Fate of the nucleolar vacuole during resumption of cell cycle in pea cotyledonary buds

Meristematic cells of pea cotyledonary buds blocked in G(0-1) state contain a small nucleolus with a large central clear area surrounded by a fibrilla r rim. The nucleolar structure varies according to the cell cycle from the G(0-1)-blocked state until the first mitoses occurring between 24 and 27 h after removal of the main stem. In order to better identify and understand the role of the central area in the nucleolar function, its content was inv estigated by cytochemical and terminal deoxynucleotidyl transferase-immunog old methods. The central area showed the characteristics of a vacuole commo nly constituted of the condensed chromatin, ribonucleoprotein granules, and lack of argyrophilic proteins. 3 h alter decapitation, a thickening of the fibrillar rim occurred, accompanied by an increase of granules in the vacu ole. After 6 h, the unique vacuole broke up into two to four small vacuoles in which the granules are more abundant. After 12 h the nucleolus acquired compact structure with few minute vacuoles dispersed over the fibrillar co mponent. During the whole cell cycle, the condensed chromatin is always obs erved in the vacuole. Our findings suggest that the appearance of the vacuo les is subsequent to the output of preribosomes from nucleolus. These vacuo les might play a role in condensation and decondensation of the chromatin.