Nucleolar fine structure and RNA synthesis in bovine oocytes from antral follicles

Nucleolar ultrastructural changes occurring in vivo in bovine oocytes during follicular growth were analyzed by electron microscopy. The rates of in vitro incorporation of 3 H‐uridine by oocytes of the same size class were evaluated by autoradiography. One to two large fibrillogranular, vacuolated nucleoli were present in oocytes from small to medium antral follicles 0.5–3 mm in diameter. These oocytes showed intense hnRNA and rRNA synthesis. The homogeneous, agranular nucleoli in oocytes from follicles 3–4 mm in diameter were composed of a compact fibrillar material. This morphological change was accompanied by an impairment of nucleolar transcriptional activity as well as by a decrease in hnRNA synthesis.