[Changes in the ultrastructure of inactive circular nucleoli from mature rat lymphocytes at early stages of blast transformation].

We studied the ultrastructure of lymphocyte nucleoli from rat peripheral blood within the first 6 hrs of their cultivation in the presence of phytohaemagglutinin (PHA). In the mixed population of intact lymphocytes there were observed several types of nucleoli, the localization and the ratio of individual nucleolar components being different. This seems to reflect functional peculiarities of these nucleoli. Some nucleoli are characteristic of the presence of several sites of intranucleolar condensed chromatin. However, all these nucleoli could be related to the so-called ring-shaped nucleoli with a fibrillar center surrounded by fibrillar and granular RNP-components, which points out to their low synthetic activity. The major changes in the nucleoli of stimulated lymphocytes were observed after the action of PHA within 1 hr. These changes were expressed in the elevation of electron density of the fibrillar centre and the loss by it of clear borders and correct shape, in the appearance of large zones of the dense fibrillar component with acquired an increased electron density, and also in the disappearance of condensed chromatin sites. By that time, there was revealed a peculiar shape of nucleoli with a "fragmented" fibrillar centre and the presence of vacuoles and strands consisting of fibrillar and granular RNP-components. The different types of changed nucleoli in the population of stimulated lymphocytes seem to be the consequence of a gradual transition of ring-shaped nucleoli to active nucleolonema-like ones. So, it is obvious that changes in the ultrastructure of nucleoli occur already by 1 hr after the PHA action. This points out to the intensification of pre-rRNA transcription that takes place by that time.