Normal Nuclear Fine Structure
Excellent reviews on nucleolar fine structure have been published in past and recent years, and it is not our intention to duplicate them. There are four distinct nucleolar components in most mammalian cells: (1) 150- to 200-A dense granules randomly dispersed in the nucleolus, (2) a loose fibrillar reticulum composed of 50- to 80-A fibrils, (3) an amorphous matrix of low electron opacity, and (4) nucleolus-associated chromatin with intranucleolar ramifications. The granular and fibrillar components are partially extracted with ribonuclease and disappear completely if digestion with the nuclease is followed by pepsin (Marinozzi and Bernhard, 1963). They can be referred to as granular and fibrillar ribonucleoproteins (RNP). The granular RNP are similar to the cytoplasmic ribosomes in their staining properties, but they are smaller, more irregular, and never arranged in subunits. Thin filaments about 20 A in width have been demonstrated in the granules; these filaments are RNase-sensitive (Smetana et al., 1968a).