IMMUNOCYTOCHEMICAL LOCALIZATION OF ACTIN IN THE NUCLEOLUS OF RAT OOCYTES

In order to determine the localization of actin, growing and fully gro wn rat oocytes were immunocytochemically examined using a post-embeddi ng ultrastructural protein-A gold technique. In quiescent oocytes, the nucleoplasm showed slightly lower levels of actin signal when compare d to the surrounding cytoplasm. The highest levels of labeling were fo und on nucleoli showing a reticular type morphology. In oocytes at the diakinesis stage in which nucleolar compaction had occurred, the leve ls of labeling increased by 5-6 times those found in quiescent oocytes . Except for conspicuous accumulation of actin under the plasma membra ne, compact nucleoli had significantly higher levels of labeling when compared with those found on the general cytoplasm, while the nucleopl asm with homogeneously dispersed chromatin showed significantly lower levels of associated actin signal than the general cytoplasm. In oocyt es at metaphase I, the cytoplasmic region had comparable or lower leve ls of labeling than the cytoplasm of oocytes at diakinesis. The meioti c spindle embedded in material with medium electron density showed a s imilar level of labeling as the surrounding cytoplasm. On the other ha nd, significantly higher levels of associated actin were observed on t he chromosomes of metaphase I. The actin signals were dispersed over t he chromosomes and not concentrated on a specific region. These result s suggest that nuclear actin may be involved in the process of chromos ome construction and also the formation of the compacted structure of the nucleolus.