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
observed in 50% of early diplotene oocytes. These nucleoli were functioning up to the lampbrush stage. It was
assumed that diplotene oocytes with inactivated NOR are stored as a pool for development in the ovary of
adult hens. Such a contradiction in the NOR functional activity in the oocytes of juvenile and adult hens required special studies. Here we represent new results of a comparative study of the nucleoli in oocytes of
juvenile and adult hens. We show 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 getting inactivated. The nucleolus disintegrates into large fragments containing both prerRNA and fibrillarin. Then these larger fragments break down into small granules that do not contain prerRNA but contain fibrillarin. The results resolve the above contradiction and demonstrate a similar pattern
of NOR functioning in oocytes of adult and juvenile birds.