Fine structure of the nucleolus in normal and mutant Xenopus embryos.

Mutant and normal Xenopus laevis embryos (o-nu, i-nu, 2-nu) were examined in the electron microscope after glutaraldehyde and/or osmium-tetroxide fixation. During cleavage both o-nu and wild-type embryos contain multiple small nucleolar bodies, less than 1 ft in diameter, composed mainly of a fibrous material. By the end of cleavage or beginning of gastrulation, granular caps develop on the fibrous nucleolar bodies. In 1and 2-nu cells, the multiple nucleolar bodies are replaced during gastrula and neurula stages by definitive nucleoli (2-5 ft in diameter) which contain abundant small (150 A) granules intermingled with fibrous material. In o-nu cells, one or two pseudonucleoli (1-3 ft in diameter) appear at about the same time that definitive nucleoli develop in wild-type cells. The multiple small nucleolar bodies disappear as the pseudonucleoli enlarge. Pseudonucleoli differ from definitive nucleoli in having a much smaller amount of the granular component, which is located as a cap on the periphery of the fibrous component and not mingled with it. The granular component of the o-nu pseudonucleoli, however, is not distinguishable in its fine structure from the same component of normal nucleoli. In many o-nu tadpoles at stage 41, the granular component of the nucleolus is entirely absent and the fibrillar component is very prominent. Both granular and fibrous components of the o-nu pseudonucleoli contain RNA as judged by RNase sensitivity and staining affinity for basic dyes.