Morphologic and functional studies of immature rat oocyte-cumulus complexes after cryopreservation

The effects of freezing and thawing (F/T) on functional activity of immature rat oocyte-cumulus complexes (OCC) were studied. The OCC were divided into three groups according to the number of cumulus-cell layers surrounding them. The OCC were then frozen and thawed (F/T), with dimethyl sulfoxide (DMSO) as cryoprotectant. The survival rates after thawing increased significantly (P less than 0.001) as the number of cumulus-cell layers increased. Germinal vesicle breakdown (GVBD) was evaluated in F/T oocytes. After 2 hours, significantly (P less than 0.05) fewer oocytes demonstrated GVBD than did those in the control group. There was no difference, however, after 4 hours of culture. A significant (P less than 0.05) decrease in follicle-stimulating hormone (FSH)-dependent cyclic adenosine monophosphate (cAMP) accumulation was observed in the F/T group. However, the amount of cAMP produced was sufficient to maintain the oocyte in meiotic arrest. There was a borderline significant decrease of the coupling between cumulus cells and the oocyte in F/T OCC, as evaluated by the transport of 3H-uridine into the oocyte. It is concluded that immature rat oocytes can be successfully cryopreserved when they are surrounded by five or more layers of cumulus cells. FSH responsiveness and intercellular communication were essentially maintained. There was a slight delay in GVBD, which needs further clarification.