Ultrastructure, protein phosphorylation and mRNA status of equine oocytes matured in vivo and in vitro

Equine oocytes were collected by follicle aspiration in vivo or by dissection of material obtained from an abattoir, and the ultrastructure, protein phosphorylation and mRNA status of the oocytes were evaluated. Electron microscopy studies indicated that the nucleus had a smooth membrane in oocytes with a compact cumulus, whereas the nuclear membrane was undulated in all other groups. Oocytes with compact cumuli had only a few microvilli, whereas those with expanded cumuli had more microvilli. There were only small numbers of cortical granules close to the oolemma in oocytes with compact cumuli and clusters of mitochondria were in the peripheral ooplasm. The number of mitochondria and cortical granules increased in oocytes with expanded cumuli and the Golgi complexes were smaller than in other oocytes. Oocytes were observed at 10, 20 and 30 h of in vitro maturation. During maturation, the mitochondria migrated centrally and the number of cortical granules immediately below the oolemma increased progressively. Membrane-bound smooth endoplasmic reticulum became progressively less predominant. Phosphorylated proteins of molecular mass ranging from 20 to 150 kDa were found in oocytes and cumulus cells. The pattern of phosphorylated proteins was different in oocytes developed in vivo compared with oocytes cultured for 16 and 32 h in vitro. Cells of different cumulus types did not have distinct bands of phosphorylated proteins. Oocytes with compact cumuli had mainly repressed mRNAs, whereas the translationally active form was found in oocytes with expanded cumuli.