TRANSCRIPTIONAL REGULATION OF OOCYTE DEVELOPMENT, FERTILIZATION AND INITIAL EMBRYONIC-DEVELOPMENT IN CATTLE

At the end of the oocyte growth phase the nucleoli of the oocyte, whic h until then are reticulated fibrillogranular, develop a large vacuole and become compact fibrillar. A concommitant decrease in rRNA and pre sumptive hnRNA transcription is seen. Before the onset of final oocyte maturation the compact fibrillar nucleoli become re-vacuolated and tr anscription of hnRNA occurs up to about 2 h after initiation of final maturation- At oocyte nucleus breakdown the nucleoli disperse, but fol lowing fertilization compact fibrillar nucleoli are reassembled in the pronuclei. At the 2-cell stage transcription of alpha-amanitin insens itive RNA is initiated, and at the 4-cell stage hnRNA is transcribed. At the 4- to 8- cell stage the compact fibrillar nucleoli are vacuolat ed again, and at the late 8-cell stage they are transformed into retic ulated fibrillo-granular nucleoli concommitant with the activation of rRNA-transcription.