Effects of actinomycin D and cycloheximide on the nucleolar ultrastructure of porcine oocytes.

In early antral follicles 0.5 mm in diameter, pig oocytes were intensively engaged in rRNA synthesis. Actinomycin D caused degranulation of their nucleoli which became entirely compact; the fibrillar centers that remained adjacent to the nucleolar surface also increased in size. In contrast, oocytes from preantral follicles treated under the same conditions failed to complete nucleolar compaction and were engaged in a special process of vacuolation. The rate of [3H]-uridine incorporation, evaluated by light-optical autoradiography, indicated that, even at high concentrations, cycloheximide impaired nucleolar transcriptional activity of the oocytes without suppressing it. The nucleoli of such treated oocytes were either unchanged or incompletely transformed, fibrillo-granular regions remaining adjacent to large compact areas. This study provides evidence that the process of nucleolar compaction in pig oocytes depends directly on the inhibition of rRNA synthesis. This process, which only occurs in oocytes from antral follicles, might represent an important step in the acquisition of oocyte capacity to initiate meiotic maturation.