: To evaluate the effect of seasonality on the nuclear maturation and quality of oocytes selected by the BCB test. Methods: The cumulus-oophorus complexes (COCs) were obtained in summer and winter of 2010 and 2011. Selected COCs were maintained for 90 min at 38.5 °C in a CO 2 incubator, in TCM 199 medium containing 10% fetal bovine serum and antibiotics, and supplemented with 26 μM brilliant cresyl blue. Afterwards, they were divided according to the ooplasm staining (BCB+ —blue; BCB− —unstained). Subsequently, COCs were matured for 22 h. Nuclear maturation was evaluated at 22 h of culture. Results: The proportion of BCB− oocytes was higher in the winter of 2010, but there was no increase in this group in the winter of 2011. The percentage of oocytes that reached metaphase II was higher in control and BCB+ groups in relation to oocytes from BCB− group. Conclusion: The season of the year influences the percentage of oocytes best suited for embryonic production in situations in which oocyte donors receive pasture-based feeding, since the method was effective in determining the effect of seasonality on the competence of bovine oocytes to reach nuclear maturation. Keywords: colorimetric test , cumulus-oophorus complexes , in vitro maturation , pasture , seasons. Resumen Antecedentes: La tincion con azul cresil brillante (BCB) es un metodo no invasivo para seleccionar ovocitos aptos para el desarrollo embrionario. Por tanto, es una herramienta util para selecionar los ovocitos de mejor calidad en temporadas de restriccion de forraje. Objetivo: Evaluar el efecto de la estacionalidad sobre la maduracion nuclear y calidad de los ovocitos seleccionados por el test BCB. Metodos: Los complejos cumulus-oophorus (CCOs) fueron obtenidos durante el verano y el invierno de 2010 y 2011. Los CCOs seleccionados se mantuvieron durante 90 min a 38,5 oC en una incubadora de CO 2 en un medio TCM 199 con 10% de suero fetal bovino y antibioticos, suplementado con 26 μM de azul cresil brillante. Luego se separaron segun el color del citoplasma (BCB+ —azul y BCB− —incoloro). Posteriormente, los CCOs se maduraron durante 22 h. La evaluacion de la maduracion nuclear se realizo a las 22 h de cultivo. Resultados: La proporcion de ovocitos BCB− fue mayor en el invierno de 2010, pero no hubo un aumento de ese grupo en el invierno de 2011. El porcentaje de ovocitos que alcanzaron la etapa de metafase II fue mayor en el grupo control y BCB+ con respecto al grupo BCB−. Conclusion: La estacion del ano influye en el porcentaje de ovocitos mas aptos para la produccion de embriones en situaciones donde las donadoras de ovocitos reciben alimentacion a base de pastos, ya que este metodo fue eficaz para determinar el efecto de la estacionalidad en la competencia de ovocitos bovinos en alcanzar la maduracion nuclear. Palabras clave: complejos de cumulus-oophorus , estaciones , maduracion in vitro , pastura , test colorimetrico. Resumo Antecedentes: O metodo do azul cresil brilhante (BCB) nao e invasivo e seleciona ovocitos mais aptos ao desenvolvimento embrionario. Portanto e ferramenta util para selecionar ovocitos de melhor qualidade em epocas do ano que ocorre restricao de pastagem. Objetivo: Avaliar o efeito da sazonalidade sobre a maturacao nuclear e a qualidade dos ovocitos selecionados pelo teste BCB. Metodos: Os complexos cumulus oophorus (CCOs) foram obtidos no verao e inverno de 2010 e 2011. Os CCOs selecionados foram mantidos por 90 min, a 38,5 °C, em incubadora de CO 2 , em meio TCM 199 contendo 10% de soro fetal bovino e antibioticos, e suplementado com 26 μM de azul cresil brilhante. Em seguida, estes foram divididos de acordo com a coloracao do citoplasma (BCB+ —azuis e BCB− —incolores). Entao os CCOs foram maturados durante 22 h. A avaliacao da maturacao nuclear foi realizada as 22 h de cultivo. Resultados: A proporcao dos ovocitos BCB− foi maior no inverno de 2010, mas nao houve aumento desse grupo no inverno de 2011. O percentual de ovocitos que atingiu o estagio de metafase II foi maior no controle e no grupo BCB+ em relacao ao grupo BCB−. Conclusao: A estacao do ano influencia o percentual de ovocitos mais aptos a producao de embrioes, em situacoes onde as doadoras de ovocitos recebem alimentacao baseada em pastagens, ja que o metodo se mostrou efetivo para determinacao do efeito da sazonalidade sobre a competencia de ovocitos bovinos em atingirem a maturacao nuclear. Palavras chave: complexos de cumulus-oophorus , estacoes do ano , maturacao in vitro , pastagem , teste colorimetrico.