Morphological and cytochemical changes during nuclear maturation in Bufo arenarum oocytes

The morphological and cytochemical changes undergone by the oocyte during progesterone-induced nuclear maturation were analyzed in fully grown follicle oocytes of Bufo arenarum during the breeding period. The distribution and the staining properties of the nuclear material and of the perinuclear cytoplasm were studied. Our observations indicate that, during this complex process, the germinal vesicle (GV) undergoes migration and dissolution. The nuclear sap, which in the control has an homogeneous aspect, during maturation adopts a fibrillar appearance in the basal zone, showing basophilia and positive reaction to PAS, Feulgen and Methyl green-Pyronin staining. When the GV breakdown occurs, the nuclear contents become mixed with the cortical cytoplasm, developing a different cytoplasmic mass that disappears about 12 h after the beginning of incubation. The perinuclear cytoplasm exhibits metachromasia and orthochromasia with Toluidine Blue at pH 5.6 and a strong basophilia with PAS, suggesting the existence of acid compounds. The presence of nucleic acids such as RNA and DNA is a strong possibility because of the positive reaction to Methyl green-Pyronin, Feulgen and DAPI. This cytoplasmic mass remains until the end of the meiotic resumption process.