Evolution of the nucleoli during oogenesis in Xenopus laevis studied by electron microscopy.

Xenopus laevis tadpoles and toads were killed at several ages. The structure of the nuclei of the germinal cells has been observed by light and electron microscopes. We distinguish 11 successive stages in nucleolar structure: (1) a single, essentially granular nucleolus in the oogonium (10 /tm diameter), (2) a reticulated nucleolus in the leptotene oocyte, (3) fragmentation of this nucleolus into a few smaller nucleoli, (4) multiple tiny nucleoli appearing in the cap of the pachytene oocyte, (5) enrichment in the fibrillar constituent of these intra-cap nucleoli, (6) grouped spherical nucleoli, with well segregated granular and fibrillar constituents, as the disintegration of the cap is going on (diplotene A oocyte, 30 fim diameter), (7) dispersion of those nucleoli in the nuclear sap (diplotene B oocyte, 50 /im diameter), (8) formation of long, ribboned nucleoli with multiple DNA-rich spots (diplotene C oocyte, 100/im diameter), (9) fragmentation of the nucleolar ribbons into multiple spherical nucleoli with excentric fibrillar core and granular cortex (diplotene D oocyte, 150 /im diameter), (10) multiple purely fibrillar nucleoli (diplotene E oocyte, between 150 and 400/im diameter), and (11) multiple classical nucleoli with concentric fibrillar core and granular cortex (diplotene F oocyte, between 400 and 1000/im diameter).