Nucleolar fibrillar centers in mouse spermatid nucleoli.

The nucleoli of young spermatids of mice are described. They exhibit a very special shape resembling a "padlock" in which three different areas can be distinguished: (a) a compact zone corresponding to the fibrillar component, (b) the granular component and (c) a fibrillar center of low density. Fibrillar and granular components usually appear segregated. This nucleolus has been reconstructed based on serial sectioning. When the silver impregnation technique is employed, both fibrillar and granular components show a positive reaction, although the fibrillar center is free of granules. The morphology of the fibrillar center seems to be similar to that reported in other cells. The possibility that these fibrillar centers correspond to the nucleolar organizer is discussed.