Nucleolus Organizer Region Functioning in Chicken Growing Oocytes: A Revision of the Existing Views

It is believed that the nucleolus organizer region (NOR) in the ovary of adult birds is inactivated at all stages of oocyte growth. However, in the ovary of juvenile chicken Gallus g. domesticus, one or two nucleoli were found in half of the oocytes of the early diplotene stage, which functioned up to the stage of developed lamp brushes. It was assumed that diplotenic oocytes with inactivated NOR in a chick are preserved as a reserve for development in the ovary of an adult female. Such a discrepancy in the functional activity of NOR in chicken oocytes and an adult female required special studies. New results of comparative analysis of the nucleolus in oocytes in chickens and in mature hens are presented here. It is shown for the first time that, in the oocytes of laying hens, the nucleolus appears at the early diplotene stage and the pre-rRNA synthesis continues up to the lampbrush stage. At the lampbrush stage, the NOR is inactivated. The nucleolus disintegrates into large fragments containing both pre-rRNA and fibrillarin. Then these larger fragments break down into small granules that do not contain pre-rRNA but contain fibrillarin. The results resolve the above contradiction and demonstrate a similar pattern of NOR functioning in oocytes of adult and juvenile birds.