The relationship between nuclear maturation and cortical granule distribution in cultured porcine oocytes

The state of nuclear maturation and the state of cortical granule (CG) distribution were observed in porcine oocytes cultured for various periods and also in those treated with olomoucine, an inhibitor of cyclin-dependent kinase, and the relationship between nuclear maturation and changes in CG distribution was investigated. CGs were distributed over the cortical cytoplasm (type I) in 98% of porcine oocytes soon after collection and in 90% of those cultured for 8 hrs after collection, and nuclei of these oocytes were all in the germinal vesicle (GV) stage. When cultured for 22 hrs, type I oocytes markedly decreased, while oocytes having CGs in the cortical cytoplasm and also immediately beneath the plasma membrane (type II) increased to 22%, and oocytes having CGs immediately beneath the plasma membrane (type III) appeared at 72%. Nuclei of type II oocytes were in the GV to M I stages, while most nuclei of type III oocytes were in the M I stage. When cultured for 32 and 44 hrs, type III oocyte populations were observed as 70 and 87% of the whole, respectively, and nuclei of these type III oocytes were all in the MI to MII stages. In the oocytes cultured for 22 hrs with olomoucine, nuclei were all in the GV stage, and 83% of the treated oocytes showed the type I distribution of CGs. On the other hand, 91% of control oocytes showed the type II and type III distribution of CGs, and type I oocyte populations were observed only as 10% of the whole. From these results, it was inferred in porcine oocytes that the start and the completion of CG movement into the cytoplasm immediately beneath the plasma membrane are closely related to the time at which GV breaks down and nuclei reach the M I stage, respectively.