Implications for the Function-Structure Relationship in the Nucleolus After Immunolocalization of DNA in Onion Cells

Abstract

The architecture of the nucleolus in onion proliferating meristematic root cells is made up of the same structural components commonly described in other cell systems: fibrillar centers (FCs), dense fibrillar component (DFC), granular component (GC) and nucleolar matrix as basic and constant components, as well as two types of nucleolar vacuoles which appear occasionally, associated with certain stages of the cell cycle. However, the arrangement and morphometrical parameters of these components are somewhat different from the most widely known nucleolar model, that of mammalian cells. The active nucleolus of onion cells has many small l, discrete FCs immersed in an abundant and compact DFC which, in sections, may appear as several masses surrounded, in turn, by the GC Furthermore, in low-active stages, FCs are larger and contain some inclusions of condensed chromatin; they are called heterogeneous FCs (see Risueno and Medina, 1986, for review). The main cause for these differences is the number of ribosomal genes from which the nucleolus is formed, around 35 times more in the onion than in mammals (see Hadjiolov, 1985). All these genes are capable of being transcribed, though some of them may be transitorily locked for transcription when the rate of nucleolar synthesis drops. These locked genes account for the condensed chromatin inclusions in heterogeneous FCs (Risueno and Medina, 1986). Thus, nucleolar chromatin can be found, in onion cells, in three different morphofunctional states: a) locked for transcription; b) unlocked, but non—transcribing; and c) active in pre—rRNA synthesis.