Ultrastructural distribution of DNA within the ring-shaped nucleolus of human resting T lymphocytes.

The precise distribution of DNA within the resting human T lymphocyte nucleolus has been investigated, at the ultrastructural level, by cytochemical and molecular immunocytochemical techniques. The nucleolus is partially enveloped by a layer of condensed chromatin which, at several places, penetrates into the nucleolar body until in close contact with the fibrillar centers. Morphometric analysis reveals that 32% of the fibrillar center surface is essentially occupied by condensed chromatin. Using the in situ terminal deoxynucleotidyl transferase-immunogold procedure for detecting DNA, we further show that evident label is exclusively found over the condensed chromatin and over the fibrillar centers, whereas no significant label is detected over the dense fibrillar component of the nucleolus.